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The Limits of Organic Life in Planetary Systems
FIGURE 6.1 Different functional groups, but analogous mechanisms, could be used to form new C—C bonds in different solvents. In water, the C=O unit would provide the necessary reactivity. In ammonia, the C=N unit would provide the necessary reactivity. In sulfuric acid, the C=C unit is sufficient to provide the necessary reactivity.
are liquid even at the temperature of Titan, are a potential biosolvent. They are abundant in the cosmos, they have a wide temperature range of liquidity, and they are not bad as solvents.
22.214.171.124 Sulfuric Acid as a Possible Solvent
Ammonia is not the only polar solvent that might serve as an alternative to water. For example, sulfuric acid is a reasonably good solvent that supports chemical reactivity.7 Sulfuric acid is known to exist above Venus,8 where three cloud layers at 40 to 70 km are composed mostly of aerosols of sulfuric acid, about 80 percent in the upper layer and 98 percent in the lower layer. The temperature (about 310 K at about 50 km altitude, at about 1.5 atm) is consistent with stable carbon-carbon covalent bonds. Many authors have discussed the possibility of early life on Venus in its acidic environment.9-13 The surface temperature of Venus is approximately 740 K. Sagan and Morowitz even considered organisms that float above the hot surface using hydrogen “float bladders”14 analogous to those found in terran aquatic organisms, although others have raised questions about how to explain the prior evolution of a float bladder that would let an organism control its altitude.a Schulze-Makuch et al. argued for sample return from the venusian atmosphere to address the possibility of life there.15
Hypotheses about metabolism are abundant for the hypothetical life in acidic aerosols. In strong acid, the C=C bond is reactive as a base and can support a metabolism as an analog of the C=O unit. This type of chemical reactivity is exemplified in some terran biochemistry. For example, acid-based reactions of the C=C unit have been used by plants as they synthesize fragrant molecules.16
The 2006 National Research Council letter report “Assessment of Planetary Protection Requirements for Venus Missions” (The National Academies Press, Washington, D.C.) provides arguments for why life is unlikely to have originated in the clouds of Venus.