NEAR Science Update

The images we have seen to date provide a tantalizing
glimpse of what lies in store for us as NEAR begins its
year-long study of Eros. From the images-of-the-day for
February 14-17, we have many hints of an underlying
global fabric – the nearly parallel markings, the chains
of small craters or pits, the grooves – but our
exploration has barely begun. Our image resolution will
improve by more than ten times over the best we have
achieved to date, and we have yet to bring several of
our instruments to bear on the asteroid. One
interpretation of the global fabric is that it is the
surface manifestation of stratigraphy – that is, layered
structure – which was produced while Eros was part of a
much larger parent body that was disrupted long ago by a
monstrous collision. In other words, the global fabric
may be a signature of ancient geologic activity (for
example, volcanism) on the parent body of Eros. The size
of this putative parent body, and the time at which it
would have disrupted, are not known, and all we can say
is that it must have been much larger than the present
maximum diameter of Eros (33 km). It may or may not have
been larger than the present-day 500-km asteroid Vesta.
Another interpretation is that the global fabric results
from large-scale fractures within Eros caused by a giant
impact, which may have occurred on the parent body of
Eros (so we would not necessarily see the huge crater on
present day Eros). We will learn much more from higher
resolution images, spatially resolved spectral maps,
composition data, and altimetry later in the mission.

Another important early result from the Eros images is
reported in the image-of-the-day for February 21, which
shows that the surface of Eros is old like the surface
of Ida. This graphic summarizes initial results of
crater counts, which are made using data like that in
the image-of-the-day February 16A. The February 21 plot
gives the number of craters of a given size range found
per unit area of Eros. Geometric saturation means that
craters of a given size basically cover the surface. Of
course, much more will be done with crater counting as
higher resolution images are obtained. However, it is
already clear that Eros does not have a high density of
giant craters as found on Mathilde, where the giant
craters approach geometric saturation (by giant craters
we mean those that have a diameter close to or bigger
than the radius of Mathilde itself). It is also clear
that Eros does not have a paucity of craters, especially
large ones, as does Gaspra. The NEAR data show that Eros
has an older surface than Gaspra – Gaspra is less
heavily cratered because it has not been exposed to
bombardment as long. However, since we are still
uncertain about collision rates in the asteroid belt at
present and in the distant past, we cannot actually
state an age for either asteroid.

Andy Cheng

NEAR Project Scientist