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Unraveling the History of Eros

even on Eros rocks still roll "downhill"
by Bruce Moomaw
Cameron Park - Sept. 3, 2001
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.

  • Part  One - Two - Three

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    Impacts Shaped Eros Topography
    Laurel - April 24, 2001
    NEAR mission science team members have concluded that the majority of the small features that make up the surface of asteroid Eros more likely came from an unrelenting bombardment from space debris than internal processes.



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