Date: 3/24/1997 - "In Praise of Idleness: RELAXing with Meteorites from Mars"
Public Lecture (free) Presenter: Professor Grenville Turner, Department of Earth Sciences, University of Manchester, Manchester, U.K. Main Lecture Hall, Chemistry Building, 3:30 PM
Later this year two NASA spacecraft will go into orbit around Mars and a lander will explore its surface. This will be the first major exploration of Mars since the memorable Viking landings more than 20 years ago. Early in the next century we anticipate that actual rock and soil samples will be returned to Earth for detailed analysis and interpretation. But as everyone now knows, these will not be the first samples from the red planet to be put under the geologist's microscope. Of the sevaral thousand meteorite specimens stored in museums across the globe, about a dozen are now known to have been ejected from the martian surface as a result of small asteroid impacts. The meteorites in question are known as the SNC meteorites after three representative meteorite types, Shergotty, Chassigny and Nakhla. Age determinations showed that they formed as a result of volcanic activity less than 1,300 million years ago. Volcanic activity cannot have occurred this recently on asteroids, which are the source of the more common meteorites. A much larger planetary body is required for radiactive heating to produce the temperatures needed for melting to occur and Mars was therefore implicated. The discovery of gases, matching, chemically and isotopically, those analysed by Viking in the martian atmosphere, clinched the identification, while the discovery of lunar rocks in the Antarctic ice sheet proved that the impact ejection mechanism was feasible. The public interest in the SNCs went into orbit with the discovery of possible indicators of "life on Mars" in a recently identified martian meteorite, ALH84001. Three lines of evidence - polycyclic aromatic hydrocarbons, coexisting microscopic magnetite and sulphide grains, and bacteria like morphologies - provided hints of former life but not yet proof. So what of the title of my talk, which is adapted from the title of a series of essays by Bertrand Russell? In Manchester we are studying the martian meteorites, including ALH84001, by way of the inert (German, idle) gases using the relatively new technique of resonance ionisation mass spectrometry. Our instrument, RELAX (Refrigerator Enhanced Laser Analyser for Xenon), is two orders of magnitude more sensitive than conventional mass spectrometers, capable of analysing samples of xenon as small as a few hundred atoms. Xenon is an especially useful element for detecting martian atmospheric gases in the minerals of SNC meteorites in that it shows a two-fold enrichment of 129Xe/132Xe compared to the Earth. In addition to inert gas studies, we use an ion microprobe to study oxygen and carbon isotopes in a search for clues to formation temperatures of minerals and possible signatures of organic compounds. Only the gases are idle.
Arkansas Center for Space and Planetary Sciences
202 Old Museum Building, University of Arkansas
Fayetteville, Arkansas 72701, USA
Tel. 479-575-7625 Fax. 479-575-7778 csaps@uark.edu
Later this year two NASA spacecraft will go into orbit around Mars and a lander will explore its surface. This will be the first major exploration of Mars since the memorable Viking landings more than 20 years ago. Early in the next century we anticipate that actual rock and soil samples will be returned to Earth for detailed analysis and interpretation. But as everyone now knows, these will not be the first samples from the red planet to be put under the geologist's microscope. Of the sevaral thousand meteorite specimens stored in museums across the globe, about a dozen are now known to have been ejected from the martian surface as a result of small asteroid impacts. The meteorites in question are known as the SNC meteorites after three representative meteorite types, Shergotty, Chassigny and Nakhla. Age determinations showed that they formed as a result of volcanic activity less than 1,300 million years ago. Volcanic activity cannot have occurred this recently on asteroids, which are the source of the more common meteorites. A much larger planetary body is required for radiactive heating to produce the temperatures needed for melting to occur and Mars was therefore implicated. The discovery of gases, matching, chemically and isotopically, those analysed by Viking in the martian atmosphere, clinched the identification, while the discovery of lunar rocks in the Antarctic ice sheet proved that the impact ejection mechanism was feasible. The public interest in the SNCs went into orbit with the discovery of possible indicators of "life on Mars" in a recently identified martian meteorite, ALH84001. Three lines of evidence - polycyclic aromatic hydrocarbons, coexisting microscopic magnetite and sulphide grains, and bacteria like morphologies - provided hints of former life but not yet proof. So what of the title of my talk, which is adapted from the title of a series of essays by Bertrand Russell? In Manchester we are studying the martian meteorites, including ALH84001, by way of the inert (German, idle) gases using the relatively new technique of resonance ionisation mass spectrometry. Our instrument, RELAX (Refrigerator Enhanced Laser Analyser for Xenon), is two orders of magnitude more sensitive than conventional mass spectrometers, capable of analysing samples of xenon as small as a few hundred atoms. Xenon is an especially useful element for detecting martian atmospheric gases in the minerals of SNC meteorites in that it shows a two-fold enrichment of 129Xe/132Xe compared to the Earth. In addition to inert gas studies, we use an ion microprobe to study oxygen and carbon isotopes in a search for clues to formation temperatures of minerals and possible signatures of organic compounds. Only the gases are idle.