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. Is Eros An Ancient Planetesimal Leftover From Sol's Birth

Eros in unmasked as an early desktop. Wallpaper version available
by Dr Tony Phillips
NASA Space Science
Huntsville - March 5, 2001
When NASA's Near Earth Asteroid Rendezvous (NEAR) spacecraft left for asteroid 433 Eros five years ago, scientists weren't certain what they would find when the probe arrived. Was Eros a 30-km fragment from a planet that broke apart billions of years ago? Or perhaps a jumble of space boulders barely held together by gravity? Was Eros young or old, tough or fragile ... no one knew for sure.

But now, after a year in orbit and a daring landing on the asteroid itself, NEAR Shoemaker has beamed back data that could confirm what many scientists have lately come to believe: Asteroid Eros is not a piece of some long-dead planet or a loose collection of space debris. Instead, it's a relic from the dawn of our solar system, one of the original building blocks of planets that astronomers call "planetesimals."

As NEAR Shoemaker was heading for its historic landing on Feb. 12, 2001, team members hoped the spacecraft --which was designed to orbit, not land-- would simply survive. When it did survive, they set their sights a little higher. From its perch on the surface of the asteroid, NEAR's gamma-ray spectrometer can detect key chemical signatures of a planetesimal -- data that scientists are anxious to retrieve.

"The gamma-ray instrument is more sensitive on the ground than it was in orbit," says Goddard's Jack Trombka, team leader for the GRS. "And the longer we can accumulate data the better." NASA recently gave the go-ahead for NEAR's mission to continue through Feb. 28th, tacking four days onto an extension granted just after the spacecraft landed.

Click for chart of Gamma-Ray Readings from Eros

To do its work the GRS relies partly on cosmic rays, high-energy particles accelerated by distant supernova explosions. When cosmic rays hit Eros, they make the asteroid glow, although it's not a glow you can see with your eyes; the asteroid shines with gamma-rays.

"Cosmic rays shatter atomic nuclei in the asteroid's soil," explains Trombka. Neutrons that fly away from the cosmic ray impact sites hit other atoms in turn. "These secondary neutrons can excite atomic nuclei (by inelastic scattering) without breaking them apart." Such excited atoms emit gamma-rays that the GRS can decipher to reveal which elements are present.

"We can detect cosmic-ray excited oxygen, iron and silicon, along with the naturally radioactive elements potassium, thorium and uranium," says Trombka. Measuring the abundances of these substances is an important test of the planetesimal hypothesis.

Planetesimals came to be when the solar system was just a swirling interstellar cloud, slowly collapsing to form the Sun and planets. Dust grains condensed within that primeval gas.

The grains were small, but by hitting and sticking together they formed pebble-sized objects that fell into the plane of the rotating nebula. The pebbles accumulated into boulders, which in turn became larger bodies, 1 to 100 km wide. These were planetesimals -- the fundamental building blocks of the planets.

For reasons unknown Eros was never captured by a growing protoplanet. It remained a planetesimal even as other worlds in the solar system grew and matured.

Dust grains are accumulating into asteroid-sized planetesimals, the building blocks of planets.

Fully-developed planets like Earth are chemically segregated -- that is, they have heavier elements near their cores and lighter ones at the surface. Planetary scientists call this "differentiation." If Eros were a chip from a planet that broke apart, perhaps in the asteroid belt, it would exhibit chemical signatures corresponding to some layer from a differentiated world.

For example, Eros might be iron-rich if it came from the core of such a planet or silicon-rich if it came from the crust.

Instead, "orbital data from the x-ray spectrometer (a low-energy cousin of the GRS) showed Eros is very much like a type of undifferentiated meteorite we find on Earth called ordinary chondrites," says Andrew Cheng, the NEAR project scientist at Johns Hopkins University Applied Physics Laboratory (APL), which manages the mission for NASA.

Eros seems to harbor a mixture of elements that you would only find in a solar system body unaltered by melting (an unavoidable step in the process of forming rocky planets). But, says Cheng, there is a possible discrepancy.

"The abundance of the element sulfur on Eros is less than we would expect from an ordinary chondrite. However, the x-ray spectra tell us only about the uppermost hundred microns of the surface, and we do not know if the sulfur depletion occurs only in a thin surface layer or throughout the bulk of the asteroid."

The GRS can go deeper, as much as 10 cm below the surface. Although the instrument can't detect sulfur, it is sensitive to gamma-ray emissions from other elements such as radioactive potassium that are indicators of melting. Like sulfur, potassium is a volatile element -- it easily evaporates when a rock is heated. Finding plenty of potassium would strengthen the conclusion that Eros is an unmelted and primitive body.

On the other hand, a widespread dearth of "volatiles" would hint that Eros isn't so primitive after all.

It might sound like an ivory-tower question, but knowing the makeup of this asteroid -- both its internal structure and its chemical composition-- has a practical application.

The solar system is littered with space rocks more or less like Eros, and many come uncomfortably close to Earth. One day we may need to blow one apart (or deflect one without blowing it apart) to avoid an unpleasant collision.

Near-Earth asteroids are also potential mining resources as humans expand into space. In either case, knowing more about them is a good idea!

"Our first four data sets are here and they look great," says Jack Trombka. "John Goldsten, the lead engineer for the gamma-ray spectrometer at the Johns Hopkins Applied Physics Laboratory, has done a fabulous job making the instrument work on the surface, which is a different environment than orbit.

"We're just hoping to get as much data as we can before the mission ends."

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A new meteorite study is rekindling a scientific debate over the creation of our solar system. The study, published in the March 2 issue of the journal Science, is based on the microscopic analysis of two rare meteorites recently discovered in Antarctica and Africa.
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