May 31, 2000 ITHACA, N.Y. -- Shortly before the Memorial Day weekend, NASA's mission to orbit and study a distant asteroid presented researchers with a glimpse of the birth of the solar system.
The spacecraft orbiting the asteroid Eros nearly 100 million miles from Earth was, for the first time, able to reveal, through a brief "snapshot," the chemical composition of an asteroid. And the data indicate that Eros is a primitive relic of the emergence of the solar system from a cloud of gas and dust.
The team operating one of the five instruments on board the Near Earth Asteroid Rendezvous spacecraft (known as NEAR Shoemaker), say they have strong evidence that the 21-mile-long Eros is a very primitive body that has remained largely unchanged since the initial formation of the solar system. "By looking at an asteroid like Eros we are looking at one of the solar system's basic building blocks," says Steven Squyres, professor of astronomy at Cornell University and one of the four members of X-ray/gamma-ray spectrometer (XGRS) science team.
The discovery, one of the most significant scientific results to date from NEAR Shoemaker, was announced today at the spring meeting of the American Geophysical Union in Washington, D.C.
The event that led to the sudden findings was a powerful solar flare, or explosion, on the sun May 4. For 30 minutes, X-rays produced by the flare struck the asteroid, which in turn produced a "glow," called X-ray fluorescence. The XGRS recorded the different elements on the asteroid's surface "glowing" at different wavelengths, enabling researchers to take a snapshot of elements in an area 3.7 miles (6 kilometers) across. Three of the XGRS's four detectors were able, in particular, to separate out the elements magnesium, silicon and aluminum.
Since the spacecraft went into orbit around Eros on Feb. 14, a question to be solved has been: Is the asteroid more like Earth and the moon, a complex body that has gone through intense melting in which a crust and a dense core have been formed; or is it a primitive body that has escaped heating and melting, remaining largely unchanged?
Eros is classified as an S-type asteroid, the most common classification of asteroids. Squyres says this latest evidence provides "a key piece of information suggesting that many other S asteroids also might show this primitive characteristic." Although measurements from one small area of Eros, he says, "don't prove that point, it's the first time we've been able to measure the elemental composition of part of an S asteroid." But, he cautions, because asteroids are puzzling and bizarre objects, "one should extrapolate from Eros to the rest of the S asteroids with great caution."
Although the XGRS data obtained so far do not answer such essential geological questions as whether Eros is a rubble pile or a collision fragment, they do clearly indicate that the asteroid is not composed of basalt or any kind of igneous, or once-melted, rock, says Squyres. Its elemental composition, he says, is instead similar to meteorites called chondrites, believed to have been formed by condensation from the solar nebula, the gas and dust from which the solar system emerged.
"If you look at the ratios of elements in chondritic meteorites, they are very similar to the ratios of many elements in the sun," says Squyres. "In Eros we are seeing the ratios of silicon, magnesium and aluminum -- produced from the cloud of gas from which planets condensed -- all basically preserved as they were in the solar nebula, and nothing much has happened since the changes. So we are seeing into the very distant past here."
The XGRS team is hoping that the analysis of X-rays from the small area of the asteroid (limited by the spacecraft's narrow field of view) will in coming months be extended over the entire body. In the first week of May, NEAR Shoemaker dropped to an orbit of 31 miles (50 kilometers) above Eros' center, and on July 7 the spacecraft will begin a gradual descent to an orbit of just 22 miles (35 kilometers). These low orbits will enable XGRS to map the composition of the rest of the asteroid.
The latest observation allows researchers for the first time to make a solid chemical linkage between an asteroid and meteorites found on Earth, says Squyres. "If our future observations confirm this data, we can go to our meteorites and find those with the exact same elemental ratios. It vastly narrows down the range of possibilities of what meteorites could be representative of. It will be like holding a piece of Eros in our hands -- in fact we may be holding a piece of Eros."
Other institutions on the XGRS team are Goddard Space Flight Center, the University of Arizona and the Max Planck Institute for chemistry.
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