home *** CD-ROM | disk | FTP | other *** search
- 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.
-