Half-baked asteroids have Earth-like crust

Jan 07, 2009
Field image of the achondrite meteorite GRA 06129, found in blue ice of the Graves Nunatak region of the Antarctica during the ANSMET 2006/2007 field-season. GRA 06129 and its pair, GRA 06128, are achondrite meteorites with compositions unlike any previously discovered Solar System materials. Image courtesy of the Antarctic Search for Meteorites (PI - Ralph Harvey, Case Western Reserve University)

Asteroids are hunks of rock that orbit in the outer reaches of space, and scientists have generally assumed that their small size limited the types of rock that could form in their crusts. But two newly discovered meteorites may rewrite the book on how some asteroids form and evolve. Researchers from the Carnegie Institution, the University of Maryland, and the University of Tennessee report in the January 8th edition of Nature that these meteorites are ancient asteroid fragments consisting of feldspar-rich rock called andesite. Similar rocks were previously known only from Earth, making these samples the first of their kind from elsewhere in the Solar System.

The two meteorites were discovered during the Antarctic Search for Meteorites (ANSMET) 2006/2007 field season in a region of the Antarctic ice known as the Graves Nunatak icefield. The light-colored meteorites, designated GRA 06128 and GRA 06129, were immediately recognized as being different from previously known meteorites.

"What is most unusual about these rocks is that they have compositions similar to Earth's andesite continental crust - what makes up the ground beneath our feet," says University of Maryland's James Day, lead author of the study. "No meteorites like this have ever been seen before."

Andesite is an igneous rock common on Earth in areas where colliding tectonic plates generate volcanoes, such as those of the Andes mountain range. The meteorites contain minerals thought to require large-scale processes such as plate tectonics to concentrate the right chemical ingredients. In view of this, some researchers had suggested that the meteorites were fragments of a planet or the Moon, not an asteroid. But analysis of the meteorites' oxygen isotopes at the Carnegie Institution's Geophysical Laboratory by Douglas Rumble ruled out that possibility.

"A number of solar system objects including parent bodies of meteorites, planets, moons, and asteroids have their own oxygen isotope signatures," says Rumble. "Just by analyzing 16O-17O-18O ratios we can tell if a meteorite came from Mars, from the Moon, or from a particular asteroid. One extensively studied parent is the asteroid 4 Vesta. In the majority of cases the actual location of the parent body is unknown, but a particular group of meteorites may be assigned to the same parent body based on the isotope ratios even if the specific location of the body isn't known. When the ratios in meteorites are plotted against one another the result is mutually parallel lines offset from one another. The GRA 06128 and GRA 06129 meteorites, and some similar ones called brachinites, plot below Earth-Moon rocks and are nearly coincident with meteorites from 4 Vesta."

The meteorites' age, more than 4.5 billion years, suggests that they formed very soon after the birth of the solar system. This makes it unlikely that they came from the crust of a differentiated planet. The chemical signature of some rare precious metals, notably osmium, in the meteorites also points to their origin on an asteroid that was not fully differentiated.

The researchers hypothesize that that the asteroid had a diameter somewhat larger than 100 kilometers, which would be sufficient to hold enough heat for the asteroid's rocks to partially, but not completely, melt. The asteroid would remain undifferentiated, but the melted portions could erupt on the asteroid's surface to form the andesitic crust.

"Our work illustrates that the formation of planet-like andesite crust has occurred by processes other than plate tectonics on solar system bodies," says Day. "Ultimately this may shed light on how evolved crust forms on planets, including Earth, during the earliest stages of their birth."

Source: Carnegie Institution

Explore further: New state map from Indiana Geological Survey makes use of high-res imaging

Related Stories

What is the Kuiper Belt?

Jun 17, 2015

Dr. Mike Brown is a professor of planetary astronomy at Caltech. He's best known as the man who killed Pluto, thanks to his team's discovery of Eris and other Kuiper Belt Objects. We asked him to help us ...

The difference between asteroids and meteorites

Jun 03, 2015

Asteroids, meteors, and meteorites … It might be fair to say these rocks from space inspire both wonder and fear among us Earthlings. But knowing a bit more about each of them and how they differ may eliminate ...

Researchers determine the origin of Annama meteorite

Apr 08, 2015

An international team led by the Spanish National Research Council (CSIC) has determined the orbit of Annama, a new characterized meteorite from a fireball occurred on April 19th 2014 at the Kola Peninsula ...

What is lunar regolith?

May 29, 2015

When you're walking around on soft ground, do you notice how your feet leave impressions? Perhaps you've tracked some of the looser earth in your yard into the house on occasion? If you were to pick up some ...

Ceres bright spots sharpen but questions remain

May 25, 2015

The latest views of Ceres' enigmatic white spots are sharper and clearer, but it's obvious that Dawn will have to descend much lower before we'll see crucial details hidden in this overexposed splatter of ...

Recommended for you

Creating a stopwatch for volcanic eruptions

49 minutes ago

We've long known that beneath the scenic landscapes of Yellowstone National Park sleeps a supervolcano with a giant chamber of hot, partly molten rock below it.

Can lightning strike an indoor pool?

11 hours ago

Two swimming pool weather policies have surprised me in recent years. One was when I showed up to swim laps at an outdoor pool as it was beginning to drizzle. "Come on in," I was told; as long as there was no lightning, the ...

User comments : 2

Adjust slider to filter visible comments by rank

Display comments: newest first

Grendel
not rated yet Jan 08, 2009
I wonder if it might be a piece of the Mars-sized object whose collision with Earth resulted in the Moon's creation?
smiffy
not rated yet Jan 08, 2009
Fair point - but the researchers seem to have ruled this possibility out due to the oxygen isotope analysis -
some researchers had suggested that the meteorites were fragments of a planet or the Moon, not an asteroid. But analysis of the meteorites' oxygen isotopes at the Carnegie Institution's Geophysical Laboratory by Douglas Rumble ruled out that possibility.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.