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Thursday, March 21, 2024

History of electric power transmission

Electric power transmission, the tools and means of moving electricity far from where it is generated, date back to the late 19th century. They include the movement of electricity in bulk (formally called "transmission") and the delivery of electricity to individual customers ("distribution"). In the beginning, the two terms were used interchangeably.

Early transmission

Berlin, 1884. With double the brilliance of gaslight, arc lamps were in high demand for stores and public areas. Arc lighting circuits used up to thousands of volts with arc lamps connected in series.

Prior to electricity, various systems had been used for transmission of power across large distances. Chief among them were telodynamic (cable in motion), pneumatic (pressurized air), and hydraulic (pressurized liquid) transmission. Cable cars were the most frequent example of telodynamic transmission, whose lines could extend for several miles for a single section. Pneumatic transmission was used for city power transmission systems in Paris, Birmingham, Rixdorf, Offenbach, Dresden and Buenos Aires at the beginning of the twentieth century. Cities in the 19th century also used hydraulic transmission using high pressure water mains to deliver power to factory motors. London's system delivered 7,000 horsepower (5.2 MW) over a 180-mile (290 km) network of pipes carrying water at 800 pounds per square inch (5.5 MPa). These systems were replaced by cheaper and more versatile electrical systems, but by the end of the 19th century, city planners and financiers were well aware of the benefits, economics, and process of establishing power transmission systems.

In the early days of electric power usage, widespread transmission of electric power had two obstacles. First, devices requiring different voltages required specialized generators with their own separate lines. Street lights, electric motors in factories, power for streetcars and lights in homes are examples of the diversity of devices with voltages requiring separate systems. Secondly, generators had to be relatively near their loads (a mile or less for low voltage devices). It was known that longer distance transmission was possible the higher the voltage was raised, so both problems could be solved if transforming voltages from a single universal power line could be done efficiently.

Specialized systems

Streetcars created enormous demand for early electricity. This Siemens Tram from 1884 required 500 V direct current, which was typical.

Much of early electricity was direct current, which could not easily be increased or decreased in voltage either for long-distance transmission or for sharing a common line to be used with multiple types of electric devices. Companies simply ran different lines for the different classes of loads their inventions required. For example, Charles Brush's New York arc lamp systems required up to 10 kV for many lamps in a series circuit, Edison's incandescent lights used 110 V, streetcars built by Siemens or Sprague required large motors in the 500 volt range, whereas industrial motors in factories used still other voltages. Due to this specialization of lines, and because transmission was so inefficient, it seemed at the time that the industry would develop into what is now known as a distributed generation system with large numbers of small generators located near their loads.

Early high voltage exterior lighting

High voltage was of interest to early researchers working on the problem of transmission over distance. They knew from elementary electricity principle that the same amount of power could be transferred on a cable by doubling the voltage and halving the current. Due to Joule's Law, they also knew that the power lost from heat in a wire is proportional to the square of the current traveling on it, regardless the voltage, and so by doubling the voltage, the same cable would be capable of transmitting the same amount of power four times the distance.

At the Paris Exposition of 1878, electric arc lighting had been installed along the Avenue de l'Opera and the Place de l'Opera, using electric Yablochkov arc lamps, powered by Zénobe Gramme alternating current dynamos. Yablochkov candles required high voltage, and it was not long before experimenters reported that the arc lamps could be powered on a 14-kilometre (8.7 mi) circuit. Within a decade scores of cities would have lighting systems using a central power plant that provided electricity to multiple customers via electrical transmission lines. These systems were in direct competition with the dominant gaslight utilities of the period.

Brush Electric Company's central power plant dynamos powered arc lamps for public lighting in New York. Beginning operation in December 1880 at 133 West Twenty-Fifth Street, it powered a 2-mile (3.2 km) long circuit.

The idea of investing in a central plant and a network to deliver energy produced to customers who pay a recurring fee for service was familiar business model for investors: it was identical to the lucrative gaslight business, or the hydraulic and pneumatic power transmission systems. The only difference was the commodity being delivered was electricity, not gas, and the "pipes" used for delivering were more flexible.

The California Electric Company (now PG&E) in San Francisco in 1879 used two direct current generators from Charles Brush's company to supply multiple customers with power for their arc lamps. This San Francisco system was the first case of a utility selling electricity from a central plant to multiple customers via transmission lines. CEC soon opened a second plant with 4 additional generators. Service charges for light from sundown to midnight was $10 per lamp per week.

Grand Rapids Electric Light & Power Company, established in March 1880 by William T. Powers and others, began operation of the world's first commercial central station hydroelectric power plant, Saturday, July 24, 1880, getting power from Wolverine Chair and Furniture Company's water turbine. It operated a 16-light Brush electric dynamo lighting several storefronts in Grand Rapids, Michigan. It is the earliest predecessor of Consumers Energy of Jackson, Michigan.

In December 1880, Brush Electric Company set up a central station to supply a 2-mile (3.2 km) length of Broadway with arc lighting. By the end of 1881, New York, Boston, Philadelphia, Baltimore, Montreal, Buffalo, San Francisco, Cleveland and other cities had Brush arc lamp systems, producing public light well into the 20th century. By 1893 there were 1500 arc lamps illuminating New York streets.

Direct current lighting

Early arc lights were extremely bright and the high voltages presented a sparking/fire hazard, making them too dangerous to use indoors. In 1878 inventor Thomas Edison saw a market for a system that could bring electric lighting directly into a customer's business or home, a niche not served by arc lighting systems. After devising a commercially viable incandescent light bulb in 1879, Edison went on to develop the first large scale investor-owned electric illumination "utility" in lower Manhattan, eventually serving one square mile with 6 "jumbo dynamos" housed at Pearl Street Station.When service began in September 1882, there were 85 customers with 400 light bulbs. Each dynamo produced 100 kW – enough for 1200 incandescent lights, and transmission was at 110 V via underground conduits. The system cost $300,000 to build with installation of the 100,000 feet (30,000 m) of underground conduits one of the most expensive parts of the project. Operating expenses exceeded income in the first two years and fire destroyed the plant in 1890. Further, Edison had a three wire system so that either 110 V or 220 V could be supplied to power some motors.

Availability of large-scale generation

Availability of large amounts of power from diverse locations would become possible after Charles Parsons' production of turbogenerators beginning 1889. Turbogenerator output quickly jumped from 100 kW to 25 megawatts in two decades. Prior to efficient turbogenerators, hydroelectric projects were a significant source of large amounts of power requiring transmission infrastructure.

Transformers and alternating current

When George Westinghouse became interested in electricity, he quickly and correctly concluded that Edison's low voltages were too inefficient to be scaled up for transmission needed for large systems. He further understood that long-distance transmission needed high voltage and that inexpensive conversion technology only existed for alternating current. Transformers would play the decisive role in the victory of alternating current over direct current for transmission and distribution systems. In 1876, Pavel Yablochkov patented his mechanism of using induction coils to serve as a step up transformer prior to the Paris Exposition demonstrating his arc lamps. In 1881, Lucien Gaulard and John Dixon Gibbs developed a more efficient device which they dubbed the secondary generator, namely an early step down transformer whose ratio could be adjusted by configuring the connections between a series of wired bobbins around a spindle, from which an iron core could be added or removed as necessary to vary the power output. The device was subject to various critisims and was occasionally misunderstood as only providing a 1:1 turn ratio.

The first demonstrative long-distance (34 km, 21 mi) AC line was built for the 1884 International Exhibition of Turin, Italy. It was powered by a 2-kV, 130-Hz Siemens & Halske alternator and featured several Gaulard secondary generators with their primary windings connected in series, which fed incandescent lamps. The system proved the feasibility of AC electric power transmission over long distances. Between 1884 and 1885, Hungarian engineers Zipernowsky, Bláthy, and Déri from the Ganz company in Budapest created the efficient "Z.B.D." closed-core coils, as well as the modern electric distribution system. The three had discovered that all former coreless or open-core devices were incapable of regulating voltage, and were therefore impractical. Their joint patent described two versions of a design with no poles: the "closed-core transformer" and the "shell-core transformer". Ottó Bláthy suggested the use of closed-cores, Károly Zipernowsky the use of shunt connections, and Miksa Déri performed the experiments. The new ZBD transformers were 3.4 times more efficient than the open-core bipolar devices of Gaulard and Gibbs.

In the closed-core transformer the iron core is a closed ring around which the two coils are wound. In the shell type transformer, the windings are passed through the core. In both designs, the magnetic flux linking the primary and secondary windings travels almost entirely within the iron core, with no intentional path through air. The core consists of iron strands or sheets. These revolutionary design elements would finally make it technically and economically feasible to provide electric power for lighting in homes, businesses and public spaces. Ottó Bláthy also discovered the transformer formula, Vs/Vp = Ns/Np. Electrical and electronic systems the world over rely on the principles of the original Ganz transformers. The inventors are also credited with the first use of the word "transformer" to describe a device for altering the EMF of an electric current.

A very first operative AC line was put into service in 1885 in via dei Cerchi, Rome, Italy, for public lighting. It was powered by two Siemens & Halske alternators rated 30 hp (22 kW), 2 kV at 120 Hz and used 200 series-connected Gaulard 2-kV/20-V step-down transformers provided with a closed magnetic circuit, one for each lamp. Few months later it was followed by the first British AC system, which was put into service at the Grosvenor Gallery, London. It also featured Siemens alternators and 2.4-kV/100-V step-down transformers, one per user, with shunt-connected primaries.

The concept that is the basis of modern transmission using inexpensive step up and step down transformers was first implemented by Westinghouse, William Stanley, Jr. and Franklin Leonard Pope in 1886 in Great Barrington, Massachusetts, resorting also to European technology. In 1888 Westinghouse also licensed Nikola Tesla's induction motor which they would eventually develop into a usable (2-phase) AC motor. The modern 3-phase system was developed by Mikhail Dolivo-Dobrovolsky and Allgemeine Elektricitäts-Gesellschaft and Charles Eugene Lancelot Brown in Europe, starting in 1889.

The International Electro-Technical Exhibition of 1891, in Frankfurt, Germany, featured the long-distance transmission of high-power, three-phase electric current. It was held between 16 May and 19 October on the disused site of the three former “Westbahnhöfe” (Western Railway Stations) in Frankfurt am Main. The exhibition featured the first long-distance transmission of high-power, three-phase electric current, which was generated 175 km away at Lauffen am Neckar. It successfully operated motors and lights at the fair. When the exhibition closed, the power station at Lauffen continued in operation, providing electricity for the administrative capital, Heilbronn, making it the first place to be equipped with three-phase AC power. Many corporate technical representatives (including E.W. Rice of Thomson-Houston Electric Company (what became General Electric)) attended. The technical advisers and representatives were impressed. As a result of the successful field trial, three-phase current, as far as Germany was concerned, became the most economical means of transmitting electrical energy.

The simplicity of polyphase generators and motors meant that besides their efficiency they could be manufactured cheaply, compactly and would require little attention to maintain. Simple economics would drive the expensive, bulky and mechanically complex DC dynamos to their ultimate extinction. As it turned out, the deciding factor in the war of the currents was the availability of low cost step up and step down transformers that meant that all customers regardless of their specialized voltage requirements could be served at minimal cost of conversion. This "universal system" is today regarded as one of the most influential innovations for the use of electricity.

High voltage direct current transmission

The case for alternating current was not clear at the turn of the century and high voltage direct current transmission systems were successfully installed without the benefit of transformers. Rene Thury, who had spent six months at Edison's Menlo Park facility, understood his problem with transmission and was convinced that moving electricity over great distances was possible using direct current. He was familiar with the work of Marcel Deprez, who did early work on high voltage transmission after being inspired by the capability of arc lamp generators to support lights over great distances. Deprez avoided transformers by placing generators and loads in series as arc lamp systems of Charles F. Brush did. Thury developed this idea into the first commercial system for high-voltage DC transmission. Like Brush's dynamos, current is kept constant, and when increasing load demands more pressure, voltage is increased. The Thury System was successfully used on several DC transmission projects from Hydro generators. The first in 1885 was a low voltage system in Bözingen, and the first high voltage system went into service in 1889 in Genoa, Italy, by the Acquedotto de Ferrari-Galliera company. This system transmitted 630 kW at 14 kV DC over a circuit 120 km long. The largest Thury System was the Lyon Moutiers project that was 230 km in length, eventually delivering 20 megawatts, at 125 kV.

Victory for AC

Ultimately, the versatility of the Thury system was hampered by the fragility of series distribution, and the lack of a reliable DC conversion technology that would not show up until the 1940s with improvements in mercury arc valves. The AC "universal system" won by force of numbers, proliferating systems with transformers both to couple generators to high-voltage transmission lines, and to connect transmission to local distribution circuits. By a suitable choice of utility frequency, both lighting and motor loads could be served. Rotary converters and later mercury-arc valves and other rectifier equipment allowed DC load to be served by local conversion where needed. Even generating stations and loads using different frequencies could also be interconnected using rotary converters. By using common generating plants for every type of load, important economies of scale were achieved, lower overall capital investment was required, load factor on each plant was increased allowing for higher efficiency, allowing for a lower cost of energy to the consumer and increased overall use of electric power.

By allowing multiple generating plants to be interconnected over a wide area, electricity production cost was reduced. The most efficient available plants could be used to supply the varying loads during the day. Reliability was improved and capital investment cost was reduced, since stand-by generating capacity could be shared over many more customers and a wider geographic area. Remote and low-cost sources of energy, such as hydroelectric power or mine-mouth coal, could be exploited to lower energy production cost.

The first transmission of three-phase alternating current using high voltage took place in 1891 during the international electricity exhibition in Frankfurt. A 15 kV transmission line connected Lauffen on the Neckar and Frankfurt am Main, 175 km (109 mi) apart.

Willamette Falls to Niagara Falls

In 1882, the German Miesbach–Munich Power Transmission used 2kV DC over 57 km (35 mi). In 1889, the first long-distance transmission of DC electricity in the United States was switched on at Willamette Falls Station, in Oregon City, Oregon. In 1890, a flood destroyed the power station. This unfortunate event paved the way for the first long-distance transmission of AC electricity in the world when Willamette Falls Electric company installed experimental AC generators from Westinghouse in 1890.

That same year, the Niagara Falls Power Company (NFPC) and its subsidiary Cataract Company formed the International Niagara Commission composed of experts, to analyze proposals to harness Niagara Falls to generate electricity. The commission was led by Sir William Thomson (later Lord Kelvin) and included Eleuthère Mascart from France, William Unwin from England, Coleman Sellers from the US, and Théodore Turrettini from Switzerland. It was backed by entrepreneurs such as J. P. Morgan, Lord Rothschild, and John Jacob Astor IV. Among 19 proposals, they even briefly considered compressed air as a power transmission medium, but preferred electricity. They could not decide which method would be best overall.

By 1893 the Niagara Falls Power Company had rejected the remaining proposals from a half dozen companies and awarded the generating contract to Westinghouse with further transmission lines and transformer contracts awarded to General Electric. Work began in 1893 on the Niagara Falls generation project: 5,000 horsepower (3,700 kW) was to be generated and transmitted as alternating current, at a frequency of 25 Hz to minimize impedance losses in transmission (changed to 60 Hz in the 1950s).

Westinghouse also had to develop a system based on rotary converters to allow them to supply all the needed power standards including single phase and polyphase AC and DC for street cars and factory motors. Westinghouse's initial customer for the power from the hydroelectric generators at the Edward Dean Adams Station at Niagara in 1895 were the plants of the Pittsburgh Reduction Company which needed large quantities of cheap electricity for smelting aluminum. On November 16, 1896, electrical power transmitted to Buffalo began powering its street cars. The generating plants were built by Westinghouse Electric Corporation. The scale of the project had General Electric also contributing, building transmission lines and equipment. That same year Westinghouse and General Electric signed a patent sharing agreement, ending some 300 lawsuits the companies were involved in over their competing electrical patents, and giving them monopolistic control over the US electric power industry for years to come.

Initially transmission lines were supported by porcelain pin-and-sleeve insulators similar to those used for telegraphs and telephone lines. However, these had a practical limit of 40 kV. In 1907, the invention of the disc insulator by Harold W. Buck of the Niagara Falls Power Corporation and Edward M. Hewlett of General Electric allowed practical insulators of any length to be constructed for higher voltages.

Early 20th century

The first 110 kV transmission line in Europe was built around 1912 between Lauchhammer and Riesa, German Empire. Original pole.

Voltages used for electric power transmission increased throughout the 20th century. The first "high voltage" AC power station, rated 4-MW 10-kV 85-Hz, was put into service in 1889 by Sebastian Ziani de Ferranti at Deptford, London. The first electric power transmission line in North America operated at 4000 V. It went online on June 3, 1889, with the lines between the generating station at Willamette Falls in Oregon City, Oregon, and Chapman Square in downtown Portland, Oregon stretching about 13 miles. By 1914 fifty-five transmission systems operating at more than 70,000 V were in service, and the highest voltage then used was 150 kV. The first three-phase alternating current power transmission at 110 kV took place in 1907 between Croton and Grand Rapids, Michigan. Voltages of 100 kV and more were not established technology until around 5 years later, with for example the first 110 kV line in Europe between Lauchhammer and Riesa, Germany, in 1912.

In the early 1920s the Pit RiverCottonwood – Vaca-Dixon line was built for 220 kV transporting power from hydroelectric plants in the Sierra Nevada to the San Francisco Bay Area, at the same time the Big CreekLos Angeles lines were upgraded to the same voltage. Both of those systems entered commercial service in 1923. On April 17, 1929 the first 220 kV line in Germany was completed, running from Brauweiler near Cologne, over Kelsterbach near Frankfurt, Rheinau near Mannheim, Ludwigsburg–Hoheneck near Austria. This line comprises the North-South interconnect, at the time one of the world's largest power systems. The masts of this line were designed for eventual upgrade to 380 kV. However the first transmission at 380 kV in Germany was on October 5, 1957 between the substations in Rommerskirchen and Ludwigsburg–Hoheneck.

The world's first 380 kV power line was built in Sweden, the 952 km HarsprångetHallsberg line in 1952. In 1965, the first extra-high-voltage transmission at 735 kV took place on a Hydro-Québec transmission line. In 1982 the first transmission at 1200 kV was in the Soviet Union.

The rapid industrialization in the 20th century made electrical transmission lines and grids a critical part of the economic infrastructure in most industrialized nations. Interconnection of local generation plants and small distribution networks was greatly spurred by the requirements of World War I, where large electrical generating plants were built by governments to provide power to munitions factories; later these plants were connected to supply civil load through long-distance transmission.

Small municipal electrical utilities did not necessarily desire to reduce the cost of each unit of electricity sold; to some extent, especially during the period 1880–1890, electrical lighting was considered a luxury product and electric power was not substituted for steam power. Engineers such as Samuel Insull in the United States and Sebastian Z. De Ferranti in the United Kingdom were instrumental in overcoming technical, economic, regulatory and political difficulties in development of long-distance electric power transmission. By introduction of electric power transmission networks, in the city of London the cost of a kilowatt-hour was reduced to one-third in a ten-year period.

In 1926 electrical networks in the United Kingdom began to be interconnected in the National Grid, initially operating at 132 kV.

Power electronics

Power electronics is the application of solid-state electronics to the control and conversion of electric power. Power electronics started with the development of the mercury arc rectifier. Invented by Peter Cooper Hewitt in 1902, it was used to convert alternating current (AC) into direct current (DC). From the 1920s on, research continued on applying thyratrons and grid-controlled mercury arc valves to power transmission. Uno Lamm developed a mercury valve with grading electrodes making them suitable for high voltage direct current power transmission. In 1933 selenium rectifiers were invented.

Comparative planetary science

Comparative planetary science or comparative planetology is a branch of space science and planetary science in which different natural processes and systems are studied by their effects and phenomena on and between multiple bodies. The planetary processes in question include geology, hydrology, atmospheric physics, and interactions such as impact cratering, space weathering, and magnetospheric physics in the solar wind, and possibly biology, via astrobiology.

Comparison of multiple bodies assists the researcher, if for no other reason than the Earth is far more accessible than any other body. Those distant bodies may then be evaluated in the context of processes already characterized on Earth. Conversely, other bodies (including extrasolar ones) may provide additional examples, edge cases, and counterexamples to earthbound processes; without a greater context, studying these phenomena in relation to Earth alone may result in low sample sizes and observational biases.

Background

The term "comparative planetology" was coined by George Gamow, who reasoned that to fully understand our own planet, we must study others. Poldervaart focused on the Moon, stating "An adequate picture of this original planet and its development to the present earth is of great significance, is in fact the ultimate goal of geology as the science leading to knowledge and understanding of earth's history."

Geology, geochemistry, and geophysics

All terrestrial planets (and some satellites, such as the Moon) are essentially composed of silicates wrapped around iron cores. The large outer Solar System moons and Pluto have more ice, and less rock and metal, but still undergo analogous processes.

Volcanism

Volcanism on Earth is largely lava-based. Other terrestrial planets display volcanic features assumed to be lava-based, evaluated in the context of analogues readily studied on Earth. For example, Jupiter's moon Io displays extant volcanism, including lava flows. These flows were initially inferred to be composed mostly of various forms of molten elemental sulfur, based on analysis of imaging done by the Voyager probes. However, Earth-based infrared studies done in the 1980s and 1990s caused the consensus to shift in favor of a primarily silicate-based model, with sulfur playing a secondary role.

Much of the surface of Mars is composed of various basalts considered analogous to Hawaiian basalts, by their spectra and in situ chemical analyses (including Martian meteorites). Mercury and Earth's Moon similarly feature large areas of basalts, formed by ancient volcanic processes. Surfaces in the polar regions show polygonal morphologies, also seen on Earth.

In addition to basalt flows, Venus is home to a large number of pancake dome volcanoes created by highly viscous silica-rich lava flows. These domes lack a known Earth analogue. They do bear some morphological resemblance to terrestrial rhyolite-dacite lava domes, although the pancake domes are much flatter and uniformly round in nature.

Certain regions further out in the Solar System exhibit cryovolcanism, a process not seen anywhere on earth. Cryovolcanism is studied through laboratory experiments, conceptual and numerical modeling, and by cross-comparison to other examples in the field. Examples of bodies with cryovolcanic features include comets, some asteroids and Centaurs, Mars, Europa, Enceladus, Triton, and possibly Titan, Ceres, Pluto, and Eris.

The trace dopants of Europa's ice are currently postulated to contain sulfur. This is being evaluated via a Canadian sulfate spring as an analogue, in preparation for future Europa probes. Small bodies such as comets, some asteroid types, and dust grains, on the other hand, serve as counterexamples. Assumed to have experienced little or no heating, these materials may contain (or be) samples representing the early Solar System, which have since been erased from Earth or any other large body.

Some extrasolar planets are covered entirely in lava oceans, and some are tidally locked planets, whose star-facing hemisphere is entirely lava.

Cratering

The craters observed on the Moon were once assumed to be volcanic. Earth, by comparison, did not show a similar crater count, nor a high frequency of large meteor events, which would be expected as two nearby bodies should experience similar impact rates. Eventually this volcanism model was overturned, as numerous Earth craters (demonstrated by e. g., shatter cones, shocked quartz and other impactites, and possibly spall) were found, after having been eroded over geologic time. Craters formed by larger and larger ordnance also served as models. The Moon, on the other hand, shows no atmosphere or hydrosphere, and could thus accumulate and preserve impact craters over billions of years despite a low impact rate at any one time. In addition, more searches by more groups with better equipment highlighted the great number of asteroids, presumed to have been even more numerous in earlier Solar System periods. As on Earth, a low crater count on other bodies indicates young surfaces. This is particularly credible if nearby regions or bodies show heavier cratering. Young surfaces, in turn, indicate atmospheric, tectonic or volcanic, or hydrological processing on large bodies and comets, or dust redistribution or a relatively recent formation on asteroids (i. e., splitting from a parent body).

Examination of the cratering record on multiple bodies, at multiple areas in the Solar System, points to a Late Heavy Bombardment, which in turn gives evidence of the Solar System's early history. However, the Late Heavy Bombardment as currently proposed has some issues and is not completely accepted.

One model for Mercury's exceptionally high density compared to other terrestrial planets is the stripping off of a significant amount of crust and/or mantle from extremely heavy bombardment.

Differentiation

As a large body, Earth can efficiently retain its internal heat (from its initial formation plus decay of its radioisotopes) over the long timescale of the Solar System. It thus retains a molten core, and has differentiated- dense materials have sunk to the core, while light materials float to form a crust.

Other bodies, by comparison, may or may not have differentiated, based on their formation history, radioisotope content, further energy input via bombardment, distance from the Sun, size, etc. Studying bodies of various sizes and distances from the Sun provides examples and places constraints on the differentiation process. Differentiation itself is evaluated indirectly, by the mineralogy of a body's surface, versus its expected bulk density and mineralogy, or via shape effects due to slight variations in gravity. Differentiation may also be measured directly, by the higher-order terms of a body's gravity field as measured by a flyby or gravitational assist, and in some cases by librations.

Edge cases include Vesta and some of the larger moons, which show differentiation but are assumed to have since fully solidified. The question of whether Earth's Moon has solidified, or retains some molten layers, has not been definitively answered. Additionally, differentiation processes are expected to vary along a continuum. Bodies may be composed of lighter and heavier rocks and metals, a high water ice and volatiles content (with less mechanical strength) in cooler regions of the Solar System, or primarily ices with a low rock/metal content even farther from the Sun. This continuum is thought to record the varying chemistries of the early Solar System, with refractories surviving in warm regions, and volatiles driven outward by the young Sun.

The cores of planets are inaccessible, studied indirectly by seismometry, gravimetry, and in some cases magnetometry. However, iron and stony-iron meteorites are likely fragments from the cores of parent bodies which have partially or completely differentiated, then shattered. These meteorites are thus the only means of directly examining deep-interior materials and their processes.

Gas giant planets represent another form of differentiation, with multiple fluid layers by density. Some distinguish further between true gas giants, and ice giants further from the Sun.

Tectonics

In turn, a molten core may allow plate tectonics, of which Earth shows major features. Mars, as a smaller body than Earth, shows no current tectonic activity, nor mountain ridges from geologically recent activity. This is assumed to be due to an interior that has cooled faster than the Earth (see geomagnetism below). An edge case may be Venus, which does not appear to have extant tectonics. However, in its history, it likely has had tectonic activity but lost it. It is possible tectonic activity on Venus may still be sufficient to restart after a long era of accumulation.

Io, despite having high volcanism, does not show any tectonic activity, possibly due to sulfur-based magmas with higher temperatures, or simply higher volumetric fluxes. Meanwhile, Vesta's fossae may be considered a form of tectonics, despite that body's small size and cool temperatures.

Europa is a key demonstration of outer-planet tectonics. Its surface shows movement of ice blocks or rafts, strike-slip faults, and possibly diapirs. The question of extant tectonics is far less certain, possibly having been replaced by local cryomagmatism. Ganymede and Triton may contain tectonically or cryovolcanically resurfaced areas, and Miranda's irregular terrains may be tectonic.

Earthquakes are well-studied on Earth, as multiple seismometers or large arrays can be used to derive quake waveforms in multiple dimensions. The Moon is the only other body to successfully receive a seismometer array; "marsquakes" and the Mars interior are based on simple models and Earth-derived assumptions. Venus has received negligible seismometry.

Gas giants may in turn show different forms of heat transfer and mixing. Furthermore, gas giants show different heat effects by size and distance to the Sun. Uranus shows a net negative heat budget to space, but the others (including Neptune, farther out) are net positive.

Geomagnetism

Two terrestrial planets (Earth and Mercury) display magnetospheres, and thus have molten metal layers. Similarly, all four gas giants have magnetospheres, which indicate layers of conductive fluids. Ganymede also shows a weak magnetosphere, taken as evidence of a subsurface layer of salt water, while the volume around Rhea shows symmetrical effects which may be rings or a magnetic phenomenon. Of these, Earth's magnetosphere is by far the most accessible, including from the surface. It is therefore the most studied, and extraterrestrial magnetospheres are examined in light of prior Earth studies.

Still, differences exist between magnetospheres, pointing to areas needing further research. Jupiter's magnetosphere is stronger than the other gas giants, while Earth's is stronger than Mercury's. Mercury and Uranus have offset magnetospheres, which have no satisfactory explanation yet. Uranus' tipped axis causes its magnetotail to corkscrew behind the planet, with no known analogue. Future Uranian studies may show new magnetospheric phenomena.

Mars shows remnants of an earlier, planetary-scale magnetic field, with stripes as on Earth. This is taken as evidence that the planet had a molten metal core in its prior history, allowing both a magnetosphere and tectonic activity (as on Earth). Both of these have since dissipated. Earth's Moon shows localized magnetic fields, indicating some process other than a large, molten metal core. This may be the source of lunar swirls, not seen on Earth.

Geochemistry

Apart from their distance to the Sun, different bodies show chemical variations indicating their formation and history. Neptune is denser than Uranus, taken as one piece of evidence that the two may have switched places in the early Solar System. Comets show both high volatile content, and grains containing refractory materials. This also indicates some mixing of materials through the Solar System when those comets formed. Mercury's inventory of materials by volatility is being used to evaluate different models for its formation and/or subsequent modification.

Isotopic abundances indicate processes over the history of the Solar System. To an extent, all bodies formed from the presolar nebula. Various subsequent processes then alter elemental and isotopic ratios. The gas giants in particular have enough gravity to retain primary atmospheres, taken largely from the presolar nebula, as opposed to the later outgassing and reactions of secondary atmospheres. Differences in gas giant atmospheres compared to solar abundances then indicate some process in that planet's history. Meanwhile, gases at small planets such as Venus and Mars have isotopic differences indicating atmospheric escape processes.{argon isotope ratio planet meteorite}{neon isotope ratio meteorite}

The various modifications of surface minerals, or space weathering, is used to evaluate meteorite and asteroid types and ages. Rocks and metals shielded by atmospheres (particularly thick ones), or other minerals, experience less weathering and fewer implantation chemistries and cosmic ray tracks. Asteroids are currently graded by their spectra, indicating surface properties and mineralogies. Some asteroids appear to have less space weathering, by various processes including a relatively recent formation date or a "freshening" event. As Earth's minerals are well shielded, space weathering is studied via extraterrestrial bodies, and preferably multiple examples.

Kuiper Belt Objects display very weathered or in some cases very fresh surfaces. As the long distances result in low spatial and spectral resolutions, KBO surface chemistries are currently evaluated via analogous moons and asteroids closer to Earth.

Aeronomy and atmospheric physics

Earth's atmosphere is far thicker than that of Mars, while far thinner than Venus'. In turn, the envelopes of gas giants are a different class entirely, and show their own gradations. Meanwhile, smaller bodies show tenuous atmospheres ("surface-bound exospheres"), with the exception of Titan and arguably Triton. Comets vary between negligible atmospheres in the outer Solar System, and active comas millions of miles across at perihelion. Exoplanets may in turn possess atmospheric properties known and unknown in the Milky Way Galaxy.

Aeronomy

Atmospheric escape is largely a thermal process. The atmosphere a body can retain therefore varies from the warmer inner Solar System, to the cooler outer regions. Different bodies in different Solar System regions provide analogous or contrasting examples. The atmosphere of Titan is considered analogous to an early, colder Earth; the atmosphere of Pluto is considered analogous to an enormous comet.

The presence or absence of a magnetic field affects an upper atmosphere, and in turn the overall atmosphere. Impacts of solar wind particles create chemical reactions and ionic species, which may in turn affect magnetospheric phenomena. Earth serves as a counterexample to Venus and Mars, which have no planetary magnetospheres, and to Mercury, with a magnetosphere but negligible atmosphere.

Jupiter's moon Io creates sulfur emissions, and a feature of sulfur and some sodium around that planet. Similarly, Earth's Moon has trace sodium emissions, and a far weaker tail. Mercury also has a trace sodium atmosphere.

Jupiter itself is assumed to have some characteristics of extrasolar "super Jupiters" and brown dwarves.

Seasons

Uranus, tipped on its side, is postulated to have seasonal effects far stronger than on Earth. Similarly, Mars is postulated to have varied its axial tilt over eons, and to a far greater extent than on Earth. This is hypothesized to have dramatically altered not only seasons but climates on Mars, for which some evidence has been observed. Venus has negligible tilt, eliminating seasons, and a slow, retrograde rotation, causing different diurnal effects than on Earth and Mars.

Clouds and haze layers

From Earth, a planetwide cloud layer is the dominant feature of Venus in the visible spectrum; this is also true of Titan. Venus' cloud layer is composed of sulfur dioxide particles, while Titan's is a mixture of organics.

The gas giant planets display clouds or belts of various compositions, including ammonia and methane.

Circulation and winds

Venus and Titan, and to a lesser extent Earth, are super-rotators: the atmosphere turns about the planet faster than the surface beneath. While these atmospheres share physical processes, they exhibit diverse characteristics.

Hadley cells, first postulated and confirmed on Earth, are seen in different forms in other atmospheres. Earth has Hadley cells north and south of its equator, leading to additional cells by latitude. Mars' Hadley circulation is offset from its equator. Titan, a far smaller body, likely has one enormous cell, flipping polarity from northerly to southerly with its seasons.

The bands of Jupiter are thought to be numerous Hadley-like cells by latitude.

Storms and cyclonic activity

The large storms seen on the gas giants are considered analogous to Earth cyclones. However, this is an imperfect metaphor as expected, due to the large differences in sizes, temperature, and composition between Earth and the gas giants, and even between gas giants.

Polar vortices were observed on Venus and Saturn. In turn, Earth's thinner atmosphere shows weaker polar vorticity and effects.

Lightning and aurorae

Both lightning and aurorae have been observed on other bodies after extensive study at Earth. Lightning has been detected on Venus, and may be a sign of active volcanism on that planet, as volcanic lightning is known on Earth. Aurorae have been observed on Jupiter and its moon Ganymede.

Comparative climatology

An understanding of the evolutionary histories and current states of the Venus and Mars climates is directly relevant for studies of the past, present and future climates of Earth.

Hydrology

A growing number of bodies display relict or current hydrological modification. Earth, the "ocean planet," is the prime example. Other bodies display lesser modifications, indicating their similarities and differences. This may be defined to include fluids other than water, such as light hydrocarbons on Titan, or possibly supercritical carbon dioxide on Mars, which do not persist in Earth conditions. Ancient lava flows in turn may be considered a form of hydrological modification, which may be confounded with other fluids. Io currently has lava calderas and lakes. Fluid modification may have occurred on bodies as small as Vesta; hydration in general has been observed.

If fluids include groundwater and vapor, the list of bodies with hydrological modification includes Earth, Mars, and Enceladus, to a lesser extent comets and some asteroids, likely Europa and Triton, and possibly Ceres, Titan, and Pluto. Venus may have had hydrology in its early history, which would since have been erased.

Fluid modification and mineral deposition on Mars, as observed by the MER and MSL rovers, is studied in light of Earth features and minerals. Minerals observed from orbiters and landers indicates formation in aqueous conditions; morphologies indicate fluid action and deposition.

Extant Mars hydrology includes brief, seasonal flows on slopes; however, most Martian water is frozen into its polar caps and subsurface, as indicated by ground penetrating radars and pedestal craters. Antifreeze mixtures such as salts, peroxides, and perchlorates may allow fluid flow at Martian temperatures.

Analogues of Mars landforms on Earth include Siberian and Hawaiian valleys, Greenland slopes, the Columbian Plateau, and various playas. Analogues for human expeditions (e.g. geology and hydrology fieldwork) include Devon Island, Canada, Antarctica, Utah, the Euro-Mars project, and Arkaroola, South Australia.

The Moon, on the other hand, is a natural laboratory for regolith processes and weathering on anhydrous airless bodies- modification and alteration by meteoroid and micrometeoroid impacts, the implantation of solar and interstellar charged particles, radiation damage, spallation, exposure to ultraviolet radiation, and so on. Knowledge of the processes that create and modify the lunar regolith is essential to understanding the compositional and structural attributes of other airless planet and asteroid regoliths.

Other possibilities include extrasolar planets completely covered by oceans, which would lack some Earthly processes.

Dynamics

Earth, alone among terrestrial planets, possesses a large moon. This is thought to confer stability to Earth's axial tilt, and thus seasons and climates. The closest analogue is the Pluto-Charon system, though its axial tilt is completely different. Both the Moon and Charon are hypothesized to have formed via giant impacts.

Giant impacts are hypothesized to account for both the tilt of Uranus, and the retrograde rotation of Venus. Giant impacts are also candidates for the Mars ocean hypothesis, and the high density of Mercury.

Most giant planets (except Neptune) have retinues of moons, rings, ring shepherds, and moon Trojans analogous to mini-solar systems. These systems are postulated to have accreted from analogous gas clouds, and possibly with analogous migrations during their formation periods. The Cassini mission was defended on the grounds that Saturn system dynamics would contribute to studies of Solar System dynamics and formation.

Studies of ring systems inform us of many-body dynamics. This is applicable to the asteroid and Kuiper Belts, and the early Solar System, which had more objects, dust, and gas. It is relevant to the magnetospherics of those bodies. It is also relevant to the dynamics of the Milky Way galaxy and others. In turn, though the Saturnian system is readily studied (by Cassini, ground telescopes, and space telescopes), the simpler and lower mass ring systems of the other giants makes their explanations somewhat easier to fathom. The Jupiter ring system is perhaps more completely understood at present than any of the other three.

Asteroid families and gaps indicate their local dynamics. They are in turn indicative of the Kuiper Belt, and its hypothesized Kuiper cliff. The Hildas and Jupiter Trojans are then relevant to the Neptune Trojans and Plutinos, Twotinos, etc.

Neptune's relative lack of a moon system suggests its formation and dynamics. The migration of Triton explains the ejection or destruction of competing moons, analogous to Hot Jupiters (also in sparse systems), and the Grand Tack hypothesis of Jupiter itself, on a smaller scale.

The planets are considered to have formed by accretion of larger and larger particles, into asteroids and planetesimals, and into today's bodies. Vesta and Ceres are hypothesized to be the only surviving examples of planetesimals, and thus samples of the formative period of the Solar System.

Transits of Mercury and Venus have been observed as analogues of extrasolar transits. As Mercury and Venus transits are far closer and thus appear "deeper," they can be studied in far finer detail. Similarly, analogues to the Solar System's asteroid and Kuiper belts have been observed around other star systems, though in far less detail.

Astrobiology

Earth is the only body known to contain life; this results in geologic and atmospheric life signatures apart from the organisms themselves. Methane observed on Mars has been postulated but cannot be definitively ascribed as a biosignature. Multiple processes of non-biological methane generation are seen on Earth as well.

The detection of biomarkers or biosignatures on other worlds is an active area of research. Although oxygen and/or ozone are generally considered strong signs of life, these too have alternate, non-biological explanations.

The Galileo mission, while performing a gravity assist flyby of Earth, treated the planet as an extraterrestrial one, in a test of life detection techniques. Conversely, the Deep Impact mission's High Resolution Imager, intended for examining comets starting from great distances, could be repurposed for exoplanet observations in its EPOXI extended mission.

Conversely, detection of life entails identification of those processes favoring or preventing life. This occurs primarily via study of Earth life and Earth processes, though this is in effect a sample size of one. Care must be taken to avoid observation and selection biases. Astrobiologists consider alternative chemistries for life, and study on Earth extremophile organisms that expand the potential definitions of habitable worlds.

Planetary science

From Wikipedia, the free encyclopedia
Photograph from Apollo 15 command module Endeavour of the rilles in the vicinity of the crater Aristarchus on the Moon.

Planetary science (or more rarely, planetology) is the scientific study of planets (including Earth), celestial bodies (such as moons, asteroids, comets) and planetary systems (in particular those of the Solar System) and the processes of their formation. It studies objects ranging in size from micrometeoroids to gas giants, aiming to determine their composition, dynamics, formation, interrelations and history. It is a strongly interdisciplinary field, which originally grew from astronomy and Earth science, and now incorporates many disciplines, including planetary geology, cosmochemistry, atmospheric science, physics, oceanography, hydrology, theoretical planetary science, glaciology, and exoplanetology. Allied disciplines include space physics, when concerned with the effects of the Sun on the bodies of the Solar System, and astrobiology.

There are interrelated observational and theoretical branches of planetary science. Observational research can involve combinations of space exploration, predominantly with robotic spacecraft missions using remote sensing, and comparative, experimental work in Earth-based laboratories. The theoretical component involves considerable computer simulation and mathematical modelling.

Planetary scientists are generally located in the astronomy and physics or Earth sciences departments of universities or research centres, though there are several purely planetary science institutes worldwide. Generally, planetary scientists study one of the Earth sciences, astronomy, astrophysics, geophysics, or physics at the graduate level and concentrate their research in planetary science disciplines. There are several major conferences each year, and a wide range of peer reviewed journals. Some planetary scientists work at private research centres and often initiate partnership research tasks.

History

The history of planetary science may be said to have begun with the Ancient Greek philosopher Democritus, who is reported by Hippolytus as saying

The ordered worlds are boundless and differ in size, and that in some there is neither sun nor moon, but that in others, both are greater than with us, and yet with others more in number. And that the intervals between the ordered worlds are unequal, here more and there less, and that some increase, others flourish and others decay, and here they come into being and there they are eclipsed. But that they are destroyed by colliding with one another. And that some ordered worlds are bare of animals and plants and all water.

In more modern times, planetary science began in astronomy, from studies of the unresolved planets. In this sense, the original planetary astronomer would be Galileo, who discovered the four largest moons of Jupiter, the mountains on the Moon, and first observed the rings of Saturn, all objects of intense later study. Galileo's study of the lunar mountains in 1609 also began the study of extraterrestrial landscapes: his observation "that the Moon certainly does not possess a smooth and polished surface" suggested that it and other worlds might appear "just like the face of the Earth itself".

Advances in telescope construction and instrumental resolution gradually allowed increased identification of the atmospheric as well as surface details of the planets. The Moon was initially the most heavily studied, due to its proximity to the Earth, as it always exhibited elaborate features on its surface, and the technological improvements gradually produced more detailed lunar geological knowledge. In this scientific process, the main instruments were astronomical optical telescopes (and later radio telescopes) and finally robotic exploratory spacecraft, such as space probes.

The Solar System has now been relatively well-studied, and a good overall understanding of the formation and evolution of this planetary system exists. However, there are large numbers of unsolved questions, and the rate of new discoveries is very high, partly due to the large number of interplanetary spacecraft currently exploring the Solar System.

Disciplines

Planetary science studies observational and theoretical astronomy, geology (astrogeology), atmospheric science, and an emerging subspecialty in planetary oceans, called planetary oceanography.

Planetary astronomy

This is both an observational and a theoretical science. Observational researchers are predominantly concerned with the study of the small bodies of the Solar System: those that are observed by telescopes, both optical and radio, so that characteristics of these bodies such as shape, spin, surface materials and weathering are determined, and the history of their formation and evolution can be understood.

Theoretical planetary astronomy is concerned with dynamics: the application of the principles of celestial mechanics to the Solar System and extrasolar planetary systems. Observing exoplanets and determining their physical properties, exoplanetology, is a major area of research besides Solar System studies. Every planet has its own branch.

Planetary geology

In planetary science, the term geology is used in its broadest sense, to mean the study of the surface and interior parts of planets and moons, from their core to their magnetosphere. The best-known research topics of planetary geology deal with the planetary bodies in the near vicinity of the Earth: the Moon, and the two neighboring planets: Venus and Mars. Of these, the Moon was studied first, using methods developed earlier on the Earth. Planetary geology focuses on celestial objects that exhibit a solid surface or have significant solid physical states as part of their structure. Planetary geology applies geology, geophysics and geochemistry to planetary bodies.

Planetary geomorphology

Geomorphology studies the features on planetary surfaces and reconstructs the history of their formation, inferring the physical processes that acted on the surface. Planetary geomorphology includes the study of several classes of surface features:

  • Impact features (multi-ringed basins, craters)
  • Volcanic and tectonic features (lava flows, fissures, rilles)
  • Glacial features
  • Aeolian features
  • Space weathering – erosional effects generated by the harsh environment of space (continuous micrometeorite bombardment, high-energy particle rain, impact gardening). For example, the thin dust cover on the surface of the lunar regolith is a result of micrometeorite bombardment.
  • Hydrological features: the liquid involved can range from water to hydrocarbon and ammonia, depending on the location within the Solar System. This category includes the study of paleohydrological features (paleochannels, paleolakes).

The history of a planetary surface can be deciphered by mapping features from top to bottom according to their deposition sequence, as first determined on terrestrial strata by Nicolas Steno. For example, stratigraphic mapping prepared the Apollo astronauts for the field geology they would encounter on their lunar missions. Overlapping sequences were identified on images taken by the Lunar Orbiter program, and these were used to prepare a lunar stratigraphic column and geological map of the Moon.

Cosmochemistry, geochemistry and petrology

One of the main problems when generating hypotheses on the formation and evolution of objects in the Solar System is the lack of samples that can be analyzed in the laboratory, where a large suite of tools are available, and the full body of knowledge derived from terrestrial geology can be brought to bear. Direct samples from the Moon, asteroids and Mars are present on Earth, removed from their parent bodies, and delivered as meteorites. Some of these have suffered contamination from the oxidising effect of Earth's atmosphere and the infiltration of the biosphere, but those meteorites collected in the last few decades from Antarctica are almost entirely pristine.

The different types of meteorites that originate from the asteroid belt cover almost all parts of the structure of differentiated bodies: meteorites even exist that come from the core-mantle boundary (pallasites). The combination of geochemistry and observational astronomy has also made it possible to trace the HED meteorites back to a specific asteroid in the main belt, 4 Vesta.

The comparatively few known Martian meteorites have provided insight into the geochemical composition of the Martian crust, although the unavoidable lack of information about their points of origin on the diverse Martian surface has meant that they do not provide more detailed constraints on theories of the evolution of the Martian lithosphere. As of July 24, 2013, 65 samples of Martian meteorites have been discovered on Earth. Many were found in either Antarctica or the Sahara Desert.

During the Apollo era, in the Apollo program, 384 kilograms of lunar samples were collected and transported to the Earth, and three Soviet Luna robots also delivered regolith samples from the Moon. These samples provide the most comprehensive record of the composition of any Solar System body besides the Earth. The numbers of lunar meteorites are growing quickly in the last few years – as of April 2008 there are 54 meteorites that have been officially classified as lunar. Eleven of these are from the US Antarctic meteorite collection, 6 are from the Japanese Antarctic meteorite collection and the other 37 are from hot desert localities in Africa, Australia, and the Middle East. The total mass of recognized lunar meteorites is close to 50 kg.

Planetary geophysics and space physics

Space probes made it possible to collect data in not only the visible light region but in other areas of the electromagnetic spectrum. The planets can be characterized by their force fields: gravity and their magnetic fields, which are studied through geophysics and space physics.

Measuring the changes in acceleration experienced by spacecraft as they orbit has allowed fine details of the gravity fields of the planets to be mapped. For example, in the 1970s, the gravity field disturbances above lunar maria were measured through lunar orbiters, which led to the discovery of concentrations of mass, mascons, beneath the Imbrium, Serenitatis, Crisium, Nectaris and Humorum basins.

The solar wind is deflected by the magnetosphere (not to scale)

If a planet's magnetic field is sufficiently strong, its interaction with the solar wind forms a magnetosphere around a planet. Early space probes discovered the gross dimensions of the terrestrial magnetic field, which extends about 10 Earth radii towards the Sun. The solar wind, a stream of charged particles, streams out and around the terrestrial magnetic field, and continues behind the magnetic tail, hundreds of Earth radii downstream. Inside the magnetosphere, there are relatively dense regions of solar wind particles, the Van Allen radiation belts.

Planetary geophysics includes, but is not limited to, seismology and tectonophysics, geophysical fluid dynamics, mineral physics, geodynamics, mathematical geophysics, and geophysical surveying.

Planetary geodesy

Planetary geodesy (also known as planetary geodetics) deals with the measurement and representation of the planets of the Solar System, their gravitational fields and geodynamic phenomena (polar motion in three-dimensional, time-varying space). The science of geodesy has elements of both astrophysics and planetary sciences. The shape of the Earth is to a large extent the result of its rotation, which causes its equatorial bulge, and the competition of geologic processes such as the collision of plates and of vulcanism, resisted by the Earth's gravity field. These principles can be applied to the solid surface of Earth (orogeny; Few mountains are higher than 10 km (6 mi), few deep sea trenches deeper than that because quite simply, a mountain as tall as, for example, 15 km (9 mi), would develop so much pressure at its base, due to gravity, that the rock there would become plastic, and the mountain would slump back to a height of roughly 10 km (6 mi) in a geologically insignificant time. Some or all of these geologic principles can be applied to other planets besides Earth. For instance on Mars, whose surface gravity is much less, the largest volcano, Olympus Mons, is 27 km (17 mi) high at its peak, a height that could not be maintained on Earth. The Earth geoid is essentially the figure of the Earth abstracted from its topographic features. Therefore, the Mars geoid (areoid is essentially the figure of Mars abstracted from its topographic features. Surveying and mapping are two important fields of application of geodesy.

Planetary atmospheric science

Cloud bands clearly visible on Jupiter.

An atmosphere is an important transitional zone between the solid planetary surface and the higher rarefied ionizing and radiation belts. Not all planets have atmospheres: their existence depends on the mass of the planet, and the planet's distance from the Sun – too distant and frozen atmospheres occur. Besides the four gas giant planets, three of the four terrestrial planets (Earth, Venus, and Mars) have significant atmospheres. Two moons have significant atmospheres: Saturn's moon Titan and Neptune's moon Triton. A tenuous atmosphere exists around Mercury.

The effects of the rotation rate of a planet about its axis can be seen in atmospheric streams and currents. Seen from space, these features show as bands and eddies in the cloud system and are particularly visible on Jupiter and Saturn.

Planetary oceanography

Exoplanetology

Exoplanetology studies exoplanets, the planets existing outside our Solar System. Until recently, the means of studying exoplanets have been extremely limited, but with the current rate of innovation in research technology, exoplanetology has become a rapidly developing subfield of astronomy.

Comparative planetary science

Planetary science frequently makes use of the method of comparison to give a greater understanding of the object of study. This can involve comparing the dense atmospheres of Earth and Saturn's moon Titan, the evolution of outer Solar System objects at different distances from the Sun, or the geomorphology of the surfaces of the terrestrial planets, to give only a few examples.

The main comparison that can be made is to features on the Earth, as it is much more accessible and allows a much greater range of measurements to be made. Earth analog studies are particularly common in planetary geology, geomorphology, and also in atmospheric science.

The use of terrestrial analogs was first described by Gilbert (1886).

In fiction

  • In Frank Herbert's 1965 Science Fiction Novel Dune, the major secondary character Liet-Kynes serves as the "Imperial Planetologist" for the fictional planet Arrakis, a position he inherited from his father Pardot Kynes. In this role, a planetologist is described as having skills of an ecologist, geologist, meteorologist, and biologist, as well as basic understandings of human sociology. The planetologists apply this expertise to the study of entire planets. In the Dune series, planetologists are employed to understand planetary resources and to plan terraforming or other planetary-scale engineering projects. This fictional position in Dune has had an impact on the discourse surrounding planetary science itself and is referred to by one author as a "touchstone" within the related disciplines. In one example, a publication by Sybil P. Seitzinger in the journal Nature opens with a brief introduction on the fictional role in Dune, and suggests we should consider appointing individuals with similar skills to Liet-Kyenes to help with managing human activity on Earth.

Professional activity

Journals

Professional bodies

This non-exhaustive list includes those institutions and universities with major groups of people working in planetary science. Alphabetical order is used.

Government space agencies

Major conferences

Smaller workshops and conferences on particular fields occur worldwide throughout the year.

Moon

From Wikipedia, the free encyclopedia https://en.wikipedia.org/wiki/Moon   Near side of the Moon , lunar ...