Example of a cluster of bright spots on Comet 67P/Churyumov-Gerasimenko found in the Khepry region. The bright patches are thought to be exposures of water-ice. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Rosetta’s Comet Sparkles with Ice, Blows Dust From Sinkholes

Article Updated: 23 Dec , 2015

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Comet 67P/C-G may be tiny at just 2.5 miles (4 km) across, but its diverse landscapes and the processes that shape them astound. To say nature packs a lot into small packages is an understatement.

In newly-released images taken by Rosetta’s high-resolution OSIRIS science camera, the comet almost seems alive. Sunlight glints off icy boulders and pancaking sinkholes blast geysers of dust into the surrounding coma.

Examples of six different bright patches identified on the surface of Comet 67P/Churyumov-Gerasimenko in OSIRIS narrow-angle camera images acquired in September 2014. The insets point to the broad regions in which they were discovered (not to specific locations). In total, 120 bright regions, including clusters of bright features, isolated features and individual boulders, were identified in images acquired during September 2014 when the spacecraft was between 20-50 km from the comet center. The false colour images are red-green-blue composites assembled from monochrome images taken at different times and have been stretched and slightly saturated to emphasis the contrasts of colour such that dark terrains appear redder and bright regions appear significantly bluer compared with what the human eye would normally see. Credit: SA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Examples of six different bright patches identified on the surface of 67P/C-G in images taken last September when Rosetta was 20-50 km from the comet. The center panel points to the broad regions in which they were discovered (not specific locations). 120 bright regions, including clusters of bright features, isolated features and individual boulders, were seen. The false color images were taken at different times and have been stretched and slightly saturated to emphasis color contrasts so that dark terrains appear redder and bright regions appear significantly bluer compared with what the human eye would normally see. Credit: SA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

More than a hundred patches of water ice some 6 to 15 feet across (a few meters) dot the comet’s surface according to a  new study just published in the journal Astronomy & Astrophysics. We’ve known from previous studies and measurements that comets are rich in ice. As they’re warmed by the Sun, ice vaporizes and carries away embedded dust particles that form the comet’s atmosphere or coma and give it a fuzzy appearance.

Examples of icy bright patches seen on Comet 67P/Churyumov-Gerasimenko during September 2014. The two left hand images are subsets of OSIRIS narrow-angle camera images acquired on 5 September; the right hand images were acquired on 16 September. During this time the spacecraft was about 30-40 km from the comet center. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Examples of icy bright patches and clusters seen in September 2014. The two left hand images are crops of OSIRIS narrow-angle camera images acquired on September 5; the right hand images are from September 16. During this time the spacecraft was about 19-25 miles (30-40 km) from the comet center. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Not all that fine powder leaves the comet. Some settles back to the surface, covering the ice and blackening the nucleus. This explains why all the comets we’ve seen up close are blacker than coal despite being made of material that’s as bright as snow.

True brightness comparisons of four different Solar System bodies. At top are Saturn's moon Enceladus, its ice-covered surface making it one of the brightest objects in the Solar System, and Earth. At bottom are the Moon and Comet 67P. Credit: ESA

True brightness comparisons of four different Solar System bodies. At top are Saturn’s moon Enceladus and Earth. At bottom are the Moon and Comet 67P. Enceladus’ ice-covered surface makes it one of the brightest objects in the Solar System. In contrast, 67P is one of the darkest, its icy surface coated in dark mineral dust and organic compounds. Credit: ESA

Scientists have identified 120 regions on the surface of Comet 67P/Churyumov-Gerasimenko that are up to ten times brighter than the average surface brightness. Some are individual boulders, while others form clusters of bright specks. Seen in high resolution, many appear to be boulders with exposures of ice on their surfaces; the clusters are often found at the base of overhanging cliffs and likely got there when cliff walls collapsed, sending an avalanche of icy rocks downhill and exposing fresh ice not covered by dark dust.

An individual boulder about 12 feet across with bright patches on its surface in the Hatmehit region. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

An individual boulder about 12 feet across with bright patches on its surface in the Hatmehit region. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

More intriguing are the isolated boulders found here and there that appear to have no relation to the surrounding terrain.  Scientists think they arrived George Jetson style when they were jetted from the comet’s surface by the explosive vaporization of ice only to later land in a new location. The comet’s exceedingly low gravity makes this possible. Let that image marinate in your mind for a moment.

All the ice-glinting boulders seen thus far were found in shadowed regions not exposed to sunlight, and no changes were observed in their appearance over a month’s worth of observations.

“Water ice is the most plausible explanation for the occurrence and properties of these features,” says Antoine Pommerol of the University of Bern and lead author of the study.

How do we know it’s water ice and not CO2 or some other form of ice? Easy. When the observations were made, water ice would have been vaporizing at the rate of 1 mm per hour of solar illumination. By contrast, carbon monoxide or carbon dioxide ice, which have much lower freezing points, would have rapidly sublimated in sunlight. Water ice vaporizes much more slowly in comparison.

Lab tests using ice mixed with different minerals under simulated sunlight revealed that it only took a few hours of sublimation to produce a dust layer only a few millimeters thick. But it was enough to conceal any sign of ice. They also found that small chunks of dust would sometimes break away to expose fresh ice beneath.

“A 1 mm thick layer of dark dust is sufficient to hide the layers below from optical instruments,” confirms Holger Sierks, OSIRIS principal investigator at the Max Planck Institute for Solar System Research.

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/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

It appears then that Comet 67P’s surface is mostly covered in dark dust with small exposures of fresh ice resulting from changes in the landscape like crumbling cliffs and boulder-tossing from jet activity. As the comet approaches perihelion, some of that ice will become exposed to sunlight while new patches may appear. You, me and the Rosetta team can’t wait to see the changes.

High-resolution view of active regions in Seth as seen with Rosetta’s OSIRIS narrow-angle camera on 20 September 2014 from a distance of about 26 km from the surface. The image scale is about 45 cm/pixel. The Seth_01 pit is seen close to centre and measures approximately 220 m across and 185 m deep. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

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

Ever wonder how a comet gets its jets? In another new study appearing 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. These roughly circular holes appear to be the source of the elegant jets like those seen in the photo above. The pits range in size from around 100 to 1,000 feet (30-100 meters) across with depths up to 690 feet (210 meters). For the first time ever, individual jets can be traced back to specific pits.

In specially processed photos, material can be seen streaming from inside pit walls like snow blasting from a snowmaking machine. Incredible!

Active pits detected in the Seth region of Comet 67P/Churyumov¬Gerasimenko can be seen in the lower right portion of this OSIRIS wide-angle camera image. The contrast of the image has been deliberately 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 20 October 2014 from a distance of 7 km from the surface of the comet. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

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

“We see jets arising from the fractured areas of the walls inside the pits. These fractures mean that volatiles 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.

Similar to the way sinkholes form on Earth, scientists believe pits form when the ceiling of a subsurface cavity becomes too thin to support its own weight. With nothing below to hold it place, it collapses, exposing fresh ice below which quickly vaporizes. Exiting the hole, it forms a collimated jet of dust and gas.

Pits Ma’at 1, 2 and 3 on Comet 67P/Churyumov–Gerasimenko 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 particularly steep-sided, whereas pits without any observed activity are shallower and seem to be filled with dust. Middle-aged pits tend to exhibit boulders on their floors from mass-wasting of the sides. The image was taken with the OSIRIS narrow-angle camera from a distance of 28 km from the comet surface. 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 mass-wasting of the sides.
Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

The paper’s authors suggest three ways for pits to form:

* The comet may contain voids that have been there since its formation. Collapse could be triggered by either vaporizing ice or seismic shaking when boulders ejected elsewhere on the comet land back on the surface.
* Direct sublimation of pockets of volatile (more easily vaporized) ices like carbon dioxide and carbon monoxide below the surface as sunlight warms the dark surface dust, transferring heat below.
* Energy liberated by water ice changing its physical state from amorphous to its normal crystalline form and stimulating the sublimation of the surrounding more volatile carbon dioxide and carbon monoxide ices.

Graphic explaining how Comet 67P/Churyumov–Gerasimenko’s pits may form through sinkhole collapse. The graphic shows a 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 (3, red arrow). Newly exposed material in the pit walls sublimates, accounting for the observed activity (3, blue arrows).

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)

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 there is. They point to 67P/C-G’s southern hemisphere which receives more energy from the Sun than the north and at least for now, shows no 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. And these holes can really kick out the dust! It’s estimated a typical full pit collapse releases a billion kilograms of material.

With Rosetta in great health and perihelion yet to come, great things lie ahead. Maybe we’ll witness a new sinkhole collapse, an icy avalanche or even levitating boulders!

Sources: 1, 2

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UFOsMOTHER
Member
UFOsMOTHER
July 3, 2015 6:55 AM

Yes Bob exciting times ahead and the closer 67P gets to the Sun the more it will change I hope the” head” will break away from the “body” and we capture this happening and relay the pictures back to Earth before our wonderful spacecraft expire and is no more, well done ESA and Good Luck

mewo
Member
mewo
July 3, 2015 7:06 AM

If that did happen, I wonder which half ESA would choose to stay with. Probably the head, because that is where Philae is.

Aqua4U
Member
July 3, 2015 2:09 PM

Who me? I’m liking: “Lab tests using ice mixed with different minerals under simulated sunlight revealed that it only took a few hours of sublimation to produce a dust layer only a few millimeters thick. But it was enough to conceal any sign of ice.” This and the fact that methane and ethane ices have been shown to darken when exposed to sunlight, I find fascinating. The combination of the two could be what causes Ceres bright impact scars to disappear or rapidly fade?

Ps8
Member
Ps8
July 3, 2015 10:51 AM
“…vaporizing at the rate of 1 mm per hour of solar illumination.” I get the distinct impression that comets LEAK vapor, rather than “blast” out “jets” of material. It only APPEARS to “jet” out, due to the lack of significant impeding atmosphere. Likewise, “the explosive vaporization of ice” is only relatively speaking. Such “explosions” are likely to be infrequent, and of very little force. The only reason they can occasionally loft large boulders is due to the extremely low gravity. Just a little nudge will start even a large boulder moving, and its inertia will carry it quite aways before the low gravitational conditions can haul it back to the surface. (Consider the incredible hours-long and extremely slow… Read more »
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