PALEOMAGNETIC EVIDENCE OF A LOW-TEMPERATURE ORIGIN OF CARBONATE IN THE MARTIAN METEORITE ALH84001

Indirect evidence for life on Mars has been reported from the study of meteorite ALH84001. The formation temperature of the carbonates is co ntroversial; some estimates suggest 20 degrees to 80 degrees C, wherea s others exceed 650 degrees C. Paleomagnetism can be used to distingui sh between these possibilities because heating can remagnetize ferrima gnetic minerals. Study of two adjacent pyroxene grains from the crushe d zone of ALH84001 shows that each possesses a stable natural remanent magnetization (NRM), implying that Mars had a substantial magnetic fi eld when the grains cooled. However, NRM directions from these particl es differ, implying that the meteorite has not been heated significant ly since the formation of the internal crushed zone about 4 billion ye ars ago. The carbonate globules postdate this brecciation, and thus fo rmed at low temperatures.