[meteorite-list] Meteorites May Be Remnants of Destroyed Dwarf Planet

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 14 Mar 2008 15:47:10 -0700 (PDT)
Message-ID: <200803142247.PAA17793_at_zagami.jpl.nasa.gov>

http://space.newscientist.com/article/dn13456-meteorites-may-be-%20remnants-of-destroyed-dwarf-planet.html
Meteorites may be remnants of destroyed dwarf planet
David Shiga
New Scientist
13 March 2008
 
Two rocks found together in Antarctica are chunks of a dwarf planet that
was smashed apart early in the solar system's history, detailed studies
suggest. Other remnants of the proto-world may still be floating around
in the asteroid belt, and might be identifiable by the spectrum of the
sunlight they reflect.

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."
          
Ancient era

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.

Remnant fragments

"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.
Received on Fri 14 Mar 2008 06:47:10 PM PDT