Scientists at NASA’s Jet Propulsion Laboratory in Pasadena, California, have discovered why comets are encased in a hard, outer crust.
This false-color four-image mosaic comprises images taken from a distance of 28.7 km from the center of Comet 67P/Churyumov-Gerasimenko on 3 February 2015. The mosaic measures 4.2 x 4.6 km. Image credit: ESA / Rosetta / NAVCAM / CC BY-SA IGO 3.0.
Using a unique cryostat instrument called Himalaya, they show that fluffy ice on the surface of a comet would crystallize and harden as the comet heads toward the Sun and warms up.
“A comet is like deep fried ice cream. 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,” said Dr Murthy Gudipati, who is a co-author of the paper published online in the Journal of Physical Chemistry A.
“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.”
Dr Gudipati teamed up with his colleague, Dr Antti Lignell, to put together a model of crystallizing comet crust.
The lab experiments began with amorphous ice – the proposed composition of the chilliest of comets and icy moons.
In this state, water vapor molecules are flash-frozen at extremely cold temperatures of minus 243 degrees Celsius. Disorderly states are preserved: water molecules are haphazardly mixed with other molecules, such as the organics, and remain frozen in that state.
On Earth, all ice is in the crystalline form. It’s not cold enough to form amorphous ice on the planet.
The scientists used the Himalaya instrument to slowly warm their amorphous ice mixtures from minus 243 to minus 123 degrees Celsius, mimicking conditions a comet would experience as it journeys toward the Sun.
The ice had been infused with polycyclic aromatic hydrocarbons (PAHs), which are seen everywhere 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,” Dr Lignell explained.
With PAHs kicked out of the ice mixtures, the water molecules had 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. 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,” Dr Gudipati concluded.
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Antti Lignell & Murthy S. Gudipati. Mixing of the Immiscible: Hydrocarbons in Water-Ice near the Ice Crystallization Temperature. J. Phys. Chem. A, published online October 10, 2014; doi: 10.1021/jp509513s
