Bad Space Weather
Asteroids orbit the sun primarily in an orbit between Mars and Jupiter in the so-called asteroid belt. From there, pieces of debris sometimes reach earth as meteorites. However, until now it has remained a mystery why the most common asteroid variant differs in appearance from the most common meteorite type.
You can often see on the face of an old sailor that he had to brave the wind and weather on the high seas. Notches and furrows criss-cross the once-smooth skin of youth. Hardly recognizable after half a century at sea. From the point of view of an asteroid, that seems laughable – after all, it has had to withstand the weather conditions of space for several billion years, constantly exposed to meteoroid impacts, bombardment by dust particles and the solar wind.
Reason enough to let his appearance age as well, say scientists led by Takahiro Hiroi of Brown University in Providence. They believe the discrepancy between the asteroids found on Earth and those seen from afar could be an indication of ongoing change in the asteroid's surface. Researchers have suspected for some time that the "skin" of the celestial bodies is becoming darker and also changing in color as a result of so-called space erosion.
Thanks to the Japanese space probe Hayabusa, which met the asteroid Itokawa in space at the end of 2005, researchers were able to take a closer look. In the limelight of the sun, the surface of the asteroid presented itself very unevenly: light and dark areas covered the chunk. But that's not all - apparently the brightness and color of these areas were correlated.
To explore this quirk, Hiroi and his team studied infrared radiation reflected from the apparently distinct regions. They took advantage of the fact that every mineral has a characteristic reflection pattern, i.e. it reflects or absorbs different parts of the solar spectrum. For comparison, they also used the reflectance spectrum of a so-called ordinary chondrite – the most common type of meteorite.
Amazingly, all three spectra matched closely, leading the researchers to conclude that the mineralogical composition was roughly the same. However, they were not the same in terms of the intensity of the reflected radiation. The scientists around Hiroi explain this with a differently advanced degree of space erosion. Very small iron particles would accumulate on the rubble and thus cause the different appearance. But the size of the rock could also contribute.
The apparently different materials on Itokawa's surface could have been created by some of the dark and rocky regions, i.e. the already weathered materials, being blown up by impacts from smaller rock fragments and relatively light-colored, fresh material emerging, they say researcher.
Other asteroids should also be affected by the weathering phenomena and thus have changed their original appearance. Hiroi and colleagues speculate that the ordinary chondrites were once split off from the so-called S asteroids, which are now the most common in the asteroid belt. Since then, the boulder giants have aged in the harsh climate of space and become more and more different from the formerly ejected rock.
Accordingly, the years do not leave an asteroid untouched - even if it takes a little longer than for a person.
