BY DR EMILY BALDWIN
Posted: 27 April, 2009
Unlike human skin which is damaged by prolonged exposure to sunlight over a lifetime, an asteroid's surface is aged in the first instances of its life.
Of course, the time scales of the exposure are much different: for an asteroid the damage is done over a period of one million years, but this is still a very short timeframe compared with the 4.6 billion year age of the Solar System itself. "Asteroids seem to get a 'sun tan' very quickly," says Pierre Vernazza, lead author of a paper published in this week's edition of the journal Nature. "But not, as for people, from an overdose of the Sun's ultraviolet radiation, but from the effects of its powerful wind."
It is already well known that asteroid surfaces change over time thanks to space weathering reddening their surfaces, but the actual processes and timescales involved were controversial.
Thanks to new observations conducted using ESO's New Technology Telescope at La Silla and the Very Large Telescope at Paranal, astronomers have shed some light on this mystery. The astronomers looked at freshly exposed asteroid surfaces (caused by the collision of two asteroids) and noticed that they change colour in less than a million years.
"The charged, fast moving particles in the solar wind damage the asteroid's surface at an amazing rate," says Vernazza. The solar wind contains highly energetic particles that bombard the exposed surfaces of asteroids, eroding the molecules and crystals on the surface and rearranging them into different configurations with distinct colours and properties.
By studying different families of asteroids, the team also demonstrated that an asteroid's surface composition can influence the rate of reddening after the initial one million year burst of exposure. The observations also suggested that collisions may not be the main mechanism behind the high proportion of fresh surfaces seen among near-Earth asteroids. Instead, they may be caused by gravitational interaction with a planet, where the tug of a planet has "shaken" the asteroid, exposing unaltered material.
This fundamental work will enable astronomers to better relate the surface of an asteroid to its history.