Have scientists found a new category of asteroid? The evidence for basalt of a hitherto unseen composition on two small objects in the outer asteroid belt points to the possibility. An igneous rock, basalt would indicate that the asteroids were once part of a larger body, one that underwent some form of internal heating. The problem is that basalt is unusual for this part of the asteroid belt, nor is it clear whether the two fragments under study came from the same parent body. Add to that an unusual reflectance spectrum and the picture gets interesting indeed.
The asteroids in question are (7472) Kumakiri and (10537) 1991 RY16, and therein lies a tale. The two were chosen from a group of six candidate asteroids thought to be classified as V-type, a name deriving from Vesta, the second largest of the asteroids. Not long ago it was thought that all basaltic V-type asteroids were simply fragments of Vesta, but in the past few years several V-type objects not belonging to this family have been found in the outer asteroid belt. That alone gets our attention because we’d like to know what happened to the original body.
But the two asteroids now under investigation, and reported at the European Planetary Science Congress in Potsdam, show reflectance spectra that indicate a shallow absorption band around the wavelength of red visible light. No other V-type asteroids have exhibited such spectra, an indication that we may be dealing with a new category of object. Possible sources for the spectral dip include impacts with other asteroids or even comets, or the findings may indicate the presence of olivine. Whatever the case, the asteroids are in for another round of scrutiny.
Rene Duffard (Instituto de Astrofisica de Andalucia), who presented this material in Potsdam, says his team will now look at both objects in the near-infrared to confirm whether or not their surface is basaltic:
“We appear to have detected basalt on the surface of these asteroids, which is very unusual for this part of the asteroid belt. We do not know whether we have discovered two basaltic asteroids with a very particular and previously unseen mineralogical composition or two objects of non basaltic nature that have to be included in a totally new taxonomic class.”
As we wait for NASA’s Dawn mission to provide us with close-up information about Vesta, note the significance of the population of V-type asteroids. Most have orbital elements that suggest their origin as part of Vesta’s crust (the huge impact crater on Vesta’s southern hemisphere may well be the impact site). Few basaltic objects have been detected in the outer asteroid belt, the first being discovered in 2001. If basalt is confirmed on the two asteroids under study here, the question of where they come from opens up a challenging new investigation. And if they turn out not to be basaltic, a new asteroid class could be in the making.
The paper is Duffard and Roig, “Two new basaltic asteroids in the Outer Main Belt,” with preprint available.