[meteorite-list] NEAR Shoemaker Science Update - December 28, 2000

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:37:38 2004
Message-ID: <200012281753.JAA24030_at_zagami.jpl.nasa.gov>


          NEAR Shoemaker Science Update
          December 28, 2000

          More Mysteries

          We are planning to devote the last two months of the
          mission to low altitude observations. What we have seen
          so far in the low orbits has merely whetted our appetite
          for more. We went up close to have a better look at the
          surface than ever before, but we now see things we do
          not understand, and we need more information. That has
          been the story of the NEAR mission, and that is why we
          are going back to low orbit despite the rough ride that
          the irregular gravity field of Eros will give us.

          The craters on Eros provide several examples of
          mysteries that we are working on. Craters are the
          records of impacts that have largely shaped the surface
          of Eros, of other asteroids we have seen, and of objects
          from Mercury to the moons of Neptune. From the beginning
          of the mission, we saw two large concavities on Eros,
          for which we have proposed the names Himeros and Psyche.
          In the early images Himeros appeared saddle-shaped, and
          we could not be sure if it was indeed an impact crater,
          but Psyche displayed from the start the classical bowl
          shape of an impact crater. Although it was not
          immediately apparent, Himeros was actually not
          saddle-shaped at all, but bowl-shaped. Careful mapping
          of its topography by the NEAR Laser Rangefinder and by
          the imager shows that as far as Eros' gravity field is
          concerned, its depth is consistent with impact
          excavation. Still, if it is an impact crater, it is
          oddly shaped.

          Another mystery is that the interior surface of Himeros
          is relatively smooth and much less heavily cratered than
          typical areas on Eros, and so it must be relatively
          young. The same is true for the interior of Psyche.
          However, the largest impact features on a body are most
          likely the oldest. Moreover, there is a third global
          scale depression on Eros that is actually larger than
          Psyche in diameter. We have proposed to name this third
          depression Shoemaker Regio, and it too may be an
          ancient, degraded impact crater or as many as three
          degraded craters side-by-side. The interior of Shoemaker
          Regio is young like the interiors of Himeros and Psyche,
          because it is lightly cratered, but it is also the most
          boulder-rich area on Eros and very different from the
          relatively smooth interiors of Himeros and Psyche. What
          has happened?

          We do not know. Eros is a body without atmosphere or
          ocean, without large-scale volcanism (Eros has never
          melted completely, but some partial melting may have
          occurred in the past), and without plate tectonics, but
          it has ongoing geologic activity. What could be
          sculpting the surface except impacts? Much the same can
          be said for the Moon, although the Moon did have
          extensive magmatic activity (releases of lava on a
          global scale) billions of years ago. On the Moon, the
          primary process shaping the surface is cratering. In the
          lunar highlands, for example, we see that the continuing
          rain of projectiles has produced a state that approaches
          what we call "equilibrium saturation", where each new
          impact on the average erases as many pre-existing
          craters as it makes new ones (each projectile makes a
          primary crater but can make additional craters if it
          produces ejecta that fall back to the surface at high
          speed). In the equilibrium state, we find that the
          density of craters on the surface obeys a characteristic
          relation. Namely, if we count craters of a given size
          range, say from 10 km to 14 km diameter in a certain
          region on the Moon, and we ask what is the total area
          covered by craters of this size in this region, we find
          empirically that about one fifth of the area is covered.
          The same is roughly true for craters in other size
          ranges (say from 20 to 28 km), as long as the minimum
          and maximum diameters of the size range stay in the same
          ratio, and provided that the craters are not too large.
          This distribution implies that the total number of
          craters smaller than some diameter scales roughly as the
          inverse square of this diameter, up to some maximum
          size. That is, the total number of craters smaller than
          2 km is four times as many as the total number smaller
          than 4 km, and the number smaller than 1 km is four
          times as many again. Similar distributions are found on
          heavily cratered bodies throughout the solar system,
          although there are deviations from the simple power law
          that reflect the geologic histories of the individual

          The crater size distribution records how many
          projectiles of various sizes hit the Moon, which
          interests us because the distribution of projectiles
          that bombarded the Moon must also hit the Earth.
          Although there are complications - it is not completely
          straightforward to relate the distribution of craters to
          that of projectiles - this is why horrific impacts like
          the one at Chicxulub, which ended the age of the
          dinosaurs on Earth, are much less frequent than minor
          impacts like the one that made Barringer Meteor Crater
          (and we are thankful). This is also why the largest
          impact craters on a body, like Psyche or Himeros (if it
          is one), are likely to be the oldest. Larger impacts
          occur less frequently, so it is unlikely for a large
          impact to have occurred very recently. Moreover, large
          impacts create large volumes of ejecta and produce large
          seismic disturbances, both of which tend to erase small
          craters around them (by covering or obliterating them).
          A very large impact, like Psyche on Eros, may be able to
          erase small craters globally. Perhaps if Psyche formed
          after Himeros, it could have 'reset' the surface on Eros
          by erasing small craters, but then how was the interior
          of Psyche also reset?

          In any case, when we saw heavily cratered surfaces on
          Eros, we were not surprised, and we expected an
          equilibrium saturation distribution to apply. However,
          the distribution of craters that we actually see at Eros
          is very different. An equilibrium saturation
          distribution would mean that if we are able to see
          smaller craters, we should find more of them,
          approximately as the inverse square of the size. This is
          not true at Eros. We went to low altitudes and looked
          for smaller craters, but found that craters below about
          a hundred meters in diameter are markedly depleted.
          Furthermore, the smaller the size of crater we look for,
          the fewer we find relative to what we would expect from
          an equilibrium distribution.

          So, again we ask, what is happening? Perhaps it will not
          be us, but some future scientists, who will unravel some
          of the mysteries we are studying. In any case, we are
          working hard to understand the surface of Eros.

     Andrew Cheng
     NEAR Project Scientist
Received on Thu 28 Dec 2000 12:53:57 PM PST

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