Solar wind tans young asteroids
Wednesday, April 22, 2009
This is an artist's impression of
how the solar wind makes young asteroids look old. After undergoing
a catastrophic collision, the color of an asteroid gets modified
rapidly by the solar wind so that it resembles the mean color of
extremely old asteroids. After the first million years, the surface
"tans" much more slowly. At that stage, the color depends more on
composition than on age. Credit: ESO/M. Martins
"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 Earth1 — but the actual processes
of this "space weathering" and the timescales involved were
controversial.
Thanks to observations of different families of asteroids2 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 rate3", 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.
Thanks to these results, astronomers will now be able to understand
better how the surface of an asteroid — which often is the only
thing we can observe — reflects its history.
###
ESO
Thanks to ESO for this article.