Evolution\nof Nucleobases under Asteroidal Aqueous\nAlteration

Nucleobases such as guanine, xanthine, and uracil have\nbeen detected\nin numerous carbonaceous chondrites (CCs), with isotope signatures\nindicating their extraterrestrial origin. A correlation between nucleobase\nconcentrations and the alteration degree of parent bodies is often\nreported, highlighting the importance of understanding the influence\nof aqueous alteration on nucleobase evolution to decode early solar\nsystem organic materials. Therefore, a laboratory investigation of\nthe evolution of guanine, uracil, and xanthine was performed under\naqueous alteration conditions typical of CCs. Given the intimate association\nof organic molecules with inorganic materials in CC matrices, we also\nincluded a set of inorganic phases in this study to discuss the influence\nof minerals on nucleobase evolution during aqueous alteration. Results\nshowed that guanine and uracil exhibit high stability, while xanthine\nundergoes decarboxylation. Saponite traps about 25% of xanthine, 53%\nof uracil, and nearly all guanine, promoting nucleobase preservation.\nThe isotope values (δ<sup>13</sup>C and δ<sup>15</sup>N) of nucleobases remained constant through aqueous alteration, suggesting\nthat their isotope signatures are related to synthesis in cold environments\nin the proto solar nebula or the parent molecular cloud. The relative\nstability of guanine during aqueous alteration explains its abundance\nin meteorites. Nevertheless, other processes such as (geo)chromatographic\neffects may account for the observed decrease in guanine abundance\nwith the increasing degree of alteration in CCs. The reported concentrations\nof uracil and xanthine in CCs are not correlated with the alteration\ndegree; hence, differences in relative abundance are either related\nto different parent body reservoirs or synthesis/migration during\naqueous alteration events.