Thu, 13 Mar 2008 19:01 UTC
In the solar system's first few tens of millions of years, collisions between rocky objects and the decay of radioactive isotopes melted the interiors of large objects. Magma oceans - perhaps hundreds of kilometres deep - lapped over the Moon, the Earth, and other large bodies, allowing dense material to settle towards their centres in a process called differentiation.
The two meteorite pieces, called GRA 06128 and GRA 06129 after the Graves Nunataks area of Antarctica where they were found together in 2006, show evidence of such differentiation - which suggests they came from a massive body.
That's because the two objects are made mostly of a mineral called feldspar, which constitutes about 75 to 90% of their volume.
Feldspar is even more abundant in some lunar rocks. That is thought to be the result of crystals of feldspar solidifying from the early magma ocean on the Moon. Because feldspar is a relatively lightweight mineral, it would have floated to the top of the magma ocean, allowing it to form a highly concentrated layer of the mineral.
The amount of feldspar in the two meteorite fragments suggests they are remnants of a very large body that differentiated in a similar way, according to Allan Treiman of the Lunar and Planetary Institute in Houston, Texas, US, who led a study of one of the fragments.
'Strange new world'
Other studies of the meteorite, including one led by Richard Ash of the University of Maryland in College Park, another headed by Chip Shearer of the University of New Mexico in Albuquerque, and a third helmed by Ryan Zeigler of Washington University in St Louis, Missouri, all in the US, agree that the parent body must have been massive enough to have separated into layers.
The feldspar concentrations suggest that body was probably smaller than the 3500-kilometre-wide Moon but larger than Vesta, the third largest asteroid in the solar system at 578 kilometres across, says Treiman.
That's because meteorites believed to be from Vesta contain solidified lava, but not large concentrations of feldspar. That suggests that Vesta was massive enough to melt, but not so massive that it differentiated to form a distinct layer of the mineral.
"This is a piece of a dwarf-planet size body that apparently no longer exists," Treiman told New Scientist. "We have here a sample of a strange new world, a sample we've never seen before."
Zeigler, however, says the newly studied meteorites share similarities with a class of meteorites called brachinites, whose parent body appears to have been large enough to partially melt. "I think we can make a case that [the new discovery] is from the brachinite parent body [but] I don't think we can say it definitively yet," he says.
The meteorites' composition has led scientists to rule out the possibility that they are chips off of the Moon, Mars or Venus. And the ratio of iron to manganese does not match that of Earth, ruling out the possibility that it is an old chunk blasted off our planet's surface that later returned.
By measuring the radioactive decay of elements in the meteorite, scientists led by Richard Ash have shown that the rock must have formed around 4.5 billion years ago, when Earth and the other planets were coalescing.
Studying these fragments of a now-vanished object from that era provides a rare window into the early solar system, Treiman says. At that time, a lot of dwarf-planet size objects were flying around the solar system. Some would have been flung out of the solar system through gravitational interactions with other objects, while others collided to help build the planets present in the solar system today.
"We're looking maybe at a part of solar system history when dwarf planets were all over the place and forming the terrestrial planets," Treiman says.
But exactly what happened to the parent object of GRA 06128 and GRA 06129 is not known. If it was destroyed in a collision, there may be fragments of it still out there floating around the solar system as asteroids. Treiman says such fragments might be identified by their light spectra.
Some aspects of the meteorite, such as the high abundance of sodium in some of its minerals, hint that the parent body may have contained a lot of water, according to another study of the meteorite by Tomoko Arai of the National Institute for Polar Research in Tokyo, Japan.
The research from the five teams was presented on Wednesday at the Lunar and Planetary Science Conference in Houston, Texas, US.