The formation of comets can be compared to producing deep-fried ice cream, a new paper from Jet Propulsion Laboratory (JPL) reports.
The Rosetta spacecraft has been studying the comet 67P/Churyumov-Gerasimenko, known to astronomers simply as 67/P. Among the many surprises astronomers saw there was that the surface of the body is covered in a hard layer, even darker than charcoal.
Himalaya, an instrument used to perform extremely low-temperature experiments, was utilized for laboratory studies of the surface of 67/P. Astronomers theorize that any fluffy ice outside the comet would crystallize as the comet heads toward the Sun, and warms due to solar radiation. Organic material trapped in ice within the comet could then rise to the surface, creating the dark surface layer.
The robotic explorer was accompanied by the Philae lander, which bounced off the surface of the comet, suggesting the outer layer is harder than researchers believed.
"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," Murthy Gudipati from Jet Propulsion Laboratory said.
Comets were already known to have a porous center, surrounded by a hard outer layer, due to previous investigations by Rosetta and the earlier Deep Impact spacecraft.
A new experiment looked at how ice can crystallize in comets, utilizing porous ice. This material is flash-frozen by temperatures around 405 degrees below zero Fahrenheit. This preserves random orientations of molecules within the body, forming a light structure with space in between, similar to the structure of cotton candy. This type of ice is not found anywhere on Earth, as temperatures on our planet do not become cold enough for this process to happen.
Mixed in with the ice were polycyclic aromatic hydrocarbons (PAH's), which are commonly found throughout space. When researchers simulated the approach of a comet to the Sun, they found that these organic materials rose to the surface, a result that was unexpected by investigators.
"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," Antti Lignell of CalTech, said.
Warming of the cometary material brought the substance up to a balmy 190 degrees below zero Fahrenheit.
Simulation of the warming of comets and study of how the process could result in the observed structure of 67/P was published in The Journal of Physical Chemistry.
