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Proving Origin of Martian Meteorites

Dateline: 03/20/00

By Alan Bruzel

How Many Meteorites Have Been Found on Earth?

Approximately 20,000, of which 19 are of lunar origin and 14 of martian origin. Some of these samples possibly derive from a meteorite breaking into several pieces during its descent to Earth, so that the number of unique (that is, unbroken) lunar and martian meteorites is a somewhat smaller number. Meteorites are believed to result from meteors, comets or asteroids striking a larger parent body, such as the Moon or Mars; some of the ejected material then collides with Earth.

How Are Lunar Meteorites Identified?

Three hundred and eighty-two kilograms (842 pounds) of rock and soil from six lunar locations were brought back to Earth by the Apollo missions. Meteorites found on Earth matching the mineral composition, isotopic patterns, texture, etc. of these lunar samples are deemed of lunar origin. No samples (as yet) have been purposely brought to Earth from Mars, yet it is still possible to identify suspected martian meteorites.

Establishing Martian Origins of Meteorites

In 1976, the Viking landers relayed the composition of the martian atmosphere. Earth-based observations of the planet also provide atmospheric data. The analyses reveal that in addition to its notable atmospheric makeup (95% carbon dioxide, 2.7% nitrogen, 1.6% argon, 0.13% oxygen, 0.03% water vapor), the heavy isotopes of all gaseous elements, from hydrogen to xenon, are much more plentiful on Mars than are the lighter isotopes of these elements. Among all bodies studied in the solar system, Mars is unique in having this isotopic signature. Gases trapped in a putative martian meteorite need to match this signature to establish martian credentials.

What About Martian Meteorite ALH84001?

ALH84001, a 1,940 gram (4.28 pound) meteorite found in 1984 in the Allan Hills of Antarctica, has become famous because of the controversy surrounding possible martian life (nannobacteria) found within it. But biological considerations aside, ALH84001 is instructive in illustrating many methodologies used in meteorite analysis:

1.) The gases trapped inside ALH84001 are enriched in those heavy isotopes consistent with the martian atmospheric gas signature.

2.) ALH84001 crystallized from molten rock about 4.5 billion years ago – a date given by both the samarium-neodymium and the rubidium-strontium radioactive dating systems. It would thus appear that the heavy isotopic abundances peculiar to martian atmospheric gases had appeared very early in the history of Mars. (This date also makes ALH84001 the oldest martian meteorite yet examined.)

3.) ALH84001 was ejected from Mars into space about 16 million years ago, as estimated by measuring levels of helium-3, neon-21 and argon-38 – isotopes indicative of cosmic ray bombardment during a meteorite's journey through space.

4.) In addition to the above three isotopes, carbon-14 is also produced from cosmic ray interaction with a meteorite's atomic constituents. After landing in Antarctica, ALH84001's exposure to cosmic rays markedly diminished, and carbon-14 could no longer be formed. Measurement of the remaining carbon-14 activity indicated that this martian meteorite plummeted into the Earth about 13,000 years ago.

What the Web Has to Say about:
Proving Origin of Martian Meteorites

Atmosphere of Mars
An article from this Web site describing composition of the martian atmosphere.

Isotopes from Space
Radioisotope dating of geological samples. From About.com's Geology Guide.

James Whitby's Home Page
Using mass spectrometry to analyze gases in meteorites. From the Isotope Geochemistry Group, University of Manchester.

Life's 'Signature' Not Found In Martian Meteorite According To New Research
University of California at San Diego study shows non-biological origin of carbonates in ALH84001.

Mars Meteorites
Latest news about martian meteorites. From Ron Baalke, Jet Propulsion Laboratory, NASA.

Meteorites from Mars!
Pictures, descriptions, and conjectures regarding their origins. From Johnson Space Center, NASA.

Nannobacteria: Minerals or Microbes
An article from this Web site examining putative terrestrial and extraterrestrial microfossils.

Recent Scientific Papers on ALH84001 Explained
Allan Treiman, Lunar and Planetary Institute, critiques and summarizes scientific analyses of martian meteorite ALH84001. Also, see his More Recent Research on ALH84001 and Technical Discussions on ALH84001.

Shocked Carbonates May Spell N-O L-I-F-E in Martian Meteorite ALH84001
A geological explanation for martian meteorite carbonates. From Edward R.D. Scott, University of Hawai'i.

The Case for Relic Life on Mars
In a Scientific American article, Everett K. Gibson, Jr. and others argue for biological origin of carbon found in ALH84001.

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