A Medley of Potpourri

iso	NA	half-life	DM	DE (MeV)	DP
¹⁸⁰W	0.12%	1.8×10¹⁸ y	α	2.516	¹⁷⁶Hf
¹⁸¹W	syn	121.2 d	ε	0.188	¹⁸¹Ta
¹⁸²W	26.50%	>1.7×10²⁰ y	(α)	1.772	¹⁷⁸Hf
¹⁸³W	14.31%	>8×10¹⁹ y	(α)	1.680	¹⁷⁹Hf
¹⁸⁴W	30.64%	>1.8×10²⁰ y	(α)	1.123	¹⁸⁰Hf
¹⁸⁵W	syn	75.1 d	β⁻	0.433	¹⁸⁵Re
¹⁸⁶W	28.43%	>4.1×10¹⁸ y	(α)	1.656	¹⁸²Hf
¹⁸⁶W	28.43%	>4.1×10¹⁸ y	(β⁻β⁻)	–	¹⁸⁶Os

Decay modes in parentheses are predicted, but have not yet been observed

Tungsten, also known as wolfram, is a chemical element with symbol W and atomic number 74. The word tungsten comes from the Swedish language tung sten directly translatable to heavy stone,^[3] though the name is volfram in Swedish to distinguish it from Scheelite, which in Swedish is alternatively named tungsten.

A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as a metal in 1783. Its important ores include wolframite and scheelite. The free element is remarkable for its robustness, especially the fact that it has the highest melting point of all the elements. Also remarkable is its high density of 19.3 times that of water, comparable to that of uranium and gold, and much higher (about 1.7 times) than that of lead.^[4] Polycrystalline tungsten is an intrinsically brittle^[5]^[6] and hard material due to its weak grain boundaries, making it difficult to work. However, pure single-crystalline tungsten is more ductile, and can be cut with a hard-steel hacksaw.^[7]

Tungsten's many alloys have numerous applications, most notably in incandescent light bulb filaments, X-ray tubes (as both the filament and target), electrodes in TIG welding, superalloys, and radiation shielding. About half is used in the form of tungsten carbide, a durable carbon alloy. Tungsten's hardness and high density give it military applications in penetrating projectiles. Tungsten compounds are also often used as industrial catalysts.

Tungsten is the only metal from the third transition series that is known to occur in biomolecules, where it is used in a few species of bacteria and archaea. It is the heaviest element known to be used by any living organism. Tungsten interferes with molybdenum and copper metabolism and is somewhat toxic to animal life.^[8]^[9]

Characteristics

Physical properties

In its raw form, tungsten is a hard steel-grey metal that is often brittle and hard to work. If made very pure, tungsten retains its hardness (which exceeds that of many steels), and becomes malleable enough that it can be worked easily.^[7] It is worked by forging, drawing, or extruding. Tungsten objects are also commonly formed by sintering.

Of all metals in pure form, tungsten has the highest melting point (3,422 °C, 6,192 °F), lowest vapor pressure (at temperatures above 1,650 °C, 3,000 °F) and the highest tensile strength.^[10] Although carbon remains solid at higher temperatures than tungsten, carbon sublimes, rather than melts, so tungsten is considered to have a higher melting point. Tungsten has the lowest coefficient of thermal expansion of any pure metal. The low thermal expansion and high melting point and tensile strength of tungsten originate from strong covalent bonds formed between tungsten atoms by the 5d electrons.^[11] Alloying small quantities of tungsten with steel greatly increases its toughness.^[4]

Tungsten exists in two major crystalline forms: α and β. The former has a body-centered cubic structure and is the more stable form. The structure of the β phase is called A15 cubic; it is metastable, but can coexist with the α phase at ambient conditions owing to non-equilibrium synthesis or stabilization by impurities. Contrary to the α phase which crystallizes in isometric grains, the β form exhibits a columnar habit. The α phase has one third of the electrical resistivity^[12] and a much lower superconducting transition temperature T_C relative to the β phase: ca. 0.015 K vs. 1–4 K; mixing the two phases allows obtaining intermediate T_C values.^[13]^[14] The T_C value can also be raised by alloying tungsten with another metal (e.g. 7.9 K for W-Tc).^[15] Such tungsten alloys are sometimes used in low-temperature superconducting circuits.^[16]^[17]^[18]

Isotopes

Naturally occurring tungsten consists of five isotopes whose half-lives are so long that they can be considered stable. Theoretically, all five can decay into isotopes of element 72 (hafnium) by alpha emission, but only ¹⁸⁰W has been observed^[19] to do so with a half-life of (1.8 ± 0.2)×10¹⁸ years; on average, this yields about two alpha decays of ¹⁸⁰W in one gram of natural tungsten per year.^[20] The other naturally occurring isotopes have not been observed to decay, constraining their half-lives to be:

¹⁸²W, t_1⁄2 > 1.7×10²⁰ years

¹⁸³W, t_1⁄2 > 8×10¹⁹ years

¹⁸⁴W, t_1⁄2 > 1.8×10²⁰ years

¹⁸⁶W, t_1⁄2 > 4.1×10¹⁸ years

Another 30 artificial radioisotopes of tungsten have been characterized, the most stable of which are ¹⁸¹W with a half-life of 121.2 days, ¹⁸⁵W with a half-life of 75.1 days, ¹⁸⁸W with a half-life of 69.4 days, ¹⁷⁸W with a half-life of 21.6 days, and ¹⁸⁷W with a half-life of 23.72 h.^[20] All of the remaining radioactive isotopes have half-lives of less than 3 hours, and most of these have half-lives below 8 minutes.^[20] Tungsten also has 4 meta states, the most stable being ^179mW (t_1⁄2 6.4 minutes).

Chemical properties

Elemental tungsten resists attack by oxygen, acids, and alkalis.^[21]

The most common formal oxidation state of tungsten is +6, but it exhibits all oxidation states from −2 to +6.^[21]^[22] Tungsten typically combines with oxygen to form the yellow tungstic oxide, WO₃, which dissolves in aqueous alkaline solutions to form tungstate ions, WO2−
4.

Tungsten carbides (W₂C and WC) are produced by heating powdered tungsten with carbon. W₂C is resistant to chemical attack, although it reacts strongly with chlorine to form tungsten hexachloride (WCl₆).^[4]

In aqueous solution, tungstate gives the heteropoly acids and polyoxometalate anions under neutral and acidic conditions. As tungstate is progressively treated with acid, it first yields the soluble, metastable "paratungstate A" anion, W
7O6–
24, which over time converts to the less soluble "paratungstate B" anion, H
2W
12O10–
42.^[23] Further acidification produces the very soluble metatungstate anion, H
2W
12O6–
40, after which equilibrium is reached. The metatungstate ion exists as a symmetric cluster of twelve tungsten-oxygen octahedra known as the Keggin anion. Many other polyoxometalate anions exist as metastable species. The inclusion of a different atom such as phosphorus in place of the two central hydrogens in metatungstate produces a wide variety of heteropoly acids, such as phosphotungstic acid H₃PW₁₂O₄₀.

Tungsten trioxide can form intercalation compounds with alkali metals. These are known as bronzes; an example is sodium tungsten bronze.

History

In 1781, Carl Wilhelm Scheele discovered that a new acid, tungstic acid, could be made from scheelite (at the time named tungsten). Scheele and Torbern Bergman suggested that it might be possible to obtain a new metal by reducing this acid.^[24] In 1783, José and Fausto Elhuyar found an acid made from wolframite that was identical to tungstic acid. Later that year, in Spain, the brothers succeeded in isolating tungsten by reduction of this acid with charcoal, and they are credited with the discovery of the element.^[25]^[26]

In World War II, tungsten played a significant role in background political dealings. Portugal, as the main European source of the element, was put under pressure from both sides, because of its deposits of wolframite ore at Panasqueira. Tungsten's resistance to high temperatures and its strengthening of alloys made it an important raw material for the arms industry.^[27]^[28]

Etymology

The name "tungsten" (from the Swedish tung sten, "heavy stone") is used in English, French, and many other languages as the name of the element, but not in the Nordic countries. Tungsten was the old Swedish name for the mineral scheelite. The other name "wolfram" (or "volfram"), is used in most European (especially Germanic and Slavic) languages, and is derived from the mineral wolframite, which is the origin of its chemical symbol, W.^[7] The name "wolframite" is derived from German "wolf rahm" ("wolf soot" or "wolf cream"), the name given to tungsten by Johan Gottschalk Wallerius in 1747. This, in turn, derives from "Lupi spuma", the name Georg Agricola used for the element in 1546, which translates into English as "wolf's froth", and is a reference to the large amounts of tin consumed by the mineral during its extraction.^[29]

Occurrence

Tungsten is found in the minerals wolframite (iron–manganese tungstate, (Fe,Mn)WO₄), scheelite (calcium tungstate, (CaWO₄), ferberite (FeWO₄), and hübnerite (MnWO₄).

In 2010, world production of tungsten was about 61,000 tonnes. The main producers were as follows (data in tonnes): ^[30]

Country	Production (2010)	Production (2011)	Reserves
Austria	900	1,000	10,000
Bolivia	1,000	1,100	53,000
Canada	2,000	300	120,000
China	51,000	52,000	1,900,000
Portugal	900	950	4,200
Russia	2,500	2,500	250,000
Other countries	3,000	3,300	400,000

There is additional production in the U.S., but the amount is proprietary company information. U.S. reserves are 140,000 tonnes.^[30]

Tungsten is considered to be a conflict mineral due to the unethical mining practices observed in the Democratic Republic of the Congo.^[31]^[32]

There is a large deposit of tungsten ore on the edge of Dartmoor in the United Kingdom, which was exploited during World War I and World War II as the Hemerdon Mine. With recent increases in tungsten prices, this mine may be reactivated. ^[33]

Production

Wolframite, scale in cm

Tungsten output in 2005

About 61,300 tonnes of tungsten concentrates were produced in the year 2009.^[34] Tungsten is extracted from its ores in several stages. The ore is eventually converted to tungsten(VI) oxide (WO₃), which is heated with hydrogen or carbon to produce powdered tungsten.^[24] Because of tungsten's high melting point, it is not commercially feasible to cast tungsten ingots. Instead, powdered tungsten is mixed with small amounts of powdered nickel or other metals, and sintered. During the sintering process, the nickel diffuses into the tungsten, producing an alloy.

Tungsten can also be extracted by hydrogen reduction of WF₆:

WF₆ + 3 H₂ → W + 6 HF

or pyrolytic decomposition:^[35]

WF₆ → W + 3 F₂ (ΔH_r = +)

Tungsten is not traded as a futures contract and cannot be tracked on exchanges like the London Metal Exchange. The prices are usually quoted for tungsten concentrate or WO₃. If converted to the metal equivalent, they were about US$19 per kilogram in 2009.^[34]

Applications

Close-up of a tungsten filament inside a halogen lamp

Tungsten carbide ring (jewelry)

Approximately half of the tungsten is consumed for the production of hard materials – namely tungsten carbide – with the remaining major use being in alloys and steels. Less than 10% is used in other chemical compounds.^[36]

Hard materials

Tungsten is mainly used in the production of hard materials based on tungsten carbide, one of the hardest carbides, with a melting point of 2770 °C. WC is an efficient electrical conductor, but W₂C is less so. WC is used to make wear-resistant abrasives, and "carbide" cutting tools such as knives, drills, circular saws, milling and turning tools used by the metalworking, woodworking, mining, petroleum and construction industries.^[4] Carbide tooling is actually a ceramic/metal composite, where metallic cobalt acts as a binding (matrix) material to hold the WC particles in place. This type of industrial use accounts for about 60% of current tungsten consumption.^[37]

The jewelry industry makes rings of sintered tungsten carbide, tungsten carbide/metal composites, and also metallic tungsten.^[38] WC/metal composite rings use nickel as the metal matrix in place of cobalt because it takes a higher luster when polished. Sometimes manufacturers or retailers refer to tungsten carbide as a metal, but it is a ceramic.^[39] Because of tungsten carbide's hardness, rings made of this material are extremely abrasion resistant, and will hold a burnished finish longer than rings made of metallic tungsten. Tungsten carbide rings are brittle, however, and may crack under a sharp blow.^[40]

Alloys

The hardness and density of tungsten are applied in obtaining heavy metal alloys. A good example is high speed steel, which can contain as much as 18% tungsten.^[41] Tungsten's high melting point makes tungsten a good material for applications like rocket nozzles, for example in the UGM-27 Polaris submarine-launched ballistic missile.^[42] Tungsten alloys are used in a wide range of different applications, including the aerospace and automotive industries and radiation shielding.^[43]
Superalloys containing tungsten, such as Hastelloy and Stellite, are used in turbine blades and wear-resistant parts and coatings.

Armaments

Tungsten, usually alloyed with nickel and iron or cobalt to form heavy alloys, is used in kinetic energy penetrators as an alternative to depleted uranium, in applications where uranium's radioactivity is problematic even in depleted form, or where uranium's additional pyrophoric properties are not required (for example, in ordinary small arms bullets designed to penetrate body armor). Similarly, tungsten alloys have also been used in cannon shells, grenades and missiles, to create supersonic shrapnel. Tungsten has also been used in Dense Inert Metal Explosives, which use it as dense powder to reduce collateral damage while increasing the lethality of explosives within a small radius.^[44]

Chemical applications

Tungsten(IV) sulfide is a high temperature lubricant and is a component of catalysts for hydrodesulfurization.^[45] MoS₂ is more commonly used for such applications.^[46]

Tungsten oxides are used in ceramic glazes and calcium/magnesium tungstates are used widely in fluorescent lighting. Crystal tungstates are used as scintillation detectors in nuclear physics and nuclear medicine. Other salts that contain tungsten are used in the chemical and tanning industries.^[10]

Tungsten oxide (WO₃) is incorporated into selective catalytic reduction (SCR) catalysts found in coal-fired power plants. These catalysts convert nitrogen oxides (NO_x) to nitrogen (N₂) and water (H₂O) using ammonia (NH₃). The tungsten oxide helps with the physical strength of the catalyst and extends catalyst life.^[47]

Niche uses

Applications requiring its high density include weights, counterweights, ballast keels for yachts, tail ballast for commercial aircraft, and as ballast in race cars for NASCAR and Formula One; depleted uranium is also used for these purposes, due to similarly high density. 75-kg blocks of tungsten were used as "cruise balance mass devices" on the entry vehicle portion of the 2012 Mars Science Laboratory spacecraft. It is an ideal material to use as a dolly for riveting, where the mass necessary for good results can be achieved in a compact bar. High-density alloys of tungsten with nickel, copper or iron are used in high-quality darts^[48] (to allow for a smaller diameter and thus tighter groupings) or for fishing lures (tungsten beads allow the fly to sink rapidly). Some types of strings for musical instruments are wound with tungsten wires.

Sodium tungstate is used in Folin-Ciocalteu's reagent, a mixture of different chemicals used in the "Lowry Assay" for protein content analysis.

Gold substitution

Its density, similar to that of gold, allows tungsten to be used in jewelry as an alternative to gold or platinum.^[7]^[49] Metallic tungsten is hypoallergenic, and is harder than gold alloys (though not as hard as tungsten carbide), making it useful for rings that will resist scratching, especially in designs with a brushed finish.

Because the density is so similar to that of gold (tungsten is only 0.36% less dense), tungsten can also be used in counterfeiting of gold bars, such as by plating a tungsten bar with gold,^[50]^[51]^[52] which has been observed since the 1980s,^[53] or taking an existing gold bar, drilling holes, and replacing the removed gold with tungsten rods.^[54] The densities are not exactly the same, and other properties of gold and tungsten differ, but gold-plated tungsten will pass superficial tests.^[50]

Gold-plated tungsten is available commercially from China (the main source of tungsten), both in jewelry and as bars.^[55]

Electronics

Because it retains its strength at high temperatures and has a high melting point, elemental tungsten is used in many high-temperature applications,^[56] such as light bulb, cathode-ray tube, and vacuum tube filaments, heating elements, and rocket engine nozzles.^[7] Its high melting point also makes tungsten suitable for aerospace and high-temperature uses such as electrical, heating, and welding applications, notably in the gas tungsten arc welding process (also called tungsten inert gas (TIG) welding).

Tungsten electrode used in a Gas tungsten arc welding torch

Because of its conductive properties and relative chemical inertness, tungsten is also used in electrodes, and in the emitter tips in electron-beam instruments that use field emission guns, such as electron microscopes. In electronics, tungsten is used as an interconnect material in integrated circuits, between the silicon dioxide dielectric material and the transistors. It is used in metallic films, which replace the wiring used in conventional electronics with a coat of tungsten (or molybdenum) on silicon.^[35]

The electronic structure of tungsten makes it one of the main sources for X-ray targets,^[57]^[58] and also for shielding from high-energy radiations (such as in the radiopharmaceutical industry for shielding radioactive samples of FDG). It is also used in gamma imaging as a material from which coded apertures are made, due to its excellent shielding properties. Tungsten powder is used as a filler material in plastic composites, which are used as a nontoxic substitute for lead in bullets, shot, and radiation shields. Since this element's thermal expansion is similar to borosilicate glass, it is used for making glass-to-metal seals.^[10]

Biological role

Tungsten, at atomic number 74, is the heaviest element known to be biologically functional, with the next heaviest being iodine (Z = 53). It is used by some bacteria, but not in eukaryotes. For example, enzymes called oxidoreductases use tungsten similarly to molybdenum by using it in a tungsten-pterin complex with molybdopterin (molybdopterin, despite its name, does not contain molybdenum, but may complex with either molybdenum or tungsten in use by living organisms). Tungsten-using enzymes typically reduce carboxylic acids to aldehydes.^[59] The tungsten oxidoreductases may also catalyse oxidations. The first tungsten-requiring enzyme to be discovered also requires selenium, and in this case the tungsten-selenium pair may function analogously to the molybdenum-sulfur pairing of some molybdenum cofactor-requiring enzymes.^[60] One of the enzymes in the oxidoreductase family which sometimes employ tungsten (bacterial formate dehydrogenase H) is known to use a selenium-molybdenum version of molybdopterin.^[61] Although a tungsten-containing xanthine dehydrogenase from bacteria has been found to contain tungsten-molydopterin and also non-protein bound selenium, a tungsten-selenium molybdopterin complex has not been definitively described.^[62]

In soil, tungsten metal oxidizes to the tungstate anion. It can be selectively or non-selectively imported by some prokaryotic organisms and may substitute for molybdate in certain enzymes. Its effect on the action of these enzymes is in some cases inhibitory and in others positive.^[63] The soil's chemistry determines how the tungsten polymerizes; alkaline soils cause monomeric tungstates; acidic soils cause polymeric tungstates.^[64]

Sodium tungstate and lead have been studied for their effect on earthworms. Lead was found to be lethal at low levels and sodium tungstate was much less toxic, but the tungstate completely inhibited their reproductive ability.^[65]

Tungsten has been studied as a biological copper metabolic antagonist, in a role similar to the action of molybdenum. It has been found that tetrathiotungstates may be used as biological copper chelation chemicals, similar to the tetrathiomolybdates.^[66]

Precautions

Because tungsten is rare and its compounds are generally inert, the effects of tungsten on the environment are limited.^[67] The median lethal dose LD₅₀ depends strongly on the animal and the method of administration and varies between 59 mg/kg (intravenous, rabbits)^[68]^[69] and 5000 mg/kg (tungsten metal powder, intraperitoneal, rats).^[70]^[71]

Patent claim

Tungsten is unique amongst the elements in that it has been the subject of patent proceedings. In 1928, a US court rejected General Electric's attempt to patent it, overturning U.S. Patent 1,082,933 granted in 1913 to William D. Coolidge.^[72]^[73]

Atmosphere of Mars

From Wikipedia, the free encyclopedia

Atmosphere of Mars

Chemical species	Mole fraction^[1]
Carbon dioxide	96.0%
Argon	1.9%
Nitrogen	1.9%
Oxygen	0.145%
Carbon monoxide	0.0557%

The atmosphere of Mars is, like that of Venus, composed mostly of carbon dioxide though far thinner. There has been renewed interest in its composition since the detection of traces of methane in 2003^[2]^[3] that may indicate life but may also be produced by a geochemical process, volcanic or hydrothermal activity.^[4]
The atmospheric pressure on the Martian surface averages 600 pascals (0.087 psi), about 0.6% of Earth's mean sea level pressure of 101.3 kilopascals (14.69 psi) and only 0.0065% that of Venus's 9.2 megapascals (1,330 psi). It ranges from a low of 30 pascals (0.0044 psi) on Olympus Mons's peak to over 1,155 pascals (0.1675 psi) in the depths of Hellas Planitia. This pressure is well below the Armstrong limit for the unprotected human body.
Mars's atmospheric mass of 25 teratonnes compares to Earth's 5148 teratonnes with a scale height of about 11 kilometres (6.8 mi) versus Earth's 7 kilometres (4.3 mi).

The Martian atmosphere consists of approximately 96% carbon dioxide, 1.9% argon, 1.9% nitrogen, and traces of free oxygen, carbon monoxide, water and methane, among other gases,^[1] for a mean molar mass of 43.34 g/mol.^[5]^[6] The atmosphere is quite dusty, giving the Martian sky a light brown or orange-red color when seen from the surface; data from the Mars Exploration Rovers indicate that suspended dust particles within the atmosphere are roughly 1.5 micrometres across.^[7]

On 16 December 2014, NASA reported detecting an unusual increase, then decrease, in the amounts of methane in the atmosphere of the planet Mars; as well as, detecting Martian organic chemicals in powder drilled from a rock by the Curiosity rover. Also, based on deuterium to hydrogen ratio studies, much of the water at Gale Crater on Mars was found to have been lost during ancient times, before the lakebed in the crater was formed; afterwards, large amounts of water continued to be lost.^[8]^[9]^[10]

Structure

Pressure comparison
Where	Pressure
Olympus Mons summit	0.03 kilopascals (0.0044 psi)
Mars average	0.6 kilopascals (0.087 psi)
Hellas Planitia bottom	1.16 kilopascals (0.168 psi)
Armstrong limit	6.25 kilopascals (0.906 psi)
Mount Everest summit^[11]	33.7 kilopascals (4.89 psi)
Earth sea level	101.3 kilopascals (14.69 psi)

Mars's atmosphere is composed of the following layers:

Lower atmosphere: A warm region affected by heat from airborne dust and from the ground.
Middle atmosphere: The region in which Mars's jetstream flows
Upper atmosphere, or thermosphere: A region with very high temperatures, caused by heating from the Sun. Atmospheric gases start to separate from each other at these altitudes, rather than forming the even mix found in the lower atmospheric layers.
Exosphere: Typically stated to start at 200 km (120 mi) and higher, this region is where the last wisps of atmosphere merge into the vacuum of space. There is no distinct boundary where the atmosphere ends; it just tapers away.

There is also a complicated ionosphere,^[12] and a seasonal ozone layer over the south pole.^[13]

Observations and measurement from Earth

Comparison of the atmospheric compositions of Venus, Mars, and the past and present Earth.

In 1864, William Rutter Dawes observed "that the ruddy tint of the planet does not arise from any peculiarity of its atmosphere seems to be fully proved by the fact that the redness is always deepest near the centre, where the atmosphere is thinnest."^[14] Spectroscopic observations in the 1860s and 1870s^[15]^[16] led many to think the atmosphere of Mars is similar to Earth's. In 1894, though, spectral analysis and other qualitative observations by William Wallace Campbell suggested Mars resembles the Moon, which has no appreciable atmosphere, in many respects.^[15]

In 1926, photographic observations by William Hammond Wright at the Lick Observatory allowed Donald Howard Menzel to discover quantitative evidence of Mars's atmosphere.^[17]^[18]

Composition

Most abundant gases on Mars.

Carbon dioxide

The main component of the atmosphere of Mars is carbon dioxide (CO₂) at 95.9%. Each pole is in continual darkness during its hemisphere's winter, and the surface gets so cold that as much as 25% of the atmospheric CO₂ condenses at the polar caps into solid CO₂ ice (dry ice). When the pole is again exposed to sunlight during summer, the CO₂ ice sublimes back into the atmosphere. This process leads to a significant annual variation in the atmospheric pressure and atmospheric composition around the Martian poles.

Argon

Argon isotope ratios are a signature of atmospheric loss on Mars.^[19]^[20]

The atmosphere of Mars is enriched considerably with the noble gas argon, in comparison to the atmosphere of the other planets within the Solar System. Unlike carbon dioxide, the argon content of the atmosphere does not condense, and hence the total amount of argon in the Mars atmosphere is constant. However, the relative concentration at any given location can change as carbon dioxide moves in and out of the atmosphere. Recent satellite data shows an increase in atmospheric argon over the southern pole during its autumn, which dissipates the following spring.^[21]

Water

Some aspects of the Martian atmosphere vary significantly. As carbon dioxide sublimes back into the atmosphere during the Martian summer, it leaves traces of water. Seasonal winds sweep off the poles at speeds approaching 400 kilometres per hour (250 mph) and transport large amounts of dust and water vapor giving rise to Earth-like frost and large cirrus clouds. These clouds of water-ice were photographed by the Opportunity rover in 2004.^[22] NASA scientists working on the Phoenix Mars mission confirmed on July 31, 2008 that they had indeed found subsurface water ice at Mars's northern polar region.

Methane

Volatile gases on Mars.

Trace amounts of methane (CH₄), at the level of several parts per billion (ppb), were first reported in Mars's atmosphere by a team at the NASA Goddard Space Flight Center in 2003.^[3]^[23] In March 2004, the Mars Express Orbiter and ground-based observations by three groups also suggested the presence of methane in the atmosphere with a mole fraction of about 10 ppb.^[24]^[25]^[26] Large differences in the abundances were measured between observations taken in 2003 and 2006, which suggested that the methane was locally concentrated and probably seasonal.

Because methane on Mars would quickly break down due to ultraviolet radiation from the Sun and chemical reactions with other gases, its reported persistent presence in the atmosphere also necessitates the existence of a source to continually replenish the gas. Current photochemical models alone can not explain the rapid variability of the methane levels.^[27]^[28] It had been proposed that the methane might be replenished by meteorites entering the atmosphere of Mars,^[29] but researchers from Imperial College London found that the volumes of methane released this way are too low to sustain the measured levels of the gas.^[30]

Research suggests that the implied methane destruction lifetime is as long as ~4 Earth years and as short as ~0.6 Earth years.^[31]^[32] This lifetime is short enough for the atmospheric circulation to yield the observed uneven distribution of methane across the planet. In either case, the destruction lifetime for methane is much shorter than the timescale (~350 years) estimated for photochemical (UV radiation) destruction.^[31] The rapid destruction (or "sink") of methane suggests that another process must dominate removal of atmospheric methane on Mars, and it must be more efficient than destruction by light by a factor of 100 to 600.^[32]^[31] This unexplained fast destruction rate also suggests a very active replenishing source.^[33] A possibility is that the methane is not consumed at all, but rather condenses and evaporates seasonally from clathrates.^[34] Another possibility is that methane reacts with tumbling surface sand quartz (SiO
2) and olivine to form covalent Si–CH
3 bonds.^[35]

Although the methane could stem from a geological source, the lack of current volcanism, hydrothermal activity or hotspots are not favorable for a geological explanation. Living microorganisms, such as methanogens, are another possible source, but no evidence exists for the presence of such organisms anywhere on Mars. Roscosmos and ESA are planning to look for companion gases that may suggest which sources are most likely.^[36]^[37] In the Earth's oceans, biological methane production tends to be accompanied by ethane, whereas volcanic methane is accompanied by sulfur dioxide.^[37] Several studies of trace gases in the Martian atmosphere have found no evidence for sulfur dioxide in the Martian atmosphere, which makes volcanism unlikely to be the source of methane.^[38]^[39]

Possible methane sources and sinks on Mars.

The principal candidates for the origin of Mars methane include non-biological processes such as water–rock reactions, radiolysis of water, and pyrite formation, all of which produce H₂ that could then generate methane and other hydrocarbons via Fischer–Tropsch synthesis with CO and CO₂.^[40] It has also been shown that methane could be produced by a process involving water, carbon dioxide, and the mineral olivine, which is known to be common on Mars.^[41] The required conditions for this reaction (i.e. high temperature and pressure) do not exist on the surface, but may exist within the crust.^[42] A detection of the mineral by-product serpentinite would suggest that this process is occurring. An analog on Earth suggests that low-temperature production and exhalation of methane from serpentinized rocks may be possible on Mars.^[43] Another possible geophysical source could be clathrate hydrates.^[44]

A group of Mexican scientists performed plasma experiments in a synthetic Mars atmosphere and found that bursts of methane can be produced when a discharge interacts with water ice. A potential source of the discharges can be the electrification of dust particles from sand storms and dust devils. The ice can be found in trenches or in the permafrost. The electrical discharge ionizes gaseous CO₂ and water molecules and their byproducts recombine to produce methane. The results obtained show that pulsed electrical discharges over ice samples in a Martian atmosphere produce about 1.41×10¹⁶ molecules of methane per joule of applied energy.^[45]^[46]

In contrast to the findings described above, studies by Kevin Zahnle, a planetary scientist at NASA's Ames Research Center, and two colleagues concluded that "there is as yet no compelling evidence for methane on Mars". They argued that the strongest reported observations of the gas to date have been taken at frequencies where interference from methane in Earth's atmosphere is particularly difficult to remove, and are thus unreliable. Additionally, they claimed that the published observations most favorable to interpretation as indicative of Martian methane are also consistent with no methane being present on Mars.^[47]^[48]^[49]

In 2011, NASA scientists reported a comprehensive search using ground-based high-resolution infrared spectroscopy for trace species (including methane) on Mars, deriving sensitive upper limits for methane (<7 ppbv), ethane (<0.2 ppbv), methanol (<19 and="" others="" ppbv="" sub="">2

CO, C₂H₂, C₂H₄, N₂O, NH₃, HCN, CH₃Cl, HCl, HO₂ – all limits at ppbv levels).^[50] The data were acquired over a period of 6 years and span different seasons and locations on Mars, suggesting that if organics are being released into the atmosphere, these events were extremely rare or currently non-existent, considering the expected long lifetimes for some of these species.^[51]
In August 2012, the Curiosity rover landed on Mars. The rover's instruments are capable of making precise abundance measurements that also distinguish between different isotopologues of methane.^[52] Efforts to identify the sources of terrestrial methane have found that measurements of different methane isotopologues do not necessarily distinguish between possible geologic and biogenic sources, but the abundances of other cogenerated gases, such as ethane (C₂H₆), relative to methane do: the ethane–methane abundance ratio is < 0.001 for biogenic sources, whereas other sources produce nearly equivalent amounts of methane and ethane.^[53]

Methane measurements in the atmosphere of Mars by the Curiosity rover.

The first measurements with Curiosity's Tunable Laser Spectrometer (TLS) indicated that there were less than 5 ppb of methane at the landing site.^[54]^[55]^[56]^[57] On 19 July 2013, NASA scientists published the results of a new analysis of the atmosphere of Mars, reporting a lack of methane around the landing site of the Curiosity rover. In addition, the scientists found evidence that Mars "has lost a good deal of its atmosphere over time", based on the abundance of isotopic compositions of gases, particularly those related to argon and carbon.^[58]^[59]^[60] On 19 September 2013, NASA scientists used further measurements from Curiosity to report a non-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). As a result, they concluded that current methanogenic microbial activity on Mars is extremely unlikely.^[61]^[62]^[63]

On 16 December 2014, NASA reported that Curiosity had detected a tenfold increase ('spike') in methane in the atmosphere around it in late 2013 and early 2014. Four measurements taken over two months in this period averaged 7 ppb. Before and after that, readings averaged around one-tenth that level.^[8]^[9]^[10]

The Indian Mars Orbiter Mission, which entered orbit around Mars on 24 September 2014, is equipped with a Fabry–Pérot interferometer to measure atmospheric methane at a level of several ppb.^[64] The ExoMars Trace Gas Orbiter planned to launch in 2016 would further study the methane, as well as its decomposition products such as formaldehyde and methanol.^[65]^[66]

Sulfur dioxide

Sulfur dioxide in the atmosphere is thought to be a tracer of current volcanic activity. It has become especially interesting due to the long-standing controversy of methane on Mars. If methane on Mars were being produced by volcanoes (as it is in part on Earth) we would expect to find sulfur dioxide in large quantities. Several teams have searched for sulfur dioxide on Mars using the NASA Infrared Telescope Facility. No sulfur dioxide was detected in these studies, but they were able to place stringent upper limits on the atmospheric concentration of 0.2 ppb.^[38]^[39] In March 2013, a team led by scientists at NASA Goddard Space Flight Center reported a detection of SO₂ in Rocknest (Mars) soil samples analyzed by the Curiosity rover.^[67]

Ozone

Rotation of Mars near opposition. Ecliptic south is up.

As reported by the European Space Agency (ESA) on September 29, 2013, a new comparison of spacecraft data with computer models explains how global atmospheric circulation creates a layer of ozone above Mars's southern pole in winter. Ozone was most likely difficult to detect on Mars because its concentration is typically 300 times lower than on Earth, although it varies greatly with location and time. In recent years, the SPICAM UV spectrometer on board Mars Express has shown the presence of two distinct ozone layers at low-to-mid latitudes. These comprise a persistent, near-surface layer below an altitude of 30 km, a separate layer that is only present in northern spring and summer with an altitude varying from 30 to 60 km, and another separate layer that exists 40–60 km above the southern pole in winter, with no counterpart above the Mars's north pole. This third ozone layer shows an abrupt decrease in elevation between 75 and 50 degrees south. SPICAM detected a gradual increase in ozone concentration at 50 km until midwinter, after which it slowly decreased to very low concentrations, with no layer detectable above 35 km. The authors of the paper in Nature Geoscience think that the observed polar ozone layers are the result of the same atmospheric circulation pattern that creates a distinct oxygen emission recently identified in the polar night and also present in Earth's atmosphere. This circulation takes the form of a huge Hadley cell in which warmer air rises and travels poleward before cooling and sinking at higher latitudes. Mars is on a quite elliptical orbit and has a large axial tilt, which causes extreme seasonal variations in temperature amongst the northern and southern hemispheres. Mars's temperature difference greatly influences the amount of water vapor in the atmosphere, because warmer air can contain more moisture. This, in turn, affects the production of ozone-destroying hydrogen radicals.

Potential for use by humans

The atmosphere of Mars is a resource of known composition available at any landing site on Mars. It has been proposed that human exploration of Mars could use carbon dioxide (CO₂) from the Martian atmosphere to make rocket fuel for the return mission. Mission studies that propose using the atmosphere in this way include the Mars Direct proposal of Robert Zubrin and the NASA Design reference mission study. Two major chemical pathways for use of the carbon dioxide are the Sabatier reaction, converting atmospheric carbon dioxide along with additional hydrogen (H₂), to produce methane (CH₄) and oxygen (O₂), and electrolysis, using a zirconia solid oxide electrolyte to split the carbon dioxide into oxygen (O₂) and carbon monoxide (CO).

History

Mars's atmosphere is thought to have changed over the course of the planet's lifetime, with evidence suggesting the possibility that Mars had large oceans a few billion years ago.^[68] As stated in the Mars ocean hypothesis, atmospheric pressure on the present-day Martian surface only exceeds that of the triple point of water (6.11 hectopascals (0.0886 psi)) in the lowest elevations; at higher elevations water can exist only in solid or vapor form. Annual mean temperatures at the surface are currently less than 210 K (−63 °C; −82 °F), significantly lower than that needed to sustain liquid water.
However, early in its history Mars may have had conditions more conducive to retaining liquid water at the surface. In 2013, scientists published that Mars might have had an "oxygen-rich" atmosphere billions of years ago.^[69]^[70]

Possible causes for the depletion of a previously thicker Martian atmosphere include:

Gradual erosion of the atmosphere by solar wind,^[71] possibly helped by Mars's magnetic-field irregularities;^[72]
Catastrophic collision by a body large enough to blow away a significant percentage of the atmosphere;^[72]
Mars's low gravity allowing the atmosphere to "blow off" into space by Jeans escape.^[73]

Mars's escaping atmosphere—carbon, oxygen, hydrogen—made by MAVEN UV spectrograph).^[74]

Images

Mars's thin atmosphere, visible on the horizon.
Mars Pathfinder – Martian sky with water ice clouds.

Martian sunset by Spirit rover at Gusev crater (May, 2005).

Martian sunset by Pathfinder at Ares Vallis (July, 1997).

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Sunday, March 1, 2015

Tungsten

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