What elegant beauty! Just look at those fountains of dust. Ever wonder where they come from? We’ve known for some time that comets are made of mostly ice, but it’s not like the pure crystal goodness that comes out of your refrigerator. Comet ice is dark and gross like the knobby black gunk lining your wheel wells in mid-winter.
As a comet approaches the Sun, it’s warmed by sunlight. With next to no atmosphere, the coldest ices — carbon dioxide (dry ice) and carbon monoxide — typically vaporize first followed by water ice as temperatures rise. As ice turns to vapor (gas), the dust is liberated as jets that feed the comet’s thin atmosphere called a coma and ultimately blown back to make its tail.
Thanks to high resolution photos just released this week by the European Rosetta spacecraft orbiting comet 67P/C-G, we now know exactly how at least some of those jets get their start. They begin in all places inside collapsing sinkholes in the comet’s crust.
In a new study published in the science journal Nature, a team of researchers report that 18 active pits or sinkholes have been identified in the comet’s northern hemisphere. The pits range in size from around 100 to 1,000 feet (30-100 meters) in diameter with depths up to 690 feet (210 meters). For the first time ever, individual jets can be traced back to specific pits.
Images processed to bring out faint detail show multiple fountain of dust shooting at high speed from the inside the pits like snow from a snowmaking machine.
“We see jets arising from the fractured areas of the walls inside the pits. These fractures mean that volatiles (easily vaporized materials) trapped under the surface can be warmed more easily and subsequently escape into space,” said Jean-Baptiste Vincent from the Max Planck Institute for Solar System Research, lead author of the study in a recent press release.
Scientists think the pits got there similar to the way sinkholes form on Earth. Pits may form when the ceiling of a subsurface cavity becomes too thin to support its own weight. With nothing below to hold it place, the “roof” collapses, exposing fresh ice below. Much of the comet’s ice is buried under a thin scree of insulating dust and not as likely to vaporize, but pristine, exposed ice can is an easy target for solar heating and sublimates with suddenness. Exiting the hole, it forms a collimated jet of dust and gas.
We know from Rosetta measurements of the comet’s density that it’s as porous as the Syrian border. Those cavernous spaces could provide the right sort of underground terrain conducive to sinkhole formation.
The researchers think they can use the appearance of the sinkholes to age-date different parts of the comet’s surface — the more pits there are in a region, the younger and less processed the surface is. 67P/C-G’s southern hemisphere receives more energy from the Sun than the north and at least for now, shows far fewer pit structures.
The most active pits have steep sides, while the least show softened contours and are filled with dust. It’s even possible that a partial collapse might be the cause of the occasional outbursts when a comet suddenly brightens and enlarges as seen from Earth. Rosetta observed just such an outburst this past April. It’s estimated a typical full pit collapse releases a billion kilograms of material.
Besides sinkholes, Rosetta’s high-resolution OSIRIS camera has picked up the glint of icy boulders. Many are in clusters, some of which lie at the base of cliffs probably after the cliff walls collapsed. Broken boulders would show freshly-exposed ice surfaces. Others are isolated with no connection to their surroundings. Intriguingly, scientists believe they arrived after being lofted into space by active jets. Crazy as it sounds, such levitating boulders are not impossible on the comet, which is only 2.5 miles (4 km) across and has but a minuscule gravitational pull.
As 67P/C-G approaches the Sun, you, me and all the scientists are eager to see what sort of changes will happen before our eyes. Will new pits form with outbursts to follow? Will Rosetta capture pictures of flying boulders? Will fissures in the comet cause it to break to pieces? I vote for all of the above.