Comets are sometimes described as “dirty snowballs,” but
a close flyby of one by NASA’s Deep Space 1 spacecraft last
fall detected no frozen water on its surface.

Comet Borrelly has plenty of ice beneath its tar-black
surface, but any exposed to sunlight has vaporized away, say
scientists analyzing data from Deep Space 1, managed by NASA’s
Jet Propulsion Laboratory, Pasadena, Calif.

“The spectrum suggests that the surface is hot and dry.
It is surprising that we saw no traces of water ice,” said Dr.
Laurence Soderblom of the U.S. Geological Survey’s Flagstaff,
Ariz., station, lead author of a report on the Borrelly flyby
results appearing in the online edition of the journal

“We know the ice is there,” he said. “It’s just well-
hidden. Either the surface has been dried out by solar heating
and maturation or perhaps the very dark soot-like material
that covers Borrelly’s surface masks any trace of surface

The Deep Space 1 science team released pictures and other
initial findings days after the spacecraft flew within 2,171
kilometers (1,349 miles) of the comet’s solid nucleus on
September 22, 2001. This week’s report provides additional
details about the nucleus and the surrounding coma of gases
and dust coming off of the comet as measured by one of Deep
Space 1’s scientific instruments.

“Comet Borrelly is in the inner solar system right now,
and it’s hot, between 26 and 71 degrees Celsius (80 and 161
degrees Fahrenheit), so any water ice on the surface would
change quickly to a gas, ” said Dr. Bonnie Buratti, JPL
planetary scientist and co-author of the paper. “As the
components evaporate, they leave behind a crust, like the
crust left behind by dirty snow.”

Borrelly is unusually dark for an object in the inner
solar system. The comet’s surface is about as dark as a blot
of photocopy toner, possibly the darkest surface in the solar
system. It is more like objects in the outer solar system such
as the dark side of Saturn’s moon Iapetus and the rings of

“It seems to be covered in this dark material, which has
been loosely connected with biological material.” Buratti
said. “This suggests that comets might be a transport
mechanism for bringing the building blocks of life to Earth.”
Comets may have played an important role in supplying organic
materials that are required for life to originate.

Soderblom points out that Borrelly’s old, mottled
terrain with dark and very dark spots — different shades of
black — are apparently inactive. Ground-based observations
estimated that 90 percent of Borrelly’s surface might be
inactive, and the observations taken by Deep Space 1 show that
this is indeed true.

“It’s remarkable how much information Deep Space 1 was
able to gather at the comet, particularly given that this was
a bonus assignment for the probe,” said Dr. Marc Rayman,
project manager of the mission. Deep Space 1 completed its
original goal to test 12 new space technologies and then
earned extra credit by achieving additional goals, such as the
risky Borrelly flyby. “It’s quite exciting now as scientists
working with this rich scientific harvest turn data into

Images of comet Borrelly from Deep Space 1 are available
at .

More information on the Deep Space 1 mission is available


Deep Space 1 was launched in October 1998 as part of
NASA’s New Millennium Program, which is managed by JPL for
NASA’s Office of Space Science, Washington, D.C. The
California Institute of Technology, Pasadena, manages JPL for