Stability of biomolecules under energetic processes on Mars

The detection and identification of organic compounds on Mars is one of the main goals of the NASA Mars 2020 and ESA ExoMars exploration programs. It is therefore important to understand the environment in which organic matter evolves on the Martian surface. In particular, minerals may play a crucial role in the processes experienced by organic molecules on Mars, influencing their chemical evolution. The preservation state of organic molecules is often controlled by their interaction with the mineral phase in which they are embedded. Therefore, the detection of organic molecules on the Martian surface requires a complementary approach with the instruments on board the rovers. One aspect that is of great interest to the scientific community is the interaction between organic and inorganic matter. Martian rocks may, in fact, play multiple roles in both preserving organic molecules over time and, conversely, promoting their degradation. The thin Martian atmosphere allows ionizing particle and UV radiation to reach the surface, affecting the stability of organic compounds.

We know that in several paleoenvironments on Earth, the long-term preservation of terrestrial biosignatures has been attributed to sedimentary materials, particularly phosphates, silica, clays, carbonates and metalliferous materials. It is also thought that the most suitable conditions for the preservation of organic compounds on Mars are found in the subsurface, protected from radiation. To aid interpretation of the data that ExoMars/Rosalind Franklin rover will provide, we have begun a comprehensive investigation of the catalytic and protective properties of various Martian analogue minerals. However, it is not possible to simply classify Martian minerals as catalytic or protective, because the behaviour of minerals under Martian conditions depends strongly on the organic molecules involved and their specific interactions with the mineral surface sites. It is therefore important to study the response of specific molecule-mineral complexes to UV and ion irradiation. In this sense, and to maximise the chances of detecting organic compounds on Mars, it is crucial to study the effect of ionising radiation on organic compounds adsorbed in minerals that may represent Martian soils. This may also help in the identification of the molecules adsorbed on the minerals, and may also help in understanding the nature of the interactions between the mineral and the biomolecule to better understand the mechanisms involved in the preservation of biomolecules in different types of minerals against UV radiation. This is important for identifying mineral targets in Mars missions that are more likely to have preserved the organic molecules.

An important parameter affecting the way molecules interact with mineral surfaces is the acidity of the solution. The pH is responsible for the protonation state of the molecules and the charge of the mineral surface, and hence the molecule-mineral interactions and spectroscopic properties. Different possible pH-dependent protonation states can influence the nature of the binding of biomolecules to the mineral. It was observed that adenine was almost completely intercalated in the clay montmorillonite when adsorbed at acidic pH in the positively charged state, whereas almost no adenine was intercalated in the mineral when adsorbed at a pH where adenine was neutral. The same result was obtained by our group with L-histidine adsorbed in saponite, where the positively charged amino acid (acidic pH) is adsorbed in the interlayer by cation exchange, while the negatively charged amino acid (basic pH) is bound to the edge groups of the mineral. In addition, at basic pH, the organic molecules are rapidly desorbed and released from the interlayer of the mineral, which on Mars means greater exposure to Martian UV radiation due to the loss of mechanical shielding by the clay mineral.

We will show infrared and Raman spectroscopy of different classes of biomolecules, such as amino acids, nucleobases, fatty acids, PAHs adsorbed at variable pHs on clays, sulfates and phosphates, with the aim of obtaining a spectroscopic database that may be useful for correctly interpreting spectra collected on Mars. We will also present recent results on the preservation of biomolecules despite Martian chemical weathering by high-energy irradiation, which could be observed by analytical techniques on board the Rosalind Franklin rover.