Vesta, a protoplanet the size of Arizonz, has
been witness to an eventful past: images taken by the framing camera
onboard NASA's space probe Dawn show two enormous craters in the
southern hemisphere. The images (above) were obtained during Dawn's
year-long visit to Vesta that ended in September 2012. These huge
impacts not only altered Vesta's shape, but also its surface
composition. Scientists under the lead of the Max Planck Institute for
Solar System Research in Katlenburg-Lindau in Germany have shown that
impacting small asteroids delivered dark, carbonaceous material to the
protoplanet. In the early days of our solar system, similar events may
have provided the inner planets such as Earth with carbon, an essential
building block for organic molecules.
Vesta is remarkable in many respects. With a diameter of approximately
530 kilometres, Vesta is the one of the few protoplanets in our solar
system still intact today. Like other protoplanets, Vesta underwent
complete melting approximately 4.5 billion years ago. However, most of
the volcanic activity on Vesta is thought to have ceased within a few
million years making it a time capsule from the early solar system. Dawn
observations of Vesta have shown a surface with diverse brightness
variations and surface composition. There is bright material on Vesta
that is as white as snow and dark material on Vesta as black as coal.
The enigmatic dark material holds the key to understanding the impact
environment around Vesta early in its evolution. Research led by
scientists at the Max Planck Institute in Katlenburg-Lindau has shown
that this dark material is not native to Vesta but was delivered by
impacting asteroids.
“The evidence suggests that the dark material on Vesta is rich in
carbonaceous material and was brought there by collisions with smaller
asteroids,” explains. Vishnu Reddy from the Max Planck Institute for
Solar System Research and the University of North Dakota, the lead
author of the paper. In the journal Icarus, he and his colleagues now
present the most comprehensive analysis of this material so far.
Compositional analysis, mapping, and modelling of dark material
distribution on Vesta suggest that it was delivered during the
formation of giant impact basins on Vesta.
“First, we created a map showing the distribution of dark material on
Vesta using the framing camera data and found something remarkable,”
explains Lucille Le Corre from the Max Planck Institute for Solar
System Research one of the lead authors of the study.
Dark material was preferentially spread around the edges of the giant
impact basins in the southern hemisphere of Vesta suggesting a link to
one of the two large impact basins. A closer examination showed that the
dark material was most probably delivered during the formation of the
older Veneneia basin when a slow impacting asteroid collided with Vesta.
Dark material from this two to three billion year old basin was covered
up by the impact that subsequently created the Rheasilvia basin.
“We believe that the Veneneia basin was created by the first of two
impacts two to three billion years ago,” says Reddy. In fact, impact
modelling presented in the paper reproduces the distribution of dark
material from such a low velocity impact.
Evidence for dark material is also found in the HED meteorites that
come from Vesta. Some of the meteorites show dark inclusions that are
carbon-rich. Colour spectra of dark material on Vesta are identical to
these carbon-rich inclusions in HED meteorites. The link between dark
material on Vesta and dark clasts in HED meteorites provides us with
direct evidence that these meteorites are indeed from Vesta.
“Our analysis of the dark material on Vesta and comparisons with
laboratory studies of HED meteorites for the first time proves directly
that these meteorites are fragments from Vesta”, says Le Corre.
“The aim of our efforts was not only to reconstruct Vesta's history,
but also to understand the conditions in the early solar system,” says
Holger Sierks, co-investigator of the Dawn mission at the Max Planck
Institute for Solar System Research.
The Dawn mission was launched approximately five years ago and
entered orbit around Vesta on July 16th, 2011. In 2015, Dawn will arrive
at its second destination, the dwarf planet Ceres, that like Vesta
orbits the Sun between the orbits of Mars and Jupiter within the
so-called asteroid belt.
Source: The Daily Galaxy via http://www.mpg.de
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