Notice: This is an archived and unmaintained page. For current information, please browse

2013 Annual Science Report

NASA Jet Propulsion Laboratory - Titan Reporting  |  SEP 2012 – AUG 2013

Task 3.5.1: Titan as a Prebiotic Chemical System

Project Summary

Six years ago, NASA sponsored a National Academies report that asked whether life might exist in environments outside of the traditional habitable zone, where “weird” genetic molecules, metabolic processes, and bio‐structures might avoid the water‐based biochemistry that is found across the terran biosphere. In pursuit of this “big picture” question, we turned to Titan, which has exotic solvents both on its surface (methane‐hydrocarbon) and sub‐surface (perhaps super‐cooled ammonia‐rich water). This work sought genetic molecules that might support Darwinian evolution in both environments, including non‐ionic polyether molecules in the first and biopolymers linked by exotic oxyanions (such as phosphite, arsenate, arsenite, germanate) in the second. Further, we asked about the possibility that Titan might inform our understanding of prebiotic chemical processes, including those on “warm Titans”. Our experimental activities found few possibilities for non‐phosphate-based genetics in subsurface aqueous environments, even if they are rich in ammonia at very low temperatures. Further, we showed that polyethers are insufficiently soluble in hydrocarbons at very low temperatures, such as the 90‐100 K found on Titan’s surface. However, we did show that “warm Titans” could exploit propane as a biosolvent for certain of these “weird” alternative genetic biopolymers; propane has a huge liquid range (far larger than water). Further, we integrated this work with other work that allows reduced molecules to appear as precursors for more standard genetic biomolecules, especially through interaction with various mineral species.

4 Institutions
3 Teams
6 Publications
0 Field Sites
Field Sites

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.

Figure 1.
Figure 1. The repeating backbone charge of DNA drives DNA to unfold in water. Backbone repulsion directs strand-strand interactions as far away from the backbones as possible, allowing Watson-Crick rules to govern nucleobase pairing. Most important for Darwinian evolution, the repeating polyelectrolyte dominates the physical properties of DNA, allowing changes in the structure to change the information carried without dramatically altering its properties, allowing it to robustly support adaptation.
Figure 2. A polyelectrolyte (such as DNA) cannot dissolve hydrocarbon liquid, including methane liquids at low temperatures on Titan’s surface. As an alternative hypothesis, however, a genetic biopolymer that has a repeating backbone dipole where the same end of the dipole protrudes throughout might allow the biopolymer to be soluble in hydrocarbons and robustly support Darwinian evolution there. The two types of polyether examined in this work are shown.

    Steven Benner
    Project Investigator

    McLendon Christopher

    Myong Jung Kim

    Jeffrey Opalko

    Objective 1.1
    Formation and evolution of habitable planets.

    Objective 1.2
    Indirect and direct astronomical observations of extrasolar habitable planets.

    Objective 2.2
    Outer Solar System exploration

    Objective 3.1
    Sources of prebiotic materials and catalysts

    Objective 3.2
    Origins and evolution of functional biomolecules

    Objective 4.1
    Earth's early biosphere.

    Objective 4.2
    Production of complex life.

    Objective 5.3
    Biochemical adaptation to extreme environments

    Objective 6.2
    Adaptation and evolution of life beyond Earth

    Objective 7.1
    Biosignatures to be sought in Solar System materials

    Objective 7.2
    Biosignatures to be sought in nearby planetary systems