[meteorite-list] A Curiouser and Curiouser KBO (2003 EL61)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Oct 19 12:31:08 2006
Message-ID: <200610191631.JAA25825_at_zagami.jpl.nasa.gov>

http://skytonight.com/news/4398671.html

A Curiouser and Curiouser KBO
by David Tytell
Sky & Telescope
October 18, 2006

With the controversy over the classification of Eris, Pluto, and "dwarf
planets," a lesser known Kuiper Belt object (KBO) is vying to be the
most interesting solar system object beyond Neptune. Soon after they
found it, astronomers learned that 2003 EL61 is shockingly strange. Its
spin period, just 3.9 hours, makes it the fastest rotating known body in
our solar system larger than 100 kilometers (60 miles) across. Its shape
is even weirder - it looks like a squashed American football, and at its
widest point the highly elongated body likely exceeds even Eris's
diameter. Astronomers also found two moons of it: an inner one with a
35-day non-circular orbit, and a brighter one in a 49-day circular
orbit. Moreover, the object's density and rapid rotation imply that it
is solid rock, with only the thinnest of icy veneers coating its surface.

Given those characteristics, it seems clear that something plowed into
2003 EL61, sped up its rotation, and left debris in the form of moons.
However, in research presented at last week's American Astronomical
Society Division for Planetary Sciences meeting in Pasadena, California,
Kristina Barkume (Caltech) suggests that there are many more fragments
from the collision than just the two satellites.

Barkume has been spectroscopically surveying KBOs between 500 and 1,000
kilometers in diameter to learn about their compositions and physical
characteristics. She found something peculiar: at least 5 KBOs share
similar orbits and a common, dominant spectral signature of water ice.
What's most intriguing is that 2003 EL61's largest moon nicely fits into
this grouping. "They have much stronger absorptions of water ice than
all the KBOs for which we have data," says Barkume. "In particular, a
majority of KBOs have no detectable water ice."

The conclusion is that when the parent, 2003 EL61, got hit, it spewed
fragments - a KBO family. The parent body is a remnant rocky core, and
the children look alike. "It's as if Mercury had a family," says
dynamicist Alessandro Morbidelli (Observatoire de la Cote d'Azur, France).

Another fascinating aspect of this family is its age. The collection
presents some tantalizing, albeit circumstantial, evidence that the
collision happened very recently, geologically speaking. Barkume is
quick to note, "We can't say its age at the moment." But the fact that
the ice signature is so strong, she adds, could mean that it is fresh.

Furthermore, the close similarity of the objects' orbits could say
something about the collision that formed them. According to team
leader, Michael E. Brown (Caltech), conventional thinking is that when
an impact occurs in the Kuiper Belt, the event either scatters objects
to far-flung distances or creates satellites - not both. To see moons
and a group of free objects so closely related is odd. Perhaps the
impact happened with just enough energy to send objects beyond the
gravitational influence of 2003 EL61, but not much more.

For now, the EL61 family tree includes: 1999 OY3, 1995 SM55, 1996 TO66,
2002 TX300, 2003 OP32, and the moons of 2003 EL61. But Barkume hopes to
examine many more family candidates in the coming months.
Received on Thu 19 Oct 2006 12:31:05 PM PDT