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Meteorites
Kevin Righter

Meteorites are found all over the world. U.S. and Japanese government-funded expeditions to ice fields in Antarctica alone have found nearly 30,000 meteorites. Most meteorites are chondrites, which represent material thought to come from the primitive solar nebula and contain glass- and crystal-bearing spherules called chondrules. A smaller percentage (10 to 15 percent) of meteorites, called achondrites, represent pieces of differentiated (partially melted or crystallized) planetary objects — including asteroids Earth's Moon, and the planet Mars. Research in the past year in meteoritics has focused on both chondritic and achondritic meteorites, and several more exciting areas.

The Moon

Studies of Apollo lunar samples revealed that there may have been a Moon-wide impact event at 3.9 billion years ago termed the "Lunar Cataclysm." That event manifests itself as a spike in age histograms. Currently, there are approximately 25 meteorites in worldwide collections thought to be pieces of the Moon. Because lunar meteorites may provide a more representative sampling of the entire lunar surface compared to the limited area covered by returned Apollo samples, detecting evidence for such a spike in lunar meteorite samples would perhaps strengthen this idea. Using argon dating of impact melt glasses in a suite of lunar meteorites, Barbara Cohen and colleagues have tried to detect such a spike. As reported in the Journal of Geophysical Research, they found that the spike is present in the lunar meteorite population; although they also found younger ages present in some of the glasses. Resolution of this topic may ultimately require return of additional samples from a region of the Moon we have not yet sampled, such as its far side.

Mars

As with the Moon, we currently have close to 25 meteorite samples thought to be pieces of Mars. Many researchers have studied the evolution of Mars compared to Earth, especially because some research indicates there may have once been life on Mars. Two issues central to understanding the ability of a planet to sustain life — age and oxidation state — have been addressed in the last year with meteorite studies. Because oxidation-reduction equilibria control the stability of many minerals at the surface and deep within a planet, knowledge of the oxidation state of Mars, as well as the age of its rocks, is important to understanding the potential for life there.

Several new studies have set out to determine the oxidation state of the martian interior, each using a novel analytical approach. By measuring the compositions of oxides, olivines and pyroxenes in a suite of martian meteorites, Chris Herd and colleagues showed that the oxygen pressure of the martian interior is similar to that of mid-ocean ridge basalt on Earth. Meenakshi Wadhwa, Donald Musselwhite and John Jones have examined the redox state of europium (2+ or 3+) in pyroxenes, which allows an assessment of magmatic oxygen pressure; their results define a range similar to the mineralogical studies of Herd. Finally, using the redox state of iron (2+ or 3+) in pigeonitic pyroxene, Molly McCanta and colleagues are providing further constraints on the oxidation state of Mars' interior.
The age of accretion of Mars has recently been determined by application of the Hafnium-Tungsten chronometer. Hafnium-182 decays to tungsten-182 with a half-life of 9 million years, and hafnium stays in the mantle of a planet during differentiation while tungsten goes into the core; therefore, this element pair offers constraints on the timescale of differentiation and accretion in terrestrial planets and asteroids. Application to Mars through tungsten isotopic analysis of martian meteorites has yielded an age of accretion and differentiation of 15 million years. This is rapid compared to the time required for the entire assembly of the Earth — 100 million years.

Integration of the age and redox values with other astronomical and geological information we have about Mars — such as water content, atmospheric composition and volatile inventories — will aid planetary scientists in assessing whether Mars supported life in the past.

Primitive solar nebula

Most chondrites fall into three classes based on their mineralogy and composition — carbonaceous, enstatite and ordinary. With more and more meteorites being returned from Antarctica, several unusual chondritic meteorite groups have been identified. One of these is a general class of chondrites that are metal-rich, containing up to 70 percent volume metal. Even the most metal rich of the reduced enstatite chondrites contain only 30 percent. These meteorites have many unusual features that will certainly enhance our understanding of conditions in the primitive solar nebula. The most striking feature of these meteorites is the presence of zoned metal grains of a composition consistent with their condensation from nebular gas at high temperatures.

Recent research summarized by Sasha Krot and colleagues in Meteoritics and Planetary Science indicates that these meteorites also hold information about the formation of calcium aluminum inclusions (CAIs) — which are among the oldest materials in the solar system — and chondrules in spatially separate oxygen-16-rich and oxygen-16-poor (respectively) nebular regions. They also provide evidence for the heterogeneous distribution of short-lived aluminum-26 in the solar nebula. Studying this growing group of meteorites will almost certainly lend insight into nebular conditions that created our solar system.

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Righter is curator of Antarctic meteorites at the NASA Johnson Space Center in Houston. He researches the Moon, Mars and the asteroid 4 Vesta through meteorite samples, as well as the origin of Earth and Moon, and the origin of basaltic volcanism in arcs on Earth. Email: kevin.righter-1@nasa.gov.

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