Methane in the Solar System

Metano en el Sistema Solar

Andrés Guzmán-Marmolejo¹, Antígona Segura^2,*

¹ Posgrado en Ciencias de la Tierra, Instituto de Geofísica, Universidad Nacional Autónoma de México, Ciudad Universitaria, Coyoacán. C.P. 04510, D.F., México.

² Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, Circuito exterior C.U. Apartado Postal 70-543. Coyoacán, C. P. 04510, D.F., México. ^* antigona@nucleares.unam.mx

Manuscript received: April 30, 2014.
Corrected manuscript received: February 9, 2015.
Manuscript accepted: February 12, 2015.

Abstract

This paper reviews the distribution of methane (CH₄) in our Solar System, as well as its sources and sinks in the atmospheres of the main Solar System bodies. Methane is widely distributed in the Solar System. In general, the inner planets are methane-poor, being Earth a unique exception, whereas the outer planets have CH₄-rich atmospheres. In general, the atmospheric chemistry of this compound is dominated by the solar radiation although in O₂-rich atmospheres this compound participates in a reaction system that removes atmospheric CH₄. In our planet most of the atmospheric CH₄ is produced by lifeforms, reason why scientists have proposed that the simultaneous detection of methane signal along with oxygen (O₂) or ozone (O₃) signals in the atmospheric spectra of planets may be good evidence of life. Therefore, the study of this gas at planetary level is important for understanding the chemical reactions that control its abundance on the exoplanetary atmospheres and to classify possible inhabited planets.

Keywords: methane, biosignatures, Solar System.

Resumen

Palabras clave: metano, bioseñales, sistema solar.

1. Distribution of methane in the Solar System

The Solar System was formed by the gravitational collapse of a primordial gas nebula. The center of this nebula collapsed faster than its outer edge, forming the Sun at the center and a protoplanetary disc around, latter processes formed planets from the dust (Cloutier, 2007). The temperature in the inner protoplanetary disc near the Sun was high enough to evaporate volatiles like methane, which is decomposed by photolysis and it is dragged later by the solar wind. In the outer regions of the protoplanetary disc, the low temperatures allowed that ices and volatiles could be preserved. The result is CH₄-poor terrestrial planets in the inner Solar System and CH₄-rich big planets in the outer (Cloutier, 2007).

Methane is preserved in ices called clathrates, these solids present structures that can capture methane in their interior. They play an important role in the stabilization and dispersion of molecules in the Solar System because they are present in many kinds of environments with a wide range of pressures and temperatures (Miller, 1961; Thompson et al., 1987). Here, we review the CH₄abundances in planets and small bodies of the Solar System.

2. Inner planets

Inner planets are the four closest planets to the Sun: Mercury, Venus, Earth, and Mars. They are small planets composed of silicates and iron. Volatiles in inner planet atmospheres as Mercury, Venus, Earth and Mars, are mainly the result of degassing from their interiors (Cloutier, 2007).

Mercury has a tiny atmosphere mainly formed by He, H₂, O₂, Na, Ca, K and water vapor (Broadfoot et al., 1974; Potter and Morgan, 1985, 1986). Measurements with the Mercury Laser Altimeter —MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER), confirmed the long held idea that Mercury contains impact-derived deposits of volatiles than may include organics (Neumann et al., 2013; Paige et al., 2013). These deposits are located in permanently shadowed zones of the north polar region where the regolith has temperatures similar to those of the icy Galilean satellites, allowing the cold-trapping of materials from comets and rich-volatile meteorites (Neumann et al., 2013). Methane is present in comets but thermal stability models do not predict its presence in cold-traps due to its higher volatility compared to water (Zhang and Paige, 2009). Gibson (1977) proposed a volatile cycle for Mercury, starting with the production of simple molecules (H₂, H₂O, CH₄, NH₃, etc.) by solar-wind ions implanted into the planet's silicate surface. These chemical species would be outgassed and then cold-trapped in colder regions of the planet. Until now, no detection of methane has been reported for this planet.

Venus has a thick atmosphere mainly formed by CO₂ (96 %) and N₂ (3 %) (Niemman et al., 1980). The Pioneer spacecraft instruments detected CH₄ in Venus atmosphere (1000 – 6000 ppm) and many other gases (Oyama et al., 1980). Nevertheless, measurements of CH₃D/CH₄ ratio of 5×10^-3 caused controversy because atmospheric evolution models predicted a CH₃D/CH₄ ratio of 9×10^-2. A plausible explanation is that the CH₄ was the result of the reaction between some highly deuterated molecules in Venus atmosphere and terrestrial CH₄that contaminated the instruments (Donahue and Hodges, 1993).

Based on the detection of NH₃, HCl, and H₂O in the Venus atmosphere, along with the fact that there is a strong possibility of electrical discharge in the atmosphere as a result of thermal convective turbulence, Otroshchenko and Surkov (1974) proposed that organic compounds could be formed in the atmosphere. Their hypothesis was experimentally tested, finding CH₄ and other low-mass molecules. The studies of Otroshchenko and Surkov (1974) show that presence of organic compounds in the Venus atmosphere is a strong possibility.

2.2. Earth

CH₄ levels in the atmosphere are currently around 1.6 – 1.8 ppmv, the enhanced greenhouse effect caused by a molecule of methane is about 8 times that of a molecule of CO₂ (Houghton, 2005). CH₄ is homogeneously mixed in the troposphere while in the upper atmosphere the highest concentrations are at the Ecuador (http://earthobservatory.nasa.gov). CH₄ levels have changed over the history of the Earth, before the emergence of life, CH₄ sources were geological.

The emergence of life increased the levels of CH₄ in an atmosphere without free oxygen, where CH₄ could have lifetimes of 5000 – 10000 years and reach concentrations of 1000 ppmv (Kasting and Siefert, 2002). Then, when oxygenic photosynthesis increased O₂ levels in the atmosphere, CH₄ decreased because of a set of reactions that will be described at the end of this section. Numerical models show that, today, the thermodynamic equilibrium value for CH₄ is > 10^-35, in volume fraction, however its abundance is approximately 1.7×10^-6 (Sagan et al., 1993). CH₄ is almost totally produced by biological sources and the abiotic sources represent less than 10 % (Levine et al., 2010). The pristine ice cores store a record of CH₄ concentrations of thousands of years. Analysis of these cores show that CH₄ abundances ranged from 0.35 ppmv to 0.8 ppmv corresponding to glacial and interglacial periods (e.g., Legrand et al., 1988; Chappellaz et al., 1990; Raynaud et al., 1993; Brook et al., 1996; Petit et al., 1999; Spahni et al., 2005; Loulergue et al., 2008). CH₄ has increased its atmospheric concentration since pre-industrial time to be relatively constant around 1.7 ppmv (Dlugokencky et al., 2003). Recently, CH₄ is calling the attention of scientists studying climate change due to its capability as greenhouse gas. Today near to 50 % of CH₄ global emissions are produced by human activity (mining, industry, farming, and ranching) causing an imbalance between their sources and sinks of 30 Tg year^-1, approximately, contributing from 4 % to 9 % of greenhouse effect (Lelieveld et al., 1998; Wuebbles and Hayhoe, 2002; Houghton, 2005).

It is estimated that all CH₄ sources produce 600 Tg yr^-1, approximately. There are no chemical reactions forming CH₄ in atmospheres such as Earth (Levine et al., 1985). Here, almost all CH₄ is produced by methanogen microorganisms. Methanogens can form CH₄ by two ways: 1) using CO₂ in the reaction CO₂ + 4H₂ → CH₄ + 2H₂O (Thauer, 1998) or, 2) using organic molecules as electron terminal acceptors, for example acetic acid, methanol, or methylamine, in the reaction CH₃COOH → CH₄ + CO₂ (Fukuzaki et al., 1990). Table 1 summarizes the CH₄sources. The major biological sources of methane are wetlands, followed by digestion of animals such as ruminants and decomposition of biomass. An important source, linked to human activity, is the production of energy. Other minor sources are the animal activity such as arthropods and decomposition of sediments and bacterial activity in marine environments. While the only known abiotic source is the serpentinization process and contributes with 3 % of methane emissions.

Serpentinization takes place in hydrothermal systems similar to the Lost City, located in the middle of Atlantic Ocean. In these sites, it is commonly said that CH₄ is formed by serpentinization but in fact, CH₄ is byproduct of a Fischer–Tropsch type reaction after to serpentinization process. In the Fischer–Tropsch reaction, CO₂ is reduced by H₂ forming CH₄: CO₂ + 4H₂→ CH₄ + 2H₂O. This reaction needs metal catalysts such as Fe, Co, and Ni, and temperatures and pressures in the range of 200 °C to 350 °C and 20 bars to 30 bars (Schulz, 1999). The H₂ used in the Fischer–Tropsch reaction is a product of serpentinization. In serpentinization, hydrolysis of olivine minerals ((Mg,Fe)2SiO₄) form serpentine (Mg₃Si₂O₅(OH)₄), brucite (Mg(OH)₂), magnetite (Fe₃O4), and H₂:

3Fe₂SiO₄ + 2H₂O → 3SiO₂ + 2Fe₃O₄ + 2H_2(ac)

3Mg₂SiO₄ + SiO₂ + 4H₂O → 2Mg₃Si₂O₅(OH)₄

2Mg₂SiO₄ + 3H₂O → Mg₃Si₂O₅(OH)₄ + Mg(OH)₂

Serpentinization reactions are possible from 1 bar to 5 kbars, temperatures from 0 °C to 500 °C, and Fe²⁺ abundances from 1% to 50% (Oze and Sharma, 2005).

In contrast to the numerous CH₄ sources, there are only three sinks. Table 2 summarizes the sinks of CH₄. The major of those occur in the troposphere where the reaction of oxidation of CH₄ by hydroxyl radical (OH) leads mainly formaldehyde (CH₂O); such reaction is responsible for removing almost 90 % of atmospheric CH₄. OH radical is byproduct in photolysis of O₃ by UV-B radiation (Rohrer y Berresheim, 2006). OH radical rapidly reacts with CH₄removing it from the atmosphere:

O₂ + hv(180 - 240 nm) → O + O

]]> O₂ + O → O₃

O₃ + hv (200 - 300 nm) → O(¹D) + O₂

O(¹D) + H₂O → 2OH

CH₄ + OH → CH₃ + H₂O

Where hv is the energy of a photon with frequency v and h is the Panck constant. The remaining CH₄ is removed trough soil oxidation, and transport to the stratosphere (Wuebbless and Hayhoe, 2002; Houweling et al., 2006; Anderson et al., 2010). After being produced, either by biological activity or serpentinization, methane may be stored in clathrates. Gas hydrates belong to a general class of inclusion compounds commonly known as clathrates. Clathrates owe their existence to the ability of H₂O molecules to assemble via hydrogen bonding and form polyhedral cavities. Molecules like methane or carbon dioxide are of an appropriate size such that they fit within cavities formed by the host material (e.g., Kvenvolden, 1993). Methane hydrates are particularly important (Mahajan et al., 2007). Within clathrates there are no chemical bond involved between the water molecules and the gas molecules other than Van der Waals forces, but the presence of guest molecules inside the ice crystals makes the structure more stable. In fact, the guest molecules stabilize the structure enough for raising the melting point of the ice to several degrees above 0 °C (Miller, 1961). There are two different reservoirs for clathrates. They can be found both within and under permafrost in arctic regions and also within a few hundred meters of the seafloor on continental slopes and in deep seas and lakes (Hester and Brewer, 2008).The permafrost is soil, sediment, or rock that is continuously frozen (temperature < 0 °C) for at least two consecutive years (Anderson et al., 2010). Permafrost is the largest CH₄ reservoir in Earth. Estimates of the global inventory of methane clathrate may be 3×10¹⁸ g of carbon (Buffett and Archer, 2004). Permafrost acts as an impermeable lid, preventing CH₄ escape through the seabed. Moreover, sub-sea permafrost is potentially more vulnerable to thawing than terrestrial permafrost. A consequence of climate warming is the partial thawing and failure of sub-sea permafrost and thus an increased permeability for gases. Shakhova et al. (2010a) estimate the total amount of carbon preserved within permafrost, only in the East Siberian Arctic Shelf (ESAS), to be ~1.4×10¹⁵ g. Shakhova et al. (2010b) estimated the annual outgassing from the shallow ESAS of 7.98 Tg CH₄. This amount is of the same magnitude as existing estimates of total methane emissions from the entire world ocean (e.g., Anderson et al., 2010).

Because methane is also a greenhouse gas, release of even a small percentage of total deposits could have a serious effect on Earth's atmosphere. A conservative estimate by Boswell and Collett (2011) for the global gas hydrate inventory is ~1.8×10¹⁵ g C, corresponding to a CH₄ volume of ~3.0×10¹⁵ m³ if CH₄ density is considered to be 0.717 kg m^-3. In the unlikely event that 0.1 % (1.8 Tg C) of this CH₄ were instantaneously released to the atmosphere, CH₄concentrations would increase to ~2900 ppb from the 2005 value of ~1774 ppb (IPCC, 2007).

2.3. Mars

In 2010, Fonti and Marzo made distribution map of methane on the Martian surface. They identify three localized sources on the Martian surface, related to probable underground water reservoirs. Their analyses suggest that CH₄abundances vary throughout seasonal cycles.

There are some hypotheses about the sources and sinks of CH₄ in Mars. For example, Krasnopolsky et al. (2004) considered that degassing from the interior of the planet is unlikely due to the lack of geologic activity. Bar-Nun and Dimitrov (2007) proposed that photolysis of H₂O in the presence of CO can generate CH₄, however Krasnopolsky (2007) argues that it is not possible due to the kinetic chemistry of Mars. Serpentinization has also been proposed (e.g. Oze and Sharma, 2005; Lyons et al., 2005; Szponar et al., 2013; Etiope et al., 2013), this hypothesis is supported by the spatial correlation of underground water reservoirs and volcanoes where serpentinization may be possible. The origin of CH₄ on Mars is still not clear, some authors have proposed biogenic sources such as methanogenesis via metabolic pathways (e.g. Weiss et al., 2000; Chapelle et al., 2002; Jakosky et al., 2003; Varnes et al., 2003; Buford, 2010). CH₄ lifetime is 340 years and methane should be uniformly mixed in the atmosphere. Heterogeneous loss of atmospheric methane is probably negligible, while the sink of CH₄ during its diffusion through the regolith may be significant. There are no processes of CH₄ formation in the atmosphere, so the photochemical loss must therefore be balanced by its sources (Krasnopolsky et al., 2004). It was thought that the main sink of CH₄ was its direct photolysis around 80 km from the surface. Other sink is the reaction between CH₄ and Martian soil, but theoretical studies calculate the collision probability between CH₄ y O of 2×10^-11. Therefore, this reaction is negligible versus its direct photolysis (Krasnopolsky et al., 2004).

3. Outer planets (Jupiter, Saturn, Uranus and Neptune)

3.1. Jupiter and Saturn

They are giant planets with atmospheres mainly constituted by H₂ (> 80 %) and He as the second more important constituent. Their composition and chemistry are relatively similar in those planets. Jupiter is the largest planet in the Solar System with 318 M. Methane is the most abundant species in the upper Jovian troposphere after hydrogen and helium, accounting for approximately 0.2 % of the molecular abundance (Taylor et al., 2005). Different calculations estimate that CH₄/H₂ ratio is from 1.9×10^-3 to 2.3×10^-3 (Hanel et al., 1979; Gautier et al., 1982; Wong et al., 2004). Methane does not condense at the temperatures found on Jupiter, and is chemically stable except in the upper atmosphere (P < 1 mbar), where it is dissociated by solar ultraviolet radiation. Higher hydrocarbons are produced from methane by photochemical processes in the upper atmosphere of Jupiter (Taylor et al., 2005). Photolysis of CH₄ is the only sink (Moses et al., 2000), but it is not an effective way to destroy it in the Jupiter's atmosphere because the large excess of H₂ that suggests that radicals like CH₃, byproducts of the CH₄ photolysis, react with the H radical reforming CH₄ (McNesby, 1969). Saturn is the second largest planet in our solar system. Observations from the Cassini spacecraft suggest mole fractions of CH₄ of 4.7×10^-3 (Fletcher et al., 2009). The chemistry of CH₄in Saturn is similar to Jupiter.

3.2. Uranus and Neptune

The only known photochemically active volatile in the atmosphere of Uranus is methane. From observations of the Ultraviolet Spectrometer in the Voyager spacecraft, the calculated abundance for CH₄ is 10^-4 near 0.1 mbar. Other species normally present in the atmospheres of Jupiter and Saturn are not likely to be gaseous in the photolytic regime of the upper troposphere and stratosphere of Uranus due to the low tropopause temperature (Atreya et al., 1991). The stratospheric CH₄ is photolyzed forming acetylene, methyl-acetylene, ethane, and ethylene (Orton et al., 1987; Bézard et al., 1991; Schulz et al., 1999; Meadows et al., 2008). However, CH₄ photolysis is relatively inefficient on Uranus. Only 10 to 15 % of CH₄ molecules, which absorb ultraviolet photons, produce higher hydrocarbons resulting in a loss rate of 6×10⁶ CH₄ molecules cm^-2 s^-1 at the equator. For comparison, the loss rate on Jupiter is 30 % (Atreya et al., 1991).

4. Methane in small bodies

4.1. Pluto

Pluto's atmosphere is the result of the sublimation of superficial ices, in consequence, it is expected that the atmosphere is in vapor-pressure equilibrium with the surface (e.g., Young et al., 1997). Owen et al. (1993) estimated that the surface contains 1.5 % of solid CH₄. Later, Young et al. (1997) detected gaseous methane in Pluto for the first time, calculating a partial pressure of 0.072 µbar. In 2008 and 2012 this body was observed using the CRIRES instrument in the Very Large Telescope (VLT) to constrain the spatial and vertical distribution of methane in Pluto's atmosphere (Lellouch et al., 2015). From these observations, the calculated methane-mixing ratio is 0.44 % with negligible longitudinal variations. Because Pluto has not yet been observed with any spacecraft, all its parameters have been inferred using instruments on the ground. In 2015, the mission New Horizons will be able to characterize the surface and atmosphere of Pluto and its satellite, Charon.

4.2. Triton

The Voyager 2 spacecraft observed Triton (Neptune's largest moon) in 1989 and it has been later studied using instruments on Earth's surface and the Hubble Space Telescope (Buratti et al., 2011 and references therein). Similar to Pluto, its atmosphere is the result of the sublimation of the more volatile ices on its surface. The surface of Triton contains approximately 0.05 % CH₄ in ices (Tyler et al., 1989; Cruikshank et al., 1993) and its atmospheric mixing ratio was calculated to be 10^-4 from the Voyager observations (Tyler et al., 1989) and confirmed by the VLT/CRIRES instrument (Lellouch et al., 2011).

4.3. Titan

In 2005, the Huygens spacecraft descended to the surface of Titan measuring in situ the CH₄ mole fraction when it descended. Huygens found that the CH₄ mole fraction is relatively constant in the stratosphere; it increases between 32 and 8 km, and remains constant near the surface (Atreya et al., 2006). Titan has pressures and temperatures near to the methane triple-point, for this reason the CH₄ can evaporate from the surface to atmosphere, where it can condense and rain, forming a CH₄cycle similar to water on Earth (Roe, 2009; Lunine, 2012).

Mathematical models based on the Voyager's measurements suggest that the lifetime of CH₄ is from 10 to 100 millions of years (e.g. Yung et al., 1984; Lara et al., 1996; Lebonnois et al., 2001; Wilson and Atreya, 2004). In the stratosphere CH₄ is photolized to CH₃, CH₂ or CH, forming ethane, propane, and benzene (Strobel, 1974). There are not reactions to generate CH₄ in the Titan's atmosphere, so it is proposed that CH₄ may come from clathrates formed in the subnebula that originated the satellite (Mousis et al., 2002, Davies et al., 2013). Other authors proposed the activity of bacteria as a likely CH₄ source (e.g. McKay and Smith, 2005; Schulze-Makuch and Grinspoon, 2005), nevertheless there is not evidence about it (Atreya et al., 2006). Another possibility is the serpentinization (Niemann et al., 2005) but according to Mousis et al. (2009) this source of methane is not able to reproduce the deuterium over hydrogen (D/H) ratio observed at present in methane in its atmosphere.

4.4. Comets

These icy bodies have been studied with flyby missions and ground infrared and radio observations. Methane is a primary volatile in comets, this means that it is stored as ice in the cometary nucleus and released as gas into the coma. This compound has been detected in eleven comets and its abundance relative to water ranges from ~0.4 % to 2 % (Allen et al., 1988; Drapatz et al., 1987; Mumma et al., 1996; Bockelée-Morvan et al., 2000; Gibb et al., 2003; Mumma and Charnley, 2011).

5. Final comments

The study of methane is relevant to understand the process of synthesis and distribution of organic molecules during the formation of the Solar System. On potentially habitable planets around other stars its presence maybe the result of geological or biological activity. The bodies of our Solar System, especially Earth, serve as benchmarks for understanding the origin, sources and reservoirs of this compound to identify possible inhabitable worlds around other stars.

We acknowledge the support of the project PAPIIT IN119709-3.

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