Last July, as NASA’s Dawn spacecraft began orbiting Vesta, scientists on the mission began noticing that the 500-kilometre wide asteroid, one of the lightest-coloured objects in the Solar System, was not uniformly bright. In December, as the spacecraft settled into its lowest orbit, they noticed blotchy areas of extreme brightness, as well as areas enriched in dark material. Could the brightest material be something special — such as water ice?
Probably not. At a press conference today at the Lunar and Planetary Science Conference in The Woodlands, Texas, principal investigator Chris Russell, of the University of California at Los Angeles, gave his explanation for the differences in colour. The bright stuff is likely to be Vesta’s most pristine, pyroxene-rich material that either has been lucky enough not to be disturbed by an impact or was recently exhumed. The darkest materials are likely to represent foreign material — the residue of impacts from other carbon-rich asteroids. And everything else on Vesta’s heavily tilled surface is a mixture. “The final story is, it’s not a special material, it’s just special handling,” Russell says.
Also on hand was Brett Denevi, a participating scientist at Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, who described a puddle of impact melt observed in Marcia crater — the first known instance of impact melt on an asteroid. Planetary scientists were unsure whether impactors in the asteroid belt could deliver the kind of punch required to melt rock.
In July, the spacecraft will fire its ion thrusters and move on to Ceres, the largest asteroid in the Solar System.
Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/UMD