Rapid Degradation of the Organic Molecules in Martian Surface Rocks Due to Exposure to Cosmic Rays. Severe Implications to the Search of the “extinct”

Introduction: Until recently, long-term exposure to cosmic rays (CRs) has not been recognized as a major environmental factor, which can alter and destroy organic molecules in the Martian surface rocks. This topic is critical because the entire strategy of the Mars Exploration Program’s search for the extinct life on Mars is based on the assumption that some original complex organic molecules would be able to survive for hundreds of millions billions of years in the ancient Martian outcrops. Moreover, current MSL, future Mars2020 will sample only the top few cm of the surface Martian rocks. Organic molecules at those depths are essentially unprotected against cosmic ray radiation. Recent modeling studies [e.g. 1] suggested that organic molecules with masses >100 amu would be effectively destroyed in less than 1 billion years in the top 5 cm of the Martian rocks. However, Pavlov et al calculated the fraction of the survived organic molecules using conservative radiolysis constants derived from the gamma irradiation experiments on pure dry amino acid mixtures [2]. Cosmic rays can produce oxidative radicals in the immediate vicinity of the organic molecules within the rocks and increase the rate of organic degradation. Methods: To evaluate the rate of organic degradation by cosmic rays in Martian rocks we exposed aminoacids (AAs) and carboxylic acids (CAs) mixed with SiO2 powder to gamma ray ionizing radiation (an analogue of CRs). For the analysis of AAs abundance and distribution in SiO2 powder we used the extraction procedure and liquid chromatography time of flight mass spectrometry techniques from [3]. Analysis of CAs abundance and distribution in SiO2 powder were conducted using a modified thermochemolysis gas chromatography mass spectrometry (i.e., a reactive pyrolysis) method [4]. The effects of accumulated dosage of up to 2 MGy were investigated by comparing the amount of organic compounds in control (nonirradiated) samples relative to irradiated materials. New radiolysis constants for aminoacids and carboxylic acids were derived. Radiation accumulation rates in the Martian rocks were derived with the stateof-the-art GEANT4 code. Newly derived radiolysis constants were then combined with the radiation accumulation rates to determine the rate of organic destruction and alteration by Galactic Cosmic Rays (GCRs) and Solar Cosmic Rays (SCRs) on Mars. Results: 1) Aminoacids. The destruction rate of aminoacids (AAs) in silicate powder mixtures is dramatically higher than in pure dry amino acid mixtures (Fig 1). Therefore, all aminoacid molecules, which were either produced (by biosphere) or deposited (by meteorites) on Mars earlier than 100 million years ago would have very little chance of survival in the surface Martian rocks.