Research suggests that the composition of comets can be explained by what happens when breaded ice cream is dropped in oil.
Ice cream is very cold. It is so cold that when breaded and dropped, very briefly, into a deep fryer the outer part of the ice cream ball crystalizes while leaving the interior cold.
Using an icebox-like instrument called Himalaya researchers at NASA's Jet Propulsion Laboratory (JPL) and the California Institute of Technology (Caltech) in Pasadena, California have demonstrated that fluffy ice on the surface of a comet behaves in roughly the same way as it approaches the sun and warms up.
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Water-ice crystals form and become denser and more ordered. When this happens carbon molecules on the surface are expelled. The end result, say the researchers, is a "crunchy chomet" with organic sprinkles.
"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," said Murthy Gudipati of NASA's Jet Propulsion Laboratory in a statement.
Gudipati is corresponding author, along with lead author Antti Lignell, a postdoctoral scholar at CalTech, of a recent study appearing in The Journal of Physical Chemistry.
Based on data from NASA's Deep Impact and the European Space Agency's Rosetta spacecraft, researchers already knew that comets have soft, porous interiors and hard crusts. Last November the Philae probe, launched by Rosetta, landed roughly on the surface of 67P/Churyumov-Gerasimenko, confirming that comets have a hard surface. Deep Impact had also registered the soot-like coating on comets but their exact composition remained unknown.
For the new study, the JPL and CalTech researchers turned to labs on Earth to try to simulate crystalizing comet dust. They began with porous ice, which is thought to compose the coldest comets and icy moons.
To create the right type of ice water vapours are flash frozen at minus 405 degrees Fahrenheit (minus 243 Celsius). At this temperature the state of molecules is disordered and water molecules are mixed with other molecules such as organics. If undisturbed, the molecules remain frozen in that state.
According to Gudipati, Amorphous or porous ice is "like cotton candy", light and fluffy with pockets of space. On Earth, all ice is in crystalline form including loose snow.
Using the Himalaya cryostat instrument, the researchers slowly warmed their cotton-candy ice mixtures to minus 190 degrees Fahrenheit (minus 123 Celsius), mimicking an approach to the sun. The ice had been infused with polycyclic aromatic hydrocarbons (PAHs) which are organics commonly 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 Lignell.
Whe the PAH's removed, the water molecules in the ice were able to link up, forming tightly packed crystalline ice, similar to what is found on Earth.
"What we saw in the lab — a crystalline comet crust with organics on top — matches what has been suggested from observations in space. 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," said Gudipati.
Understanding the composition and behavior of comets is important to scientists as they struggle to understand their role in the formation of the Earth and other planets. Results from the Rosetta mission show that comets could be carriers of the ingredients for life, but the debate is still ongoing.
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