In the News this month... how asteroids get a sun tan
When two asteroids collide, they create a family of fragments with newly exposed surfaces. As these fragments age, they become redder in colour, but the actual process, and the timescales over which they act, have been heavily debated. A team of researchers, led by Pierre Vernazza of the European Space Agency, have observed asteroids from different groups with various ages and compositions, and concluded that the ageing process is far more rapid than thought. The research, published in Nature (PDF) during April, not only shows that asteroid surfaces age and redden in less than one million years, but that the solar wind is the most likely cause of this weathering.
Artist’s impression of how the solar wind makes young asteroids look old. After undergoing a catastrophic collision, the colour of an asteroid gets modified rapidly by the solar wind so that it resembles the mean colour of extremely old asteroids. After the first million years, the surface “tans” much more slowly. At that stage, the colour depends more on composition than on age. CREDIT: ESO/M. Martins
While human skin is damaged over time by repeated exposure to the Sun's ultraviolet light, it is the highly energetic particles in the solar wind which damages the outer layers of an asteroid, destroying the molecules and crystals on the surface and forming a thin crust of material with distinctive properties. By studying different families of asteroids, the team found that the composition of an asteroid is an important factor in determining how red its surface becomes. After the initial rapid reddening during the first million years, the surface ages more slowly with the colour determined more by composition than by age.
Their research also showed that collisions alone cannot account for the high proportion of fresh-looking surfaces seen on near-Earth asteroids: roughly 10% of 1-km size near Earth objects appear to have unreddened surfaces. Instead, they suggest that these may be the result of planetary encounters where tidal shaking could expose fresh, unaltered material. The authors point out that, if this is the case, then a comparison of the colours of near-Earth asteroids with similar asteroids in the main asteroid belt should show a greater abundance of redder objects away from the Earth in the main belt.