Tracking HCN Molecules in Crater Melt Ponds on Titan

Abstract:

Titan is one of the most chemically rich places in the solar system. Its dense nitrogen atmosphere with small amounts of methane is constantly ionized and dissociated; overtime, the carbon, hydrogen and nitrogen recombine and form a range of complex organics known as tholins. Neish et al. (2010) showed that Titan’s tholins produce amino acids when exposed to liquid water, providing a natural experiment in prebiotic chemistry. This biological potential is a primary motivation for the newly selected Dragonfly mission to Titan. Dragonfly will directly sample and analyze the chemistry frozen in Titan’s ice as well as its atmosphere and surface. One of the prime targets for Dragonfly’s astrobiological investigations are frozen water melt ponds in impact craters. For Dragonfly to adequately sample the solidified melt sheet for prebiotic (or possibly biological) products, the team must understand the geologic history of these water-organic mixtures. In this work, we use the planetary ice model of Buffo et al. (2020) to track the concentrations of organic molecules within Titan’s melt ponds as they freeze. We track the concentration evolution of hydrogen cyanide (HCN), a common molecule on Titan, within the forming ice and the residual melt. This model has previously been used to model saltwater freezing into terrestrial sea ice and the Europan crust. In the model, impurity dynamics are driven by gravity drainage due to buoyancy differences. However, HCN is less dense than water, which suggests that it will be rejected from the floor of the melt pond while at its roof HCN will stay suspended and become trapped in the ice at a level equivalent to the bulk concentration of the underlying melt pond. The concentration will increase with depth as rejected impurities concentrate in the melt. Therefore, the ideal location Dragonfly should focus on sampling is the middle portion of the impact crater’s melt sheet, but Dragonfly will ultimately be limited to sampling ice that has been exposed through processes like fluvial erosion by rivers.