火星のアストロバイオロジー探査はどこまで進んだか

抄録

Now, many planetary bodies in our Solar System are of strong astrobiological interests, but Mars still continues to be the most feasible planet to look for life. Mars scientific exploration has focused on water, habitability (i.e., climate), and signs of life. This paper reviews major questions about these three issues and discuss an outlook of future Mars missions. The questions addressed in this paper are: (1) Did Mars have warm and wet climate in the past? (2) Is there methane in the Mars atmosphere? (3) Why hasn't organics been found on Mars? Current answers based on a review of recent researches found in the literature are the following. First, we are obtaining more and more data indicating that Mars had wet and warm climate, but the duration of such climate is very unclear. One possibility is that warm and wet climate may have been short lived (perhaps tens of years to tens of thousands of years each time) but occurred many times during the first several hundred million years in the Mars history. Second, the recent criticisms on the reports for methane discovery on Mars that they may have been errors due to telluric ¹³CH₄ absorption lines pose a serious question about these reports, although the criticisms do not necessarily account for all the reported data. Thus, methane on Mars should be regarded as a possibility at present and needs further investigation with much higher fidelity. Third, the discovery of perchlorate on Mars by the Phoenix mission opened up the possibility that organics on Mars surface may have been oxidized by O₂ released by perchlorate during pyrolysis and form chlorocarbons as detected by Viking landers. This oxidation process by perchlorate in pyrolysis may have been the reason why no organics have been found on Mars. Following these progresses, Mars exploration is stepping forward to a sample return mission. Although this will be an extremely exciting mission, it may make Mars exploration programs rather inflexible over the next decade or more. Thus, well-focused very small Mars probes, which can be relatively easily launched by a nation that have not landed on Mars before, such as Japan, may be able to make important contributions to Mars astrobiology.