Scientists Discover Why Comets are like Deep Fried Ice Cream

Comets are a bit like deep fried ice cream--at least that's according to astronomers who have taken a closer look at the composition of comets. They've examined the structure of a comet and have discovered why they're encased in a hard, outer crust.

Researchers have long known that comets have soft interiors and seemingly hard crusts. In fact, the Rosetta spacecraft's Philae probe bounced to a landing on the surface of the comet 67P/Churyumov-Gerasimenko, which confirmed that comets have a hard surface. In addition, the black, soot-like coats of comets, made up of organic molecules and dust, had also been seen before the Deep Impact mission. Yet the composition of the comet crust and how it forms has remained unclear.

In order to find this out, the researchers turned to labs to put together a model of crystallizing comet crust. The scientists began with porous ice; in this state, water vapor molecules are flash-frozen at extremely cold temperatures so that the disorderly states are preserved. The water molecules are haphazardly mixed with other molecules, such as the organics, and remain frozen in that state.

The researchers then used an instrument to slowly warm the ice mixtures, mimicking the conditions that a comet would experience as it journeys toward the sun. The ice itself was infused with organics called polycyclic aromatic hydrocarbons (PAHs), which are seen in deep space.

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

Once the PAHs were kicked out of the ice mixtures, the water molecules had room to link up and form the more tightly packed structures of crystalline ice. This, in turn, created a hard surface.

"A comet is like deep fried ice cream," said Murthy Gudipati, one of the researchers. "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 findings reveal a bit more about the composition of a comet and show exactly how these structures develop their hard surfaces.