Asteroid Eros poses a magnetic puzzle

A space probe called NEAR-Shoemaker made history last February when it became the first craft to land on an asteroid, a potato-shaped rock called 433 Eros. Not only did the 2,400-kilogram craft survive the descent onto Eros (SN: 2/17/01, p. 103), but measurements with its magnetometer a few days after landing confirmed a major mystery. Eros, indeed, has no detectable magnetic field.

Boulder-filled depression on Eros. NEAR Mission/JHU APL

That’s puzzling because meteorites, which are believed to be fragments of asteroids, possess magnetic fields. How could a chip of an asteroid be magnetic if the parent asteroid isn’t?

Gunther Kletetschka of the Catholic University of America in Washington, D.C., and his collaborators suggest a way out of the dilemma. The research team, including Mario Acuna of NASA’s Goddard Space Flight Center in Greenbelt, Md., presented its findings this week at the spring meeting of the American Geophysical Union in Boston.

In reviewing data indicating that meteorites are magnetized, Kletetschka’s team found that the rocks could have acquired at least some of their magnetism after they journeyed to Earth. For example, our planet’s magnetic field could have induced the magnetization in meteorites during their long earthly residence–typically thousands to millions of years–the team suggests.

The recent study offers a further solution to the magnetic puzzle. By examining four meteorites with compositions similar to that of Eros, the researchers found that the larger the meteorite, the smaller its magnetization. These rocks ranged in length from millimeters to centimeters.

The new finding suggests that when the smallest bits of rocky material first coalesced, most likely from the disk of gas and dust that enveloped the young sun, they were indeed magnetized. As these tiny fragments coalesced into larger rocks, the fragments’ individual magnetic fields could have ended up in random orientations. In this way, a large rock could easily have a near-zero magnetic field, says Kletetschka. According to this scenario, if pieces chipped off Eros are small enough, they would display an intrinsic magnetic field.

Eros is the first asteroid for which researchers have measured a magnetic field close up, notes Acuna. The asteroid’s apparent absence of a magnetic field may call into question the measurement of fields reported for two asteroids–951 Gaspra and 9969 Braille–similar in composition to Eros. Those detections were made when the Galileo craft passed 1,600 kilometers from Gaspra, and Deep Space 1 flew 26 km from Braille. These data had prompted researchers to make the magnetic measurements on Eros.

Because Eros’ composition is uniform, the asteroid is probably a pristine relic from the early solar system. Other asteroids, Acuna notes, may have formed later and under different conditions, or they could have at one time melted, altering their magnetic properties. “There may be many families of asteroids and meteorites, and some may be highly magnetized,” he says.

The interpretation by Kletetschka’s team is certainly plausible, says Richard P. Binzel of the Massachusetts Institute of Technology. “There’s a lot we don’t understand about magnetization in the solar system,” he adds.

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