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Project Progress

This project collected experimental data related to the possibility of life on modern Titan in two of its liquid phases:
(a) On the surface, mixtures of methane and other hydrocarbons (ethane, propane) that are “cryogenic” (ca. 95 K), and
(b) Beneath the surface, which contained (according to some models) perhaps ammoniarich water, where the antifreeze properties of ammonia allowed temperatures as low as 200 K. These environments might permit genetic biopolymers to have bridging exotic species that are not sufficiently stable in water at terran temperatures to support genetics (e.g. arsenate, arsenite, phosphite, germanate).

This work exploited a “synthetic biology” strategy, where molecules believed (for theoretical reasons) to have genetic potential were synthesized in the laboratory and examined in various solvent mixtures relevant to those found on Titan. For the subsurface environments, we used waterammonia mixtures to examine (as alternatives for the phosphate esters in terran biology) esters of the more reduced phosphite, arsenate and arsenite, germanate, tellurate, and other exotic species. Two publications [Neveu et al. 2013][Benner 2013] discuss these results, which under- scored the special properties of phosphate that make it extraordinarily well suited as a linking atom in the backbone of genetic molecules.

For the surface environments, the “polyelectrolyte theory of the gene” (Fig. 1), regarded as being applicable universally in aqueous mixtures, cannot apply in cryosolvents. Here, we examined polyethers, which replace the repeating backbone charge, so useful to support Darwinian evolution in water, by a repeating dipole that has its negative end pointing outwards (Fig. 2). We found these to be insufficiently soluble in hydrocarbons at the 90-100 K on Titan’s surface to be plausible genetic biopolymers there.

However, these experiments drove us to appreciate the huge liquid range of propane, a range that is far larger than water in Earth-like gravitational fields. This forced us to conclude that “warm Titans” might exploit propane as a biosolvent, where the polyether genetic materials that we prepared are adequately soluble to support “weird” alternative genetics.

In the process, we integrated this work with other work that allows reduced organic molecules (albeit those that contain more oxygen than found in the accessible organics on Titan) to be used as precursors for more standard genetic biomolecules, especially through interaction with various mineral species.

The mineral elements that we found most useful (boron, molybdenum) are scarce; therefore, they must be concentrated to be effective in prebiotic chemistry. This, in turn, requires that they not be diluted into a planetary ocean.

Many models for the water inventory on early Earth require such an ocean, creating the possibility that such prebiotic chemistry might not have been possible on early Earth. Recognizing that the inventory of water on early Mars was never sufficient to create a planetary ocean there, we have noted that this “water problem” in prebiotic chemistry might be solved by having terran life originate on Mars, to be later transferred to Earth. This concept, of course, need not be restricted to Mars, but to any planet with sufficiently low gravitational pull as to allow ejection of material via meteoritic impact. As our understanding of extra-solar planetary systems develops over the coming years, the pairing of a “dry” and a “wet” rocky planets might identify certain systems as more likely to generate biology than those with isolated rocky planets.