Astronomers reveal this week that the terrain of Eros - far from being smooth and featureless - is dominated by dust-covered impact craters and scattered rocks. Two teams of US scientists are piecing together evidence from the most detailed pictures ever taken of an asteroid, which were captured as the NEAR-Shoemaker spacecraft skimmed the surface of Eros in October 2000. The history of the asteroid is expected to shed light on the early evolution of the solar system (J Veverka et al 2001 Science 292 484; A F Cheng et al 2001 Science 292 488).
Joseph Veverka of Cornell University in the US and his team studied photographs of the asteroid’s gently undulating landscape taken as the NEAR-Shoemaker probe passed within 6.4 km of the body. Many of the craters that pock-mark the terrain – which range in diameter from a few metres to over a kilometre – are eroded and partially filled in. Veverka and colleagues believe that the rocks scattered across the asteroid’s surface were ejected by the same impacts that produced the craters. The extremely angular nature of some rocks compared with other more rounded rocks suggests that the ejected rocks have different structures and constituents. The probe imaged the surface with a resolution of 1 metre per pixel – only the Moon and Phobos, the tiny satellite of Mars, have been imaged in such detail.
The team also found that there are around 100 times as many large rocks than impact craters on Eros. Ververka and co-workers speculate that fragments could be more easily ejected – and then propelled further – than on Earth or the Moon because less work must be done against gravity. They suggest that Eros may be very fragmented because it suffered so many collisions in the early solar system when it was in the main asteroid belt between Mars and Jupiter. This fragmentation could make the rocks easier to eject.
Meanwhile, in a separate investigation, Andrew Chang of the Johns Hopkins University in the US and co-workers used a laser range finder on board the NEAR-Shoemaker probe to characterize some of the small features on the surface of the asteroid. The device had a range precision of 1 metre and measured many craters and boulders as it traced a path a few kilometres long. Chang and colleagues believe that the fractal-like distribution of structural features on the asteroid’s surface supports the idea that a single process – probably early impacts – shaped the landscape of Eros.
