Detection of methane in the martian atmosphere: evidence for life?
Introduction
Half a century ago, seasonal variations of the reflectivity of Mars at low and middle latitudes were ascribed to vegetation, and attempts were made to detect the chlorophyll bands and other “bands of life.” G.A. Tikhov, a Russian scientist, suggested the term astrobotany in 1945 for that new field and published a book with this title in 1949 and followed it in 1953, with a book entitled “Astrobiology.” It was recognized subsequently that the observed seasonal variations are caused by windblown dust. However, even after the careful and detailed but unsuccessful search for life by the Viking instruments, Mars still represents our greatest if not only hope to achieve a nonhypothetical subject for astro- or exobiology.
It is currently thought that methanogenesis is a highly likely metabolic pathway for possible microbial life on Mars McCollom, 1999, Weiss et al., 2000, Jakosky et al., 2003, Varnes et al., 2003. The probable existence of subsurface liquid water on Mars (Mellon and Phillips, 2001) would provide protected habitats for such organisms, perhaps similar to the microbial communities thriving in many places deep beneath the Earth's surface (Chapelle et al., 2002). The detection of atmospheric methane is a way to deduce the existence of this type of life.
The strictest upper limit to CH4 in the martian atmosphere was obtained by summing up 1747 infrared spectra from the Mariner 9 orbiter (Maguire, 1977). No absorption of Mars thermal radiation was observed at the CH4 band at 1306 cm−1, and this resulted in an upper limit of 20 parts per billion (ppb). This limit corresponds to the absorption at a level of twice the noise equivalent radiance, that is, 0.1%, measured with resolving power (ν is wavenumber).
An indication of a possible presence of methane in the martian atmosphere was obtained by Krasnopolsky et al. (1997). To search for HDO, they observed a spectrum of Mars at 2650–2800 cm−1 with resolving power of 270,000 using the Fourier transform spectrometer at the Kitt Peak National Observatory. Methane was a by-product in that search, and a weak signal from possible methane at was below the two-sigma limit.
A spectrum of Mars at 2.4–45 μm was observed with the short-wavelength spectrometer at the Infrared Space Observatory with resolving power of 2000 near 3.3 μm. No methane absorption was detected, and the upper limit to methane was 50 ppb (Lellouch et al., 2000).
We report here our observations which resulted in a detection of methane on Mars. We will discuss sources and sinks of methane on Mars and its relevance to the problem of life. An abstract of this work with its basic results was submitted to and published by the European Geosciences Union (Krasnopolsky et al., 2004), and the submission deadline, 11 January 2004, was on the second day of operation of the Planetary Fourier Spectrometer (PFS) on board the Mars Express orbiter (Formisano et al., 2004). Later, Kerr (2004) reported that the PFS team also announced a detection of methane at a press conference in March 2004. Attempts to detect methane on Mars were made even earlier by Mumma et al. (2003). However, no results of their observations have been published in that abstract.
Section snippets
Observation
We searched for CH4 on Mars using the P-branch of its strongest band at 3020 cm−1. This band is stronger than the band at 1306 cm−1, which was used by Maguire (1977) to establish his upper limit, by a factor of 2.2. Reflection of solar light by Mars with the mean surface albedo of 0.1 (Erard and Calvin, 1997) dominates the planet's spectrum near 3020 cm−1 and further doubles the effect of absorption because of the two-way path in the atmosphere. However, our observations were ground-based and
Detection of methane
All possible martian CH4 lines of interest are on the blue wings of the strong telluric CH4 lines. To search for methane, we choose those CH4 lines, which are weakly contaminated by other telluric lines at the expected positions of the martian lines. We consider all available methane lines with strengths exceeding a given limit, which we choose at and . The numbers of lines exceeding the two limits are 5 and 15, respectively. The peak opacity from a line with
Photochemical loss of methane
The observed abundance of methane is too small to affect the abundances of major photochemical species. We found four reactions for the photochemical loss of methane on Mars (Table 1). The main process is direct photolysis by solar Lyman-alpha radiation. The mean Lyman-alpha photon flux is at 1 AU (Woods et al., 1996), and the CH4 and CO2 cross sections at 1216 Å are and (Huebner, http://espsun.space.swri.edu/amop). Absorption of
Diffusion of methane through regolith
Diffusion of gas through the martian regolith (Weiss et al., 2000) is of the Knudsen type, when molecular collisions with the walls of the pores dominate in the transport process. The calculated diffusion coefficient (Weiss et al., 2000) may be approximated as , and μ is the molecular mass in atomic units.
The equation of diffusion from a thin layer at depth d to the surface may be integrated to give the diffusion time Here is the CH4 flow and
Hydrothermal and magmatic methane
There are no processes of methane formation in the atmosphere, so the loss of must therefore be balanced by abiogenic and biogenic sources. Welhan (1988) considered sources of methane in the hydrothermal systems on the Earth. He discussed two abiogenic sources of methane, from magmatic outgassing and high-temperature chemical reactions within a hydrothermal fluid. For example, methane may form in the following equilibria: CO2 + 2H2O = CH4 + 2O2, 8FeO + 2H2O + CO2 = 4Fe2O3 + CH4,
Methanogenesis on Mars
We are therefore led to consider a biogenic origin of methane on Mars. “Natural gas” on Earth is related to the extinct macroscopic life forms, which were and are impossible on Mars, given the exceedingly low limits on organic matter in the soil set by the Viking landers (Biemann et al., 1977) and the dry recent history of the planet. The major source of atmospheric methane on the Earth (Yavitt, 1992) is the microbial methanation of dead macroscopic biota: 2CH2O → CH4 + CO2. There are
Conclusions
We observed a spectrum of Mars at the P-branch of the strongest CH4 band at 3.3 μm with resolving power of 180,000 for the apodized spectrum. Summing up the spectral intervals at the expected positions of the strongest Doppler-shifted martian lines, we detected methane at a level which is slightly above the standard detection criterion of 3 sigma. The observed CH4 abundance is . Photochemical loss of methane is and corresponds to its lifetime of 340 years.
Outgassing from
Acknowledgment
We are grateful to G.R. Gladstone for some helpful comments.
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