Mars science and telecommunications orbiter

science goals

Background

Mars has intrigued planetary scientists for many years, and especially so since 1971 when Mariner 9 returned photographic evidence of fluvial features on the Martian surface. Ever since there has been ongoing debate about how the channels formed and what the climatic conditions must have been to produce them. Because most of the channels are found on heavily cratered parts of Mars' surface, most authors agree that they formed prior to ~3.8 billion years ago (the end of the so-called “heavy bombardment period” of solar system history). A warm, wet, essentially Earth-like Mars prior to that time has been favored by some authors (e.g., Mangold, et al., 2004; Pollack, et al., 1987; Sagan, et al., 1973), whereas others (e.g., Clifford, 1993; Peale, et al., 1975; Segura, et al., 2002; Squyres and Kasting, 1994) have argued that the fluvial features could have formed under much colder conditions. Recent evidence from the MER rover mission (Squyres, et al., 2004a; Squyres, et al., 2004b) supports the case for standing liquid water, although this interpretation has already been challenged (Knauth, et al., 2005; McCollum and Hynek, 2005).

The papers cited above represent only a small fraction of the literature on Martian atmospheric and climate history. Although progress continues to be made with each new Mars mission, there is still no consensus view as to how the Martian atmosphere evolved and how it helped shape the surface that we see today. Understanding Mars’ atmospheric evolution is thus a high priority for the NASA planetary science program. This question is of great interest to astrobiologists also, as the potential for life on or near Mars’ surface depends critically on the planet’s climate history, along with other environmental factors.

Approaches to studying Martian atmospheric evolution

Two different approaches have proved useful in studying the evolution of Mars’ atmosphere and climate. One approach is to simply ask what is possible from a climatic standpoint. One-dimensional, globally averaged climate models have been used to investigate the magnitude of the greenhouse effect on early Mars to see what, if any, type of atmospheric composition would have been needed to warm its surface substantially. An interesting result has emerged from these calculations: The Sun is thought to have been only about 75 percent as bright as today at the time when most of the channels formed, ~3.8 Ga. (billions of years before present). Because of this, and because CO₂ condenses at low temperatures, a gaseous CO₂-H₂O atmosphere could not have produced Earth-like conditions, regardless of the amount of CO₂ that was originally present (Kasting, 1991). If these calculations are correct, such an atmosphere could have produced a mean surface temperature no higher than 225 K at or prior to 3.8 Ga. This is probably too cold to have allowed the formation of fluvial channels, unless transient heating by impacts is invoked as their cause (Segura, et al., 2002). Additional radiative heating could have been provided by CO₂ ice clouds (Forget and Pierrehumbert, 1997; Mischna, et al., 2000), but near 100 percent cloud cover would be required to produce global mean surface temperatures above freezing, and this is considered unrealistic. Venus and Titan are completely shrouded in clouds, but in both cases the clouds are produced photochemically. CO₂ clouds on Mars, like H₂O clouds on Earth, are formed by condensation and are unlikely to cover a planet’s entire surface. So, a CO₂-H₂O atmosphere on early Mars would have been cold.

One possible way to get around this problem is if the early Martian atmosphere contained additional greenhouse gases besides CO₂ and H₂O. Sagan and Mullen (1972) (see also Sagan and Chyba, 1997) suggested NH₃ as the additional greenhouse gas; however, NH₃ is known to be photochemically unstable with respect to conversion to N₂ and H₂ (Atreya, 1979), even in the presence of a UV-shielding organic haze (Pavlov, et al., 2001). CH₄ is a more plausible alternative (Kasting, 1997). CH₄ is itself photochemically unstable; however, as discussed further below, its lifetime is much longer than that of NH₃. CH₄ is thought to have been abundant in Earth’s atmosphere prior to about 2.4 Ga and may have provided much of the warming required to keep the early Earth warm (Pavlov, et al., 2000). Definitive calculations showing that CH₄ could have kept early Mars warm have not been published, but this remains a viable possibility. As discussed below, the putative discovery of CH₄ in the present Martian atmosphere lends credence to the idea that CH₄ could have been abundant in Mars’ distant past.

An alternative approach to studying Martian atmospheric evolution is to start from the present atmosphere and to back-calculate what it was like in the past. This approach itself has two components: (i) Current isotopic ratios in different atmospheric gases can be used to show that Mars has lost an appreciable amount of its atmosphere over time. For example, the elevated ¹⁵N/¹⁴N ratio in Mars’ atmosphere, 1.7 times the ratio on Earth, shows that Mars has lost substantial amounts of N₂ (Fox, 1993b). The exact amount is model dependent, but it is probably of the order of 90 percent or more of Mars’ original inventory (Jakosky and Phillips, 2001). Similar heavy isotope enrichments are seen for H, C, O, and non-radiogenic Ar. (ii) Theoretical models also suggest that Mars has lost substantial amounts of atmosphere over time.

Some models suggest that the dominant loss process for molecules heavier than H₂ is sputtering by solar wind particles (Kass and Yung, 1995; Luhmann, et al., 1992). Kass and Yung (1995) concluded that Mars may have lost as much as 3 bars of CO₂ by this sputtering mechanism during the course of its history. This prediction is consistent with the observed enrichments in heavy isotopes, as well as with the need for a greenhouse gas-rich atmosphere although, as pointed out above, an enhanced CO₂ concentration cannot by itself explain how Mars’ climate could have been warm early in its history. Other processes such as ionosphere initiated outflow (Ma, et al., 2004) and ion-neutral chemistry processes (Fox, 1993b) yield similar results.

Collaborative studies of Phobos and Mars Global Surveyor (MGS) data have demonstrated that there is a thin, sharp boundary at ≈ 1.2 Mars radii (similar in shape, but not in physics, to a magnetopause), where the solar wind proton flux terminates and is replaced by planetary heavy ions (Nagy, et al., 2004). Thus, the removal of atmospheric components is not via direct sputtering, but rather by a mass loading process.

In any case, all of the models for how the Martian atmosphere evolved are at best poorly constrained by the available data. Based on the above discussion, better answers are needed to the important questions: is there a good reason to believe that CH₄ was present in the early Martian atmosphere, what processes have contributed to atmospheric escape over Mars’ history, and how much of the original volatile inventory has been lost in this manner? These questions can be addressed, in part, by making detailed measurements of Mars’ present atmosphere. The MSTO orbiter is ideally suited to examine both issues. A high-resolution spectrometer could be used not only to determine definitively whether CH₄ is present in Mars’ lower atmosphere, but also to map its vertical profile and spatial distribution. The vertical profile would provide a valuable test of current models of the chemistry of the atmosphere. And from an elliptical orbit MSTO could be used to make detailed measurements of Mars’ upper atmosphere and ionosphere and study their interaction with the solar wind. This will lead to a much better understanding of current atmospheric escape processes and allow a more reliable extrapolation back into Mars’ past.