Formation of Prebiotic Molecules in Liquid Water Environments on the Surface of Titan

Abstract

Saturn’s moon Titan represents a unique locale for studying prebiotic chemistry. Reactions occurring in its thick nitrogen-methane atmosphere produce a wide variety of carbon, hydrogen, and nitrogen containing organic molecules. If these molecules are exposed to liquid water, they may react further to produce oxygen-containing species, a key step in the formation of terrestrial biomolecules. On average, Titan's surface is too cold for liquid water. However, models indicate that melting caused by impacts and/or cryovolcanism may lead to its episodic availability. One possible cryovolcanic dome, Ganesa Macula, was identified in early observations by the Cassini spacecraft. In this work, I estimate the height and morphology of this feature using a synthetic aperture radar (SAR) image. I then use a thermal conduction code to calculate the freezing timescale for an initially liquid dome, yielding freezing timescales of ~10² - 10⁵ years. To determine how far aqueous organic chemistry can proceed in liquid water environments on Titan, I measure the rate coefficients of Titan analogue organic molecules ("tholins") with low temperature aqueous solutions to produce oxygenated species. These reactions display first-order kinetics with half-lives between 0.4 and 7 days at 273 K (in water) and between 0.3 and 14 days at 253 K (in 13 wt. % ammonia-water). Tholin hydrolysis in aqueous solutions is thus very fast compared to the freezing timescales of impact melts and volcanic sites on Titan, which take hundreds to thousands of years to freeze. The fast incorporation of oxygen, along with new chemistry made available by the introduction of ammonia, may lead to the formation of molecules of prebiotic interest in these transient liquid water environments. This chemistry makes impact craters and cryovolcanoes important targets for future missions to Titan.