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@jhuapl.edu or
Dr. Carey Lisse
(240) 228-0535 or (443) 778-0535
carey.lisse@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."

###
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