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, NASA said in a statement.

"A comet is like deep fried ice cream. 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," explained Murthy Gudipati of NASA's Jet Propulsion Laboratory in Pasadena, California.

Researchers already know that comets have soft interiors and seemingly hard crusts. In the new study, Gudipati and Antti Lignell, post-doctoral scholar at the California Institute of Technology, put together a model of crystallising comet crust.

Gudipati and team used their Himalaya instrument to slowly warm their amorphous ice mixtures to minus 123 degrees Celsius, 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 (PAHs) which are seen everywhere in deep space."What we saw in the lab - a crystalline comet crust with organics on top - matches what has been suggested from observations in space," Gudipati noted.

"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," he pointed out.

The study appeared in the Journal of Physical Chemistry.


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