[1] Detection of the organic matter on Mars is one of the main goals of the future Martian landing missions. Yet, the degradation of organic molecules by cosmic ray irradiation on Mars is often ignored. We calculate the radiation dose accumulation rates from solar and galactic cosmic rays at various depths in the shallow Martian subsurface. We demonstrate that a 1-billion-year outcrop on Mars accumulates the dosage of $500 MGy in the top 0-2 cm and $50 MGy at 5-10 cm depths. We show that the preservation of ancient complex organic molecules in the shallow ($10 cm depth) subsurface of rocks could be highly problematic if the exposure age of a geologic outcrop would exceed 300 Myr. We demonstrate that more simple organic molecules with masses $100 amu should have a good chance to survive in the shallow subsurface of rocks. Implications to the sampling strategy for the oncoming Martian missions are discussed. Citation: Pavlov, A. A., G. Vasilyev, V. M. Ostryakov, A. K. Pavlov, and P. Mahaffy (2012), Degradation of the organic molecules in the shallow subsurface of Mars due to irradiation by cosmic rays, Geophys. Res. Lett., 39, L13202,

We suggest an explanation of a sharp increase in the abundance of cosmogenic radiocarbon found in tree rings dated AD 775. The increase could originate from high-energy irradiation of the atmosphere by a galactic gamma-ray burst. We argue that, unlike a cosmic ray event, a gamma-ray burst does not necessarily result in a substantial increase in long-lived 10 Be atmospheric production. At the same time, the 36 Cl nuclide would be generated in the amounts detectable in the corresponding ice core samples from Greenland and Antarctica. These peculiar features allow experimental discrimination of nuclide effects caused by gamma-ray bursts and by powerful proton events.

No abstract