Larry Nittler and former postdoctoral associate Nicole Foley are featured in this week’s issue of Science in an article entitled, “Planetary Science: Peeling Back One More Layer of Asteroid Mystery,” that highlights their work on uncovering the composition of the asteroid Eros, findings that will appear in this month’s Icarus. From the time Eros was reached by NASA’s Near Earth Asteroid Rendezvous (NEAR) Shoemaker spacecraft six years ago, scientists had hoped it would answer whether or not ordinary chondrites—the most common meteorites that fall on Earth—originate from S-type asteroids—the most common type in the inner asteroid belt. Though the original analysis of NEAR data showed that Eros has an elemental composition similar to ordinary chondrites, the element sulfur appears to be less than half as abundant in the body as it should be.

Foley, Nittler, and colleagues set out to test possible scenarios, such as space weathering of sulfur in the outermost micrometers of Eros, or the possibility that Eros melted early in its history, carrying much of its sulfur into its interior. The group studied three trace elements on Eros: chromium, manganese, and nickel--all less volatile than sulfur and none susceptible to space weathering. However, according to Nittler, these trace elements would be “strongly affected by any differentiation." A new analysis of NEAR Shoemaker’s x-ray spectrometer readings revealed that the three trace elements are as abundant in Eros as in chondrites, thus solidifying the case that space weathering was responsible for the depletion of sulfur and tying Eros to ordinary chrondrites.

Though this argument is strong, some researchers point out that Eros might still have melted slightly, thus ruling it out as a source of ordinary chondrites. Others point out that there might be discrepancies in NEAR Shoemaker’s mineral-identifying near-infared spectrometer data. These questions will probably remain unanswered until samples from near-Earth asteroids are retruned tto terrestrial laboratories.