Early asteroids unexpectedly crusty

Mineralogy of two meteorites chronicles formation of crust on an ancient asteroid

Two meteorites retrieved from West Antarctica contain a type of rock commonly found in Earth’s crust but previously unseen in meteorites. Analyses suggest that the meteorites, the oldest rocks of their type yet found, are fragments of an asteroid that coalesced early in the solar system’s history.

CRUSTY CLUES Two meteorites found in Antarctica (one shown at left) are a relatively light type of rock commonly found in Earth’s crust but never before seen in meteorites. Analyses of the meteorites, including viewing slices in polarized light (right), indicate that the rocks are chock-full of crystals rich in sodium and other volatile elements. R. Harvey, Case Western Reserve University; Day

COLD STORAGE Special scientific expeditions to Antarctica find many meteorites atop the continent’s ice sheet. Two found there offer new insights on asteroids of the early solar system. J. Day

When asteroids collide, fragments of their surfaces spray into space and can make their way to Earth, where they fall as meteorites. The composition of many such objects indicates that, like Earth and other rocky planets of the solar system, some asteroids formed crusts of dense, basaltic rock akin to cooled lava soon after they coalesced, says James Day, a geochemist at the University of Maryland in College Park. Now, in the Jan. 8 Nature, Day and his colleagues describe the first meteorites discovered that are composed of a relatively light type of rock similar to those that make up Earth’s continents.

Chemical analyses indicate that the meteorites came from the same parent body and “are clearly unique,” says Day. “This is what Earth’s earliest crust may have looked like,” he adds. While remnants of Earth’s crust from that era have been consumed by erosion and tectonic activity, asteroids were small enough to essentially be frozen in time, their makeups still reflecting their early composition. Furthermore, the meteorites are the first hard evidence that asteroids could form an Earth-like crust.

Minerals rich in volatile elements such as sodium, potassium and sulfur make up more than 75 percent of the two meteorites. This recipe means that, overall, the chemical composition of the rocks is similar to that found in silicate rocks generated by Earth’s tectonic activity. The ratios of lead isotopes in the objects hint that their minerals cooled about 4.52 billion years ago, less than 50 million years after the solar system formed.

The extreme age of the meteorites strongly suggests that they didn’t originate on any of the solar system’s rocky planets, says Day.

For example, scientists estimate that Mercury’s crust is less than 4.4 billion years old, and the average age of the crust on Venus is less than 1 billion years. Furthermore, high concentrations of rare metallic elements such as platinum and osmium in the meteorites hint that their parent body wasn’t large enough to completely melt and thereby form a dense, metal-rich core.

Day and his colleagues speculate that the meteorites’ unusual mineralogy resulted from a two-stage process. First, the team suggests, materials in the objects’ parent asteroid partially melted when it first coalesced, a process that allowed relatively light silicate minerals to rise to the surface of the asteroid. Then, after the asteroid cooled somewhat, the heat released by the decay of short-lived radioactive isotopes such as aluminum-26 baked the rocks further to produce additional minerals.

The presence in the meteorites of micrometer-sized crystals of a mineral called pyroxene indicates that these rocks formed at a relatively shallow depth, between 15 and 20 meters, says Day — additional evidence that the meteorites are fragments of an ancient asteroid crust.

The team’s new report “is a plausible, comprehensive explanation of where these [meteorites] may have come from,” says Robert Hazen, a geophysicist at the Carnegie Institution for Science in Washington, D.C.

It’s not surprising why such meteorites haven’t been described before, Hazen continues. First of all, the fraction of asteroids that could have produced such meteorites is very tiny — “a very small part of the solar system inventory,” he notes. Plus, the veneer of crust that would have formed on such planetesimals is “not terribly thick, probably no more than a few tens of meters,” he notes.

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