Kari Fluegel March 15, 1993

Johnson Space Center

Pam Thompson

Lunar and Planetary Institute

Release No. 93-019

OF PLANET'S ATMOSPHERE

Several lines of evidence, such as relatively young crystallization ages and the composition of trapped volatiles, have convinced scientists that the so-called SNC meteorites are actually pieces of the surface of Mars that were blasted off the planet to eventually arrive on Earth as meteorites.

A team of researchers has painstakingly analyzed oxygen isotopes in the water and carbon dioxide extracted from a suite of SNC meteorites and will present their findings at the 24th Annual Lunar and Planetary Science Conference this week at the Johnson Space Center.

The meteorite samples are known as SNCs for Shergotty-Nakhla-Chassigny because they are "type" representative for the chemical and mineralogical classes thought to be from Mars. Researchers hope to learn about the origin and evolution of the ancient Martian atmosphere and to determine whether Mars once had flowing water--an ancient hydrosphere that carved the many channels observed by Mariner 9 and the Viking probes.

Recently, Dr. Haraldur Karlsson of Texas Tech University; Robert Clayton and Toshiko Mayeda of the University of Chicago; A.J.T. Jull of the University of

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Arizona; and Dr. Everett Gibson of JSC, concluded that some of the water in the meteorites is of extraterrestrial origin and is not in isotopic equilibrium with the host meteorite. The latter finding suggests that the Martian lithosphere and hydrosphere formed from two separate oxygen isotope reservoirs, one contributing the oxygen to the silicates in the rock and one contributing a component to the water.

In conference paper, the team describes its examination of oxygen isotopes and carbon isotopes in carbon dioxide extracted from the Martian meteorites. The results suggest that there also may have been separate reservoirs of carbon isotopes for the lithosphere and the atmosphere. One possibility is that the source of atmospheric carbon may have been non-Martian -- perhaps from comets.

The inhomogeneous isotopes support the contention that Mars lacks plate tectonics. On Earth, where plate tectonic activity cycles oceans through the crust at mid-ocean ridges, isotope ratios are relatively constant in both the lithosphere and atmosphere. Separate reservoirs are not inconceivable on a "one-plate" planet like Mars, and further study may provide insight into this style of planetary evolution.

The 24th Annual Lunar and Planetary Science Conference, set for March 15-19, is an annual gathering that gives scientists from around to world the opportunity to discuss the latest research in a variety of research areas. It is co-sponsored by the Lunar and Planetary Institute and JSC.

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