NEAR Shoemaker Science Update 1 June 2000

NEAR Shoemaker continues its detailed exploration of Eros from 50 km orbit,
a process which resembles in some ways an archaeological expedition. We have
to map our site globally, pick through it carefully to find any interesting
specimens, study some of these in detail back in the laboratory, and then
attempt a synthesis and a placing of results in context. To date, NEAR
Shoemaker has completed about one quarter of its planned orbital operations,
including our Northern Hemisphere mapping campaigns with the imager and the
near infrared spectrometer, and our primary global mapping campaign with the
laser altimeter. However, our Southern Hemisphere mapping campaign with the
imager is scheduled to start in September of this year, once this region
emerges into sunlight, and the global mapping campaigns with the x-ray and
gamma ray spectrometers have just begun. So, our exploration of Eros is in
progress, but it is far from complete. We have many fascinating specimens in
hand, but are still picking our way through the site.

Some of our findings have been featured in recent images of the day. Three
images, those for the days May
16,
22
and
23,
have shown areas with numerous
boulders that protrude above the surrounding surface (in NEAR images,
features protruding above the surface are lit on the right side and shadowed
on the left, whereas the opposite is true for features that are depressions,
such as craters). The boulders are mysterious in many ways. Some appear
angular
(May 22),
and others appear rounded
(May 16
and
May 23),
suggesting
various origins or histories – are they collisional fragments? products of
comminution (grinding)? are any of them actually clods of dirt and not
rocks at all? where do they come from? The boulders are distributed in a
nonuniform way that does not appear obviously correlated with any of the
large craters or with gravitational lows on the asteroid.

Several images of the day display linear features on Eros, which are the
grooves and ridges that we have previously described as comprising a global
fabric. This fabric has turned out to be quite complex, suggesting a
complicated and interesting geologic history. Several different styles of
grooves – elongated, linear depressions – have been identified on Eros. Some
look like chains of pits or craters – we don’t know which – as seen on
May 19.
Some appear as elongated craters or furrows, although we have to be
careful because even a perfectly round crater may appear as an oval when
viewed obliquely. The grooves often appear in the form of nearly parallel
systems, cutting across one another and even cutting into and out of
craters, as on
May 26.
In a few places, such as the ‘saddle’ region
(May 9, and
25)
and the ‘paw’ crater
(May 23),
the grooves are wider and deeper
based on laser altimetry; the alternating light and dark bands
in these images can result from topography and/or albedo variations.
Also the density of craters
(meaning the number of craters of a given size within a given area) is much
lower, suggesting a younger surface and/or a process that has smoothed out
craters, such as motion of regolith. Boulders are ubiquitous. Ridges are
likewise ubiquitous and are often associated with nearly parallel systems of
grooves as on
May 26.
One particularly interesting ridge is the one
shown on
May 24,
which is several kilometers long and is crosscut by another
system of grooves. On Earth, deformation can produce ridges and associated
fractures or faults. Many of us are asking if the grooves and ridges on Eros
indicate the presence of underlying fractures, and if so, which of these may
have resulted from processes (such as impacts) on Eros in roughly its
present form, and which were pre-existing and therefore may have formed
while Eros was part of a much larger parent body.

To answer these questions, as well as others about the boulders and about
features that we haven’t discussed today (such as, the bright areas in the
crater walls), we have to complete our survey of the site and finish picking
through it. We have to construct detailed maps of the site, and specifically
in our case, map the distributions of boulders, craters, and linear
features. We also have to map the orientations of the grooves and ridges and
study the crosscutting relations (which are a clue to relative ages). We can
then begin to sort out the history of Eros. Until then, we will not even
know how sensible our questions are.

Andrew Cheng
NEAR Project Scientist