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Fwd = Near Shoemaker Science Update - March 31, 2000

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  • Frits Westra
    Forwarded by: fwestra@hetnet.nl Originally from: baalke@jpl.nasa.gov Original Subject: Near Shoemaker Science Update - March 31, 2000 Original Date:
    Message 1 of 1 , Apr 3, 2000
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      Forwarded by: fwestra@...
      Originally from: baalke@...
      Original Subject: Near Shoemaker Science Update - March 31, 2000
      Original Date: Mon, 3 Apr 2000 13:01:27 -0700 (PDT)

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      Near Shoemaker Science Update
      March 31, 2000

      As NEAR Shoemaker continues orbital mapping
      from its 200 km orbit, more and more details
      are emerging about the geologic features on
      Eros's surface. Three examples of what we are
      seeing were released as images of the day for
      March 17, March 20 and March 21. In all cases
      we are looking at the north polar region of
      Eros, but under different viewing and
      illumination conditions. By comparing sets of
      images as these, looking for features that are
      viewed in multiple images, we can infer
      whether portions of a feature look dark
      because of true brightness or reflectivity
      variations, as opposed to looking dark because
      of oblique illumination and/or viewing. In
      this way we sort out the detailed shapes of
      the features and any reflectivity variations
      that may be present, so we can classify the
      features and map their distributions across
      the asteroid. Mapping the features is a
      challenging task, especially on an irregularly
      shaped object like Eros, but it is critical to
      piecing together the story of how Eros came to
      be what it is. It is by mapping that we can
      study the spatial relationships - the degrees
      to which linear features line up and the ways
      in which features meet and/or cross one
      another - which provide clues to the nature of
      the underlying geologic processes. We shall
      return to the topic of mapping later.

      In the image of the day for March 17 we can
      see three boulders at the top of the image -
      these are 80 m boulders sitting on the eastern
      rim of the large depression we have been
      calling the 'saddle'. We know these are
      boulders, protruding above the surface,
      because of where the shadows fall (toward the
      bottom of the image, whereas for craters the
      shadowed side is toward the top). At the lower
      left of this image, where the Empire State
      Building is outlined for scale, is the 5.5 km
      crater we have been calling the 'paw' (the
      reason for that name is not evident in these
      images). The 'paw' is the same crater with the
      prominent rim that appears in the top of the
      March 20 image (toward the left) and that I
      wrote about on February 8 and February 15. Now
      that we are oriented, we can examine the large
      crater on the far left of the March 17 image
      which has light and dark areas within its
      walls. This same large crater appears in all
      three images. It is in the center-left, below
      the paw, in the March 20 image, and it is on
      the horizon, at the lower left, in the March
      21 image. We note especially that the pattern
      of light and dark areas is reproduced
      consistently in the three images. This is
      convincing evidence that there are true
      brightness variations in the materials lining
      the walls of this crater. We have seen
      consistently that the "bright spots" mentioned
      previously (see, for example, the February 15
      update) are areas of higher reflectivity in
      the walls of craters.

      Another important feature that appears in all
      three images is a long ridge that extends much
      of the way around the asteroid at its waist.
      Again, we know this feature is a ridge,
      standing above the surface, because of the
      side that the shadow falls on. In the March 17
      image, the portion of the ridge that we can
      see begins at the left edge of the image
      immediately above the crater we have just
      discussed, and it points toward the boulders
      in the saddle. We can then find the ridge in
      the March 21 image, left side, and in the
      March 20 image, cutting diagonally across the
      lower left corner. This same corner is where
      an unusually smooth area, that may be
      relatively young, can be seen. Both this
      smooth area and the ridge have important
      stories to tell about the geology of Eros, but
      our studies of these features have barely

      Our focus on lighting conditions and their importance for
      understanding the images may seem surprising, but the lighting
      geometry is much more important on airless bodies like Eros than it is
      on Earth. Our atmosphere scatters light and creates diffuse lighting
      (where light comes from many directions at once). Diffuse lighting is
      especially prevalent indoors, as most of us make an effort to achieve
      that condition by putting shades and diffusers on lamps. With diffuse
      lighting, the effects of shadows and oblique illumination are reduced,
      but there is very little diffuse light on the surface of Eros. Most of
      us who live on Earth have no experience of such lighting - the only
      people who have actually seen it in nature are the lucky few who
      walked on the Moon.

      Andrew Cheng
      NEAR Project Scientist

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