A new NASA study reveals how comets are just like deep fried ice cream.
While messing around with an icebox-like instrument nicknamed Himalaya, scientists may have revealed why comets are encased in a "crispy" outer crust.
The study shows that "fluffy" ice on the surface of a comet could crystalize and harden as the object moves closer to the blazing hot Sun. As the hard water-ice crystals form, carbon-containing molecules would also be expelled to the comet's surface resulting in a "crunchy comet crust sprinkled with organic dust," NASA reported.
"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."
Scientists have believed that comets are composed of soft insides encased in hard crusts for a while now, but recent findings from Rosetta's Philae probe (which landed on the surface of comet 67P/Churyumov-Gerasimenko in November) confirmed these beliefs.
Researchers are still not sure of the composition of comet crust, or exactly how it forms. In the recent study researchers put together a model of crystallizing comet crust in a lab setting. They started out with porous (amorphous) ice that was similar to what would be seen in the interior of a comet. There is not type of ice does not exist naturally on our planet because it can only form at a frigid temperature of negative 405 degrees Fahrenheit. The team used the Himalaya cryostat instrument to slowly warm the amorphous ice from 30 Kelvin to 150 Kelvin (negative 190 degrees Fahrenheit), mimicking the condition that would be found on a comet that is approaching the Sun. The ice was infused with polycyclic aromatic hydrocarbons (PAH's) which can be found in deep space.
"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 lead author of the study is Antti Lignell, a postdoctoral scholar at the California Institute of Technology in Pasadena.
When PAHs were eliminated from the ice mixture the water molecules had more room to "link up" and formed even more tightly-packed 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," Gudipati said. "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 findings could help scientists gain insight into how comets delivered water to Earth in its molten-hot early days.
"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," Gudipati concluded.
