SEEDING THE PREGENETIC EARTH: METEORITIC ABUNDANCES OF NUCLEOBASES AND POTENTIAL REACTION PATHWAYS

Abstract

Carbonaceous chondrites are a class of meteorite known for having a high\ncontent of water and organics. In this study, abundances of the nucleobases,\ni.e., the building blocks of RNA and DNA, found in carbonaceous chondrites are\ncollated from a variety of published data and compared across various meteorite\nclasses. An extensive review of abiotic chemical reactions producing\nnucleobases is then performed. These reactions are then reduced to a list of 15\nindividual reaction pathways that could potentially occur within meteorite\nparent bodies. The nucleobases guanine, adenine and uracil are found in\ncarbonaceous chondrites in the amounts of 1$-$500 ppb. It is currently unknown\nwhich reaction is responsible for their synthesis within the meteorite parent\nbodies. One class of carbonaceous meteorites dominate the abundances of both\namino acids and nucleobases$-$the so-called CM2 (e.g. Murchison meteorite). CR2\nmeteorites (e.g. Graves Nunataks) also dominate the abundances of amino acids,\nbut are the least abundant in nucleobases. The abundances of total nucleobases\nin these two classes are $330 \\pm250$ ppb and $16 \\pm13$ ppb respectively.\nGuanine most often has the greatest abundances in carbonaceous chondrites with\nrespect to the other nucleobases, but is 1$-$2 orders of magnitude less\nabundant in CM2 meteorites than glycine (the most abundant amino acid). Our\nsurvey of the reaction mechanisms for nucleobase formation suggests that\nFischer-Tropsch synthesis (i.e. CO, H$_2$ and NH$_3$ gases reacting in the\npresence of a catalyst such as alumina or silica) is the most likely for\nconditions that characterize early states of planetesimals.\n