The radiation stability of glycine in solid CO2 - in situ laboratory measurements with applications to Mars

TL;DR

This study investigates how glycine, an organic molecule, degrades under radiation in solid CO2 environments similar to Mars, revealing it is less stable there than in water ice, with implications for detecting life on Mars.

Contribution

The paper provides new in situ kinetic data on glycine radiolytic destruction in CO2 ice at Martian temperatures, highlighting its reduced stability compared to water ice.

Findings

Glycine in CO2 ice has destruction rate constants 20-40 times higher than in H2O ice.

Half-lives of glycine in CO2 ice are less than 100-200 million years at a few meters depth on Mars.

Glycine's stability is significantly affected by the radiation environment in CO2-rich conditions.

Abstract

The detection of biologically important, organic molecules on Mars is an important goal that may soon be reached. However, the current small number of organic detections at the Martian surface may be due to the harsh UV and radiation conditions there. It seems likely that a successful search will require probing the subsurface of Mars, where penetrating cosmic rays and Solar energetic particles dominate the radiation environment, with an influence that weakens with depth. Toward the goal of understanding the survival of organic molecules in cold radiation-rich environments on Mars, we present new kinetics data on the radiolytic destruction of glycine diluted in frozen carbon dioxide. Rate constants were measured in situ with infrared spectroscopy, without additional sample manipulation, for irradiations at 25, 50, and 75 K with 0.8-MeV protons. The resulting half-lives for glycine in CO2-ice are compared to previous results for glycine in H2O-ice and show that glycine in CO2-ice is much less stable in a radiation environment, with destruction rate constants ~ 20-40 times higher than glycine in H2O-ice. Extrapolation of these results to conditions in the Martian subsurface results in half-lives estimated to be less than 100-200 million years even at depths of a few meters.