NEAR Science Update 19 June 2000

NEAR Shoemaker has by now imaged the entire surface of Eros at least once,
but our task is not yet complete. The last portion of Eros to be imaged was
the south pole, because this region has been in shadow until this month.
Previously, during the southern winter, the sun never rose above the horizon
at the south pole, but now that the sun has passed directly above the Eros
equator heading south, it rises briefly each Eros day. However, the sun
remains low in the sky even at local noon, and the terrain is heavily
shadowed. As we need to obtain images under a variety of lighting
conditions, we shall be imaging this area again in the coming months. We
also need to fill in our color coverage of the far southern latitudes.
Still, we can now be assured that the south polar region of Eros is not
grossly dissimilar from other regions on Eros. We are convinced that the
surface of Eros is covered with regolith, which is fragmented, particulate
material; that most of the surface displays systems of grooves and ridges;
that boulders are ubiquitous in the regolith; and that a variety of crater
shapes and crater densities are seen. We are still attempting to synthesize
the information we have gathered – to assemble an integrated picture of Eros
the asteroid from the myriad pieces of the puzzle we have collected so far.
We have not yet reached the halfway mark of the Eros orbital mission, and
many pieces of the puzzle may still be missing.

Still, after having seen all of the surface at least once, we are obliged to
begin asking hard questions. One of these is whether the regolith material
is resting loosely upon the surface, as opposed to being fixed in place with
appreciable strength. That is, if we were to visit the surface, would we be
able to scoop up material easily and pick up rocks, or would we find a hard
surface that we would need to chisel or drill into? On the Moon, the
regolith does rest loosely upon the surface, and it has the consistency of a
very fine, dry dust. The lunar regolith can be scooped up easily. On Mars
also, the regolith rests loosely upon the surface and has the consistency of
fine dust; when the wind picks up on Mars, the dust is often lifted off the
surface and can be blown all around the planet. However, just below the
surface of the Martian regolith, there is a hard, cemented layer that the
Viking landers were not able to dig through.

On Eros, there is no atmosphere and no wind, but it is too early to say that
the regolith must be like that of the Moon. NEAR Shoemaker has no direct
means of measuring the strength of the surface, but we can make inferences.
One approach is to study images at the highest possible resolution. We are
often asked if the grooves on Eros could have been made by boulders as they
come to rest after having been set into motion somehow – that is, could the
grooves be troughs plowed up by boulders sliding or rolling along the
surface? Examination of the images shows that this is not a viable
mechanism. We have found no examples of grooves with boulders at one end,
where the size of the boulder is plausibly consistent with making the
groove. However, we are still puzzled by the boulders. We are asking, for
example, whether they were emplaced as a result of any of the impacts that
created the craters we now see on Eros. Alternatively, were the boulders
pre-existing on Eros, and are they now being exposed by removal of regolith
from above and around them?

However, images are not the only tools that NEAR Shoemaker can use to make
inferences about the regolith. Another approach is to use shape and gravity
measurements of Eros to determine the strength and the direction of the
acceleration of gravity at the surface. In other words, we ask whether loose
material on the surface would fly away or slide off. Does material need to
be cemented in place to maintain the surface slopes? To make this
evaluation, we have to remember that Eros is rotating on its axis, so that
any material resting on the surface would feel a centrifugal force that is
directed away from the rotation axis and that tends to oppose gravity,
especially at the equator. This centrifugal force is proportional to the
distance from the rotation axis, so it is greatest at the ends of an
elongated object like Eros. The acceleration of gravity also tends to be
weakest at the ends, which are the farthest regions from the center. These
forces generally do not oppose each other exactly, because the acceleration
of gravity does not point exactly toward the center of an irregular body,
whereas the centrifugal force does always point directly away from the
rotation axis. Although this is a topic for another time, the rotation axis
for an isolated body like Eros must always pass through the center of mass.

The weight of an object on the surface is just the acceleration of gravity
multiplied by the mass. On Earth, one’s weight is slightly reduced by the
action of the centrifugal force created by Earth’s rotation. We do not
normally notice this centrifugal force because Earth’s rotation rate is
small – only one rotation every 24 hours. Eros rotates somewhat more
quickly, once every 5.27 hours. This difference is enough to make
centrifugal force much more important relative to gravity for Eros than for
Earth. If we form the ratio of the gravitational acceleration at the equator
to the centrifugal force, we find that this ratio is proportional to the
density and to the square of the rotation period. The average radius of the
body is irrelevant! As we have discussed before, the average density of Eros
is about half that of Earth, so this factor reduces the relative importance
of gravity by about a factor of two at Eros. The shorter length of the Eros
day, or the more rapid rotation, further enhances centrifugal force relative
to gravity by a factor of (24/5.27)x(24/5.27) = 21. Combining these factors,
we find that the centrifugal force is some 40 times larger, relative to the
gravity, at Eros than on Earth. This is not an exact result, because we have
not included the irregular shape of Eros properly, but it gives the correct
idea.

On Eros, one’s weight at the poles would be about 2000 times less than it
would be at the surface of Earth. One could ‘lose’ about half one’s weight
at Eros simply by moving to either end of the asteroid, where the
acceleration of gravity is reduced and the opposing effect of centrifugal
force is increased. It is of course also true at the Earth, that Santa Claus
could ‘lose’ weight just by moving from the North Pole to the equator, but
on Earth his weight loss would be less than a percent (and he would ‘gain’
the weight back as soon as he returned to the Pole, assuming that his mass
did not change – when I travel my mass actually increases). There is a
complication we have not discussed, which is that centrifugal force distorts
the shape of the Earth itself by creating an equatorial bulge; this
distortion also affects Santa’s weight change when he moves to the equator.

In summary, we must consider the detailed shape of Eros, its gravity and its
rotation to determine whether loose material could sit stably on the
surface. If slopes are anywhere too steep, loose material would be able to
slide away, but where are the steep slopes in relation to surface features
such as bright and dark patches or boulders? And just how steep does a slope
have to be before regolith material should start to slide? These are among
the questions we are now thinking about.