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 -- and 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.

On the average, the smaller an orbiting chunk of asteroidal rock is, the more likely it is to have been broken loose from a larger one by a relatively recent collision, and so the less it will be weathered.

Thus the little chunks of rock that make up Earth's meteorites show very little space weathering -- and any they did acquire has been scoured off them during their fiery entries into Earth's atmosphere.

There's another consequence of this theory: the smaller asteroids are, the less weathered they are. Our instruments still aren't sensitive enough to get good near-IR spectra of little asteroids out in the Main Belt, which would explain why we haven't detected ordinary-chondrite asteroids.

But over the past decade we've finally begun getting good near-IR mineral spectra of the little asteroids that wander into the inner Solar System -- and, sure enough, for the first time we're finding large numbers of asteroids whose near-IR spectra really do match those of ordinary-chondrite meteorites. But while the space-weathering explanation of the Ordinary Chondrite Mystery is very popular among planetary geologists at this point, by no means does everyone agree with it.

Michael J. Gaffey said at the Lunar and Planetary Science Conference (LPSC) last March that even if space weathering by micrometeoroids does occur, it can't explain ALL the spectral differences between big S asteroids and ordinary-chondrite rock.

He thinks that S asteroids, from the very start, really have been made of a different kind of rock than ordinary-chondrite meteorites, richer in flecks of separate iron-nickel.

If he's right, then we're thrown back to our original puzzle: why do meteorites consist so disproportionately of ordinary chondrites? Gaffey thinks that the explanation is that most of the meteorites that reach Earth don't come from small asteroids in general, but from just a few asteroids (including the big asteroid 6 Hebe) that happen to orbit near a few narrow "chaotic resonance zones" in the Asteroid Belt.

Resonance zones are areas in the Asteroid Belt where any asteroid has an orbital period which is a fairly simple fraction of the orbital period of Mars, Saturn or (especially) Jupiter -- so that those planets exert repeated slight gravitational tugs on the asteroid at the same few points over and over in each of its orbits, gradually stretching its orbit into a more elliptical one.

These have been known since 1867, and they seem to explain several mostly-empty gaps in the Asteroid Belt where any asteroids are pulled into orbits crossing those of their neighbors and eventually knocked into new orbits a short distance outside the Zones by collisions.

Most of the original asteroids were cleared out of these zones immediately during the Solar System's formation, but collisions still knock a small stream of new strays from nearby areas into them.

But only in the past 20 years, thanks to more powerful computers, has it been mathematically discovered that there are a few such resonance zones that produce "chaotic" effects on asteroids' orbits, far more dramatically stretching their orbits in just a few million years' time so that they quickly start flying all the way into the inner Solar System.

Most of them, in fact, soon have their orbits stretched to such a degree that they either crash into the Sun or are catapulted completely out of the Solar System --but a few actually crash into the inner planets, and others make close flybys of them that modify their orbits so that they escape from the original resonance effects. This explains the long-mysterious origin of the near-Earth asteroids.

So, according to Gaffey, almost all meteorites come from the few asteroids near the borders of these chaotic resonance zones -- which, by sheer chance, happen to have compositions different from the vast majority of asteroids.

Jeffrey Bell of the University of Hawaii thinks this is seriously stretching coincidence, and has come up with a third theory. He agrees with Gaffney that the larger S asteroids really aren't made of OC rock -- but he also doesn't think that Earth's meteorites mostly just happen to come from a rare freak population of unusual-composition asteroids that just happen to have been located near the chaotic resonance zones.

Instead, he thinks they come from that vast majority of asteroid "parent bodies" during the Solar System's early days which were small.

It's universally believed now that the Asteroid Belt formed because these rocky parent bodies in that region -- which were kept from coalescing into a larger rocky planet by nearby Jupiter's constant gravitational stirrings -- then began colliding with and shattering each other.

And their fragments did the same thing, eventually producing the vast cloud of objects -- from balls of rock hundreds of kilometers across to microscopic dust particles -- that populates the Asteroid Belt today.

Before that happened, however, the larger asteroid parent bodies would tend to have their rocks partially melted by heat from traces of the short-lived and very intense radioisotope aluminum-26, which is known to have infused the rocky material of the forming Solar System (maybe due to radiation from the same nearby supernova whose shock wave may have triggered the condensation of the System out of an interstellar dust cloud in the first place).

And this melting would tend to free some of the iron and nickel from the OC material of the asteroids to form separate flecks, before the radioisotopes decayed and the rock cooled down again in the Solar System's early days.

But the bigger an object is, the smaller its surface area is compared to its volume, and so its interior stores up accumulating heat better -- and so only the bigger asteroid parent bodies heated up enough inside to undergo this partial internal melting.

(In fact, the biggest parent bodies -- several hundred km across and more -- got so hot inside that the melted iron settled to their cores, leaving them with solid metal cores which were occasionally later exposed by collisions to explain that small fraction of asteroids and meteorites that are made out of iron-nickel metal.)

The smallest asteroid parent bodies, less than about 50 km across, remained ordinary unmelted OC rock -- and, according to Bell, they're so plentiful that they actually make up most of the material in the inner Asteroid Belt, and so naturally provide most of the meteorites broken off asteroids (especially since rock fragments are also blasted off small asteroids more easily than larger asteroids, because of their lower gravity).

Why haven't we detected them in the Asteroid Belt? Simply because -- as I said -- Earth-based telescopes weren't powerful enough to get good near-IR spectra of small asteroids, and so we've been analyzing a very seriously biased sample of big, non-OC, partially melted asteroids.

The trouble is that the much bigger share of OC rock that we see among the IR spectra of those tiny near-Earth asteroids that we're finally starting to analyze could be explained by any of these three theories. So the hope was that NEAR's close up analysis of Eros could provide important new data to help settle this interminable wrangle.

Eros is an S-type asteroid whose spectra, as usual, don't match OC meteorites -- although it comes closer than some other S asteroids. NEAR's color camera and near-IR spectrometer could map the spectra of individual spots on its surface, to see if rocky areas more freshly exposed by craters matched OC spectra more closely than older patches of surface, which if true would clench the existence of space weathering and prove that S asteroids really are made of ordinary-chondrite rock.

And its X-ray and gamma-ray spectrometers could measure the actual percentages of major elements in Eros' rock -- something that Earth-based telescopes can't do, and which can't be affected by space weathering -- to see how closely they match the percentages in OC meteorites.

The actual results were initially trumpeted as firmly proving that Eros (and thus all S asteroids) really are made of ordinary-chondrite rock; and that they've been space-weathered, and thus disguised, in the upper few millimeters of their surfaces. But a closer examination of NEAR's results makes this less certain.

First, there are the results from its two element detectors. NEAR's X-ray spectrometer (XRS) measured X rays given off by the upper fraction of a millimeter of Eros' surface when it was struck by charged-particle solar radiation, and could thus measure the amounts of six major elements in it: magnesium, aluminum, silicon, sulfur, calcium and iron.

At first, the results from this instrument were advertised as showing clearly that Eros had the same makeup as ordinary-chondrite meteorites. But a closer examination raises some doubts.

While the amounts of magnesium, calcium and iron detected by the XRS on Eros did nicely match those in OC rock, it appeared at first that there was only about 50 to 75 percent of the aluminum that they usually contain.

This was still within the broad bounds of possibility for OCs -- and the XRS data has been recalibrated in the last few months, with its researchers reporting in May that the more accurate resulting data shows a normal OC-type level of aluminum after all.

More seriously, the original X-ray results seemed to show that Eros' surface had only about 10% as much sulfur in it as OC rock does -- and even the new recalibration indicates that it has only 50% as much as OCs do.

This could mean that Eros really isn't made out of OC rock -- that its larger parent asteroid had undergone some partial internal melting, making much of its original rock's iron sulfide melt and drain down into the asteroid's interior before Eros was broken off its outer crust by a collision.

But it could also be the result of another kind of space weathering, in which sulfur atoms are sputtered off its outer film of soil into space by the bombardment of micrometeorites or the solar wind -- in which case Eros may be OC rock after all.

At the March LPSC meeting, P.E. Clark presented a paper claiming that such solar-wind sputtering would easily remove that much sulfur from the top 3 millimeters of Eros' soil in just 5 million years.

In any case, the NEAR experimenters concluded that, on balance, the results from NEAR's X-ray spectra are fuzzy enough that they allow Eros to be either ordinary-chondrite, or some kinds of stony-iron meteorites -- although other types are firmly ruled out by the element measurements.

They have stuck by their initial conclusion that it is definitely an ordinary chondrite not because of their own data, but because they also think that the mineral spectra from NEAR's near-infrared spectrometer do show a ratio of the minerals olivine and pyroxene that is satisfied only by OC rock, but it's become increasingly apparent that NEAR's X-ray results were a good deal fuzzier than had been hoped, possibly because of inadequate Earth testing of the instrument.

Scientists had also awaited the results from NEAR's other element-measurement device: its gamma-ray spectrometer, which can sense both the gamma rays given off both by a radioisotope of potassium and by oxygen, magnesium, silicon and iron hit by cosmic rays.

But the results from this instrument were very disappointing -- again because of inadequate Earth testing, it had only a fraction of the sensitivity hoped for, and indeed all its measurements from orbit around Eros were virtually useless.

For this reason, its experimenters were delighted when NEAR unexpectedly survived its landing and could get 10 days of data with the gamma-ray instrument practically touching Eros' surface soil.

But the resultant data may very well still be too vague to answer the question, although it's still being analyzed and more results will be announced next month.

(They're especially interested in potassium, since that element is volatile and would also be diminished in Eros' soil if its sulfur has been sputtered away.)

In short, despite all their initial upbeat billing, NEAR's two element detectors were something of a washout. That left its two instruments that could provide mineralogical data on Eros from close-up: its multicolor camera and its near-IR spectrometer -- and they may have provided a clue much more significant than NEAR's element measurements.

The camera discovered that, while Eros' surface is quite uniform in hue outside its craters, there are many areas inside the craters where the relatively dark surface soil has slid down the slopes, exposing soil underneath which is considerably lighter in hue ("albedo").

Indeed, on the average these areas are half again lighter in albedo -- and some of them are as much as three times lighter. This is exactly what we would expect if space weathering had darkened the upper few millimeters of Eros' soil while leaving the rest untouched.

But the camera's eight color filters also revealed that the entire asteroid is extremely even in color. Most of it is the butterscotch hue to be expected for that subcategory of "S" asteroid known as "S(IV)", which bears a closer resemblance to ordinary-chondrite rock than any other kind of S asteroid.

But, to the researchers' surprise, the lighter patches of exposed soil showed almost no difference in actual color as compared to Eros' darker soil. Unfortunately, NEAR's near-IR spectrometer (NIS) failed only a month after NEAR entered its main survey orbit around Eros -- but while this kept the instrument from getting its hoped-for close up views, it did manage to obtain thousands of spectra of different spots on the asteroid's northern hemisphere, with a spatial resolution of about 1/2 kilometer.

And the patches of light soil on the slopes of Eros' biggest crater -- Psyche, which is over 5 km wide -- were big enough that even at that distance the NIS could get several hundred spectra that allow clear comparisons between the IR spectra of dark and light soil.

A team led by Beth Clark of Cornell has recently finished its analysis of these spectra -- and found that they confirm, in more detail, the indications from NEAR's color photos that there are almost no color differences between its dark and light soil.

This is completely different from the effect of space weathering on lunar soil: if it's darker, it's also comparably redder in spectral hue. The correlation between albedo and color in Eros' soil is only about one-tenth that found in the Moon's soil.

Clark's team also concluded that a difference in the size of Eros' soil grains can't be responsible for most of the difference in albedo -- for, while coarser soil is darker in hue, it's also considerably less reddish.

And they found an additional puzzle: the lighter patches of unweathered soil are comparable in albedo to ground-up rock from ordinary-chondrite meteorites -- but their IR spectra are distinctly different. They're much redder in overall hue.

Again, what in the world is going on here? The space weathering occurring on Eros must be completely different in nature than what had been predicted -- and it must be a very strange process indeed.

First, Eros must have undergone a period in which the continual rain of meteoroids of various sizes onto it ground up its rock into a layer of soil ("regolith") -- and, if Eros is made out of OC rock, space weathering must have simultaneously reddened those soil particles, but without darkening them.

Then something caused both the manufacture of soil on its surface and (if Eros is OC rock) most of its reddening to stop -- but an entirely new kind of space weathering then started up that caused the soil at the very top of the layer to dramatically darken with hardly any more reddening!

But there is a possible answer to this mystery -- and it's directly connected with the second remarkable puzzle NEAR discovered at Eros: the startling nature of its small-scale surface features.

In the next part of this report, I'll describe that new puzzle -- and then explain how a surprising sequence of physical processes may explain both mysteries at once, as well as the OC mystery in general.

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