•• ••. 2004). These measurements are all on the verge of spectroscopic believability, so one should retain some scepticism about this result. If it is correct, though, it may imply that there are methanogens (or their martian equivalents) living at depth within some subsurface aquifer. This is not inconceivable. Putting a high-resolution spectrometer into orbit around Mars to further test this possibility is an excellent idea.
In any case, even though we cannot claim to understand the climate of early Mars, we can use the results shown in Figure 8.7 to obtain a (conservative) estimate for the outer edge of the HZ. If we restrict ourselves to greenhouse warming by gas-phase CO2
and H2 O, the lowest solar flux at which Mars' mean surface temperature can be raised above freezing corresponds to S/S0® = 0.43 x 0.85 = 0.37. This, in turn, corresponds to an orbital distance of 1 AU x (1/0.37)0 5 = 1.64 AU. The presence of widespread, optically thick CO2 ice clouds could conceivably raise this value to -2.0 AU (Forget & Pierrehumbert 1997; Mischna •• ••. 2000).
Why then is Mars not presently habitable? This question is not too difficult to answer. Mars is much smaller than Earth (-0.1 Earth mass), and consequently it cooled off much faster. Thus, there is by this time little or no active volcanism or plate tectonics to recycle carbonate rocks back into gaseous CO2. An Earth-sized planet at Mars' orbital distance would probably be habitable, but Mars itself is small, dry and cold. Clearly, other factors besides orbital distance are required to ensure planetary habitability.
Finally, one can also use these results to estimate the width of the
(CHZ) around the Sun. The CHZ is the region that remains habitable over some specified time period, usually taken as the lifetime of the Solar System, 4.6 Ga. As the Sun was only 70 per cent as bright at the beginning of this time, the outer edge of the HZ would have been in at 1.64 AU x 0.70 5 = 1.37 AU (or, if one uses the more optimistic estimate of 2.0 AU for the outer edge of the present HZ, the CHZ outer edge would be at 1.67 AU). This is equivalent to saying that early Mars could have been warmed by CO2 clouds if they were sufficiently widespread.
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