[meteorite-list] Vesta A Planet?

[Sterling K. Webb]

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.