of the reactivity of carbon-containing molecules under conditions where they are stable. Rather, we would have to conclude that either life is scarce in these conditions or that there is something special, and better, about the environment that Earth presents (including its water).
Many of the most important potential solvents found in the solar system exist only in their gaseous form on Earth. They become liquids at temperatures that are (regarded by humans as being) low. Hence, they are known as cryosolvents. Low temperatures are, however, prominent throughout the cosmos, as are species that are liquid there. Therefore, cryosolvents cannot be dismissed as potential biosolvents.
The most abundant compound in the solar system is dihydrogen, the principal component (86 percent) of the upper regions of the gas giants Jupiter, Saturn, Uranus, and Neptune. The other principal component of the outer regions of the giant planets is helium (14 percent). Throughout most of the volume of gas giants where dihydrogen is stable, it is a supercritical fluid. For the gas giants, two radii can be defined. The first is the radius of the region where dihydrogen becomes supercritical. The second is where the temperature rises to a point where organic molecules are no longer stable; for this discussion, 500 K is chosen as that point. If the second radius is smaller than the first, then the gas giant has a habitable zone for life in supercritical dihydrogen. If the second radius is larger than the first, however, then the planet has no habitable zone.
If such a zone exists on Jupiter, it is narrow. Where the temperature is 300 K (clearly suitable for organic molecules), the pressure (about 8 atm) is still subcritical. At about 200 km down, where the jovian pressure is supercritical, the temperature rises above 500 K, approaching the upper limit where carbon-carbon bonds are stable.24
For Saturn, Uranus, and Neptune, the habitable zone is broader relative to the planetary radius. On Saturn, the temperature is about 300 K when dihydrogen becomes supercritical. On Uranus and Neptune, the temperature when dihydrogen becomes supercritical is only 160 K, a temperature at which organic molecules are stable.
The atmospheres of these planets convect, of course. To survive on Jupiter, any hypothetical life based on molecules containing carbon-carbon covalent bonds would have to avoid being moved by convection to positions in the atmosphere where they are not stable. This is, of course, not impossible. Even on Earth, life in the oceans must avoid being moved by convection from its particular habitable zone. Sagan and Salpeter presented a detailed discussion of what might be necessary for a “floater” to remain stable in the jovian atmosphere.25
Bains has recently discussed dinitrogen as a possible biocryosolvent.26 His article also reviews the principal issues in using a cryosolvent. The most significant is the relative insolubility of substances in cryosolvents.
Thus, little is known about the behavior of organic molecules in supercritical dihydrogen as a solvent. In the 1950s and 1960s, various laboratories studied the solubility of organic molecules (e.g., naphthalene) in compressed gases, including dihydrogen and helium.27,28 None of the environments examined in the laboratory explored high pressures and temperatures, however.
Bains considered the possibility of organic species being dissolved in dinitrogen, including the possibility that silicon-based species might have greater solubility in dinitrogen than carbon-based species.29 However, the reactivity of silanes would make them unworkable as biopolymers on today’s Earth, because water reacts with many silanes.
This is not the case for dinitrogen as a biosolvent. Dinitrogen is abundant in the cosmos, like water. Its lower freezing and boiling points, however, make it a liquid over a wider range of the cosmos. For example, liquid dinitrogen may be a bulk solvent on Triton, the largest moon of Neptune.