Boffins working for Caltech and NASA's Jet Propulsion Laboratory have created comet-like stuff in a cryostat.
Along the way, they've decided that your average comet is kind of like fried ice-cream: squishy on the inside, crusty on the outside, with organic chemicals dusted on top.
Both the ESA's Rosetta and NASA's Deep Impact comet missions had found evidence of the soft, porous interiors of comets, and the Philae lander discovered the hard surface the hard way, with a triple-bounce that surprised mission scientists and degraded some of its activities.
Fooling around with an icebox instrument called Himalaya, Antti Lignell at Caltech and Murthy Gudipati of JPL demonstrated, as the NASA announcement says, “that the fluffy ice on the surface of a comet would crystallise and harden as the comet heads towards the sun and warms up”.
(After all, it's still in the icy void of space – nobody says it gets warm enough to become liquid.)
The ice crystals are denser and more ordered than the fluffy interior, and as a result, carbon-containing organic molecules are pushed to the surface.
The fluffy interior, the researchers explain, are a form of ice not observed under normal Earth conditions. The “amorphous” porous ice forms when water vapour is flash-frozen at around 30 Kelvin, and never forms crystals.
The disorderly states of amorphous ice mean it's somewhat like fairy floss (cotton candy) – light, fluffy, and with pockets of space throughout.
To try and reproduce the process that comets go through, Gudipati and Lignell used the Himalaya cryostat to slowly warm amorphous ice, doped with polycyclic aromatic hydrocarbons (PAHs), a common deep-space molecule, from 30 Kelvin to 150 Kelvin (still seriously cold at -123°C).
“The PAHs stuck together and were expelled from the ice host as it crystallised,” Lignell explained. “This may be the first observation of molecules clustering together due to a phase transition of ice”.
That's not just about space, either: NASA and Caltech reckon as well as helping us understand how comets and asteroids may have helped bring the ingredients of life to Earth, the result has “important consequences for the chemistry and physics of ice”. ®