NEW DELHI (TIP): Nasa researchers may have discovered why comets are encased in a hard, outer crust – like ice cream that has been deep fried. Using an icebox-like instrument nicknamed Himalaya, they showed that fluffy ice on the surface of a comet would crystalize and harden as the comet heads toward the sun and warms up.
“A comet is like deep fried ice cream,” said Murthy Gudipati of Nasa’s Jet Propulsion Laboratory, 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. Last November, Rosetta’s Philae probe bounced to a landing on the surface of 67P/Churyumov-Gerasimenko, confirming that comets have a hard surface. But the exact composition of comet crust, and how it forms, was 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. At these extremely cold temperatures of around minus 243 degrees Celsius (minus 405 degrees Fahrenheit) water vapor molecules are flash-frozen and haphazardly mixed with other molecules, such as the organics. Amorphous ice is like cotton candy, explains Gudipati: light and fluffy and filled with pockets of space.
Gudipati and Lignell used their Himalaya cryostat instrument to slowly warm their amorphous ice mixtures to minus 123 degrees Celsius (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 were kicked out of the ice mixtures giving water molecules 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.