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The Science Of Eros Grows

Artist's impression of the NEAR spacecraft during its final minutes before "landing" on Eros
by Bruce Moomaw
Cameron Park - Sept. 3, 2001
The "NEAR" spacecraft has just completed a highly successful mission providing us with our first prolonged and close up view of an asteroid.

After orbiting the tiny near-Earth asteroid Eros for a year at distances as close as 19 km, and making several passes as low as 3 km to its surface, it was finally ordered to end its mission with an optional bang on Feb. 12 by making a slow-speed descent all the way to Eros' surface.

This final "landing" surpassed expectations by sending back clear pictures down to an altitude of only 120 meters, and then surviving the landing itself to transmit back 10 days of magnetic and gamma-ray compositional data from Eros' surface before finally being commanded off.

But what has it told us? Its data falls into two general categories: data on Eros' chemical composition, and photos and other data on its physical structure.

It's starting to look, however, as though the chemical data may be somewhat too ambiguous to fully answer the most important question NEAR was designed to study about Eros -- and, in fact, may actually point away from the initial conclusion that was confidently announced by the experimenters. And its physical-structure data has also revealed a fascinating new puzzle that we're not sure we understand.

The chemical puzzle may be called the Curious Affair of the Missing Chondrites, perhaps the single biggest mystery about the Asteroid Belt.

It's basically simple: meteorites are universally thought to be pieces of asteroids -- detached over the eons by high-speed collisions with the asteroids by other rock fragments, and then drifting into the inner Solar System -- and 80% of them are a type of rock called "ordinary chondrite", which seems to have undergone little heating since it originally condensed out of the material of the nebula from which the Solar System formed.

When scientists first began making near-IR spectra of the mineral makeup of asteroids in the Seventies, most of the asteroids in the inner part of the Belt could be classified as a general category named "S-type", which did indeed seem to be made of the same silicate rocks as the ordinary-chondrite meteorites.

But more detailed spectra soon showed significant differences -- virtually all the S asteroids had somewhat darker and more reddish-tinted rocks, and showed spectral indications that their rocks contained more individual flecks of iron and nickel.

Throughout the following discussion, keep in mind that when we say "redder", we're talking about differences in color almost too faint for the human eye to make out, but very easily detectable by spectrometers.

In fact -- while the spectra of the S asteroids more closely match those of the much less common "stony-iron" meteorites that do indeed contain flecks of metal -- astronomers were unable to locate any S asteroid that properly matched the spectra of the ordinary-chondrite (or "OC") meteorites. So where in the world are 80% of Earth's meteorites coming from?

There have been three rival theories. The first and most popular is that the S asteroids really are made out of ordinary chondrite, but that a process called "space weathering" has slowly altered the color of their surfaces to make it redder, darker and more metallic-looking. Such weathering really has been observed on the rocks -- and, more dramatically, the soil -- of the lunar surface.

It was originally thought to be due to due to the production of tiny specks of melted glass on them by high-speed impacts from the rain of tiny micrometeorites that has been pelting down on them for billions of years. More recent studies have indicated that such glass flecks wouldn't redden either lunar or asteroidal rocks.

But in the past few years, a team of scientists from the University of Tokyo has run ground-based experiments revealing a second possible space-weathering effect from micrometeoroids.

Some material on the surfaces of rocks is vaporized at such impacts -- and when the vaporized rock recondenses, it contains microscopic ("nanophase") flecks of metallic iron that darken and redden the surfaces of the rocks in just the right way for space weathering.

They also suggest that the vaporized rock would recondense much more efficiently onto fine soil grains on the asteroid's surface than onto larger chunks of rock, further indicating that large asteroids with an accumulation of ground-up rocky material ("regolith") on their surface would redden more quickly than small bare orbiting rocks.

It's true that micrometeorites slam into Main Belt asteroids at only about one-third the speed that they hit the Moon's surface, which could seriously weaken their ability to redden the asteroids' surfaces.

But other lab tests have shown that when OC rock is bombarded with simulated solar-wind radiation, it has a similar reddening effect by slowly "sputtering" traces of the rock's molecules off its surface, with the iron in it recondensing as similar nanophase specks.

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