Landing data confirm Eros’ primitive nature

When a spacecraft for the first time left its footprint on an asteroid this past February, it was more than an engineering feat. By landing on the city-length rock called 433 Eros (SN: 2/17/01, p. 103), the spacecraft enabled its gamma-ray spectrometer to make measurements of unprecedented accuracy.

Two gamma-ray spectra recorded by NEAR Shoemaker after it landed on 433 Eros reveal emissions from several elements. Below: Arrow shows the landing site. NEAR/APL


The data, collected over 7 days, support earlier evidence that this near-Earth asteroid is a primitive rock, unaltered since its formation at the birth of the solar system, says Jacob I. Trombka of NASA’s Goddard Space Flight Center in Greenbelt, Md. This week, Trombka, the principal investigator of the gamma-ray spectrometer aboard the NEAR (Near Earth Asteroid Rendezvous) Shoemaker spacecraft, shared some of his team’s findings with Science News. Further details will appear in the November Meteoritics & Planetary Science.

The gamma-ray spectra show that Eros has a low abundance of iron and aluminum relative to magnesium. That’s the same signature found in the sun and ordinary chondrites–meteorites that rank among the oldest relics in the solar system. X-ray spectra recorded during NEAR Shoemaker’s yearlong orbit of Eros had already indicated that the rock was a well-preserved chunk of the early solar system (SN: 6/10/00, p. 375), but the gamma rays reveal the elemental composition beneath the asteroid’s surface–to a depth of about 10 centimeters.

The gamma rays were recorded from just one spot on Eros. Other regions could have different compositions, says Trombka although that’s unlikely. The X-ray spectra collected from several regions indicate that Eros has a uniform surface composition, he notes. Moreover, the craft’s motion around the asteroid also reveals, that the asteroid has a uniform density.

The findings suggest that Eros remains much as it was when it formed some 4.5 billion years ago. Had it undergone significant melting and cooling, for example, the asteroid’s density and composition would now vary. Data from any one experiment doesn’t prove that the asteroid is an unaltered relic, but the combined data make a compelling case, says Trombka.

The gamma-ray data reveal, however, that Eros has less abundant sulfur than do other bodies that formed early in the solar system’s life. Trombka speculates that numerous collisions with space debris could have heated the surface and caused the sulfur to evaporate.

Having a primitive body [such as Eros] come close enough to study . . . brings us a little closer to understanding how our solar system formed, says Zoe M. Leinhardt of the University of Maryland in College Park.

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