Rosetta Spies Shiny Ice And Giant Sinkholes On Comet 67P

Comet 67P/C-G on June 21, 2015. The nucleus is a mixture of frozen ices and dust. As the comet approaches the Sun, sunlight warms its surface, causing the ices to boil away. This gas streams away carrying along large amounts of dust, and together they build up the coma. Copyright: ESA/Rosetta/NavCam – CC BY-SA IGO 3.0
Comet 67P/Churyumov-Gerasimenkon photographed by the orbiting Rosetta spacecraft on June 21, 2015. The nucleus is a mixture of frozen ices and dust. As the comet approaches the Sun, sunlight warms its surface, causing the ices to boil away. This gas streams away carrying along large amounts of dust, and together they build up the coma. Copyright: ESA/Rosetta/NavCam – CC BY-SA IGO 3.0

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.

Active pits detected in the Seth region of the comet. The contrast of the image has been stretched to reveal the details of the fine-structured jets against the shadow of the pit, which are interpreted as dusty streams rising from the fractured wall of the pit. The image was acquired on October 20, 2014 from a distance of 4.3 miles (7 km) from the surface of the comet. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
Active pits seen in the Seth region of the comet. The contrast of the image has been stretched to reveal the details of the fine-structured jets against the shadow of the pit, which scientists think are dusty streams rising from the fractured wall of the pit. The image was taken last October from a distance of 4.3 miles (7 km) from the surface of the comet. ‘Sublimation’ is the technical term for ice that goes directly from solid to gas, skipping the liquid state. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

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.

High-resolution view of an active pit photographed last September from a distance of about 16 miles (26 km) from the comet’s surface in the Seth region. The image scale is about 45 cm a pixel. The Seth_01 pit measures approximately 720 feet (220 m) across and 605 feet (85 m) deep. Note the smooth deposits of dust around the pit. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
Close up view of an active pit photographed last September from a distance of about 16 miles (26 km) from the comet’s surface in the Seth region. The Seth_01 pit measures approximately 720 feet (220 m) across and 605 feet (85 m) deep. Note the smooth deposits of dust around the pit. They may have settled out there after falling back to the surface like a fine, mineral snow. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

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.

Names and locations of the pits found on Comet 67P/C-G. Nearly all are in the "sunnier" northern hemisphere. Credit: Credit: SA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
Names and locations of the 18 pits found on Comet 67P/C-G. Credit: Credit: SA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

“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.

Graphic showing how pits may form through sinkhole collapse in the comet’s dusty surface layer covering a mixture of dust and ices. 1. Heat causes subsurface ices to sublimate (blue arrows), forming a cavity. 2.When the ceiling becomes too weak to support its own weight, it collapses, creating a deep, circular pit (orange arrow). Newly exposed material in the pit walls sublimates (blue arrows). Credit: ESA/Rosetta/J-B Vincent et al (2015)
Graphic showing how pits may form through sinkhole collapse. 1. Heat causes subsurface ices to sublimate (blue arrows), forming a cavity. 2.When the ceiling becomes too weak to support its own weight, it collapses, creating a deep, circular pit (orange arrow). Newly exposed material in the pit walls sublimates (blue arrows). Credit: ESA/Rosetta/J-B Vincent et al (2015)

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.

Pits Ma’at 1, 2 and 3 show differences in appearance that may reflect their history of activity. While pits 1 and 2 are active, no activity has been observed from pit 3. The young, active pits are very steep-sided; pits without any observed activity are shallower and seem to be filled with dust. Middle-aged pits tend to have boulders on their floors from mass-wasting of the sides. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
Pits Ma’at 1, 2 and 3 show differences in appearance that may reflect their history of activity. While pits 1 and 2 are active, no activity has been observed from pit 3. The young, active pits are very steep-sided; pits without any observed activity are shallower and seem to be filled with dust. Middle-aged pits tend to have boulders on their floors from slumping of the pit’s sides.
Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

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.

Don't slip! Bright patches seen in 67P/C-G’s Khepry region appear to be boulders with exposed surfaces of water ice. Scale bar is 50 meters or 164 feet. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
Watch your step! Bright patches seen in 67P/C-G’s Khepry region appear to be boulders with exposed surfaces of water ice. Scale bar is 50 meters or 164 feet. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

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.

2 Responses

    1. astrobob

      Paul,
      I couldn’t agree more. These new photos which not only reveal ice but get to the heart of what powers jets are simply fascinating and add a new dimension to our understanding of comets.

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