Yale Lock on Martian Ages

alh_meteorite
The ALH Meteorite, about the size of a softball and one of more than two dozen Mars samples available for study on Earth today. [ALH84001] was found at Allen Hills, Antarctica. Around 28 Mars meteorites have been identified so far.
Image Credit: NASA/ Johnson Space Center

Scientists at Yale University have devised a method to precisely date the timing and temperature of a meteorite impact on Mars that led to ejection of a piece of the planet into space and its eventual impact on Earth.

Meteorites are the main source of mass exchange between planets and carry with them characteristic clues about the nature and history of the planets or planetesimals where they originated, the impacts that dislodged them, and the time they spent in space.

Kyoungwon Kyle Min, postdoctoral fellow in geology, reported an innovation for determining the timing and temperatures of ancient impacts that liberate meteorites from extraterrestrial bodies such as Mars.

To measure both the age and thermal history of the piece of Martian rock, Min assayed the natural radioactive decay of uranium and thorium to the gas helium in these meteorites, and combined it with knowledge of how temperature affects helium loss over time. This (U-Th)/He dating method, used on single grains of minerals in the "Los Angeles" Martian meteorite gave a far more accurate picture than the conventional method of analyzing chunks of meteorite. The "helium age" of about three million years corresponds with the estimated cosmogenic space exposure age.

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Close-up of Allen Hills meteorite Image Credit: NASA

According to co-author Assistant Professor Peter W. Reiners, "The three million-year age of this meteorite is also important because other meteorites we’re working on, including some Martian ones, are several hundred million to billions of years older. These methods allow us to better understand both the timing and dynamics of ancient impacts on other planets, and how these events relate to interplanetary material transfer."

Scientists have long looked at meteorites to answer the question of whether there is now, or once was, life on Mars. "Most Mars meteorites studied in labs on earth," said Arizona scientist, painter, and writerDr. William Hartmann, now working with the Mars Global Surveyor image team, "have clear evidence of having been exposed to moisture and salty water. One (named Lafayette) has enough weathered minerals that they could be dated by two labs (California and Arizona) and the water exposure was found to have happened 670 million years ago."

Computer modeling of half an hour after Giant Impact
Impact blasts seem to deliver martian fragments to Earth regularly, prompting comparisons to a kind of intra-solar-system ‘bus traffic’ of rocks.
Copyright William K. Hartmann

The timeline proposed for Lafayette showed a lineage that began around 700 million years ago on Mars, when some saline began to seep into it and change the rock’s mineral content. About 11 million years ago, the fragment blasted off of Mars as debris and then landed on Earth [originally in Illinois] about 2,900 years ago.

Scientists now can compare data from meteorites with the observations of space vehicles, such as future rovers, to learn more about past activities on the surface of Mars. These contextual questions will occupy comparisons of the world’s meteorite collections with what the Mars’ rovers are chemically analyzing on the surface this year. In a remarkable meteor traffic between the red planet and Earth, our planetary neighbor seems to have left behind more than a few intriguing clues about its geological history.

 

 


Stefan Nicolescu and James Greenwood from Yale University co-authored the study supported by a grant from the National Science Foundation.

Related Web Pages

NASA Mars Exploration Program
The Great Debate Series

Astrobiology Magazine: Mars Articles
Chomping on Nano-Nuggets
Evidence of Martian life dealt critical blow (NAI)
Search for Past Life on Mars (McKay, Thomas-Keptra, et al.)
Mars Meteorites (JPL)
Pyroxene and basalt in martian meteorites