NASA Astrobiology Institute (NAI)

Comment on Titan First

Andrew Pohorille (NASA-Ames)

Membranous vesicles that self-assemble in water from soap-like molecules extracted from the Murchison carbonaceous meteorite (courtesy of David Deamer). Besides water only very few other liquids have some capabilities to promote the formation of boundary structures.

In their recent, interesting paper, Robert Shapiro and Dirk Schulze-Makuch argue that Titan is the best target to search for indigenous alien life. The essence of their argument is that the environment on Titan is particularly conducive to rich organic, carbon-based chemistry. I agree that this, indeed, appears to be the case. Abundance of organic compounds, however, does not imply the existence of life or even a possibility of the emergence of life.

To become animated, organic matter has to separate itself from the environment and self-organize into functional structures capable of responding to environmental changes and self-reproduction. These processes are largely driven by non-covalent interactions. Folded proteins, membranes forming cell walls and the DNA double helix are examples of structures stabilized by such interactions. Non-covalent interactions might be either specific (enzyme-substrate interactions, selective ion transport) or nonspecific (lipid-lipid and lipid-protein interactions needed for membrane integrity, fusion and division). Their strength needs to be properly tuned. If they are too weak, there might be undesired response to natural fluctuations of physical and chemical parameters. If they are too strong it could impede kinetics and energetics of life processes.

Strength of non-covalent interactions depends critically on the solvent. Among different solvents, water plays a special role not only because it is the solvent for the only life that we know, but also because it exhibits a number of traits that in combination make it uniquely suited to promote self-organization of organic matter. For example, strong electrostatic interactions between polar or charged molecules are reduced in polar liquids, such as water, compared to those in the gas phase or non-polar liquids, such that they become well balanced with other non-covalent interactions. Furthermore, water exhibits a remarkable trait of promoting hydrophobic interactions between non-polar molecules or groups rich in hydrocarbons. In biology, these interactions are responsible, for example, for self-organization of cell walls, and protein folding and aggregation. Hydrophobic interactions provide a unique mechanism for separating polar and non-polar organic material that is critical to many functions of living systems.

Currently, there is no convincing evidence for the existence of water on Titan. There is also very little evidence that other solvents that might exist on Titan can promote the self-organizing processes leading to the emergence of life. Until one of those two possibilities is demonstrated we should exercise caution in advocating an expensive mission to search for life on Titan.

Membranous vesicles that self-assemble in water from soap-like molecules extracted from the Murchison carbonaceous meteorite (courtesy of David Deamer). Besides water only very few other liquids have some capabilities to promote the formation of boundary structures.