Scientists
have long known that a major ingredient in comets
is water ice, but they were unsure whether the ice was contained mainly inside
or if it could be found on the surface as well.
A new
analysis of data from NASA's Deep
Impact mission last year provides the first evidence that water ice can
indeed exist on a comet's exterior.
In a new
study released today in an online edition of the journal Science,
researchers report that the surface of Tempel 1,
the comet targeted by Deep Impact, has three small pockets of water ice.
Tempel 1
has a surface area of roughly 45 square miles, or 1.2 billion square feet. The
area taken up by the water ice, however, is only 300,000 square feet. The rest
of the comet surface is dust.
"It's like
a seven-acre skating rink of snowy dirt," said study co-author Peter Schultz of
Brown University.
On July 4,
2005, NASA slammed a heavy copper probe called Impactor
into Tempel 1's surface while it was 83 million miles from Earth. The resulting
collision created a stadium-sized crater and flung tons of debris into space.
Impactor was one of two Deep Impact spacecrafts; the mothership, responsible
for recording and analyzing the blast, was called Flyby.
The
researchers believe Tempel 1's surface ice used to reside inside the comet and
became exposed over time. It's also thought that occasional geyser-like blasts
of dust and vapor, called jets, send the ice outward. Once ejected, the ice
crystals can become incorporated into the luminous coma, a cloud of material surrounding
the main body of the comet, or the ice can become part of its tail.
The same
team previously reported that Tempel 1's interior also contained an abundance
of organic material and suggested the comet may have originated in a region of
the solar system now occupied by Uranus and Neptune.
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