Could organic matter have been preserved on Mars for 3.5 billion years?

3.5 billion years (byr) ago, when it is thought that Mars and Earth had similar climates, biological evolution on Earth had made considerable progress, such that life was abundant. It is therefore surmised that prior to this time period the advent of chemical evolution and subsequent origin of life occurred on Earth and may have occurred on Mars. Analysis for organic compounds in the soil buried beneath the Martian surface may yield useful information regarding the occurrence of chemical evolution and possibly biological evolution. Calculations based on the stability of amino acids lead to the conclusion that remnants of these compounds, if they existed on Mars 3.5 byr ago, might have been preserved buried beneath the surface oxidizing layer. For example, if phenylalanine, an amino acid of average stability, existed on Mars 3.5 byr ago, then 1.6% would remain buried today, or 25 pg-2.5 ng of C g-1 Martian soil may exist from remnants of meteoritic and cometary bombardment, assuming that 1% of the organics survived impact.