False-colour map showing variation in Vesta's surface composition.
ASTEROID VESTA
 
Vesta, one of the first asteroids discovered, has turned out to be unusual in that it exhibits signs of volcanic activity, as well as being an identified parent of meteorites. Vesta was discovered in 1807.
 
Orbit
Vesta is a main belt asteroid, and consequently has similar orbital characteristics to thousands of other asteroids. There is nothing unusual about its orbit, which places Vesta at an average distance from the Sun of 353 million kilometres. The orbital eccentricity of 0.097 gives Vesta a perihelion of 319 million kilometres.
 

The asteroids tend to orbit in the main belt.
 
Physical properties
Vesta is one of the largest asteroids, measuring 520 kilometres across. It is in fact the third largest asteroid known (Ceres and Pallas are larger). Its large size, and consequential brightness, ensured its early discovery.
 

Comparison between Vesta and Earth.
 
Atmosphere
No atmosphere has been detected.
 
Magnetic field
No magnetic field has been detected.
 
Interior and surface
It is when Vesta's surface is considered that the asteroid becomes most interesting. The Hubble Space Telescope acquired images of Vesta in 1994, when Vesta was 251 million kilometres from Earth. Vesta's rotation rate of 5.342 hours allowed a time sequence of images to be constructed.
 

Hubble Space Telescope sequence showing the rotation of Vesta.
 
Light and dark areas are clearly visible on Vesta's surface, representing an active geological history. Furthermore, and most significant, a large impact crater has also been detected. The crater is over 450 kilometres in diameter, and given that Vesta is only 520 kilometres across, this is a very large crater on a very small object. To put it in perspective, this is equivalent to a 10,000 kilometre crater on Earth.
 

Light and dark areas are visible in this Hubble Space Telescope image.
 
The depth of the crater is such that it exposes Vesta's interior. Ground-based analysis of light reflected from Vesta shows that the surface material is basalt - volcanic rock. Pyroxene has been identified, a common component of lava. Within the crater olivine has been exposed, a material that constitutes Earth's mantle. The implication therefore is that Vesta is differentiated - it has a core, a mantle, and a crust. In this respect Vesta is less of an asteroid and more of a mini-planet.
 
The collision that created the crater was certainly dramatic. Nearly a million cubic kilometres of rock were ripped from Vesta and thrown into space, leaving a hole over 12 kilometres deep. Many, but not all, of the pieces from Vesta became smaller asteroids. As the rock, melted by the impact, rebounded it formed a mountain peak in the centre of the crater. The mountain is about 12 kilometres high.
 

Computer model of the shape of Vesta. Note the mountain peak at the bottom.
 
The identification of basalt is surprising. Molten lava has no place on asteroids - they're supposed to be geologically dead remnants of the Solar System's formation. Vesta may have formed from smaller asteroids and thereby incorporated material that was radio-active. Such material, as it decays, would release sufficient heat to allow the core to melt, and would allow lighter material to float to Vesta's surface. Vesta would in this way become layered, with a volcanic crust, in the same way that a planet acquires a core, mantle and crust. Once Vesta had cooled, there would be no further geological activity for four thousand million years, and Vesta would have behaved as an asteroid should.
 
Eucrites
Vesta's story now turns to Millbillillie in Western Australia. In 1960 a large fireball was observed by workers out repairing fences. In the 1970s, the crash site was found and pieces of the meteorite were recovered. The pieces were identified as members of the eucrite family of meteorites. There are now over 30 meteorites identified as eucrites. Eucrites have similar chemistry, and all appear to be related to the flow of lava. Their composition is that of crustal lava, not mantle. If the eucrites come from a shattered asteroid, there should also be mantle meteorites, and not just crustal meteorites. The fact that only crustal material has arrived on Earth shows that the parent body must still be intact, and has only lost material from its surface without being destroyed. The further requirement is that the parent asteroid must show signs of volcanic activity - it needs to have basalt.
 

Eucrite meteorites exhibit similar chemical composition to the asteroid Vesta.
 
There is only one large asteroid with a lava surface, and with a chemistry that matches the eucrites. That asteroid is without doubt Vesta. The eucrites are similar in composition, but not identical to each other. However, the surface of Vesta has a varying chemical composition, as shown in this composition map.
 

False-colour map showing variation in Vesta's surface composition.
 
The spectral signature of the eucrites matches the spectral signatures from different parts of Vesta's surface. The eucrites are therefore samples of different parts of Vesta. However, the meteorites did not travel to Earth directly from Vesta. Instead, they were thrown from Vesta into a part of the asteroid belt that is chaotic - where Jupiter's gravitational force then removes them and flings them into the inner Solar System. The eucrites are the only samples we have of another Solar System body apart from the Moon and meteorites from Mars.