NEAR mission science team members have concluded that the majority of the
small features that make up the surface of asteroid Eros more likely came
from an unrelenting bombardment from space debris than internal processes.
Details of the research from NASA’s Near Earth Asteroid Rendezvous (NEAR)
mission were published this week in Science and are based on the NEAR
Shoemaker spacecraft’s Oct. 25-26, 2000, low-altitude flyover of asteroid
Eros that brought the spacecraft to within about 3 miles of the surface of
the asteroid.

“We think that impacts to the asteroid’s surface have probably been the
single-most dominant process in shaping the surface texture of the
asteroid,” says NEAR Project Scientist Dr. Andrew Cheng of the Johns Hopkins
University Applied Physics Laboratory in Laurel, Md., which managed the
mission for NASA. “We saw surface details such as regolith [surface dust and
debris], craters and fields of small boulders in incredible detail. We also
saw things that confound us, but we now have a more in-depth picture of Eros
that will help us to decipher the asteroid’s history.”

During the flyover, simultaneous observations were taken by the spacecraft’s
multispectral imager and laser rangefinder over two tracks approximately 1
mile and 2.5 miles long that showed objects the size of a doghouse at three
to four times better resolution than previously obtained. The data revealed
an inordinate number of small boulders, a saturation of large craters and a
dearth of small ones, crater “ponds,” and unknown erosion processes.

A vast number of large craters, 1,630 to 3,280 feet (500 to 1,000 meters) in
diameter, have been imaged, but there is a surprising scarcity of boulders
large enough to make such impacts. There is more than 100 times the number
of 10- to 12-foot (3- to 4-meter) boulders than there are impact craters in
this region. Some angular or slab-like features were imaged that could
indicate they are composed of stronger material than rounded objects. Some
boulder clusters are thought to be fragments of a larger projectile that hit
the asteroid.

The flyover also yielded evidence of an unusually low number of smaller
craters. “There could be some unknown process, possibly something like
seismic shaking following impacts, which is more likely on a small body such
as Eros,” says Dr. Joseph Veverka of Cornell University, Ithaca, N.Y., who
heads the imaging team. “Other possibilities are processes that could erode
or erase smaller craters such as micro-cratering [the pummeling of the
surface by smaller objects] or thermal creep [the erosion of surface
material through normal seasonal heating and cooling of the asteroid] that
is eroding the smaller craters.”

“We do know there is a substantial amount of regolith from erosion and
impacts that is covering blocks [boulders] and craters possibly to a depth
of several meters. So it could be that many smaller craters do exist but
they’re buried under the regolith,” says Veverka. “A thick covering of fine
dust that prevents us from seeing what lies beneath might also be part of
the answer to why the asteroid has little color variation. It is possible
that parts of Eros are covered in regolith as deep as a 10-story building.”

The data also revealed ponds – flat surfaces at the bottom of craters –
formed by regolith deposits. These ponds are intriguing science team members
because of their extremely smooth surfaces. “The smoothness indicates that
there is an efficient process on Eros which is able to sort out the finest
component of the regolith from the coarser, more blocky portion and
concentrate this fine material into some low-lying areas such as crater
bottoms,” Veverka says.

Moreover, the laser altimeter found that ponded deposits are not only smooth
but also extremely horizontal – level relative to local gravity – as if
formed by fluid-like motions. “It is astonishing that the total dry regolith
of an asteroid like Eros can apparently be mobilized like a fluid,” says
Cheng. “There is no water on Eros, and there has not been any water, for
billions of years. However, seismic shaking caused by impacts may be able to
produce fluidized movement of regolith.”

“Aprons” of debris at the base of some of the larger boulders indicate
another phenomenon the researchers are studying: efficient erosion or
disintegration of ejecta boulders (boulders forced out of a crater as the
result of an impact) after they have landed on the surface. But scientists
say they need to study higher resolution images to more definitively
interpret the various forms of regolith that the low-altitude images have
provided. “What causes this efficient disintegration remains a mystery,”
Veverka says. “But one we hope to solve over the coming months by studying
the wealth of data that the NEAR mission has provided.”

More information on the NEAR mission can be found at the NEAR mission Web

Media Contacts:

Helen Worth

Johns Hopkins University Applied Physics Laboratory

(240) 228-5113

Michael Buckley

Johns Hopkins University Applied Physics Laboratory

(240) 228-7536