Print this pageThe Search for Possible Life on Mars
GIBSON, EVERETT K., DAVID S. McKAY, KATHIE THOMAS-KEPRTA, CHRISTOPHER P. ROMANEK, HOJATOLLAH VALI, RICHARD N. ZARE and SIMON J. CLEMETT
Dr. Gibson and his colleagues at NASA Houston, Stanford University, McGill University, and Univ. of Georgia published an article in the August 16, 1996 issue of Science Magazine reporting three sets of observations from meteorite ALH84001 that argue for the existence of past life on Mars: (1) occurrence of microscopic forms that resemble terrestrial bacteria and microfossils, (2) microscopic mineral grains with a composition and crystal form that on earth is associated with biogenic activity, and (3) reduced carbon in the form of polycyclic aromatic hydrocarbons (PAM) that may be associated with decomposition of bacteria. Gibson and his colleagues showed that these features are indigenous to the martian meteorite, and not due to terrestrial or laboratory contamination.
ALH84001 is a basaltic achondrite (i.e., igneous) meteorite that crystallized 4.5 billion years ago. ALH84001 was impact shocked during an early period of bombardment in the inner solar system. The sample contains intemal fractures which were infiltrated by water resulting in fommation of carbonate globules. Evidence for possible early biogenic processes is present within these carbonate globules. ALH84001 was ejected from Mars about 16 million years ago by a meteorite or cometary impact. The martian rock landed in Antarctica about 13,000 years ago and was discovered on the ice in 1984. In 1993, ALH84001 was shown to be one of twelve meteorites in the SNC (Shergotty-Nakhla-Chassigny Class) that originated on Mars.
Each of these observations, if made on a terrestrial rock, would be interpreted as evidence for biotic activity. But there are alternative, non-biotic, explanations for each observation. But taken together, they suggest the past existence of life on Mars.