Miércoles 3 de Agosto de 2005, Ip nº 120

‘Cosmic Botox’ bashes asteroid wrinkles away
Por Zeeya Merali

Meteoroid impacts can give facelifts to asteroids, knocking hundreds of millions of years off their apparent age. The finding casts doubt on the reliability of current asteroid dating methods.

The only way to date most asteroids, like other bodies in the solar system, is to infer their ages from the number and appearance of craters that pockmark their surfaces. Heavily cratered objects suggest ancient surfaces that have long been pummelled by space rocks. The only exception to the rule is the asteroid Vesta, which has been dated radiometrically using meteorites known to have originated there.

But a new Nature study by Peter Thomas of Cornell University in Ithaca, New York, and Mark Robinson of Northwestern University in Evanston, Illinois, both in the US, hints the crater-based technique may not always be accurate.

They mapped the craters on the 33-kilometre-long asteroid Eros using more than 100,000 images taken by NASA's NEAR Shoemaker spacecraft between 2000 and 2001. They noticed that 40% of the asteroid’s rocky surface bore far fewer craters than the rest of it.

Smooth and youthful
At first Thomas thought that a meteoroid impact might have created a massive crater. That would have thrown debris into the air, which then could have blanketed small craters in the area. "But there clearly wasn't enough material in the central crash site to fill so many holes by that process," he explains.

Now he believes a collision with a meteoroid a few hundred metres across caused an asteroid-quake on Eros, which appears to be coated in 100 metres of soil-like regolith. The seismic waves set Eros shaking so hard that the smallest craters within a 9 km radius of the crash site collapsed, he suggests, giving it a smoother, more youthful appearance.

“One impact can completely re-set a surface," says Thomas. "If you only look at part of an asteroid’s surface, instead of at the whole thing, you could be fooled into thinking it’s a factor of 10 younger than it is."

Erik Asphaug, a planetary scientist at the University of California in Santa Cruz, US, agrees. "This asteroidal Botox calls into question the habit of dating asteroid surfaces through their cratering record," he writes in an accompanying article.

  20/07/2005. New Scientist Magazine.