[meteorite-list] Comet Borrelly Puzzle: Darkest Object in the Solar System

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:47:15 2004
Message-ID: <200111301813.KAA20741_at_zagami.jpl.nasa.gov>


Comet Borrelly Puzzle: Darkest Object in the Solar System
By Robert Roy Britt
29 November 2001

Covered in a crust of blackness likened to the toner in a copy machine, a
5-mile-long potato-shaped comet called Borrelly has been found to be the
darkest object in the solar system, scientists announced today.

The determination should help researchers learn what comets are made of,
though one scientist said he can't figure how anything could be so dark.

Comet Borrelly reflects less than 3 percent of all the sunlight that hits
it, about 1 percent less than NASA scientists initially suggested in the
days following the Deep Space 1 spacecraft's Sept. 22 flyby when the first
pictures of Borrelly were released.

Previously, the darkest object known was comet Halley, which reflects 4
percent of the sunlight it receives.

Comets are seen as pristine messengers from the formation of the solar
system, but scientists rarely get close-up looks at them. Their interiors
remain unexplored. Even their surfaces are largely enigmatic, because when
comets are close enough to study -- anywhere near Earth -- they are also
close to the Sun, which boils away the comet surface and hides it in a cloud
of dust and vapor, called a coma.

Sunlight bounces off the particles in the coma and in a comet's tail, giving
the objects their popular bright appearance that on occasion grace the night
sky. But inside those bright halos, the story is altogether different.

Dark as asphalt

Borrelly's dark nature was revealed when Deep Space 1 took the best close-up
images ever obtained of a comet's core, or nucleus, as it zoomed through
Borrelly's coma.

"We have known for years that the surface of the earth's Moon is dark --
about as reflective as an asphalt parking lot," said Robert Nelson, a
project scientist on the Deep Space 1 mission at NASA's Jet Propulsion
Laboratory. "The nucleus of Borrelly is about half as reflective as the

Scientists call this reflectivity -- the sunlight that is not absorbed by an
object -- albedo.

"I'm not sure how you get an albedo that low," said Donald Yeomans, an
expert on comets and asteroids at JPL. "It must have to do with texture --
it can't all be color."

Yeomans suggested that fine-grained surface structures might create shadows
that can't be individually detected in the Deep Space 1 images. He and
others are amazed at the complexity of Borrelly.

"The surface looks to be fairly bizarre," Yeomans said. It is riddled with
bright and dark patches, smooth areas, jumbled hills and bright jets that
emit material from its insides.

The overall darkness indicates what materials likely make up the comet.

Carbon and iron are eligible candidates, Nelson said in a telephone
interview. But there is no way to know what lies underneath the crust of the
nucleus, he said.

Nelson presented the findings today at a meeting of the American
Astronomical Society's Division for Planetary Sciences in New Orleans. They
are part of a trickle of discoveries that promises to soon turn into a flood
as Borrelly becomes the most highly scrutinized comet ever.

"We've just scratched the surface with regard to analyzing this data set,"
Nelson said.

Separate view from Hubble

During the same week that Deep Space 1 imaged Borrelly from up close, the
Hubble Space Telescope examined the comet's entire coma. The Hubble
observations, which have not been released but will be published next year,
were led by Alan Stern of the Southwest Research Institute.

Stern told SPACE.com that Hubble showed the coma was loaded with red dust,
which he said is consistent with the dark appearance of the comet. His team
also calculated the rate at which water evaporated off the surface of the
comet, which he said was in line with predictions made in recent months of
several tons every second.

Scientists had once described comets as dirty snowballs. Now they believe
comets have less water than was previously calculated. "Icy dirtballs" is a
more apt description, they say.

Stern and his colleagues made other findings, including a confirmation of
the length of a day on Borrelly -- how long it takes to rotate on its axis.

"We measured the rotation rate, and we're still refining the number," he
said. "It's about a day."

Scientists expect another NASA mission, called Deep Impact, will answer many
of their questions about what's going on inside comets. Deep Impact's
mission plan calls for shooting a camera-packing copper probe headlong into
Comet Tempel 1 on Independence Day in 2005. The collision is expected carve
out material and provide the first ever peek inside a comet.
Received on Fri 30 Nov 2001 01:13:55 PM PST

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