[meteorite-list] Scientists Gaining Clearer Picture of Comet Makeup and Origin

[Ron Baalke]

The Johns Hopkins University Applied Physics Laboratory
Office of Communications and Public Affairs
Media Contacts: Michael Buckley
(240) 228-7536 or (443) 778-7536
michael.buckley at jhuapl.edu or
Dr. Carey Lisse
(240) 228-0535 or (443) 778-0535
carey.lisse at jhuapl.edu

July 17, 2006

FOR IMMEDIATE RELEASE

SCIENTISTS GAINING CLEARER PICTURE OF COMET MAKEUP AND ORIGIN

Scientists are getting their best understanding yet of the makeup of comets 
- not only of the materials inside these planetary building blocks, but 
also of the way they could have formed around the Sun in the solar system's 
earliest years.

When NASA's Deep Impact spacecraft slammed into comet Tempel 1 on July 4, 
2005, the collision sent tons of pristine materials into space and gave 
astronomers from around the world, using ground- and space-based 
telescopes, the first look "inside" a comet. From that sample, over the 
past several months, scientists who used the imaging spectrometer on NASA's 
Spitzer Space Telescope have refined their models of what a comet is made 
of and how it comes together.

The Spitzer observation team, led by Dr. Carey Lisse of the Johns Hopkins 
University Applied Physics Laboratory in Laurel, Md., writes about its 
findings this week on the Science Express Web site 
(http://www.sciencemag.org/sciencexpress/recent.dtl).

"Spitzer's spectral observations of the impact at Tempel 1 not only gave us 
a much better understanding of a comet's makeup, but we now know more about 
the environment in the solar system at the time this comet was formed," 
Lisse says.

 From its orbit in space, Spitzer's infrared spectrograph closely observed 
the materials ejected from Tempel 1 when Deep Impact's probe dove into the 
comet's surface. Astronomers spotted the signatures of solid chemicals 
never seen before in comets, such as carbonates (chalk) and smectite 
(clay), metal sulfides (like fool's gold), and carbon-containing molecules 
called polycyclic aromatic hydrocarbons, found in barbecue grills or 
automobile exhaust on Earth.

Lisse says the clay and carbonates were surprises because they typically 
require liquid water to make - and liquid water isn't found in the regions 
of deep space where comets form. Also surprising was the superabundance of 
crystalline silicates, material formed only at red-hot temperatures found 
inside the orbit of Mercury.

"In the same body, you have material formed in the inner solar system, 
where water can be liquid, and frozen material from out by Uranus and 
Neptune," Lisse says. "Except for the lightest elements, the total 
abundances of atoms in the comet are practically the same as makes up the 
Sun. It implies there was a great deal of churning in the primordial solar 
system, with high- and low-temperature materials mixing over great distances."

Planets, comets and asteroids were all born out of a thick and dusty mix of 
chemicals that surrounded the young Sun. Because comets formed in the 
outer, colder regions of our solar system, some of this early planetary 
material remains frozen inside them. By refining their list of comet 
ingredients, theoreticians can begin testing models of planet formation.

More than 80 telescopes on and above Earth observed Deep Impact's 
rendezvous with Tempel 1, and their findings are shedding light on the 
comet's broader history in the solar system. Lisse's team is also comparing 
Spitzer's discoveries with those from NASA's Stardust mission, which last 
January returned particles from the coma (or atmosphere) of comet Wild 2 
back to Earth.

"We can compare the inferred composition of Tempel 1 to the Stardust sample 
returns and obtain a 'ground truth,' " Lisse says. "From this we can create 
a Rosetta stone, which we'll use to better understand the materials seen in 
our own solar system as well as around other stars."

Twelve of the 14 species found by Spitzer match up with preliminary 
Stardust analyses, Lisse says, but several mysteries remain. For example, 
the Stardust samples do not yet include definitive evidence of the 
carbonate and clay minerals found in Tempel 1.

"There's no reason to think Tempel 1 represents all comets," he says. "Deep 
Impact only hit and excavated Tempel 1 in one precise location, and 
Stardust only sampled the surface of one comet at one point in its orbit. 
We'll need additional missions to comets - such as robotic landing 
spacecraft or sample-return probes - to help us complete the picture."
                                                                        
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NASA's Jet Propulsion Laboratory (JPL), Pasadena, Calif., manages the 
Spitzer Space Telescope mission for NASA's Science Mission Directorate, 
Washington. Science operations are conducted at the Spitzer Science Center. 
Spitzer's infrared array camera was built by NASA's Goddard Space Flight 
Center, Greenbelt, Md.

The University of Maryland, College Park, conducted the overall mission 
management for Deep Impact, a Discovery class NASA program. JPL handled 
project management for the Deep Impact mission. The spacecraft was built 
for NASA by Ball Aerospace & Technologies Corp., Boulder, Colo.

The Applied Physics Laboratory, a not-for-profit division of The Johns 
Hopkins University, meets critical national challenges through the 
innovative application of science and technology. For more information, 
visit http://www.jhuapl.edu