Comets are just like some people: a hard surface hiding a soft(er) heart. Learning this has left astronomers puzzled and keen to know why. Now, lab experiments appear to have replicated the formation of a comet's crust, leading to a better understanding of cometary structures, and a series of sweet-toothed analogies.
Comets, commonly described as “dirty snowballs,” are mostly ice with some dust. As they approach the sun and warm up, some of their ice sublimes. This can lead to dust particles building up on the surface.
Deep Impact and Rosetta have already gathered evidence for the soft and porous interiors of comets, but it was only last November that Philae demonstrated that beneath Comet 67P/Churyumov-Gerasimenko's thin film of dust is a very hard layer. The probe confirmed this first by bouncing and then being unable to drill more than a few millimeters into the solid ice on which it landed. This was unexpected (otherwise the probe might have been equipped with a more powerful drill) and left planetary scientists seeking answers.
To learn more, an instrument named Himalaya has been constructed at the Jet Propulsion Laboratory to try to simulate the conditions that comets experience. According to a paper published in the Journal of Physical Chemistry, when approaching the sun, fluffy surface ice forms dense, ordered crystals from which non-water molecules are expelled. The comet's interior, however, remains cold enough to stay fluffy.
Credit: Lignell/Journal of Physical Chemistry. The alignment of water molecules change between forms of ice.
"A comet is like deep fried ice cream," says JPL's Dr. Murthy Gudipati. "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."
Gudipati and Dr. Antti Lignell packed Himalaya with amorphous ice similar to that thought to make up a comet's center. This amorphous ice forms from water vapor below about -243°C and, according to Gudipati, resembles candy floss: fluffy and containing plenty of empty space with a few other molecules thrown in. Due to this, amorphous ice is even lighter than snowballs.
When the researchers used Himalaya to slowly warm the amorphous ice—which was mixed with dust and organic particles known as polycyclic aromatic hydrocarbons (PAHs)—to -123°C, Gudipati and Lignell found something unexpected.
“The PAHs stuck together and were expelled from the ice host as it crystallized,” says Lignell. “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.”
Removing the PAHs caused the water molecules to pack tightly. "What we saw in the lab—a crystalline comet crust with organics on top—matches what has been suggested from observations in space," says Gudipati. As with all cometary research, the findings could deepen our understanding of the formation of the solar system and the chemicals brought to Earth during early bombardments.