September 21, 2000


The Eros asteroid could have witnessed the formation of the Earth, according to a preliminary analysis of its surface composition using NASA's Near Earth Asteroid Rendezvous (NEAR) spacecraft. Analysis of solar X-rays from Eros' surface indicate that it has a composition similar to the chondrite meteorites found on Earth, which are believed to be primordial agglomerations of dust grains from the nebula that became the Sun and planets.

"It looks like Eros is one of the most ancient rocks in the solar system, according to the initial data from NEAR," said Dr. Jacob Trombka, NEAR X-ray Gamma-Ray Spectrometer (XGRS) Science Team Leader at NASA's Goddard Space Flight Center, Greenbelt, Md. "Exploring Eros will help us understand how the rocky planets like Earth formed, so we can learn more about how we came to be."

The result will be published in the September 22 issue of the journal Science, as part of a special edition describing the latest results from the NEAR-Shoemaker mission.

X-rays from the Sun strike Eros' surface and cause elements to fluoresce (to be stimulated, then glow) in specific X-ray "colors," invisible to the human eye but detectable by XGRS. This is similar to the way a black light uses ultraviolet light to make pop-art posters glow in various garish colors. Each element on Eros glows in unique X-ray colors, allowing XGRS to build a map of the element distribution at and just beneath the surface of Eros.

Discovering the composition of Eros is the key to unlocking the mystery of its origin. When a forming planet reaches a certain size, its heat and gravity become sufficient to cause differentiation of its elements -- the planet becomes molten, and heavy elements, like nickel and iron, tend to sink to the center, while light elements, like silicon and carbon, rise to the surface. If Eros was composed mostly of light elements, this would indicate that it was a fragment from near the surface of a larger body. If it was rich in heavy elements, it would likely have been from the center of a larger object. However, instead Eros appears to be undifferentiated, like the chondrite meteorites found on Earth, so it probably never was exposed to much heat. This implies that it is a very primitive, unprocessed object, probably an aggregate of smaller objects from the early solar system nebula.

"The result is preliminary, because we have only mapped about 20 percent of Eros' surface so far," said Trombka. "Also, the X-ray spectrometer only reveals composition right at the surface, in a layer about the thickness of a human hair. Bombardment by cosmic rays or micrometeorites could have altered the surface composition."

"However, we made the same X-ray measurements of the Moon during the Apollo missions," added Trombka. "The Apollo astronauts took core samples, and the composition of the samples agreed with our surface map. This gives us confidence that our surface map of Eros reflects its true composition. Also, later in the mission, when NEAR makes its closest approach to Eros, our Gamma-ray spectrometer will reveal the composition down to about five inches beneath the surface. This will let us see if the composition right at the surface is the same as the interior."

The NEAR mission is managed for NASA's Office of Space by the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md.

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