[meteorite-list] NEAR Shoemaker Science Update - February 20, 2001

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:41:11 2004
Message-ID: <200102212255.OAA24716_at_zagami.jpl.nasa.gov>

http://near.jhuapl.edu/news/sci_updates/01feb20.html

          NEAR Shoemaker Science Update
          Landing on Eros
          February 20, 2001

          On Monday, 12 February 2001, the NEAR spacecraft touched
          down on asteroid Eros, after transmitting 69 close-up
          images of the surface during its final descent. Watching
          that event was the most exciting experience of my life.
          I was asked immediately afterwards how I felt, and I
          mumbled something about being tired and happy, but I
          missed the point. I realized afterward what I should
          have said: it was like watching Michael Jordan on the
          basketball court, when the game is on the line and he is
          in the groove. One miracle after another unfolds, and we
          are left stunned and speechless. When we learned that
          the spacecraft had not only landed on the surface, but
          was still operational, we hardly knew what to think.

          Over the past week, we have started to come to
          our senses again and to appreciate how
          fortunate we are. The final weeks of low altitude
          operations revealed bizarre and surprising aspects of
          surface structures on Eros, including one type of
          feature we noticed for the first time in the very last
          image taken by the spacecraft (the incomplete image
          taken from a height of 120 meters, 2001 Feb 12F ). As we
          discussed previously, there are markedly fewer small,
          fresh craters on Eros than we would expect from our
          experience at the Moon, and an amazing profusion of
          boulders, likewise more than we expected. We do not know
          just what is happening on the surface of Eros to cover
          and/or obliterate craters while making and/or uncovering
          boulders. We have seen many examples of mass motion on
          Eros - loose material sliding downhill - and that is no
          doubt part of the story, but maybe not all of it. We
          also believe that at least some of the bouldery debris
          found on Eros is comprised of ejecta from impacts on
          Eros; some of these ejecta do not escape but fall back
          to the surface.

          Some of the strange features we are beginning to think
          about can be seen in the low altitude images obtained
          during the past few weeks. The new type of feature seen
          in the last image returned ( 2001 Feb 12F ) can be
          found, for example, at the bottom of the image (just
          above the vertical streaks indicating loss of signal),
          to the left of center. It appears to be a collapse
          feature, formed when support is removed from below the
          surface, and it is about the size of one's hand. Other
          strange sights are clusters of boulders (e.g., the upper
          right of 2001 Feb 12E ) - are these cases of
          disintegration in place? - and extremely flat, sharply
          delineated areas in the bottoms of some craters (e.g.,
          the two left panels of 2001 Jan 31 ). The mere existence
          of sharp boundaries, called "contacts", is surprising in
          itself, especially if the entire surface of the asteroid
          is thought to have been blanketed by debris from
          impacts. These boundaries can be incredibly sharp on
          Eros, as evidenced by the last frame, 2001 Feb 12F
          (compare the upper right and lower left of the image).

          The images tell us a tale whose outcome we don't yet
          know, but there is more: the story of Eros's composition
          is likewise still emerging. Our orbital data from the
          x-ray spectrometer showed that the abundances of key
          elements on Eros are very similar to those in the
          undifferentiated meteorites called ordinary chondrites,
          but there was a discrepancy. The abundance of the
          volatile element sulfur 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.

          Fortunately, the spacecraft is now in a position to help
          answer the question (on the surface, that is). The gamma
          ray spectrometer measures the composition to a depth of
          about ten centimeters, and it is much more sensitive on
          the surface than it was in orbit. We are now in the
          process of trying to obtain our best yet gamma ray
          spectrum of Eros. We will try to determine the
          abundances of the volatile element potassium and the
          major element iron from this spectrum, to look harder at
          the match between the compositions of Eros and the
          ordinary chondrites, and to look for evidence for bulk
          depletion of volatiles. The latter would suggest that
          Eros has undergone significant heating (a geologist
          would call it "metamorphism").

          It is sad for me to say, but the gamma ray measurement
          will be the last from NEAR - one more miracle is what we
          ask of this little spacecraft. Its job is almost done,
          but ours is just beginning.

     Andrew Cheng
     NEAR Project Scientist
Received on Wed 21 Feb 2001 05:55:16 PM PST