Mars science and telecommunications orbiter

Recommended Measurements

For measurements of the escape fluxes from each of these processes, measurements of the neutral and ionized species listed in Error: Reference source not found should be made over the altitude range from above the bowshock to as low as possible - 100 km being suggested as the desirable minimum altitude, especially during the solar maximum phase of the mission. The flux measurement should map the space around Mars, i.e., the dayside, the dawn-dusk regions and down the tail.

In addition, observations of the crustal, upper atmosphere and solar wind vector magnetic fields, and the ion and electron 3-dimensional energy distributions in the upper atmosphere and solar wind are required.

Mars lower Atmosphere

Composition

Inventory of Molecular Species

Measurements which would provide an inventory of the molecular constituents of the lowest few scale-heights of the atmosphere, i.e., from the surface to, say, 100 km altitude, hold the key to understanding the processes involved in interactions of the boundary layer with the regolith, to looking for current geological activity and, most important in the context of NASA's exploratory goals, to look for biological activity in the near-surface environment. The analogy with Earth's stratosphere above 20 km altitude is relevant: the pressure and temperature are not dissimilar (6 mb, 220 K), and the major radiatively active gases, H₂O, CO₂, O₃, play similar roles, despite their considerably smaller relative concentrations on Earth. These gases, together with nitrogen oxides, give rise to a rich variety of transient, reservoir and sink molecules which, in turn, provide a clear indication of processes occurring at lower altitudes and at the surface. A spectroscopic survey of the Earth stratosphere, which set out initially to acquire an inventory of molecular constituents at the parts per billion level, and to define the spectral radiative environment of this region of the atmosphere - the Shuttle-based ATMOS experiment - revealed the presence of more than 40 different molecular species and their isotopologues, many of which were unexpected new detections (e.g., ClNO₃, N₂O₅, CH₃Cl, HNO₄, C₂H₂, C₂H₆, SF₆; (see Farmer, 1987; Irion, 2002). In the case of Mars, it would be surprising not to find an even richer inventory of constituents resulting from contemporary geological activity, from catalytic cycles enhanced by the surface mineralogy, or from biological processes. Thus measurements of the spectral environment made at the high resolution achievable today from orbit could be expected to reveal the presence of such transient products of the major atmospheric constituents as CH₄, CH₃S, CH₃X, H₂CO, NOx, NH₃, SO₂, HCN etc. lists current upper limits for a number of other molecular species of interest for which searches have been made from both ground based and spacecraft observations.

Whether Mars is biologically or geologically active is one of the most important questions in the scientific exploration of the planet. Previous observations of Mars have demonstrated that evidence of life is neither obvious nor trivial. Nevertheless, if life ever existed on Mars, it likely exists today, having adapted as the Martian climate evolved. This assertion is supported by the ever-growing list of extremophile life forms on Earth that can be found in almost any niche where even minimally supportive conditions exist. Furthermore, geomorphologic evidence suggests that Mars may also have been volcanically active in its recent past.

Methane

One of the most intriguing questions that will be addressed by MSTO is whether or not the recent reports of methane in Mars’ atmosphere are correct. Within the past two years, three different groups have claimed detections of Martian methane (Formisano, et al., 2004; Krasnopolsky, et al., 2004; Mumma, et al., 2003). Two of these measurements (Mumma, Krasnopolsky) have been made using ground-based telescopes on Earth, combined with high resolution infrared spectroscopy; the other (Formisano) was made from ESA’s Mars Express orbiter with a relatively low-resolution spectrometer. All of them searched for absorption in the 3.3-m absorption band of CH₄. Krasnopolsky et al. measured a constant 10 ppbv of CH₄ over the surface, whereas Mumma and Formisano et al. both reported spatial gradients. Mumma’s measurements suggest a CH₄ concentration that is higher at the equator than at the poles, although these past conclusions are currently (2/23/06) being tested by a new series of measurements (private communication, M.Mumma). Formisano et al. claim orbit-to-orbit CH₄ variations that suggest longitudinal gradients in CH₄ concentration.

Implied CH₄ source

All of these measurements have been carefully made and analyzed; however, they are all close to or perhaps beyond, the limiting detectivity of CH₄ of their respective instruments. If they are correct, the measurements may have far-reaching implications for the nature of the Martian surface environment and possibly for astrobiology. Consider first what is implied by a constant 10-ppbv concentration of methane. The Martian surface pressure is about 6 mbar, and Martian gravity is 3.73 m/s², so the mass of a vertical column of atmosphere is about 16 g/cm². The corresponding column density, assuming nearly pure CO₂, is 2.210²³ molecules/cm². (Henceforth, “molecules” will be eliminated from the units, in accordance with standard atmospheric chemists’ practice.) Hence, a 10-ppbv concentration of CH₄ corresponds to a column density, N_col, of 2.210¹⁵ cm^-2. The photochemical lifetime, , of methane in Mars’ atmosphere is about 340 years (Krasnopolsky, et al., 2004), or ~10¹⁰ s. Since CH₄ has no appreciable photochemical sources in the Martian atmosphere, its concentration must be maintained by a surface flux of N_col/ = 2.210⁵ cm^-2s^-1. By comparison, Earth’s (mostly biological) methane flux is 535 Tg(CH₄)/yr (Watson, et al., 1990), or 1.2410¹¹ cm^-2s^-1. The required Martian methane flux is thus about 210^-6 times the biological methane flux on Earth.

This immediately raises the question: Could such a CH₄ flux on Mars be maintained abiotically, or would it necessitate a biological source? Again, we can turn to Earth for an analogy. Could volcanism provide this amount of methane? At first glance, it seems not. CH₄ is neither a predicted component of surface volcanic gases, nor one that is observed (Holland, 1984). The magmas from which such gases are released on Earth are relatively oxidized; they have an oxygen fugacity near the QFM (quartz-fayalite-magnetite) synthetic buffer. Furthermore, the high temperatures associated with surface melts, ~1200^oC (ibid.), favor molecules that are smaller than the 5-atom CH₄ molecule. Both of these factors conspire against emission of CH₄ from surface volcanism.

CH₄ from serpentinization reactions at midocean ridges

CH₄ may be released, however, by submarine volcanic outgassing or by related submarine processes. Substantial quantities of CH₄, ~ 2 mmol/kg, are observed in vent fluids emanating from the off-axis Lost City vent field on the Mid-Atlantic ridge (Kelley, et al., 2001; Kelley, et al., 2005). (The original measurements of Kelley, et al., 2001, were subsequently revised upwards by a factor of 10 by Kelley, et al., 2005) Kelley et al. originally suggested that this CH₄ was produced by serpentinization of peridotite within the hydrothermal circulation cells deep beneath the seafloor. Such reactions have been studied in the laboratory (e.g., Berndt, et al., 1996). Warm water in contact with ultramafic (Fe- and Mg-rich) rocks will react with the rock to produce various serpentine minerals. These minerals exclude iron; thus, the iron in the rock has to find another stable phase. Over a wide range of oxygen fugacities the stable oxide phase is magnetite, Fe₃O₄. This may be considered a combination of one ferrous ion, Fe⁺⁺ or FeO, and two ferric ions, Fe⁺³ or Fe₂O₃. By contrast, the iron in the original rock was entirely ferrous iron. Hence, for each mole of magnetite that is produced, two moles of iron are oxidized. In order to achieve redox balance, something else must be reduced. If H₂O itself is the oxidant, then H₂ is released. If the water contains dissolved CO₂, then CH₄ can be produced (Berndt, et al., 1996). So, serpentinization is one possible source for the CH₄ observed in the Lost City vent fluids. Indeed, this interpretation is supported by D/H measurements of Lost City CH₄ and H₂, which suggest that both compounds are being produced by relatively high-temperature (>110^oC), abiotic processes (Proskurowski et al., in press). There are, however, methanogens (methane-producing bacteria, or Archaea) living within the hydrothermal circulation systems. The presence of Archaeal biomarkers within the vent fluids suggests that Recent measurements of the carbon isotopic composition of the Lost City methane suggest that it at least some of the methane ismay be biogenic (Kelley, et al., 2005; J. Hayes, priv. comm..John Hayes, private communication—need to check this!). Full resolution of this question awaits C isotope measurements of a suite of higher hydrocarbons that are also observed within the vent fluids. We return below to the question of how carbon isotopes can be used to discriminate between biogenic and abiogenic CH₄ sources.
Suppose for the moment that the methane emanating from the Lost City vent fields is abiogenic. What would this imply about the global abiotic CH₄ flux? Kasting and Catling (2003) (Section 4.4) have used heat flow arguments to extrapolate the Lost City measurements to the entire globe. Their estimates used the Kelley et al. (2001) CH₄ values. If the Lost City vents are representative of other off-axis vents worldwide, they concluded that the global abiotic CH₄ flux would be ~110¹¹ mol/yr, or 1.6 Tg(CH₄)/yr. This is about 1/300^th of the biological methane flux. Ramping this up to the revised Kelley, et al. (2005) CH₄ concentrations implies an abiotic CH₄ flux of 1/30^th the biological CH­₄ flux. In atmospheric chemists’ units, this flux would be 410⁹ cm^-2s^-1. This is larger than the flux required to sustain 10 ppbv in the Martian atmosphere by a factor of ~210⁴.

Possible subsurface sources of Martian methane

What do these arguments imply about the possible source of Martian methane? Obviously, the answer is unclear. (That is part of why we need a mission to confirm the CH₄ measurements.) Liquid water probably is present in the Martian crust at depths of one to several kilometers (Clifford, 1993). How much water is present, and how vigorously it circulates is unknown. Only a small fraction of Earth’s midocean ridge methane source would be required to maintain a CH₄ concentration of 10 ppbv. The potential for generation of methane is increased by the apparent low oxygen fugacity of the Martian crust, as indicated by the composition of SNC meteorites (Wadhwa, 2001). It is conceivable that 10 ppbv of Martian methane could be sustained by serpentinization reactions deep beneath the Martian surface. It is also conceivable that methanogens could be living in the underground Martian aquifer and metabolizing hydrogen produced by serpentinization of ultramafic rock.

Distinguishing between biotic and abiotic CH₄ sources

Recent investigations of the terrestrial sources of methane suggest that distinguishing between abiotic and biotic sources of Martian methane would be most successful if based on simultaneous observations of the carbon and hydrogen isotopic composition of methane and the presence of higher alkanes. Biogenic methane on Earth is depleted in ¹³C by an average value of about 45‰ relative to the carbon source. By comparison, the CH₄ in the Lost City vent fluids is only 8.8-13.6 ‰ depleted in ¹³C relative to dissolved inorganic carbon in seawater (Kelley, et al., 2005). Martian methanogens would presumably be using CO₂ as the carbon source, so a simultaneous measurement of the ¹³C/¹²C ratio of atmospheric CO₂ and CH₄ would be a significant contribution to answering the question of source origin. The observed ranges of the terrestrial ¹³C/¹²C ratio in CH₄ (-40 to -115 ‰ for methanogenesis, and 0 to -60 ‰ for abiotic sources) do overlap to some extent, so that a ¹³C/¹²C ratio measurement alone becomes ambiguous at the low depletion end where biotic and abiotic source values overlap (Horita, 2005). Investigations of sources of terrestrial methane have found that both biogenic and abiogenic methane can have similar values of ¹³C depletion (Schoell, 1988 Onstott, 2006 #118}). Thus, an observation on Mars of a ¹³C/¹²C ratio of -100‰ relative to the carbon source would be a strong indicator of biotic origin, but one of only -60‰ would be ambiguous. In this event, additional discrimination is needed.

Considering D/H values, some measurements of CH₄ derived from low-temperature serpentinization show a depletion in deuterium about double that of biogenically-derived CH₄ (-400‰ vs -200‰ relative to mean ocean water). Therefore, since water is the source of the hydrogen in CH₄, measurements of the isotopic composition of near-surface water vapor might yield insights into the subsurface water fractionation and identification of the methane source.

Measurements of the presence of higher alkanes, e.g., ethane and propane, provide important additional information. Abiotic synthesis of organics, whether by serpentinization or by Fischer-Tropsch-type reactions (Lollar, et al., 2002) tends to generate the entire sequence of alkanes, with the abundance decreasing with carbon number. Methanogenesis starting from CO₂ (CO₂ + 4 H₂  CH₄ + 2H₂O) generates only methane. If methane is derived from the decomposition of more complex organic material, higher alkanes are produced as well, but the abundance typically increases with increasing carbon number (ibid.). Hence, a search for ethane and propane assists in separating different ways of generating methane. The abundance ratio of methane to simultaneous measurements of ethane, propane, and butane when correlated with ¹³C/¹²C values shows a clear separation between samples from extant microbial processes and abiogenic water-rock reactions (Horita et al.).

Connection with past atmospheric conditions

The possibility that methane is present on Mars is interesting for another reason. If it is present now, then regardless of whether it is biogenic or abiogenic, methane is likely to have been more abundant in the distant past. The addition of methane to a CO₂-rich paleoatmosphere could possibly have provided the additional greenhouse warming needed to bring the average surface temperature closer to the freezing point of water, and thus to explain the presence of the fluvial features observed on the ancient, heavily cratered southern highlands of Mars (Pollack, et al., 1987). Climate modelers have not yet produced a self-consistent CH₄-CO₂-H₂O greenhouse for early Mars, but the possibility of doing so still looms. If we were to verify that methane exists in the present Martian atmosphere, this would provide additional motivation for studying its possible effect in the Martian atmosphere of long ago.