[meteorite-list] A Pallas sight

From: Darren Garrison <cynapse_at_meteoritecentral.com>
Date: Thu, 08 Oct 2009 19:21:23 -0500
Message-ID: <fc0tc5h6hpvpri0ltr3bd5l2vnjovegjtn_at_4ax.com>

Video and art at the link.

http://cosmiclog.msnbc.msn.com/archive/2009/10/08/2092402.aspx

Protoplanet frozen in time
Posted: Thursday, October 08, 2009 2:01 PM by Alan Boyle

Images from the Hubble Space Telescope suggest that the asteroid Pallas should
be grouped along with two other big space rocks as protoplanets - "planetary
embryos" that were big enough to stay pretty much as they were during the
formation of the solar system, but too small to progress to the next stage of
development.

"These are the first really high-resolution images of Pallas that come from
Hubble," said Britney Schmidt, a planetary scientist at the University of
California at Los Angeles and lead author of the study in Friday's issue of the
journal Science. "This was a suite of observations that haven't been made
before."

The imagery was collected in 2007 by a camera that's no longer on the Hubble,
the Wide Field and Planetary Camera 2, or WFPC2. That instrument was replaced
with a next-generation wide-field camera during May's final Hubble servicing
mission.

Getting the pictures down from WFPC2 was just the beginning of a two-year-long
process to put together a picture of Pallas, similar to the Hubble imagery
already collected for its asteroidal siblings Ceres and Vesta.

Schmidt and her colleagues painstakingly looked at the asteroid's profile from
different perspectives, then fit all those views together using a computer
modeling program called Maya. They also analyzed the slight color differences in
the pictures and matched them up with Pallas' 3-D shape.

The result? Pallas turns out to be almost but not quite round, falling just
short of a state known as hydrostatic equilibrium. Its mean radius is 170 miles
(272 kilometers), which makes it the second-largest asteroid after Ceres, which
has a 297-mile (475-kilometer) radius. But at 449 quintillion pounds (2.04 ?
1020 kilograms), it's the third most massive asteroid, ranking behind Ceres as
well as Vesta.

The asteroid has several depressions of various sizes, including what appears to
be a monster crater about 150 miles (240 kilometers) wide. Such craters were
likely caused by early impacts that knocked loose a family of asteroidal
fragments linked to Pallas, the researchers said.

The analysis also turned up bright and dark spots in ultraviolet light,
suggesting differences in composition. "What's great to see is that
heterogeneity actually exists, because it gives us some idea that there's some
processing going on," Schmidt said.

Pallas' composition suggests that it had liquid water and an active geology at
an early point in its multibillion-year history. "There aren't going to be
volcanoes on Pallas, and there aren't going to be continents, but it's heading
in a direction where it's going to be a planet," Schmidt said.

Much of that activity was frozen in place, making Pallas something of a planet
interrupted - what the researchers call "an evolved body with planetlike
properties," or a protoplanet. Scientists believe even the biggest planets in
the solar system passed through the protoplanetary stage, gravitationally
glomming onto bigger and bigger chunks of material until they got where they are
today.

Worlds such as Ceres, Pallas and Vesta were stuck in a state of arrested
development because nearby Jupiter pushed the asteroids around and grabbed a lot
of the good stuff for itself. At least that's how the favored scenario plays
out.

Ceres had grown large enough to keep a roundish shape, even after numerous
impacts, and thus is now considered a dwarf planet alongside Pluto, Eris and
potentially scores of other worlds beyond Neptune. Pallas and Vesta, however,
aren't quite in the same league. "They're not quite perfectly round, and
potentially because of impact," Schmidt said.

The bottom line is that Pallas is, well, right on the line when it comes to the
important features dividing the solar system's big planets and dwarfs (and, for
that matter, roundish natural satellites such as our moon) from irregular
objects such as small asteroids and comets. The researchers say it's closer to a
planet than to a typical asteroid, but Schmidt said the most interesting thing
about Pallas isn't its precise classification.

"What's more interesting than just the classification is to think of the
process," Schmidt said. "What's unique about this object is that it probably
stayed almost completely intact from the early days of the solar system. It
hasn't been broken up, and there are only a few of those kinds of objects left."

Protoplanets such as Pallas - and Ceres and Vesta - can thus serve as a fossil
record for an important time in our solar system's development. "They were not
only the building blocks of planets, but they're also what planets looked like
for a short period of time," Schmidt said. "They just never really got to form
into something bigger."

For comparison's sake, Eris' mean radius is an estimated 800 miles (1,300
kilometers); Pluto's is 721 miles (1,153 kilometers); and Earth's is 3,959 miles
(6,371 kilometers). All those worlds are thus more than twice as wide as Ceres,
Pallas and Vesta.
Received on Thu 08 Oct 2009 08:21:23 PM PDT