[meteorite-list] Vesta A Planet?

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Wed, 26 Sep 2012 21:31:44 -0500
Message-ID: <000080064B5C47C8B49FCF767CDF6F1F_at_ATARIENGINE2>

List,

Interesting new analysis of Vesta:
http://www.sciencedaily.com/releases/2012/09/120926143519.htm

Sterling

[Text follows]:

ScienceDaily (Sep. 26, 2012) - Enormous troughs that
reach across the asteroid Vesta may actually be stretch
marks that hint of a complexity beyond most asteroids.
Scientists have been trying to determine the origin of
these unusual troughs since their discovery just last year.
Now, a new analysis supports the notion that the troughs
are faults that formed when a fellow asteroid smacked into
Vesta's south pole. The research reinforces the claim that
Vesta has a layered interior, a quality normally reserved for
larger bodies, such as planets and large moons.

Asteroid surface deformities are typically straightforward
cracks formed by crashes with other asteroids. Instead, an
extensive system of troughs encircles Vesta, the second
most massive asteroid in the solar system, about one-seventh
as wide as the Moon. The biggest of those troughs, named
Divalia Fossa, surpasses the size of the Grand Canyon by
spanning 465 kilometers (289 miles) long, 22 km (13.6 mi)
wide and 5 km (3 mi) deep.

The origin of these troughs on Vesta has puzzled scientists.
The complexity of their formation can't be explained by simple
collisions. New measurements of Vesta's topography, derived
from images of Vesta taken by NASA's Dawn spacecraft last
year, indicate that a large collision could have created the
asteroid's troughs. But, this would only have been possible
if the asteroid is differentiated -- meaning that it has a core,
mantle and crust -- said Debra Buczkowski of the Johns
Hopkins University Applied Physics Laboratory in Laurel,
Md. Because Vesta is differentiated, its layers have different
densities, which react differently to the force from the impact
and make it possible for the faulted surface to slide, she added.
"By saying it's differentiated, we're basically saying Vesta was
a little planet trying to happen."

Her team's research will be published online this Saturday
in Geophysical Research Letters.

Most asteroids are pretty simple. "They're just like giant rocks
in space," said Buczkowski. But previous research has found
signs of igneous rock on Vesta, indicating that rock on Vesta's
surface was once molten, a sign of differentiation. If the troughs
are made possible by differentiation, then the cracks aren't just
troughs, they're graben. A graben is a dip in the surface that
forms when two faults move apart from each other and the ground
sinks into the widening gap, such as in Death Valley in California.
Scientists have also observed graben on the Moon and planets
such as Mars.

The images from the Dawn mission show that Vesta's troughs
have many of the qualities of graben, said Buczkowski. For example,
the walls of troughs on simpler asteroids such as Eros and
Lutetia are shaped like the letter V. But Vesta's troughs have
floors that are flat or curved and have distinct walls on either
side, like the letter U -- a signature of a fault moving apart,
instead of simple cracking on the surface.

The scientists' measurements also showed that the bottoms
of the troughs on Vesta are relatively flat and slanted toward
what's probably a dominant fault, much as they are in Earth-
bound graben.

These observations indicate that Vesta is also unusually
planet-like for an asteroid in that its mantle is ductile and
can stretch under a lot of pressure. "It can become almost
silly putty-ish," said Buczkowski. "You pull it and it deforms."

Buczkowski and her colleagues' arguments for differentiation
of Vesta are interesting, said planetary scientist Geoff Collins
 of Wheaton College, in Norton, Mass, who specializes in
tectonics, the structure and motion of planetary crusts.
"On many much smaller asteroid bodies, we've seen very
narrow troughs that look just like cracks on the surface,"
said Collins, who was not involved in the new study. "But
nothing that looks like a sort of traditional terrestrial graben
that you'd find on Mars or the moon where things have
really been pulled apart."

But Collins is not yet fully convinced that Vesta's troughs
are graben. An example of rock-solid evidence of graben on
Vesta that has yet to be discovered, he said, would be an
obvious crater that had been torn in two by a trough.

There are other qualities of Vesta that could be clues to how
the troughs formed. For example, unlike the larger asteroid
Ceres, Vesta is not classified as a dwarf planet because the
large collision at its south pole knocked it out of its spherical
shape, said Buczkowski. It's now more squat, like a walnut.
But if Vesta has a mantle and core, that would mean it has
qualities often reserved for planets, dwarf planets and moons --
regardless of its shape.

The origin of that funny shape is the centerpiece of a different
hypothesis about how the troughs formed. Britney Schmidt of
the Institute for Geophysics in Austin, Texas, believes the south
pole collision knocked Vesta into its current speedy rate of
rotation about its axis of about once per 5.35 hours, which may
have caused the equator to bulge outward so far and so fast that
the rotation caused the troughs, rather than the direct power of
the impact. "It's an enigma why Vesta rotates so quickly," said
Schmidt, who was not a part of the current study.

Dawn has already left to explore Ceres, so all the data it will
retrieve on Vesta is in hand. Buczkowski said scientists will
continue to sort that data out and improve on computer simulations
of Vesta's interior. As those analyses come along, she said she
will keep an open mind toward any revelations that come to light,
but she doesn't expect her conclusion will change. "I really think
that these are graben," she said.
Received on Wed 26 Sep 2012 10:31:44 PM PDT