A Medley of Potpourri

Mars is the fourth planet from the Sun and the second smallest planet in the Solar System, after Mercury. Named after the Roman god of war, it is often described as the "Red Planet" because the iron oxide prevalent on its surface gives it a reddish appearance.^[15] Mars is a terrestrial planet with a thin atmosphere, having surface features reminiscent both of the impact craters of the Moon and the volcanoes, valleys, deserts, and polar ice caps of Earth. The rotational period and seasonal cycles of Mars are likewise similar to those of Earth, as is the tilt that produces the seasons. Mars is the site of Olympus Mons, the second highest known mountain within the Solar System (the tallest on a planet), and of Valles Marineris, one of the largest canyons. The smooth Borealis basin in the northern hemisphere covers 40% of the planet and may be a giant impact feature.^[16]^[17] Mars has two known moons, Phobos and Deimos, which are small and irregularly shaped. These may be captured asteroids,^[18]^[19] similar to 5261 Eureka, a Martian trojan asteroid.

Until the first successful Mars flyby in 1965 by Mariner 4, many speculated about the presence of liquid water on the planet's surface. This was based on observed periodic variations in light and dark patches, particularly in the polar latitudes, which appeared to be seas and continents; long, dark striations were interpreted by some as irrigation channels for liquid water. These straight line features were later explained as optical illusions, though geological evidence gathered by unmanned missions suggest that Mars once had large-scale water coverage on its surface.^[20] In 2005, radar data revealed the presence of large quantities of water ice at the poles^[21] and at mid-latitudes.^[22]^[23] The Mars rover Spirit sampled chemical compounds containing water molecules in March 2007. The Phoenix lander directly sampled water ice in shallow Martian soil on July 31, 2008.^[24]

Mars is currently host to five functioning spacecraft: three in orbit – the Mars Odyssey, Mars Express, and Mars Reconnaissance Orbiter – and two on the surface – Mars Exploration Rover Opportunity and the Mars Science Laboratory Curiosity. Defunct spacecraft on the surface include MER-A Spirit and several other inert landers and rovers such as the Phoenix lander, which completed its mission in 2008. Observations by the Mars Reconnaissance Orbiter have revealed possible flowing water during the warmest months on Mars.^[25] In 2013, NASA's Curiosity rover discovered that Mars' soil contains between 1.5% and 3% water by mass (about two pints of water per cubic foot or 33 liters per cubic meter, albeit attached to other compounds and thus not freely accessible).^[26]

Mars can easily be seen from Earth with the naked eye, as can its reddish coloring. Its apparent magnitude reaches −3.0,^[8] which is surpassed only by Jupiter, Venus, the Moon, and the Sun. Optical ground-based telescopes are typically limited to resolving features about 300 km (186 miles) across when Earth and Mars are closest because of Earth's atmosphere.

Physical characteristics

Earth compared with Mars.

Play media

Mars - animation (00:40) showing major features.

Mars has approximately half the diameter of Earth. It is less dense than Earth, having about 15% of Earth's volume and 11% of the mass. Its surface area is only slightly less than the total area of Earth's dry land.^[7] While Mars is larger and more massive than Mercury, Mercury has a higher density. This results in the two planets having a nearly identical gravitational pull at the surface – that of Mars is stronger by less than 1%. The red-orange appearance of the Martian surface is caused by iron(III) oxide, more commonly known as hematite, or rust.^[28] It can also look butterscotch,^[29] and other common surface colors include golden, brown, tan, and greenish, depending on minerals.^[29]

Internal structure

Like Earth, this planet has undergone differentiation, resulting in a dense, metallic core region overlaid by less dense materials.^[30] Current models of the planet's interior imply a core region about 1,794 km ± 65 km (1,115 mi ± 40 mi) in radius, consisting primarily of iron and nickel with about 16–17% sulfur.^[31] This iron sulfide core is partially fluid, and it has twice the concentration of the lighter elements that exist at Earth's core. The core is surrounded by a silicate mantle that formed many of the tectonic and volcanic features on the planet, but it now appears to be dormant. Besides silicon and oxygen, the most abundant elements in the Martian crust are iron, magnesium, aluminum, calcium, and potassium. The average thickness of the planet's crust is about 50 km (31 mi), with a maximum thickness of 125 km (78 mi).^[32] Earth's crust, averaging 40 km (25 mi), is only one third as thick as Mars's crust, relative to the sizes of the two planets. The InSight lander planned for 2016 will use a seismometer to better constrain the models of the interior.

Surface geology

Mars is a terrestrial planet that consists of minerals containing silicon and oxygen, metals, and other elements that typically make up rock. The surface of Mars is primarily composed of tholeiitic basalt,^[33] although parts are more silica-rich than typical basalt and may be similar to andesitic rocks on Earth or silica glass. Regions of low albedo show concentrations of plagioclase feldspar, with northern low albedo regions displaying higher than normal concentrations of sheet silicates and high-silicon glass. Parts of the southern highlands include detectable amounts of high-calcium pyroxenes. Localized concentrations of hematite and olivine have also been found.^[34] Much of the surface is deeply covered by finely grained iron(III) oxide dust.^[35]^[36]

Mars - Geologic Map
(USGS; 14 July 2014)
(full / video).^[37]^[38]^[39]

Although Mars has no evidence of a current structured global magnetic field,^[40] observations show that parts of the planet's crust have been magnetized, and that alternating polarity reversals of its dipole field have occurred in the past. This paleomagnetism of magnetically susceptible minerals has properties that are very similar to the alternating bands found on the ocean floors of Earth. One theory, published in 1999 and re-examined in October 2005 (with the help of the Mars Global Surveyor), is that these bands demonstrate plate tectonics on Mars four billion years ago, before the planetary dynamo ceased to function and the planet's magnetic field faded away.^[41]

During the Solar System's formation, Mars was created as the result of a stochastic process of run-away accretion out of the protoplanetary disk that orbited the Sun. Mars has many distinctive chemical features caused by its position in the Solar System. Elements with comparatively low boiling points, such as chlorine, phosphorus, and sulphur, are much more common on Mars than Earth; these elements were probably removed from areas closer to the Sun by the young star's energetic solar wind.^[42]

After the formation of the planets, all were subjected to the so-called "Late Heavy Bombardment". About 60% of the surface of Mars shows a record of impacts from that era,^[43]^[44]^[45] while much of the remaining surface is probably underlain by immense impact basins caused by those events. There is evidence of an enormous impact basin in the northern hemisphere of Mars, spanning 10,600 km by 8,500 km, or roughly four times larger than the Moon's South Pole – Aitken basin, the largest impact basin yet discovered.^[16]^[17] This theory suggests that Mars was struck by a Pluto-sized body about four billion years ago. The event, thought to be the cause of the Martian hemispheric dichotomy, created the smooth Borealis basin that covers 40% of the planet.^[46]^[47]

The geological history of Mars can be split into many periods, but the following are the three primary periods:^[48]^[49]

This Mars rock revealed its bluish-gray interior to Mars Science Laboratory^[50]

Noachian period (named after Noachis Terra): Formation of the oldest extant surfaces of Mars, 4.5 billion years ago to 3.5 billion years ago. Noachian age surfaces are scarred by many large impact craters. The Tharsis bulge, a volcanic upland, is thought to have formed during this period, with extensive flooding by liquid water late in the period.
Hesperian period (named after Hesperia Planum): 3.5 billion years ago to 2.9–3.3 billion years ago. The Hesperian period is marked by the formation of extensive lava plains.
Amazonian period (named after Amazonis Planitia): 2.9–3.3 billion years ago to present. Amazonian regions have few meteorite impact craters, but are otherwise quite varied. Olympus Mons formed during this period, along with lava flows elsewhere on Mars.

Some geological activity is still taking place on Mars. The Athabasca Valles is home to sheet-like lava flows up to about 200 Mya. Water flows in the grabens called the Cerberus Fossae occurred less than 20 Mya, indicating equally recent volcanic intrusions.^[51] On February 19, 2008, images from the Mars Reconnaissance Orbiter showed evidence of an avalanche from a 700 m high cliff.^[52]

Soil

Exposure of silica-rich dust uncovered by the Spirit rover

The Phoenix lander returned data showing Martian soil to be slightly alkaline and containing elements such as magnesium, sodium, potassium and chlorine. These nutrients are found in gardens on Earth, and they are necessary for growth of plants.^[53] Experiments performed by the Lander showed that the Martian soil has a basic pH of 8.3, and may contain traces of the salt perchlorate.^[54]^[55]

Streaks are common across Mars and new ones appear frequently on steep slopes of craters, troughs, and valleys. The streaks are dark at first and get lighter with age. Sometimes, the streaks start in a tiny area which then spread out for hundreds of metres. They have also been seen to follow the edges of boulders and other obstacles in their path. The commonly accepted theories include that they are dark underlying layers of soil revealed after avalanches of bright dust or dust devils.^[56] Several explanations have been put forward, some of which involve water or even the growth of organisms.^[57]^[58]

Hydrology

Microscopic photo taken by Opportunity showing a gray hematite concretion, indicative of the past presence of liquid water

Liquid water cannot exist on the surface of Mars due to low atmospheric pressure, except at the lowest elevations for short periods.^[59]^[60] The two polar ice caps appear to be made largely of water.^[61]^[62] The volume of water ice in the south polar ice cap, if melted, would be sufficient to cover the entire planetary surface to a depth of 11 meters.^[63] A permafrost mantle stretches from the pole to latitudes of about 60°.^[61]

Large quantities of water ice are thought to be trapped within the thick cryosphere of Mars. Radar data from Mars Express and the Mars Reconnaissance Orbiter show large quantities of water ice both at the poles (July 2005)^[21]^[64] and at mid-latitudes (November 2008).^[22] The Phoenix lander directly sampled water ice in shallow Martian soil on July 31, 2008.^[24]

Landforms visible on Mars strongly suggest that liquid water has at least at times existed on the planet's surface. Huge linear swathes of scoured ground, known as outflow channels, cut across the surface in around 25 places. These are thought to record erosion which occurred during the catastrophic release of water from subsurface aquifers, though some of these structures have also been hypothesized to result from the action of glaciers or lava.^[65]^[66] One of the larger examples, Ma'adim Vallis is 700 km long and much bigger than the Grand Canyon with a width of 20 km and a depth of 2 km in some places. It is thought to have been carved by flowing water early in Mars' history.^[67] The youngest of these channels are thought to have formed as recently as only a few million years ago.^[68] Elsewhere, particularly on the oldest areas of the Martian surface, finer-scale, dendritic networks of valleys are spread across significant proportions of the landscape. Features of these valleys and their distribution very strongly imply that they were carved by runoff resulting from rain or snow fall in early Mars history. Subsurface water flow and groundwater sapping may play important subsidiary roles in some networks, but precipitation was probably the root cause of the incision in almost all cases.^[69]

Along crater and canyon walls, there are also thousands of features that appear similar to terrestrial gullies. The gullies tend to be in the highlands of the southern hemisphere and to face the Equator; all are poleward of 30° latitude. A number of authors have suggested that their formation process demands the involvement of liquid water, probably from melting ice,^[70]^[71] although others have argued for formation mechanisms involving carbon dioxide frost or the movement of dry dust.^[72]^[73] No partially degraded gullies have formed by weathering and no superimposed impact craters have been observed, indicating that these are very young features, possibly even active today.^[71]

Other geological features, such as deltas and alluvial fans preserved in craters, also argue very strongly for warmer, wetter conditions at some interval or intervals in earlier Mars history.^[74] Such conditions necessarily require the widespread presence of crater lakes across a large proportion of the surface, for which there is also independent mineralogical, sedimentological and geomorphological evidence.^[75] Some authors have even gone so far as to argue that at times in the Martian past, much of the low northern plains of the planet were covered with a true ocean hundreds of meters deep, though this remains controversial.^[76]

Composition of "Yellowknife Bay" rocks – rock veins are higher in calcium and sulfur than "Portage" soil – APXS results – Curiosity rover (March, 2013).

Further evidence that liquid water once existed on the surface of Mars comes from the detection of specific minerals such as hematite and goethite, both of which sometimes form in the presence of water.^[77] Some of the evidence believed to indicate ancient water basins and flows has been negated by higher resolution studies by the Mars Reconnaissance Orbiter.^[78] In 2004, Opportunity detected the mineral jarosite. This forms only in the presence of acidic water, which demonstrates that water once existed on Mars.^[79] More recent evidence for liquid water comes from the finding of the mineral gypsum on the surface by NASA's Mars rover Opportunity in December 2011.^[80]^[81] Additionally, the study leader Francis McCubbin, a planetary scientist at the University of New Mexico in Albuquerque looking at hydroxals in crystalline minerals from Mars, states that the amount of water in the upper mantle of Mars is equal to or greater than that of Earth at 50–300 parts per million of water, which is enough to cover the entire planet to a depth of 200–1,000 m (660–3,280 ft).^[82]

On March 18, 2013, NASA reported evidence from instruments on the Curiosity rover of mineral hydration, likely hydrated calcium sulfate, in several rock samples including the broken fragments of "Tintina" rock and "Sutton Inlier" rock as well as in veins and nodules in other rocks like "Knorr" rock and "Wernicke" rock.^[83]^[84]^[85] Analysis using the rover's DAN instrument provided evidence of subsurface water, amounting to as much as 4% water content, down to a depth of 60 cm, in the rover's traverse from the Bradbury Landing site to the Yellowknife Bay area in the Glenelg terrain.^[83]

Polar caps

North polar early summer ice cap (1999)

South polar midsummer ice cap (2000)

Mars has two permanent polar ice caps. During a pole's winter, it lies in continuous darkness, chilling the surface and causing the deposition of 25–30% of the atmosphere into slabs of CO₂ ice (dry ice).^[86] When the poles are again exposed to sunlight, the frozen CO₂ sublimes, creating enormous winds that sweep off the poles as fast as 400 km/h. These seasonal actions transport large amounts of dust and water vapor, giving rise to Earth-like frost and large cirrus clouds. Clouds of water-ice were photographed by the Opportunity rover in 2004.^[87]

The polar caps at both poles consist primarily of water ice. Frozen carbon dioxide accumulates as a comparatively thin layer about one metre thick on the north cap in the northern winter only, while the south cap has a permanent dry ice cover about eight metres thick.^[88] This permanent dry ice cover at the south pole is peppered by flat floored, shallow, roughly circular pits, which repeat imaging shows are expanding by meters per year; this suggests that the permanent CO₂ cover over the south pole water ice is degrading over time.^[89] The northern polar cap has a diameter of about 1,000 kilometres during the northern Mars summer,^[90] and contains about 1.6 million cubic km of ice, which, if spread evenly on the cap, would be 2 km thick.^[91] (This compares to a volume of 2.85 million cubic km (km³) for the Greenland ice sheet.) The southern polar cap has a diameter of 350 km and a thickness of 3 km.^[92] The total volume of ice in the south polar cap plus the adjacent layered deposits has also been estimated at 1.6 million cubic km.^[93] Both polar caps show spiral troughs, which recent analysis of SHARAD ice penetrating radar has shown are a result of katabatic winds that spiral due to the Coriolis Effect.^[94]^[95]

The seasonal frosting of some areas near the southern ice cap results in the formation of transparent 1-metre-thick slabs of dry ice above the ground. With the arrival of spring, sunlight warms the subsurface and pressure from subliming CO₂ builds up under a slab, elevating and ultimately rupturing it. This leads to geyser-like eruptions of CO₂ gas mixed with dark basaltic sand or dust.
This process is rapid, observed happening in the space of a few days, weeks or months, a rate of change rather unusual in geology – especially for Mars. The gas rushing underneath a slab to the site of a geyser carves a spider-like pattern of radial channels under the ice, the process being the inverted equivalent of an erosion network formed by water draining through a single plughole.^[96]^[97]^[98]^[99]

Geography and naming of surface features

A MOLA-based topographic map showing highlands (red and orange) dominating the southern hemisphere of Mars, lowlands (blue) the northern. Volcanic plateaus delimit the northern plains in some regions, while the highlands are punctuated by several large impact basins.

Although better remembered for mapping the Moon, Johann Heinrich Mädler and Wilhelm Beer were the first "areographers". They began by establishing that most of Mars's surface features were permanent and by more precisely determining the planet's rotation period. In 1840, Mädler combined ten years of observations and drew the first map of Mars. Rather than giving names to the various markings, Beer and Mädler simply designated them with letters; Meridian Bay (Sinus Meridiani) was thus feature "a".^[100]

Today, features on Mars are named from a variety of sources. Albedo features are named for classical mythology. Craters larger than 60 km are named for deceased scientists and writers and others who have contributed to the study of Mars. Craters smaller than 60 km are named for towns and villages of the world with populations of less than 100,000. Large valleys are named for the word "Mars" or "star" in various languages; small valleys are named for rivers.^[101]

Large albedo features retain many of the older names, but are often updated to reflect new knowledge of the nature of the features. For example, Nix Olympica (the snows of Olympus) has become Olympus Mons (Mount Olympus).^[102] The surface of Mars as seen from Earth is divided into two kinds of areas, with differing albedo. The paler plains covered with dust and sand rich in reddish iron oxides were once thought of as Martian "continents" and given names like Arabia Terra (land of Arabia) or Amazonis Planitia (Amazonian plain). The dark features were thought to be seas, hence their names Mare Erythraeum, Mare Sirenum and Aurorae Sinus. The largest dark feature seen from Earth is Syrtis Major Planum.^[103] The permanent northern polar ice cap is named Planum Boreum, while the southern cap is called Planum Australe.

Mars's equator is defined by its rotation, but the location of its Prime Meridian was specified, as was Earth's (at Greenwich), by choice of an arbitrary point; Mädler and Beer selected a line in 1830 for their first maps of Mars. After the spacecraft Mariner 9 provided extensive imagery of Mars in 1972, a small crater (later called Airy-0), located in the Sinus Meridiani ("Middle Bay" or "Meridian Bay"), was chosen for the definition of 0.0° longitude to coincide with the original selection.^[104]

Since Mars has no oceans and hence no "sea level", a zero-elevation surface also had to be selected as a reference level; this is also called the areoid^[105] of Mars, analogous to the terrestrial geoid. Zero altitude was defined by the height at which there is 610.5 Pa (6.105 mbar) of atmospheric pressure.^[106] This pressure corresponds to the triple point of water, and it is about 0.6% of the sea level surface pressure on Earth (0.006 atm).^[107] In practice, today this surface is defined directly from satellite gravity measurements.

Impact topography

Bonneville crater and Spirit rover's lander

The dichotomy of Martian topography is striking: northern plains flattened by lava flows contrast with the southern highlands, pitted and cratered by ancient impacts. Research in 2008 has presented evidence regarding a theory proposed in 1980 postulating that, four billion years ago, the northern hemisphere of Mars was struck by an object one-tenth to two-thirds the size of Earth's Moon. If validated, this would make the northern hemisphere of Mars the site of an impact crater 10,600 km long by 8,500 km wide, or roughly the area of Europe, Asia, and Australia combined, surpassing the South Pole–Aitken basin as the largest impact crater in the Solar System.^[16]^[17]

Fresh asteroid impact on Mars 3°20′N 219°23′E / 3.34°N 219.38°E - before/March 27 & after/March 28, 2012 (MRO).^[111]

Mars is scarred by a number of impact craters: a total of 43,000 craters with a diameter of 5 km or greater have been found.^[112] The largest confirmed of these is the Hellas impact basin, a light albedo feature clearly visible from Earth.^[113] Due to the smaller mass of Mars, the probability of an object colliding with the planet is about half that of the Earth. Mars is located closer to the asteroid belt, so it has an increased chance of being struck by materials from that source. Mars is also more likely to be struck by short-period comets, i.e., those that lie within the orbit of Jupiter.^[114] In spite of this, there are far fewer craters on Mars compared with the Moon, because the atmosphere of Mars provides protection against small meteors. Some craters have a morphology that suggests the ground became wet after the meteor impacted.^[115]

Volcanoes

Viking orbiter view of Olympus Mons

MOLA colorized shaded-relief map of western hemisphere of Mars showing Tharsis bulge (shades of red and brown). Tall volcanoes appear white.

The shield volcano Olympus Mons (Mount Olympus) is an extinct volcano in the vast upland region Tharsis, which contains several other large volcanoes. Olympus Mons is roughly three times the height of Mount Everest, which in comparison stands at just over 8.8 km.^[116] It is either the tallest or second tallest mountain in the solar system, depending on how it is measured, with various sources giving figures ranging from about 21 to 27 km high.^[117]^[118]

Tectonic sites

The large canyon, Valles Marineris (Latin for Mariner Valleys, also known as Agathadaemon in the old canal maps), has a length of 4,000 km and a depth of up to 7 km. The length of Valles Marineris is equivalent to the length of Europe and extends across one-fifth the circumference of Mars. By comparison, the Grand Canyon on Earth is only 446 km (277 mi) long and nearly 2 km (1.2 mi) deep. Valles Marineris was formed due to the swelling of the Tharsis area which caused the crust in the area of Valles Marineris to collapse. In 2012, it was proposed that Valles Marineris is not just a graben, but also a plate boundary where 150 km of transverse motion has occurred, making Mars a planet with possibly a two-plate tectonic arrangement.^[119]^[120]

Holes

A suspected lava-tube skylight

Images from the Thermal Emission Imaging System (THEMIS) aboard NASA's Mars Odyssey orbiter have revealed seven possible cave entrances on the flanks of the volcano Arsia Mons.^[121] The caves, named after loved ones of their discoverers, are collectively known as the "seven sisters."^[122] Cave entrances measure from 100 m to 252 m wide and they are believed to be at least 73 m to 96 m deep. Because light does not reach the floor of most of the caves, it is possible that they extend much deeper than these lower estimates and widen below the surface. "Dena" is the only exception; its floor is visible and was measured to be 130 m deep. The interiors of these caverns may be protected from micrometeoroids, UV radiation, solar flares and high energy particles that bombard the planet's surface.^[123]

Atmosphere

The tenuous atmosphere of Mars, visible on the horizon in this low-orbit photo

Mars lost its magnetosphere 4 billion years ago,^[124] so the solar wind interacts directly with the Martian ionosphere, lowering the atmospheric density by stripping away atoms from the outer layer.
Both Mars Global Surveyor and Mars Express have detected ionised atmospheric particles trailing off into space behind Mars,^[124]^[125] and this atmospheric loss will be studied by the upcoming MAVEN orbiter. Compared to Earth, the atmosphere of Mars is quite rarefied. Atmospheric pressure on the surface today ranges from a low of 30 Pa (0.030 kPa) on Olympus Mons to over 1,155 Pa (1.155 kPa) in Hellas Planitia, with a mean pressure at the surface level of 600 Pa (0.60 kPa).^[126] The highest atmospheric density on Mars is equal to that found 35 km (22 mi)^[127] above the Earth's surface. The resulting mean surface pressure is only 0.6% of that of the Earth (101.3 kPa). The scale height of the atmosphere is about 10.8 km (6.7 mi),^[128] which is higher than Earth's (6 km (3.7 mi)) because the surface gravity of Mars is only about 38% of Earth's, an effect offset by both the lower temperature and 50% higher average molecular weight of the atmosphere of Mars. The atmosphere of Mars consists of about 96% carbon dioxide, 1.93% argon and 1.89% nitrogen along with traces of oxygen and water.^[7]^[129] The atmosphere is quite dusty, containing particulates about 1.5 µm in diameter which give the Martian sky a tawny color when seen from the surface.^[130]
Methane has been detected in the Martian atmosphere with a mole fraction of about 30 ppb;^[13]^[131] it occurs in extended plumes, and the profiles imply that the methane was released from discrete regions. In northern midsummer, the principal plume contained 19,000 metric tons of methane, with an estimated source strength of 0.6 kilogram per second.^[132]^[133] The profiles suggest that there may be two local source regions, the first centered near 30°N 260°W / 30°N 260°W and the second near
0°N 310°W / 0°N 310°W .^[132] It is estimated that Mars must produce 270 tonnes per year of methane.^[132]^[134]

Methane on Mars -"potential sources and sinks" (November 2, 2012).

The implied methane destruction lifetime may be as long as about 4 Earth years and as short as about 0.6 Earth years.^[132]^[135] This rapid turnover would indicate an active source of the gas on the planet. Volcanic activity, cometary impacts, and the presence of methanogenic microbial life forms are among possible sources. Methane could also be produced by a non-biological process called serpentinization^[b] involving water, carbon dioxide, and the mineral olivine, which is known to be common on Mars.^[136]

The Curiosity rover, which landed on Mars in August 2012, is able to make measurements that distinguish between different isotopologues of methane,^[137] but even if the mission is to determine that microscopic Martian life is the source of the methane, the life forms likely reside far below the surface, outside of the rover's reach.^[138] The first measurements with the Tunable Laser Spectrometer (TLS) indicated that there is less than 5 ppb of methane at the landing site at the point of the measurement.^[139]^[140]^[141]^[142] On September 19, 2013, NASA scientists, on the basis of further measurements by Curiosity, reported no detection of atmospheric methane with a measured value of 0.18±0.67 ppbv corresponding to an upper limit of only 1.3 ppbv (95% confidence limit) and, as a result, conclude that the probability of current methanogenic microbial activity on Mars is reduced.^[143]^[144]^[145] The Mars Trace Gas Mission orbiter planned to launch in 2016 would further study the methane,^[146]^[147] as well as its decomposition products such as formaldehyde and methanol.
Ammonia was also tentatively detected on Mars by the Mars Express satellite, but with its relatively short lifetime, it is not clear what produced it.^[148] Ammonia is not stable in the Martian atmosphere and breaks down after a few hours. One possible source is volcanic activity.^[148]

Climate

Dust storm on Mars.

November 18, 2012

November 25, 2012

Opportunity and Curiosity rovers are noted.

Of all the planets in the Solar System, the seasons of Mars are the most Earth-like, due to the similar tilts of the two planets' rotational axes. The lengths of the Martian seasons are about twice those of Earth's because Mars's greater distance from the Sun leads to the Martian year being about two Earth years long. Martian surface temperatures vary from lows of about −143 °C (at the winter polar caps)^[9] to highs of up to 35 °C (in equatorial summer).^[10] The wide range in temperatures is due to the thin atmosphere which cannot store much solar heat, the low atmospheric pressure, and the low thermal inertia of Martian soil.^[149] The planet is also 1.52 times as far from the Sun as Earth, resulting in just 43% of the amount of sunlight.^[150]

If Mars had an Earth-like orbit, its seasons would be similar to Earth's because its axial tilt is similar to Earth's. The comparatively large eccentricity of the Martian orbit has a significant effect. Mars is near perihelion when it is summer in the southern hemisphere and winter in the north, and near aphelion when it is winter in the southern hemisphere and summer in the north. As a result, the seasons in the southern hemisphere are more extreme and the seasons in the northern are milder than would otherwise be the case. The summer temperatures in the south can reach up to 30 kelvins warmer than the equivalent summer temperatures in the north.^[151]

Mars also has the largest dust storms in the Solar System. These can vary from a storm over a small area, to gigantic storms that cover the entire planet. They tend to occur when Mars is closest to the Sun, and have been shown to increase the global temperature.^[152]

Orbit and rotation

Mars is about 143 million miles from the Sun; its orbital period is 687 (Earth) days - depicted in red - Earth's orbit in blue.

Mars's average distance from the Sun is roughly 230 million km (1.5 AU, or 143 million miles), and its orbital period is 687 (Earth) days. The solar day (or sol) on Mars is only slightly longer than an Earth day: 24 hours, 39 minutes, and 35.244 seconds. A Martian year is equal to 1.8809 Earth years, or 1 year, 320 days, and 18.2 hours.^[7]

The axial tilt of Mars is 25.19 degrees, which is similar to the axial tilt of the Earth.^[7] As a result, Mars has seasons like the Earth, though on Mars, they are nearly twice as long given its longer year. Currently, the orientation of the north pole of Mars is close to the star Deneb.^[14] Mars passed an aphelion in March 2010^[153] and its perihelion in March 2011.^[154] The next aphelion came in February 2012^[154] and the next perihelion came in January 2013.^[154]

Mars has a relatively pronounced orbital eccentricity of about 0.09; of the seven other planets in the Solar System, only Mercury shows greater eccentricity. It is known that in the past, Mars has had a much more circular orbit than it does currently. At one point, 1.35 million Earth years ago, Mars had an eccentricity of roughly 0.002, much less than that of Earth today.^[155] The Mars cycle of eccentricity is 96,000 Earth years compared to the Earth's cycle of 100,000 years.^[156] Mars also has a much longer cycle of eccentricity with a period of 2.2 million Earth years, and this overshadows the 96,000-year cycle in the eccentricity graphs. For the last 35,000 years, the orbit of Mars has been getting slightly more eccentric because of the gravitational effects of the other planets. The closest distance between the Earth and Mars will continue to mildly decrease for the next 25,000 years.^[157]

Search for life

Viking 1 Lander - sampling arm created deep trenches, scooping up material for tests (Chryse Planitia).

The current understanding of planetary habitability – the ability of a world to develop and sustain life – favors planets that have liquid water on their surface. This most often requires that the orbit of a planet lie within the habitable zone, which for the Sun currently extends from just beyond Venus to about the semi-major axis of Mars.^[158] During perihelion, Mars dips inside this region, but the planet's thin (low-pressure) atmosphere prevents liquid water from existing over large regions for extended periods. The past flow of liquid water demonstrates the planet's potential for habitability.
Some recent evidence has suggested that any water on the Martian surface may have been too salty and acidic to support regular terrestrial life.^[159]

The lack of a magnetosphere and extremely thin atmosphere of Mars are a challenge: the planet has little heat transfer across its surface, poor insulation against bombardment of the solar wind and insufficient atmospheric pressure to retain water in a liquid form (water instead sublimates to a gaseous state). Mars is also nearly, or perhaps totally, geologically dead; the end of volcanic activity has apparently stopped the recycling of chemicals and minerals between the surface and interior of the planet.^[160]

Curiosity rover self-portrait at "Rocknest" (October 31, 2012), with the rim of Gale Crater and the slopes of Aeolis Mons in the distance.

Evidence suggests that the planet was once significantly more habitable than it is today, but whether living organisms ever existed there remains unknown. The Viking probes of the mid-1970s carried experiments designed to detect microorganisms in Martian soil at their respective landing sites and had positive results, including a temporary increase of CO₂ production on exposure to water and nutrients. This sign of life was later disputed by some scientists, resulting in a continuing debate, with NASA scientist Gilbert Levin asserting that Viking may have found life. A re-analysis of the Viking data, in light of modern knowledge of extremophile forms of life, has suggested that the Viking tests were not sophisticated enough to detect these forms of life. The tests could even have killed a (hypothetical) life form.^[161] Tests conducted by the Phoenix Mars lander have shown that the soil has a very alkaline pH and it contains magnesium, sodium, potassium and chloride.^[162] The soil nutrients may be able to support life, but life would still have to be shielded from the intense ultraviolet light.^[163]

At the Johnson Space Center lab, some fascinating shapes have been found in the meteorite ALH84001, which is thought to have originated from Mars. Some scientists propose that these geometric shapes could be fossilized microbes extant on Mars before the meteorite was blasted into space by a meteor strike and sent on a 15 million-year voyage to Earth. An exclusively inorganic origin for the shapes has also been proposed.^[164]

Small quantities of methane and formaldehyde recently detected by Mars orbiters are both claimed to be possible evidence for life, as these chemical compounds would quickly break down in the Martian atmosphere.^[165]^[166] Alternatively, these compounds may instead be replenished by volcanic or other geological means, such as serpentinization.^[136]

Habitability

The German Aerospace Center discovered that Earth lichens can survive in simulated Mars conditions.^[167] The simulation based temperatures, atmospheric pressure, minerals, and light on data from Mars probes.^[167] An instrument called REMS is designed to provide new clues about the signature of the Martian general circulation, microscale weather systems, local hydrological cycle, destructive potential of UV radiation, and subsurface habitability based on ground-atmosphere interaction.^[168]^[169] It landed on Mars as part of Curiosity (MSL) in August 2012. Microorganisms make up 80% of Earth's biomass.^[167]

Exploration missions

Panorama of Gusev crater, where Spirit rover examined volcanic basalts

In addition to observation from Earth, some of the latest Mars information comes from five active probes on or in orbit around Mars, including three orbiters and two rovers. This includes 2001 Mars Odyssey,^[170] Mars Express, Mars Reconnaissance Orbiter, Opportunity rover, and Curiosity rover.
Dozens of unmanned spacecraft, including orbiters, landers, and rovers, have been sent to Mars by the Soviet Union, the United States, Europe, and Japan to study the planet's surface, climate, and geology. The public can request images of Mars via the HiWish program.

The Mars Science Laboratory, named Curiosity, launched on November 26, 2011, reached Mars on August 6, 2012 UTC. It is larger and more advanced than the Mars Exploration Rovers, with a movement rate up to 90 m per hour.^[171] Experiments include a laser chemical sampler that can deduce the make-up of rocks at a distance of 7 m.^[172] On February 10 the Curiosity Mars rover obtained the first deep rock samples ever taken from another planetary body, using its onboard drill.^[173]

The Indian Space Research Organization launched the Mars Orbiter Mission, Mangalyaan, on November 5, 2013, with the aim of analyzing the Martian atmosphere and topography. The Mars Orbiter Mission used a Hohmann transfer orbit to escape Earth's gravitational influence and catapult into a nine-month-long voyage to Mars. The mission will be the first successful Asian interplanetary mission, if it positions itself in Mars orbit by September 2014.^[174]

Astronomy on Mars

Phobos transits the Sun (Opportunity; March 10, 2004).

Comet Siding Spring to pass near Mars on October 19, 2014 (Hubble; March 11, 2014).

With the existence of various orbiters, landers, and rovers, it is now possible to study astronomy from the Martian skies. While Mars's moon Phobos appears about one third the angular diameter of the full moon as it appears from Earth, Deimos appears more or less star-like and appears only slightly brighter than Venus does from Earth.^[175]

There are various phenomena, well-known on Earth, that have been observed on Mars, such as meteors and auroras.^[176] A transit of the Earth as seen from Mars will occur on November 10, 2084.^[177] There are also transits of Mercury and transits of Venus, and the moons Phobos and Deimos are of sufficiently small angular diameter that their partial "eclipses" of the Sun are best considered transits (see Transit of Deimos from Mars).^[178]^[179]

On October 19, 2014, Comet Siding Spring is expected to pass extremely close to Mars, so close that the coma may envelop Mars.^[180]^[181]

Viewing

Animation of the apparent retrograde motion of Mars in 2003 as seen from Earth

Because the orbit of Mars is eccentric, its apparent magnitude at opposition from the Sun can range from −3.0 to −1.4. The minimum brightness is magnitude +1.6 when the planet is in conjunction with the Sun.^[8] Mars usually appears distinctly yellow, orange, or red; the actual color of Mars is closer to butterscotch, and the redness seen is just dust in the planet's atmosphere; considering this, NASA's Spirit rover has taken pictures of a greenish-brown, mud-colored landscape with blue-grey rocks and patches of light red sand.^[182] When farthest away from the Earth, it is more than seven times as far from the latter as when it is closest. When least favorably positioned, it can be lost in the Sun's glare for months at a time. At its most favorable times – at 15- or 17-year intervals, and always between late July and late September – Mars shows a wealth of surface detail to a telescope. Especially noticeable, even at low magnification, are the polar ice caps.^[183]

As Mars approaches opposition, it begins a period of retrograde motion, which means it will appear to move backwards in a looping motion with respect to the background stars. The duration of this retrograde motion lasts for about 72 days, and Mars reaches its peak luminosity in the middle of this motion.^[184]

Closest approaches

Relative

The point at which Mars's geocentric longitude is 180° different from the Sun's is known as opposition, which is near the time of closest approach to the Earth. The time of opposition can occur as much as 8½ days away from the closest approach. The distance at close approach varies between about 54^[185] and about 103 million km due to the planets' elliptical orbits, which causes comparable variation in angular size.^[186] The last Mars opposition occurred on April 8, 2014 at a distance of about 180 million km.^[187] The average time between the successive oppositions of Mars, its synodic period, is 780 days but the number of days between the dates of successive oppositions can range from 764 to 812.^[188]

As Mars approaches opposition it begins a period of retrograde motion, which makes it appear to move backwards in a looping motion relative to the background stars. The duration of this retrograde motion is about 72 days.

Absolute, around the present time

Mars oppositions from 2003–2018, viewed from above the ecliptic with the Earth centered

Mars made its closest approach to Earth and maximum apparent brightness in nearly 60,000 years, 55,758,006 km (0.372719 AU; 34,646,400 mi), magnitude −2.88, on 27 August 2003 at 9:51:13 UT.
This occurred when Mars was one day from opposition and about three days from its perihelion, making Mars particularly easy to see from Earth. The last time it came so close is estimated to have been on September 12, 57 617 BC, the next time being in 2287.^[189] This record approach was only very slightly closer than other recent close approaches. For instance, the minimum distance on August 22, 1924 was 0.37285 AU, and the minimum distance on August 24, 2208 will be 0.37279 AU.^[156]

Historical observations

The history of observations of Mars is marked by the oppositions of Mars, when the planet is closest to Earth and hence is most easily visible, which occur every couple of years. Even more notable are the perihelic oppositions of Mars, which occur every 15 or 17 years and are distinguished because Mars is close to perihelion, making it even closer to Earth.

Ancient and medieval observations

The existence of Mars as a wandering object in the night sky was recorded by the ancient Egyptian astronomers and by 1534 BCE they were familiar with the retrograde motion of the planet.^[190] By the period of the Neo-Babylonian Empire, the Babylonian astronomers were making regular records of the positions of the planets and systematic observations of their behavior. For Mars, they knew that the planet made 37 synodic periods, or 42 circuits of the zodiac, every 79 years. They also invented arithmetic methods for making minor corrections to the predicted positions of the planets.^[191]^[192]
In the fourth century BCE, Aristotle noted that Mars disappeared behind the Moon during an occultation, indicating the planet was farther away.^[193] Ptolemy, a Greek living in Alexandria,^[194] attempted to address the problem of the orbital motion of Mars. Ptolemy's model and his collective work on astronomy was presented in the multi-volume collection Almagest, which became the authoritative treatise on Western astronomy for the next fourteen centuries.^[195] Literature from ancient China confirms that Mars was known by Chinese astronomers by no later than the fourth century BCE.^[196] In the fifth century CE, the Indian astronomical text Surya Siddhanta estimated the diameter of Mars.^[197] In the East Asian cultures, Mars is traditionally referred to as the "fire star" (火星), based on the Five elements.^[198]

During the seventeenth century, Tycho Brahe measured the diurnal parallax of Mars that Johannes Kepler used to make a preliminary calculation of the relative distance to the planet.^[199] When the telescope became available, the diurnal parallax of Mars was again measured in an effort to determine the Sun-Earth distance. This was first performed by Giovanni Domenico Cassini in 1672.
The early parallax measurements were hampered by the quality of the instruments.^[200] The only occultation of Mars by Venus observed was that of October 13, 1590, seen by Michael Maestlin at Heidelberg.^[201] In 1610, Mars was viewed by Galileo Galilei, who was first to see it via telescope.^[202] The first person to draw a map of Mars that displayed any terrain features was the Dutch astronomer Christiaan Huygens.^[203]

Martian "canals"

Map of Mars by Giovanni Schiaparelli

Mars sketched as observed by Lowell sometime before 1914. (South top)

Map of Mars from Hubble Space Telescope as seen near the 1999 opposition. (North top)

By the 19th century, the resolution of telescopes reached a level sufficient for surface features to be identified. A perihelic opposition of Mars occurred on September 5, 1877. In that year, Italian astronomer Giovanni Schiaparelli used a 22 cm (8.7 in) telescope in Milan to help produce the first detailed map of Mars. These maps notably contained features he called canali, which were later shown to be an optical illusion. These canali were supposedly long, straight lines on the surface of Mars, to which he gave names of famous rivers on Earth. His term, which means "channels" or "grooves", was popularly mistranslated in English as "canals".^[204]^[205]

Influenced by the observations, the orientalist Percival Lowell founded an observatory which had a 30 cm and 45 cm telescope (11.8 and 17.7 in). The observatory was used for the exploration of Mars during the last good opportunity in 1894 and the following less favorable oppositions. He published several books on Mars and life on the planet, which had a great influence on the public.^[206] The canali were also found by other astronomers, like Henri Joseph Perrotin and Louis Thollon in Nice, using one of the largest telescopes of that time.^[207]^[208]

The seasonal changes (consisting of the diminishing of the polar caps and the dark areas formed during Martian summer) in combination with the canals lead to speculation about life on Mars, and it was a long-held belief that Mars contained vast seas and vegetation. The telescope never reached the resolution required to give proof to any speculations. As bigger telescopes were used, fewer long, straight canali were observed. During an observation in 1909 by Flammarion with an 84 cm (33 in) telescope, irregular patterns were observed, but no canali were seen.^[209]

Even in the 1960s articles were published on Martian biology, putting aside explanations other than life for the seasonal changes on Mars. Detailed scenarios for the metabolism and chemical cycles for a functional ecosystem have been published.^[210]

Spacecraft visitation

Foothills of Aeolis Mons ("Mount Sharp") (white balanced image).

Once spacecraft visited the planet during NASA's Mariner missions in the 1960s and 70s these concepts were radically broken. In addition, the results of the Viking life-detection experiments aided an intermission in which the hypothesis of a hostile, dead planet was generally accepted.^[211]
Mariner 9 and Viking allowed better maps of Mars to be made using the data from these missions, and another major leap forward was the Mars Global Surveyor mission, launched in 1996 and operated until late 2006, that allowed complete, extremely detailed maps of the Martian topography, magnetic field and surface minerals to be obtained.^[212] These maps are now available online, for example, at Google Mars. Mars Reconnaissance Orbiter and Mars Express continued exploring with new instruments, and supporting lander missions.

In culture

Mars is named after the Roman god of war. In different cultures, Mars represents masculinity and youth. Its symbol, a circle with an arrow pointing out to the upper right, is also used as a symbol for the male gender.

The many failures in Mars exploration probes resulted in a satirical counter-culture blaming the failures on an Earth-Mars "Bermuda Triangle", a "Mars Curse", or a "Great Galactic Ghoul" that feeds on Martian spacecraft.^[213]

Intelligent "Martians"

The fashionable idea that Mars was populated by intelligent Martians exploded in the late 19th century. Schiaparelli's "canali" observations combined with Percival Lowell's books on the subject put forward the standard notion of a planet that was a drying, cooling, dying world with ancient civilizations constructing irrigation works.^[214]

Many other observations and proclamations by notable personalities added to what has been termed "Mars Fever".^[215] In 1899 while investigating atmospheric radio noise using his receivers in his Colorado Springs lab, inventor Nikola Tesla observed repetitive signals that he later surmised might have been radio communications coming from another planet, possibly Mars. In a 1901 interview Tesla said:

It was some time afterward when the thought flashed upon my mind that the disturbances I had observed might be due to an intelligent control. Although I could not decipher their meaning, it was impossible for me to think of them as having been entirely accidental. The feeling is constantly growing on me that I had been the first to hear the greeting of one planet to another.^[216]

An 1893 soap ad playing on the popular idea that Mars was populated.

Tesla's theories gained support from Lord Kelvin who, while visiting the United States in 1902, was reported to have said that he thought Tesla had picked up Martian signals being sent to the United States.^[217] Kelvin "emphatically" denied this report shortly before departing America: "What I really said was that the inhabitants of Mars, if there are any, were doubtless able to see New York, particularly the glare of the electricity."^[218]

In a New York Times article in 1901, Edward Charles Pickering, director of the Harvard College Observatory, said that they had received a telegram from Lowell Observatory in Arizona that seemed to confirm that Mars was trying to communicate with the Earth.^[219]

Early in December 1900, we received from Lowell Observatory in Arizona a telegram that a shaft of light had been seen to project from Mars (the Lowell observatory makes a specialty of Mars) lasting seventy minutes. I wired these facts to Europe and sent out neostyle copies through this country. The observer there is a careful, reliable man and there is no reason to doubt that the light existed. It was given as from a well-known geographical point on Mars. That was all. Now the story has gone the world over. In Europe it is stated that I have been in communication with Mars, and all sorts of exaggerations have spring up. Whatever the light was, we have no means of knowing. Whether it had intelligence or not, no one can say. It is absolutely inexplicable.^[219]

Pickering later proposed creating a set of mirrors in Texas, intended to signal Martians.^[220]
In recent decades, the high-resolution mapping of the surface of Mars, culminating in Mars Global Surveyor, revealed no artifacts of habitation by "intelligent" life, but pseudoscientific speculation about intelligent life on Mars continues from commentators such as Richard C. Hoagland.
Reminiscent of the canali controversy, some speculations are based on small scale features perceived in the spacecraft images, such as 'pyramids' and the 'Face on Mars'. Planetary astronomer Carl Sagan wrote:

Mars has become a kind of mythic arena onto which we have projected our Earthly hopes and fears.^[205]

Martian tripod illustration from the 1906 French edition of The War of the Worlds by H.G. Wells.

The depiction of Mars in fiction has been stimulated by its dramatic red color and by nineteenth century scientific speculations that its surface conditions might support not just life but intelligent life.^[221] Thus originated a large number of science fiction scenarios, among which is H. G. Wells's The War of the Worlds, published in 1898, in which Martians seek to escape their dying planet by invading Earth. A subsequent US radio adaptation of The War of the Worlds on October 30, 1938, by Orson Welles was presented as a live news broadcast and became notorious for causing a public panic when many listeners mistook it for the truth.^[222]

Influential works included Ray Bradbury's The Martian Chronicles, in which human explorers accidentally destroy a Martian civilization, Edgar Rice Burroughs' Barsoom series, C. S. Lewis' novel Out of the Silent Planet (1938),^[223] and a number of Robert A. Heinlein stories before the mid-sixties.^[224]

Author Jonathan Swift made reference to the moons of Mars, about 150 years before their actual discovery by Asaph Hall, detailing reasonably accurate descriptions of their orbits, in the 19th chapter of his novel Gulliver's Travels.^[225]

A comic figure of an intelligent Martian, Marvin the Martian, appeared on television in 1948 as a character in the Looney Tunes animated cartoons of Warner Brothers, and has continued as part of popular culture to the present.^[226]

After the Mariner and Viking spacecraft had returned pictures of Mars as it really is, an apparently lifeless and canal-less world, these ideas about Mars had to be abandoned, and a vogue for accurate, realist depictions of human colonies on Mars developed, the best known of which may be Kim Stanley Robinson's Mars trilogy. Pseudo-scientific speculations about the Face on Mars and other enigmatic landmarks spotted by space probes have meant that ancient civilizations continue to be a popular theme in science fiction, especially in film.^[227]

The theme of a Martian colony that fights for independence from Earth is a major plot element in the novels of Greg Bear as well as the movie Total Recall (based on a short story by Philip K. Dick) and the television series Babylon 5. Some video games also use this element, including Red Faction and the Zone of the Enders series. Mars (and its moons) were also the setting for the popular Doom video game franchise and the later Martian Gothic.

Mars surface details

Streaks on slopes in Acheron Fossae

Nanedi Valles inner channel

HiRISE view of the aftermath of an avalanche that cascaded down a steep 700 m slope at the edge of Mars' north polar layered terrain

Moons

Enhanced-color HiRISE image of Phobos, showing a series of mostly parallel grooves and crater chains, with its crater Stickney at right

Enhanced-color HiRISE image of Deimos (not to scale), showing its smooth blanket of regolith.

Mars has two relatively small natural moons, Phobos (about 14 miles in diameter) and Deimos (about 8 miles in diameter), which orbit close to the planet. Asteroid capture is a long-favored theory, but their origin remains uncertain.^[228] Both satellites were discovered in 1877 by Asaph Hall; they are named after the characters Phobos (panic/fear) and Deimos (terror/dread), who, in Greek mythology, accompanied their father Ares, god of war, into battle. Mars was the Roman counterpart of Ares.^[229]^[230] In modern Greek, though, the planet retains its ancient name Ares (Aris: Άρης).^[231]

From the surface of Mars, the motions of Phobos and Deimos appear very different from that of our own moon. Phobos rises in the west, sets in the east, and rises again in just 11 hours. Deimos, being only just outside synchronous orbit – where the orbital period would match the planet's period of rotation – rises as expected in the east but very slowly. Despite the 30 hour orbit of Deimos, 2.7 days elapse between its rise and set for an equatorial observer, as it slowly falls behind the rotation of Mars.^[232]

Orbits of Phobos and Deimos (to scale)

Because the orbit of Phobos is below synchronous altitude, the tidal forces from the planet Mars are gradually lowering its orbit. In about 50 million years, it could either crash into Mars' surface or break up into a ring structure around the planet.^[232]

The origin of the two moons is not well understood. Their low albedo and carbonaceous chondrite composition have been regarded as similar to asteroids, supporting the capture theory. The unstable orbit of Phobos would seem to point towards a relatively recent capture. But both have circular orbits, very near the equator, which is very unusual for captured objects and the required capture dynamics are complex. Accretion early in the history of Mars is also plausible, but would not account for a composition resembling asteroids rather than Mars itself, if that is confirmed.

A third possibility is the involvement of a third body or some kind of impact disruption.^[233] More recent lines of evidence for Phobos having a highly porous interior,^[234] and suggesting a composition containing mainly phyllosilicates and other minerals known from Mars,^[235] point toward an origin of Phobos from material ejected by an impact on Mars that reaccreted in Martian orbit,^[236] similar to the prevailing theory for the origin of Earth's moon. While the VNIR spectra of the moons of Mars resemble those of outer belt asteroids, the thermal infrared spectra of Phobos are reported to be inconsistent with chondrites of any class.^[235]

Mars may have additional moons smaller than 50–100 meters, and a dust ring is predicted between Phobos and Deimos.^[237]

Saturday, August 16, 2014

Evolution of sexual reproduction

The evolution of sexual reproduction is described by several competing scientific hypotheses. All sexually reproducing eukaryotic organisms derive from a common ancestor which was a single celled eukaryotic species.^[1]^[2]^[3] Many protists reproduce sexually, as do multicellular plants, animals, and fungi. There are a few species which have secondarily lost this feature, such as Bdelloidea and some parthenocarpic plants. The evolution of sex contains two related, yet distinct, themes: its origin and its maintenance. However, since the hypotheses for the origins of sex are difficult to test experimentally, most current work has been focused on the maintenance of sexual reproduction.

It seems that a sexual cycle is maintained because it improves the quality of progeny (fitness), despite reducing the overall number of offspring (the two-fold cost of sex). In order for sex to be evolutionarily advantageous, it must be associated with a significant increase in the fitness of offspring. One of the most widely accepted explanations for the advantage of sex lies in the creation of genetic variation. Another explanation is based on two molecular advantages. The first of these is the advantage of recombinational DNA repair (promoted during meiosis because homologous chromosomes pair at that time), while the second is the advantage of complementation (also known as hybrid vigor, heterosis or masking of mutations).

For the advantage due to genetic variation, there are three possible reasons this might happen. First, sexual reproduction can combine the effects of two beneficial mutations in the same individual (i.e. sex aids in the spread of advantageous traits). Also, the necessary mutations do not have to have occurred one after another in a single line of descendants.^[4]^{[unreliable source?]} Second, sex acts to bring together currently deleterious mutations to create severely unfit individuals that are then eliminated from the population (i.e. sex aids in the removal of deleterious genes). However in organisms containing only one set of chromosomes, deleterious mutations would be eliminated immediately, and therefore removal of harmful mutations is an unlikely benefit for sexual reproduction. Lastly, sex creates new gene combinations that may be more fit than previously existing ones, or may simply lead to reduced competition among relatives.

For the advantage due to DNA repair, there is an immediate large benefit of removing DNA damage by recombinational DNA repair during meiosis, since this removal allows greater survival of progeny with undamaged DNA. The advantage of complementation to each sexual partner is avoidance of the bad effects of their deleterious recessive genes in progeny by the masking effect of normal dominant genes contributed by the other partner.

The classes of hypotheses based on the creation of variation are further broken down below. It is important to realise that any number of these hypotheses may be true in any given species (they are not mutually exclusive), and that different hypotheses may apply in different species. However, a research framework based on creation of variation has yet to be found that allows one to determine whether the reason for sex is universal for all sexual species, and, if not, which mechanisms are acting in each species.

On the other hand, the maintenance of sex based on DNA repair and complementation applies widely to all sexual species.

Historical perspective

Modern philosophical-scientific thinking on the problem can be traced back to Erasmus Darwin in the 18th century; it also features in Aristotle's writings. The thread was later picked up by August Weismann in 1889, who argued that the purpose of sex was to generate genetic variation, as is detailed in the majority of the explanations below. On the other hand, Charles Darwin concluded that the effects of hybrid vigor (complementation) "is amply sufficient to account for the ... genesis of the two sexes." This is consistent with the repair and complementation hypothesis, given below under "Other explanations."

Several explanations have been suggested by biologists including W. D. Hamilton, Alexey Kondrashov, George C. Williams, Harris Bernstein, Carol Bernstein, Michael M. Cox, Frederic A. Hopf and Richard E. Michod to explain how sexual reproduction is maintained in a vast array of different living organisms.

Questions

Some questions biologists have attempted to answer include:

Why sexual reproduction exists, if in many organisms it has a 50% cost (ﬁtness disadvantage) in relation to asexual reproduction?^[5]
Did mating types (types of gametes, according to their compatibility) arise as a result of anisogamy (gamete dimorphism), or did mating types evolve before anisogamy?^[6]
Why do most sexual organisms use a binary mating system?^[7] Why do some organisms have gamete dimorphism?

Two-fold cost of sex

This diagram illustrates the twofold cost of sex. If each individual were to contribute to the same number of offspring (two), (a) the sexual population remains the same size each generation, where the (b) asexual population doubles in size each generation.

In most multicellular sexual species, the population consists of two sexes, only one of which is capable of bearing young (with the exception of simultaneous hermaphrodites). In an asexual species, each member of the population is capable of bearing young. This implies that an asexual population has an intrinsic capacity to grow more rapidly with each generation. The cost was first described in mathematical terms by John Maynard Smith.^[8] He imagined an asexual mutant arising in a sexual population, half of which comprises males that cannot themselves produce offspring. With female-only offspring, the asexual lineage doubles its representation in the population each generation, all else being equal. Technically this is not a problem of sex but a problem of some multicellular sexually reproducing organisms. There are numerous isogamous species which are sexual and do not have the problem of producing individuals which cannot directly replicate themselves.^[9] The principal costs of sex is that males and females must search for each other in order to mate, and sexual selection often favours traits that reduce the survival of individuals.^[8]

Evidence that the cost is not insurmountable comes from George C. Williams, who noted the existence of species which are capable of both asexual and sexual reproduction. These species time their sexual reproduction with periods of environmental uncertainty, and reproduce asexually when conditions are more favourable. The important point is that these species are observed to reproduce sexually when they could choose not to, implying that there is a selective advantage to sexual reproduction.^[10]

It is widely believed that a disadvantage of sexual reproduction is that a sexually reproducing organism will only be able to pass on 50% of its genes to each offspring. This is a consequence of the fact that gametes from sexually reproducing species are haploid.^[11] This, however, conflates sex and reproduction which are two separate events. The "two-fold cost of sex" may more accurately be described as the cost of anisogamy. Not all sexual organisms are anisogamous. There are numerous species which are sexual and do not have this problem because they do not produce males or females.
Yeast, for example, are isogamous sexual organisms which have two mating types which fuse and recombine their haploid genomes. Both sexes reproduce during the haploid and diploid stages of their life cycle and have a 100% chance of passing their genes into their offspring.^[9]

The two-fold cost of sex may be compensated for in some species in many ways. Females may eat males after mating, males may be much smaller or rarer, or males may help raise offspring.
Some species avoid the cost of 50% of sexual reproduction, although they have "sex" (in the sense of genetic recombination). In these species (e.g., bacteria and ciliates), "sex" and reproduction occurs separately. ^[12]

Promotion of genetic variation

August Weismann proposed in 1889^[13] an explanation for the evolution of sex, where the advantage of sex is the creation of variation among siblings. It was then subsequently explained in genetics terms by Fisher^[14] and Muller^[15] and has been recently summarised by Burt in 2000.^[16]

George C. Williams gave an example based around the elm tree. In the forest of this example, empty patches between trees can support one individual each. When a patch becomes available because of the death of a tree, other trees' seeds will compete to fill the patch. Since the chance of a seed's success in occupying the patch depends upon its genotype, and a parent cannot anticipate which genotype is most successful, each parent will send many seeds, creating competition between siblings. Natural selection therefore favours parents which can produce a variety of offspring (see lottery principle).

A similar hypothesis is named the tangled bank hypothesis after a passage in Charles Darwin's The Origin of Species:

"It is interesting to contemplate an entangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner, have all been produced by laws acting around us."

The hypothesis, proposed by Michael Ghiselin in his 1974 book, The Economy of Nature and the Evolution of Sex, suggests that a diverse set of siblings may be able to extract more food from its environment than a clone, because each sibling uses a slightly different niche. One of the main proponents of this hypothesis is Graham Bell of McGill University. The hypothesis has been criticised for failing to explain how asexual species developed sexes. In his book, Evolution and Human Behavior (MIT Press, 2000), John Cartwright comments:

"Although once popular, the tangled bank hypothesis now seems to face many problems, and former adherents are falling away. The theory would predict a greater interest in sex among animals that produce lots of small offspring that compete with each other. In fact, sex is invariably associated with organisms that produce a few large offspring, whereas organisms producing small offspring frequently engage in parthenogenesis [asexual reproduction]. In addition, the evidence from fossils suggests that species go for vast periods of [geologic] time without changing much."

In contrast to the view that sex promotes genetic variation, Heng^[17] and Gorelick and Heng^[18] reviewed evidence that sex actually acts as a constraint on genetic variation. They consider that sex acts as a coarse filter, weeding out major genetic changes, such as chromosomal rearrangements, but permitting minor variation, such as changes at the nucleotide or gene level (that are often neutral) to pass through the sexual sieve.

Spread of advantageous traits

Novel genotypes

This diagram illustrates how sex might create novel genotypes more rapidly. Two advantageous alleles A and B occur at random. The two alleles are recombined rapidly in a sexual population (top), but in an asexual population (bottom) the two alleles must independently arise because of clonal interference.

Sex could be a method by which novel genotypes are created. Because sex combines genes from two individuals, sexually reproducing populations can more easily combine advantageous genes than can asexual populations. If, in a sexual population, two different advantageous alleles arise at different loci on a chromosome in different members of the population, a chromosome containing the two advantageous alleles can be produced within a few generations by recombination. However, should the same two alleles arise in different members of an asexual population, the only way that one chromosome can develop the other allele is to independently gain the same mutation, which would take much longer. Several studies have addressed whether this model is sufficiently robust to explain the predominance of sexual versus asexual reproduction. These studies, including arguments for and against the model, were summarized by Birdsell and Wills [pp. 73–86]^[19]

Ronald Fisher also suggested that sex might facilitate the spread of advantageous genes by allowing them to escape their genetic surroundings, if they should arise on a chromosome with deleterious genes.

Supporters of these theories respond to the balance argument that the individuals produced by sexual and asexual reproduction may differ in other respects too – which may influence the persistence of sexuality. For example, in water fleas of the genus Cladocera, sexual offspring form eggs which are better able to survive the winter.

Increased resistance to parasites

One of the most widely discussed theories to explain the persistence of sex is that it is maintained to assist sexual individuals in resisting parasites, also known as the Red Queen's Hypothesis.^[11]^[20]^[21] and (pp. 113–117) of Birdsell and Wills^[19]

When an environment changes, previously neutral or deleterious alleles can become favourable. If the environment changed sufficiently rapidly (i.e. between generations), these changes in the environment can make sex advantageous for the individual. Such rapid changes in environment are caused by the co-evolution between hosts and parasites.

Imagine, for example that there is one gene in parasites with two alleles p and P conferring two types of parasitic ability, and one gene in hosts with two alleles h and H, conferring two types of parasite resistance, such that parasites with allele p can attach themselves to hosts with the allele h, and P to H. Such a situation will lead to cyclic changes in allele frequency - as p increases in frequency, h will be disfavoured.

In reality, there will be several genes involved in the relationship between hosts and parasites. In an asexual population of hosts, offspring will only have the different parasitic resistance if a mutation arises. In a sexual population of hosts, however, offspring will have a new combination of parasitic resistance alleles.

In other words, like Lewis Carroll's Red Queen, sexual hosts are continually adapting in order to stay ahead of their parasites.

Evidence for this explanation for the evolution of sex is provided by comparison of the rate of molecular evolution of genes for kinases and immunoglobulins in the immune system with genes coding other proteins. The genes coding for immune system proteins evolve considerably faster.^[22]^[23]
Further evidence for the Red Queen hypothesis were provided by observing long‐term dynamics and parasite coevolution in a "mixed" (sexual and asexual) population of snails (Potamopyrgus antipodarum). The number of sexuals, the number asexuals, and the rates of parasite infection for both were monitored. It was found that clones that were plentiful at the beginning of the study became more susceptible to parasites over time. As parasite infections increased, the once plentiful clones dwindled dramatically in number. Some clonal types disappeared entirely. Meanwhile, sexual snail populations remained much more stable over time.^[24]^[25]

However, Hanley et al.^[26] studied mite infestations of a parthenogenetic gecko species and its two related sexual ancestral species. Contrary to expectation based on the Red Queen hypothesis, they found that the prevalence, abundance and mean intensity of mites in sexual geckos was significantly higher than in asexuals sharing the same habitat.

In 2011, researchers used the microscopic roundworm Caenorhabditis elegans as a host and the pathogenic bacteria Serratia marcescens to generate a host-parasite coevolutionary system in a controlled environment, allowing them to conduct more than 70 evolution experiments testing the Red Queen Hypothesis. They genetically manipulated the mating system of C. elegans, causing populations to mate either sexually, by self-fertilization, or a mixture of both within the same population. Then they exposed those populations to the S. marcescens parasite. It was found that the self-fertilizing populations of C. elegans were rapidly driven extinct by the coevolving parasites while sex allowed populations to keep pace with their parasites, a result consistent with the Red Queen Hypothesis.^[27]^[28]

Critics of the Red Queen hypothesis question whether the constantly changing environment of hosts and parasites is sufficiently common to explain the evolution of sex. In particular, Otto and Nuismer ^[29] presented results showing that species interactions (e.g. host vs parasite interactions) typically select against sex. They concluded that, although the Red Queen hypothesis favors sex under certain circumstances, it alone does not account for the ubiquity of sex. Otto and Gerstein ^[30] further stated that “it seems doubtful to us that strong selection per gene is sufficiently commonplace for the Red Queen hypothesis to explain the ubiquity of sex.” Parker ^[31] reviewed numerous genetic studies on plant disease resistance and failed to uncover a single example consistent with the assumptions of the Red Queen hypothesis.

Deleterious mutation clearance

Mutations can have many different effects upon an organism. It is generally believed that the majority of non-neutral mutations are deleterious, which means that they will cause a decrease in the organism's overall fitness.^[32] If a mutation has a deleterious effect, it will then usually be removed from the population by the process of natural selection. Sexual reproduction is believed to be more efficient than asexual reproduction in removing those mutations from the genome.^[33]

There are two main hypotheses which explain how sex may act to remove deleterious genes from the genome.

Maintenance of mutation-free individuals

In a finite asexual population under the pressure of deleterious mutations, the random loss of individuals without such mutations is inevitable. This is known as Muller's ratchet. In a sexual population, however, mutation-free genotypes can be restored by recombination of genotypes containing deleterious mutations.

This comparison will only work for a small population - in a large population, random loss of the most fit genotype becomes unlikely even in an asexual population. [see Birdsell and Wills (pp. 86–103)^[19] for a review of this topic]

Removal of deleterious genes

Diagram illustrating different relationships between numbers of mutations and fitness. Kondrashov's model requires synergistic epistasis, which is represented by the red line^[34]^[35] - each mutation has a disproproportionately large effect on the organism's fitness.

This hypothesis was proposed by Alexey Kondrashov, and is sometimes known as the deterministic mutation hypothesis.^[33] It assumes that the majority of deleterious mutations are only slightly deleterious, and affect the individual such that the introduction of each additional mutation has an increasingly large effect on the fitness of the organism. This relationship between number of mutations and fitness is known as synergistic epistasis.

By way of analogy, think of a car with several minor faults. Each is not sufficient alone to prevent the car from running, but in combination, the faults combine to prevent the car from functioning.
Similarly, an organism may be able to cope with a few defects, but the presence of many mutations could overwhelm its backup mechanisms.

Kondrashov argues that the slightly deleterious nature of mutations means that the population will tend to be composed of individuals with a small number of mutations. Sex will act to recombine these genotypes, creating some individuals with fewer deleterious mutations, and some with more. Because there is a major selective disadvantage to individuals with more mutations, these individuals die out. In essence, sex compartmentalises the deleterious mutations.

There has been much criticism of Kondrashov's theory, since it relies on two key restrictive conditions. The first requires that the rate of deleterious mutation should exceed one per genome per generation in order to provide a substantial advantage for sex. While there is some empirical evidence for it (for example in Drosophila^[36] and E. coli^[37]), there is also strong evidence against it. Thus, for instance, for the sexual species Saccharomyces cerevisiae (yeast) and Neurospora crassa (fungus), the mutation rate per genome per replication are 0.0027 and 0.0030 respectively. For the nematode worm Caenorhabditis elegans, the mutation rate per effective genome per sexual generation is 0.036.^[38] Secondly, there should be strong interactions among loci (synergistic epistasis), a mutation-fitness relation for which there is only limited evidence. Conversely, there is also the same amount of evidence that mutations show no epistasis (purely additive model) or antagonistic interactions (each additional mutation has a disproportionally small effect).

Other explanations

Speed of evolution

Ilan Eshel suggested that sex prevents rapid evolution. He suggests that recombination breaks up favourable gene combinations more often than it creates them, and sex is maintained because it ensures selection is longer-term than in asexual populations - so the population is less affected by short-term changes.[see Eshel and Feldman^[39] and reviewed in Birdsell and Wills^[19] (pp. 85–86)]. This explanation is not widely accepted, as its assumptions are very restrictive.

It has recently been shown in experiments with Chlamydomonas algae that sex can remove the speed limit^{[clarification needed]} on evolution.^[40]

DNA repair and complementation

As discussed in the earlier part of this article, sexual reproduction is conventionally explained as an adaptation for producing genetic variation through allelic recombination. As acknowledged above, however, serious problems with this explanation have led many biologists to conclude that the benefit of sex is a major unsolved problem in evolutionary biology.

An alternative "informational" approach to this problem has led to the view that the two fundamental aspects of sex, genetic recombination and outcrossing, are adaptive responses to the two major sources of "noise" in transmitting genetic information. Genetic noise can occur as either physical damage to the genome (e.g. chemically altered bases of DNA or breaks in the chromosome) or replication errors (mutations)^[41]^[42]^[43] This alternative view is referred to as the repair and complementation hypothesis, to distinguish it from the traditional variation hypothesis.

The repair and complementation hypothesis assumes that genetic recombination is fundamentally a DNA repair process, and that when it occurs during meiosis it is an adaptation for repairing the genomic DNA which is passed on to progeny. Recombinational repair is the only repair process known which can accurately remove double-strand damages in DNA, and such damages are both common in nature and ordinarily lethal if not repaired. For instance, double-strand breaks in DNA occur about 50 times per cell cycle in human cells [see DNA damage (naturally occurring)].
Recombinational repair is prevalent from the simplest viruses to the most complex multicellular eukaryotes. It is effective against many different types of genomic damage, and in particular is highly efficient at overcoming double-strand damages. Studies of the mechanism of meiotic recombination indicate that meiosis is an adaptation for repairing DNA.^[44]^[45] These considerations form the basis for the first part of the repair and complementation hypothesis.

In some lines of descent from the earliest organisms, the diploid stage of the sexual cycle, which was at first transient, became the predominant stage, because it allowed complementation — the masking of deleterious recessive mutations (i.e. hybrid vigor or heterosis). Outcrossing, the second fundamental aspect of sex, is maintained by the advantage of masking mutations and the disadvantage of inbreeding (mating with a close relative) which allows expression of recessive mutations (commonly observed as inbreeding depression). This is in accord with Charles Darwin,^[46] who concluded that the adaptive advantage of sex is hybrid vigor; or as he put it, "the offspring of two individuals, especially if their progenitors have been subjected to very different conditions, have a great advantage in height, weight, constitutional vigor and fertility over the self fertilised offspring from either one of the same parents."

However, outcrossing may be abandoned in favor of parthogenesis or selfing (which retain the advantage of meiotic recombinational repair) under conditions in which the costs of mating are very high. For instance, costs of mating are high when individuals are rare in a geographic area, such as when there has been a forest fire and the individuals entering the burned area are the initial ones to arrive. At such times mates are hard to find, and this favors parthenogenic species.

In the view of the repair and complementation hypothesis, the removal of DNA damage by recombinational repair produces a new, less deleterious form of informational noise, allelic recombination, as a by-product. This lesser informational noise generates genetic variation, viewed by some as the major effect of sex, as discussed in the earlier parts of this article.

Libertine bubble theory

The evolution of sex can alternatively be described as a kind of gene exchange that is independent from reproduction.^[47] According to the Thierry Lodé's "libertine bubble theory", sex originated from an archaic gene transfer process among prebiotic bubbles.^[48]^[49] The contact among the pre-biotic bubbles could, through simple food or parasitic reactions, promote the transfer of genetic material from one bubble to another. That interactions between two organisms be in balance appear to be a sufficient condition to make these interactions evolutionarily efficient, i.e. to select bubbles that tolerate these interactions (“libertine” bubbles) through a blind evolutionary process of self-reinforcing gene correlations and compatibility.^[50]

The "libertine bubble theory" proposes that meiotic sex evolved in proto-eukaryotes to solve a problem that bacteria did not have,^[51] namely a large amount of DNA material, occurring in an archaic step of proto-cell formation and genetic exchanges. So that, rather than providing selective advantages through reproduction, sex could be thought of as a series of separate events which combines step-by-step some very weak benefits of recombination, meiosis, gametogenesis and syngamy.^[52] Therefore, current sexual species could be descendants of primitive organisms that practiced more stable exchanges in the long term, while asexual species have emerged, much more recently in evolutionary history, from the conflict of interest resulting from anisogamy.

Origin of sexual reproduction

In the eukaryotic fossil record, sexual reproduction first appeared by 1200 million years ago in the Proterozoic Eon.^[53] All sexually reproducing eukaryotic organisms derive from a common ancestor which was a single celled species.^[1]^[45]^[54]^[55] Many protists reproduce sexually, as do the multicellular plants, animals, and fungi. There are a few species which have secondarily lost this feature, such as Bdelloidea and some parthenocarpic plants.

Organisms need to replicate their genetic material in an efficient and reliable manner. The necessity to repair genetic damage is one of the leading theories explaining the origin of sexual reproduction. Diploid individuals can repair a damaged section of their DNA via homologous recombination, since there are two copies of the gene in the cell and one copy is presumed to be undamaged. A mutation in a haploid individual, on the other hand, is more likely to become resident, as the DNA repair machinery has no way of knowing what the original undamaged sequence was.^[41] The most primitive form of sex may have been one organism with damaged DNA replicating an undamaged strand from a similar organism in order to repair itself.^[56]

If, as evidence indicates, sexual reproduction arose very early in eukaryotic evolution, the essential features of meiosis may have already been present in the prokaryotic ancestors of eukaryotes.^[54]^[57] In extant organisms, proteins with central functions in meiosis are similar to key proteins in bacterial transformation. For example, recA recombinase, that catalyses the key functions of DNA homology search and strand exchange in the bacterial sexual process of transformation, has orthologs in eukaryotes that perform similar functions in meiotic recombination (see Wikipedia articles RecA, RAD51 and DMC1). Both bacterial transformation and meiosis in eukaryotic microorganisms are induced by stressful circumstances such as overcrowding, resource depletion and DNA damaging conditions.^[50] This suggests that these sexual processes are adaptations for dealing with stress, particularly stress that causes DNA damage. In bacteria, these stresses induce an altered physiologic state, termed competence, that allows active take-up of DNA from a donor bacterium and the integration of this DNA into the recipient genome (see Natural competence) allowing recombinational repair of the recipients’ damaged DNA.^[58] If environmental stresses leading to DNA damage were a persistent challenge to the survival of early microorganisms, then selection would likely have been continuous through the prokaryote to eukaryote transition,^[52] and adaptative adjustments would have followed a course in which bacterial transformation naturally gave rise to sexual reproduction in eukaryotes.

Sex may also have been present even earlier, in the RNA world that is considered to have preceded DNA cellular life forms.^[59] A proposal for the origin of sex in the RNA world was based on the type of sexual interaction that is known to occur in extant single-stranded segmented RNA viruses such as influenza virus, and in extant double-stranded segmented RNA viruses such as reovirus.^[60] Exposure to conditions that cause RNA damage could have led to blockage of replication and death of these early RNA life forms. Sex would have allowed re-assortment of segments between two individuals with damaged RNA, permitting undamaged combinations of RNA segments to come together, thus allowing survival. Such a regeneration phenomenon, known as multiplicity reactivation, occurs in influenza virus^[61] and reovirus^[62]

Another theory is that sexual reproduction originated from selfish parasitic genetic elements that exchange genetic material (that is: copies of their own genome) for their transmission and propagation. In some organisms, sexual reproduction has been shown to enhance the spread of parasitic genetic elements (e.g.: yeast, filamentous fungi).^[63] Bacterial conjugation, a form of genetic exchange that some sources describe as sex, is not a form of reproduction, but rather an example of horizontal gene transfer. However, it does support the selfish genetic element theory, as it is propagated through such a "selfish gene", the F-plasmid.^[56] Similarly, it has been proposed that sexual reproduction evolved from ancient haloarchaea through a combination of jumping genes, and swapping plasmids.^[64]

A third theory is that sex evolved as a form of cannibalism. One primitive organism ate another one, but rather than completely digesting it, some of the 'eaten' organism's DNA was incorporated into the 'eater' organism.^[56]

Sex may also be derived from another prokaryotic process. A comprehensive 'origin of sex as vaccination' theory proposes that eukaryan sex-as-syngamy (fusion sex) arose from prokaryan unilateral sex-as-infection when infected hosts began swapping nuclearised genomes containing coevolved, vertically transmitted symbionts that provided protection against horizontal superinfection by more virulent symbionts. Sex-as-meiosis (fission sex) then evolved as a host strategy to uncouple (and thereby emasculate) the acquired symbiont genomes.^[65]

Mechanistic origin of sexual reproduction

Though theories positing benefits that lead to the origin of sex are often problematic, several additional theories on the evolution of the mechanisms of sexual reproduction have been proposed.

Viral eukaryogenesis

The viral eukaryogenesis (VE) theory proposes that eukaryotic cells arose from a combination of a lysogenic virus, an archaeon and a bacterium. This model suggests that the nucleus originated when the lysogenic virus incorporated genetic material from the archaeon and the bacterium and took over the role of information storage for the amalgam. The archaeal host transferred much of its functional genome to the virus during the evolution of cytoplasm but retained the function of gene translation and general metabolism. The bacterium transferred most of its functional genome to the virus as it transitioned into a mitochondrion.^[66]

For these transformations to lead to the eukaryotic cell cycle, the VE hypothesis specifies a pox-like virus as the lysogenic virus. A pox-like virus is a likely ancestor because of its fundamental similarities with eukaryotic nuclei. These include a double stranded DNA genome, a linear chromosome with short telomeric repeats, a complex membrane bound capsid, the ability to produce capped mRNA, and the ability to export the capped mRNA across the viral membrane into the cytoplasm. The presence of a lysogenic pox-like virus ancestor explains the development of meiotic division, an essential component of sexual reproduction.^[67]

Meiotic division in the VE hypothesis arose because of the evolutionary pressures placed on the lysogenic virus as a result of its inability to enter into the lytic cycle. This selective pressure resulted in the development of processes allowing the viruses to spread horizontally throughout the population. The outcome of this selection was cell-to-cell fusion. (This is distinct from the conjugation methods used by bacterial plasmids under evolutionary pressure, with important consequences.)^[66] The possibility of this kind of fusion is supported by the presence of fusion proteins in the envelopes of the pox viruses that allow them to fuse with host membranes. These proteins could have been transferred to the cell membrane during viral reproduction, enabling cell-to-cell fusion between the virus host and an uninfected cell. The theory proposes meiosis originated from the fusion between two cells infected with related but different viruses which recognised each other as uninfected. After the fusion of the two cells, incompatibilities between the two viruses result in a meiotic-like cell division.^[67]

The two viruses established in the cell would initiate replication in response to signals from the host cell. A mitosis-like cell cycle would proceed until the viral membranes dissolved, at which point linear chromosomes would be bound together with centromeres. The homologous nature of the two viral centromeres would incite the grouping of both sets into tetrads. It is speculated that this grouping may be the origin of crossing over, characteristic of the first division in modern meiosis.
The partitioning apparatus of the mitotic-like cell cycle the cells used to replicate independently would then pull each set of chromosomes to one side of the cell, still bound by centromeres. These centromeres would prevent their replication in subsequent division, resulting in four daughter cells with one copy of one of the two original pox-like viruses. The process resulting from combination of two similar pox viruses within the same host closely mimics meiosis.^[67]

Neomuran revolution

An alternative theory, proposed by Thomas Cavalier-Smith, was labeled the Neomuran revolution. The designation "Neomuran revolution" refers to the appearances of the common ancestors of eukaryotes and archaea. Cavalier-Smith proposes that the first neomurans emerged 850 million years ago. Other molecular biologists assume that this group appeared much earlier, but Cavalier-Smith dismisses these claims because they are based on the "theoretically and empirically" unsound model of molecular clocks. Cavalier-Smith's theory of the Neomuran revolution has implications for the evolutionary history of the cellular machinery for recombination and sex. It suggests that this machinery evolved in two distinct bouts separated by a long period of stasis; first the appearance of recombination machinery in a bacterial ancestor which was maintained for 3 Gy,^{[clarification needed]} until the neomuran revolution when the mechanics were adapted to the presence of nucleosomes. The archaeal products of the revolution maintained recombination machinery that was essentially bacterial, whereas the eukaryotic products broke with this bacterial continuity. They introduced cell fusion and ploidy cycles into cell life histories. Cavalier-Smith argues that both bouts of mechanical evolution were motivated by similar selective forces: the need for accurate DNA replication without loss of viability.^[68]

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Saturday, August 16, 2014

Evolution of sexual reproduction

Evolution of sexual reproduction

Historical perspective

Questions

Two-fold cost of sex

Promotion of genetic variation

Spread of advantageous traits

Novel genotypes

Increased resistance to parasites

Deleterious mutation clearance

Maintenance of mutation-free individuals

Removal of deleterious genes

Other explanations

Speed of evolution

DNA repair and complementation

Libertine bubble theory

Origin of sexual reproduction

Mechanistic origin of sexual reproduction

Viral eukaryogenesis

Neomuran revolution

Inequality (mathematics)