[meteorite-list] NASA's Spitzer Gets Sneak Peak Inside Comet Holmes

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon, 13 Oct 2008 13:54:59 -0700 (PDT)
Message-ID: <200810132054.NAA08409_at_zagami.jpl.nasa.gov>

http://www.jpl.nasa.gov/news/features.cfm?feature=1901

NASA's Spitzer Gets Sneak Peak Inside Comet Holmes
Jet Propulsion Laboratory
October 13, 2008

When comet Holmes unexpectedly erupted in 2007, professional and amateur
astronomers around the world turned their telescopes toward the
spectacular event. Their quest was to find out why the comet had
suddenly exploded.

Observations taken of the comet after the explosion by NASA's Spitzer
Space Telescope deepen the mystery, showing oddly behaving streamers in
the shell of dust surrounding the nucleus of the comet. The data also
offer a rare look at the material liberated from within the nucleus, and
confirm previous findings from NASA's Stardust and Deep Impact missions.

"The data we got from Spitzer do not look like anything we typically see
when looking at comets," said Bill Reach of NASA's Spitzer Science
Center at the California Institute of Technology, Pasadena, Calif. Reach
is lead investigator of the Spitzer observations. "The comet Holmes
explosion gave us a rare glimpse at the inside of a comet nucleus." The
findings were presented at the 40th meeting of the Division of Planetary
Sciences in Ithaca, N.Y.

Every six years, comet 17P/Holmes speeds away from Jupiter and heads
inward toward the sun, traveling the same route typically without
incident. However, twice in the last 116 years, in November 1892 and
October 2007, comet Holmes exploded as it approached the asteroid belt,
and brightened a million-fold overnight.

In an attempt to understand these odd occurrences, astronomers pointed
NASA's Spitzer Space Telescope at the comet in November 2007 and March
2008. By using Spitzer's infrared spectrograph instrument, Reach was
able to gain valuable insights into the composition of Holmes' solid
interior. Like a prism spreading visible-light into a rainbow, the
spectrograph breaks up infrared light from the comet into its component
parts, revealing the fingerprints of various chemicals.

In November of 2007, Reach noticed a lot of fine silicate dust, or
crystallized grains smaller than sand, like crushed gems. He noted that
this particular observation revealed materials similar to those seen
around other comets where grains have been treated violently, including
NASA's Deep Impact mission, which smashed a projectile into comet Tempel
1; NASA's Stardust mission, which swept particles from comet Wild 2 into
a collector at 13,000 miles per hour (21,000 kilometers per hour), and
the outburst of comet Hale-Bopp in 1995.

"Comet dust is very sensitive, meaning that the grains are very easily
destroyed, said Reach. "We think the fine silicates are produced in
these violent events by the destruction of larger particles originating
inside the comet nucleus."

When Spitzer observed the same portion of the comet again in March 2008,
the fine-grained silicate dust was gone and only larger particles were
present. "The March observation tells us that there is a very small
window for studying composition of comet dust after a violent event like
comet Holmes' outburst," said Reach.

Comet Holmes not only has unusual dusty components, it also does not
look like a typical comet. According to Jeremie Vaubaillon, a colleague
of Reach's at Caltech, pictures snapped from the ground shortly after
the outburst revealed streamers in the shell of dust surrounding the
comet. Scientists suspect they were produced after the explosion by
fragments escaping the comet's nucleus.

In November 2007, the streamers pointed away from the sun, which seemed
natural because scientists believed that radiation from the sun was
pushing these fragments straight back. However, when Spitzer imaged the
same streamers in March 2008, they were surprised to find them still
pointing in the same direction as five months before, even though the
comet had moved and sunlight was arriving from a different location. "We
have never seen anything like this in a comet before. The extended shape
still needs to be fully understood," said Vaubaillon.

He notes that the shell surrounding the comet also acts peculiarly. The
shape of the shell did not change as expected from November 2007 to
March 2008. Vaubaillon said this is because the dust grains seen in
March 2008 are relatively large, approximately one millimeter in size,
and thus harder to move.

"If the shell was comprised of smaller dust grains, it would have
changed as the orientation of the sun changes with time," said
Vaubaillon. "This Spitzer image is very unique. No other telescope has
seen comet Holmes in this much detail, five months after the explosion."

"Like people, all comets are a little different. We've been studying
comets for hundreds of years ??? 116 years in the case of comet Holmes ???
but still do not really understand them," said Reach. "However, with the
Spitzer observations and data from other telescopes, we are getting
closer."

NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer
Space Telescope mission for NASA's Science Mission Directorate,
Washington. Science operations are conducted at the Spitzer Science
Center at the California Institute of Technology, also in Pasadena.
Caltech manages JPL for NASA. For more information about Spitzer, visit
http://www.spitzer.caltech.edu/spitzer and http://www.nasa.gov/spitzer .

------------------------------------------------------------------------

Media contact: Whitney Clavin/ Jet Propulsion Laboratory 818-354-4673
whitney.clavin at jpl.nasa.gov
Received on Mon 13 Oct 2008 04:54:59 PM PDT