[meteorite-list] Why Comets Are Like Deep Fried Ice Cream

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 10 Feb 2015 22:08:22 -0800 (PST)
Message-ID: <201502110608.t1B68ML6003940_at_zagami.jpl.nasa.gov>

http://www.jpl.nasa.gov/news/news.php?feature=4480

Why Comets Are Like Deep Fried Ice Cream
Jet Propulsion Laboratory
February 10, 2015

--Studying comet composition helps explain how early Earth may have received
water and organics.

--New research used "Himalaya," an icebox-like instrument.

Astronomers tinkering with ice and organics in the lab may have discovered
why comets are encased in a hard, outer crust.

Using an icebox-like instrument nicknamed Himalaya, the researchers show
that fluffy ice on the surface of a comet would crystalize and harden
as the comet heads toward the sun and warms up. As the water-ice crystals
form, becoming denser and more ordered, other molecules containing carbon
would be expelled to the comet's surface. The result is a crunchy comet
crust sprinkled with organic dust.

"A comet is like deep fried ice cream," said Murthy Gudipati of NASA's
Jet Propulsion Laboratory in Pasadena, California, corresponding author
of a recent study appearing in The Journal of Physical Chemistry. "The
crust is made of crystalline ice, while the interior is colder and more
porous. The organics are like a final layer of chocolate on top."

The lead author of the study is Antti Lignell, a postdoctoral scholar
at the California Institute of Technology in Pasadena, who formerly worked
with Gudipati at JPL.

Researchers already knew that comets have soft interiors and seemingly
hard crusts. NASA's Deep Impact and the European Space Agency's Rosetta
spacecraft both inspected comets up close, finding evidence of soft, porous
interiors. Last November, Rosetta's Philae probe bounced to a landing
on the surface of 67P/Churyumov-Gerasimenko, confirming that comets have
a hard surface. The black, soot-like coats of comets, made up of organic
molecules and dust, had also been seen before by the Deep Impact mission.

But the exact composition of comet crust -- and how it forms -- remains
unclear.

In the new study, researchers turned to labs on Earth to put together
a model of crystallizing comet crust. The experiments began with amorphous,
or porous, ice -- the proposed composition of the chilliest of comets
and icy moons. In this state, water vapor molecules are flash-frozen at
extremely cold temperatures of around 30 Kelvin (minus 243 degrees Celsius,
or minus 405 degrees Fahrenheit), sort of like Han Solo in the Star Wars
movie "The Empire Strikes Back." Disorderly states are preserved: Water
molecules are haphazardly mixed with other molecules, such as the organics,
and remain frozen in that state. Amorphous ice is like cotton candy, explains
Gudipati: light and fluffy and filled with pockets of space.

On Earth, all ice is in the crystalline form. It's not cold enough to
form amorphous ice on our planet. Even a handful of loose snow is in the
crystalline form, but contains much smaller ice crystals than those in
snowflakes.

Gudipati and Lignell used their Himalaya cryostat instrument to slowly
warm their amorphous ice mixtures from 30 Kelvin to 150 Kelvin (minus
123 degrees Celsius, or minus 190 degrees Fahrenheit), mimicking conditions
a comet would experience as it journeys toward the sun. The ice had been
infused with a type of organics, called polycyclic aromatic hydrocarbons,
or PAHs, which are seen everywhere in deep space.

The results came as a surprise.

"The PAHs stuck together and were expelled from the ice host as it crystallized.
This may be the first observation of molecules clustering together due
to a phase transition of ice, and this certainly has many important consequences
for the chemistry and physics of ice," said Lignell.

With PAHs kicked out of the ice mixtures, the water molecules had room
to link up and form the more tightly packed structures of crystalline
ice.

"What we saw in the lab -- a crystalline comet crust with organics on
top -- matches what has been suggested from observations in space," said
Gudipati. Deep fried ice cream is really the perfect analogy, because
the interior of the comets should still be very cold and contain the more
porous, amorphous ice."

The composition of comets is important to understanding how they might
have delivered water and organics to our nascent, bubbling-hot Earth.
New results from the Rosetta mission show that asteroids may have been
the primary carriers of life's ingredients; however, the debate is ongoing
and comets may have played a role. For Gudipati, comets are capsules containing
clues not only to our planet's history but to the birth of our entire
solar system.

He said, "It's beautiful to think about how far we have come in our understanding
of comets. Future missions designed to bring cold samples of comets back
to Earth could allow us to fully unravel their secrets."

Rosetta is a European Space Agency mission with contributions from its
member states and NASA. JPL, a division of the California Institute of
Technology in Pasadena, manages the U.S. contribution of the Rosetta mission
for NASA's Science Mission Directorate in Washington.

Caltech manages JPL for NASA.

Media Contact
Whitney Clavin
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-4673
whitney.clavin at jpl.nasa.gov

2015-056
Received on Wed 11 Feb 2015 01:08:22 AM PST