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

[Ron Baalke]

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