I do not know about you, but my
expectations were pretty low for Vesta. Instead we are given a nice problem in impact behavior
from the very beginning.
One quiet though thought is warranted. This object has been impacted plenty of
times, but in no manner do I get the sense that it has accumulated a lot of
mass when we compare it to the volume available in the belt itself.
In fact, one presumes that the
early solar system was just as crowded and the present theory supposes that
natural accumulation did all the heavy lifting.
I do not like that theory anymore since a better alternative exists that
avoids such orbital accumulation. The better
theory allows all accumulation to be done by Jupiter. Jupiter is operating close to the rotational
instability range that generates the ejection of emergents as the accumulation
progresses.
We have already written on the likelihood
of Venus been a recent such emergent.
All the inner planets were earlier such emergents and perhaps some of
the smaller moons and outer planets that are not gas giants.
Anyway, to our surprise Vesta is
interesting.
Close-up of Vesta poses puzzle
Astronomers keen to look into strange hole on second-largest asteroid.
Ron Cowen
Published online 12 August 2011 | Nature |
doi:10.1038/news.2011.480
News
The origin of Vesta's large crater and equatorial ridges is mysterious.NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
Planetary scientists thought they knew what to expect when NASA's Dawn
spacecraft returned the first close-up portrait of the giant asteroid Vesta
last month. Fuzzy images from the Hubble Space Telescope (HST) taken in 1996
seemed to show that something had taken a big bite out of the asteroid's south
polar region1.
The crater was posited as the source of Vesta-like fragments that
populate the asteroid belt, and of a surprisingly large fraction of the
meteorites found on Earth.
But seconds after viewing the first image, Peter Thomas of Cornell University
in Ithaca , New York
Thomas later realized he had misjudged Dawn's location when he sent
that e-mail, but his words give an idea of scientists' surprise. Vesta's huge
depression isn't like those of most impact craters: it is ringed by a wall for
only about half its circumference, says Dawn team member Paul Schenk of the
Lunar and Planetary Institute in Houston, Texas. It also has a large rounded
mound in its middle, rather than the usual conical uplift.
Perhaps strangest of all is a series of troughs ringing the asteroid's
equator, a feature not seen in any other body in the Solar System and which may
be related to the impact and its huge scale.
If it was caused by an impact, the crater is shaping up to be one of
the biggest puzzles of the mission, says Chris Russell, principal investigator
of the Dawn mission at the University of California, Los Angeles.
Looking for answers
Russell has commissioned a task force of scientists on the Dawn team to
solve the puzzle in time for two conferences in October.
New, sharper, images and spectra will help, as will maps of the
asteroid's gravity. Dawn is now orbiting Vesta at a distance of about 2,700
kilometres, some six times closer than when the initial observations were made
last month.
The task force will use the data gathered from this closer approach to
hunt for evidence of whether the hole really was caused by some sort of
collision. Tell-tale signs would include rock that has melted and resolidified
on the floor of the depression, and a mixture of broken rock and melted
material splashed out of the hole by the force of the blow.
Researchers have already come up with several possible explanations for
the hole's strange shape. These all assume that the roughly 460-kilometre-wide
crater was gouged out by a piece of space debris measuring 40-80 kilometres
across.
One idea is that Vesta, which, at 530 kilometres across is the
second-largest asteroid in the Solar System, was struck not at its south pole
but midway between the pole and the equator. Because it spins rapidly, completing
a full rotation in about five hours, Vesta would have reoriented itself so that
the gouged-out region became the rock's new south pole.
This would be the most stable configuration for the damaged asteroid,
says Schenk. "I don't think we've ever seen before a body with such a
large impact and such a high rotation rate," he says.
In January, Martin Jutzi of the University
of Bern in Switzerland
and Erik Asphaug of the University of California , Santa
Cruz
They calculate that Vesta completed an entire revolution while the
crater was forming. As a result, the debris thrown up by the impact did not
settle evenly around the crater, but fell in uneven clumps. This lopsided
excavation might explain why a wall runs around only half of the impact site.
The cause of the equatorial troughs remain a mystery, says Asphaug, but
they might be the result of material rushing back into the hole created by the
impact. "We really don't know the physics when the crater gets to be about
the size of the body [it strikes]," he says.
Russell is also hoping that Dawn will explain why Vesta is the
brightest member of the asteroid belt, reflecting some 40% of the sunlight that
hits it.
Images from the craft have already showed that some regions of the
asteroid are brighter than average, and revealed dark streaks on the inside of
craters. Compositional information recorded by Dawn's spectrometers may also
show whether the bright regions are made from different material or whether
they simply have a more crystalline structure, which scatters more light,
Russell says.
References
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