Astronomers baffled by basalt in the outer Asteroid Belt

Dr Rene Duffard, who is presenting results at the European Planetary Science Congress in Potsdam on Wednesday 22nd August, said, “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.”

The presence of basalt means that the asteroid must have melted partially at some time in the past, which implies that it was once part of a larger body which had internal heating processes. However, there do not appear to be other basaltic fragments in the region and, from spectral analysis, it is not clear whether the two are fragments of the same parent body or not.

Until recently, most of the known basaltic asteroids, which are classified as V-type, were thought to be fragments of Vesta, the second largest object in the asteroid belt. Since 2001, several V-type asteroids have been identified as not belonging to this Vesta family, including (1459) Magnya, the first basaltic object to be detected in the outer asteroid belt.

Dr Duffard, of the Instituto de Astrofisica de Andalucia in Spain, and his colleague, Dr Fernando Roig, from the Observatorio Nacional in Rio de Janeiro, Brazil, selected the two asteroids, (7472) Kumakiri and (10537) 1991 RY16, for investigation by from a group of six candidate V-type asteroids identified using photometric data from the Sloan Digital Sky Survey (SDSS).

The reflectance spectra of the two bodies seem to show the characteristics of a V-type asteroid. However, there is a shallow absorption band around the wavelength of red visible light, which has never been observed before in other V-type spectra. This means that these objects have a slightly different chemical composition and do not fit into any existing category of asteroid. The unexpected dip in the spectra could have two sources: it could be due to impacts with other asteroids or comets “shocking” iron-rich compounds into a oxidized state, or it could indicate the presence of olivine, a green mineral that is also known as the semi-precious gemstone.

Dr Duffard said, “We need now to observe both objects in the near-infrared range to confirm whether they have a basaltic surface. If they do, we will need to try and work out where they came from and the fate of their parent objects. If they do not, we will have to come up with a new class of asteroid.”