10 Habitability of Desert Worlds Most of this review has considered planetary

10 habitability of desert worlds most of this review

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10. Habitability of Desert Worlds Most of this review has considered planetary habitability on worlds that are at least as wet as the Earth, if not much wetter. The surfaces of such “aqua planets” freeze if they are located too far from the star, while enhanced water vapor photolysis leading to a moist or runaway greenhouse occurs at closer distances. However, 3D simulations find that the solar system’s inner edge for “land planets” (which are worlds with an ocean inventory that is a fraction of Earth’s) is ~0.77 AU [ 7 ]. This is much closer to the Sun than the 0.95 AU computed for the classical HZ (e.g., [ 25 ]), which widens the overall HZ [ 7 ]. These land planets have a reduced water vapor greenhouse effect that enhances thermal infrared emission to space and creates a dry stratosphere, which limits water vapor photolysis and subsequent H escape to space [ 7 ]. Ignoring the carbonate–silicate cycle and keeping Earth’s atmospheric concentration constant, Abe et al. [ 7 ] also predict that our planet would freeze at ~1.05 AU. In contrast, the smaller water inventory on the land planet weakens the ice-albedo feedback, triggering global glaciation farther away (1.14 AU), according to their model. A subsequent calculation with a single-column radiative–convective climate model found a minimum inner edge distance of 0.38 AU for land planets if the atmospheric relative humidity is reduced to 1% [ 8 ]. However, these latter calculations may be flawed because their surfaces were not in energy balance. That is, the net absorbed solar flux (solar +thermal IR) at the surface must equal the convective (latent + sensible) heat flux [ 153 , 217 ]. At 1% relative humidity, Kasting et al. [ 153 ] only found balanced solutions when the mean surface temperature was ~250 K. Thus, solutions that are in energy balance do not exist at the high mean surface temperatures (~350 K) that Zsom et al. [ 8 ] predicted were necessary to trigger a runaway greenhouse on dry planets. Indeed, Kasting et al. [ 153 ]
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Geosciences 2018 , 8 , 280 26 of 48 calculate a maximum lifetime of liquid water against evaporation for such planets of ~400 years, which implies that these worlds are likely to be dry and uninhabitable. However, perhaps liquid water could still be present on the surface in cooler areas at higher elevation [ 8 ]. Indeed, such refuges for life may also be available on the Earth in the far distant future even as the Sun gets brighter, atmospheric CO 2 concentrations decrease, and C3/C4 photosynthesis ends in ~1 billion years [ 218 220 ]. Unicellular life will be more resistant, however, lasting for 2.8 billion years from present, given the presence of sheltered, high-altitude, or high-latitude regions [ 220 ]. Thus, it is unclear whether the deserts worlds of Abe et al. [ 7 ] and Zsom et al. [ 8 ] cannot also harbor vestiges of life, at least on some regions of the planet.
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