A new study published in Nature this week reveals that asteroid surfaces age and redden much faster than previously thought - in less than a million years, the blink of an eye for an asteroid.

This study has finally confirmed that the solar wind is the most likely cause of very rapid space weathering in asteroids.

“Asteroids seem to get a ‘sun tan’ very quickly,” says lead author Pierre Vernazza. “But not, as for people, from an overdose of the Sun’s ultraviolet radiation, but from the effects of its powerful wind.”

It has long been known that asteroid surfaces alter in appearance with time - the observed asteroids are much redder than the interior of meteorites found on Earth - but the actual processes of this “space weathering” and the timescales involved were controversial.

Thanks to observations of different families of asteroids using ESO’s New Technology Telescope at La Silla and the Very Large Telescope at Paranal, as well as telescopes in Spain and Hawaii, Vernazza’s team have now solved the puzzle.

When two asteroids collide, they create a family of fragments with “fresh” surfaces. The astronomers found that these newly exposed surfaces are quickly altered and change colour in less than a million years - a very short time compared to the age of the Solar System.

“The charged, fast moving particles in the solar wind damage the asteroid’s surface at an amazing rate ”, says Vernazza. Unlike human skin, which is damaged and aged by repeated overexposure to sunlight, it is, perhaps rather surprisingly, the first moments of exposure (on the timescale considered) - the first million years - that causes most of the aging in asteroids.

By studying different families of asteroids, the team has also shown that an asteroid’s surface composition is an important factor in how red its surface can become. After the first million years, the surface “tans” much more slowly.

At that stage, the colour depends more on composition than on age. Moreover, the observations reveal that collisions cannot be the main mechanism behind the high proportion of “fresh” surfaces seen among near-Earth asteroids. Instead, these “fresh-looking” surfaces may be the results of planetary encounters, where the tug of a planet has “shaken” the asteroid, exposing unaltered material.

"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)"

Source: ESO