ISOTOPIC EVIDENCE FOR A TERRESTRIAL SOURCE OF ORGANIC-COMPOUNDS FOUNDIN MARTIAN METEORITES ALLAN-HILLS-84001 AND ELEPHANT-MORAINE-79001

Stepped-heating experiments on martian meteorites Allan Hills 84001 (A LH84001) and Elephant Moraine 79001 (EETA79001) revealed low-temperatu re (200 to 430 degrees Celsius) fractions with a carbon isotopic compo sition delta(13)C between -22 and -33 per mil and a carbon-14 content that is 40 to 60 percent of that of modern terrestrial carbon, consist ent with a terrestrial origin for most of the organic material. Interm ediate-temperature (400 to 600 degrees Celsius) carbonate-rich fractio ns of ALH84001 have delta(13)C of +32 to 140 per mil with a low carbon -14 content, consistent with an extraterrestrial origin, whereas some of the carbonate fraction of EETA79001 is terrestrial, In addition, AL H84001 contains a small preterrestrial carbon component of unknown ori gin that combusts at intermediate temperatures. This component is like ly a residual acid-insoluble carbonate or a more refractory organic ph ase.