From: Ron Baalke <BAALKE@kelvin.jpl.nasa.gov>
Subject: More on Vesta: A Science Background
SCIENCE BACKGROUND
"ASTEROID OR MINI-PLANET?
HUBBLE MAPS THE ANCIENT SURFACE OF VESTA"
VESTA: THE SIXTH TERRESTRIAL PLANET?
Vesta is the most geologically diverse of the large asteroids and the
only known one with distinctive light and dark areas--much like the
face of our Moon. Previous ground-based spectroscopy of Vesta
indicates regions that are basaltic, which means lava flows once
occurred on its surface. This is surprising evidence that the
asteroid once had a molten interior, like Earth does.
One possibility is that Vesta agglomerated from smaller material that
includes radioactive debris (such as the the isotope Aluminum-26)
that was incorporated into the core. This radioactive "shrapnel"
probably came from a nearby supernova explosion. (In fact a
supernova might have triggered the birth of our solar system.) This
hot isotope may have melted the core, causing the asteroid to
differentiate: heavier, dense material sank to the center while
lighter rock rose to the surface. This is a common structure for the
terrestrial planets. After Vesta's formation, molten rock flowed onto
the asteroid's surface. This happened more than four billion years
ago. The surface has remained unchanged since then, except for
occasional meteoroid impacts.
One or more large impacts tore away some of the crust exposing a
deeper mantle of olivine, which is believed to constitute most of the
Earth's mantle. Some of the pieces knocked off Vesta have fallen to
Earth as meteorites, which show a similar spectral fingerprint to
Vesta's surface composition.
A PIECE OF VESTA FALLS TO EARTH
In October 1960, two fence workers in Millbillillie, Western
Australia, observed a fireball heading toward the ground, and pieces
of the fallen meteorite were found ten years later. The fragments
stood out from the area's reddish sandy soil because they had a shiny
black fusion crust, produced by their fiery entry through Earth's
atmosphere.
Unlike most other meteorites, this sample can be traced to its parent
body, the asteroid Vesta. The meteorite's chemical identity points to
Vesta because it has the same unique pyroxene spectral signature.
Pyroxine is common in lava flows, meaning that the meteorite was
created in an ancient lava flow on Vesta's surface. The structure of
the meteorite's mineral grains also indicates it was molten and then
cooled. The isotopes (oxygen atoms with varying number of neutrons)
in the specimen are unlike the isotopes found for all other rocks of
the Earth, Moon and most other meteorites.
The meteorite also has the same pyroxene signature as other small
asteroids, recently discovered near Vesta, that are considered chips
blasted off Vesta's surface. This debris extends all the way to an
escape hatch region in the asteroid belt called the Kirkwood gap.
This region is swept free of asteroids because Jupiter's
gravitational pull removes material from the main belt and hurls it
onto a new orbit that crosses Earth's path around the Sun.
The Australian meteorite probably followed this route to Earth. It
was torn off Vesta's surface as part of a larger fragment. Other
collisions broke apart the parent fragment and threw pieces toward
the Kirkwood gap, and onto a collision course toward Earth.
Meteorites found in other locations on Earth are probably from Vesta
too.
THE OBSERVATION
Ben Zellner (Georgia Southern University), Alex Storrs (Space
Telescope Science Institute Baltimore, MD), Ed Wells (Computer
Sciences Corporation, Bethesda, MD), Rudi Albrecht (European
Southern Observatory in Garching bei Munchen, Germany) and
collaborators used Hubble's Wide Field and Planetary Camera 2
(WFPC 2) to collect images of Vesta in four colors of light between
November 28 and December 1, 1994. At the time Vesta was 156 million
miles (252 million km) from Earth. In late December 1994, when
Vesta was 10 million miles (16 million km) closer to Earth than a
month earlier, HST's Faint Object Camera made even higher
resolution images. These results are complemented by infrared
observations made on December 11, by Olivier Hainaut and colleagues
with an adaptive- optics camera on the European Southern
Observatory's 3.6-meter telescope in Chile. By combining Hubble and
ESO observations astronomers will be able to produce a geochemical
map of an asteroid's surface.
REPRINTED FROM ASTRONET, ISSUE 13.
FOR MORE INFORMATION, PLEASE CONTACT resource@rahul.net.
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- T o d d E. V a n H o o s e a r -
``'''vanhoose@lalaland.cl.msu.edu - vanhoose@msu.edu - vanhoose@lalaland.cl.msu.edu
(._.) Michigan State University - East Lansing, MI USA
(_) Computer Laboratory - Department of Communication
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