Rock Around the Comet Clock with Hubble

These photos, taken on April 4, 2016 over the span of 4 1/2 hours, reveal a narrow, well-defined jet of dust ejected by the comet's icy nucleus. With a diameter of only about a mile, the nucleus is too small for Hubble to see. The jet is illuminated by sunlight and changes direction like the hour hand on a clock as the comet spins on its axis. Credit: NASA, ESA, and J.-Y. Li (Planetary Science Institute)
These photos, taken on April 4, 2016 over the span of 4 1/2 hours, reveal a narrow, well-defined jet of dust ejected by the comet’s icy nucleus. With a diameter of only about a mile, the nucleus is too small for Hubble to see. The jet is illuminated by sunlight and changes direction like the hour hand on a clock as the comet spins on its axis. Credit: NASA, ESA, and J.-Y. Li (Planetary Science Institute)

Remember 252P/LINEAR? This comet appeared low in the morning sky last month and for a short time grew bright enough to see with the naked eye from a dark site. 252P swept closest to Earth on March 21, passing just 3.3 million miles away or about 14 times the distance between our planet and the moon. Since then, it’s been gradually pulling away and fading though it remains bright enough to see in small telescope during late evening hours.

252P LINEAR looks like a big fuzzy ball in this photo taken on April 30. The comet is located in Ophiuchus and rises in the eastern sky at nightfall. At this scale, the jet shown in the Hubble photos is too tiny to see. See map below to find the comet yourself. Credit: Rolando Ligustri
252P LINEAR looks like a big fuzzy ball in this photo taken on April 30. The comet is located in Ophiuchus and rises in the eastern sky at nightfall. At this scale, the jet shown in the Hubble photos is too tiny to see. See map below to find the comet yourself. Credit: Rolando Ligustri

While amateurs set their clocks to catch the comet before dawn, astronomers using NASA’s Hubble Space Telescope captured close-up photos of it two weeks after closest approach. The images reveal a narrow, well-defined jet of dust ejected by the comet’s fragile, icy nucleus spinning like a water jet from a rotating lawn sprinkler. These observations also represent the closest celestial object Hubble has observed other than the moon.

Want to get a good look at a comet's tiny nucleus and its jets of vapor and dust? Get up close in the spaceship. This photo was taken by the European Space Agency's Rosetta probe which has been orbiting Comet 67P/Churyumov-Gerasimenko since the fall of 2014. Credit: ESA
Want to get a good look at a comet’s tiny nucleus and its jets of vapor and dust? Get up close in the spaceship. This photo was taken by the European Space Agency’s Rosetta probe which has been orbiting Comet 67P/Churyumov-Gerasimenko since the fall of 2014. Credit: ESA

Sunlight warms a comet’s nucleus, vaporizing ices below the surface. In a confined space, the pressure of the vapor builds and builds until it finds a crack or weakness in the comet’s crust and blasts into space like water from a whale’s blowhole. Dust and other gases go along for the ride. Some of the dust drifts back down to coat the surface, some into space to be shaped by the pressure of sunlight into a dust tail.

This map shows the path -- marked off every five nights -- of 252P/LINEAR along the border of Ophiuchus and Hercules through the end of June. Bright stars are labeled by Greek letter or number. Stars shown to magnitude 8.5. Diagram: Bob King, source: Chris Marriott's SkyMap
This map shows the path — marked off every five nights at 11:30 p.m. CDT (4:30 UT) — of 252P/LINEAR along the border of Ophiuchus and Hercules through the end of June. Bright stars are labeled by Greek letter or number. Stars shown to magnitude 8.5. Click to enlarge. Diagram: Bob King, source: Chris Marriott’s SkyMap

You can still see 252P/LINEAR if you have a 4-inch or larger telescope. Right now it’s a little brighter than magnitude +9 as it slowly arcs along the border of Ophiuchus and Hercules. With the moon getting brighter and brighter as it fills toward full, tonight and tomorrow night will be best for viewing the comet. After that you’re best to wait till after the May 21st full moon when darkness returns to the evening sky. 252P will spend much of the next couple weeks near the 3rd magnitude star Kappa Ophiuchi, a convenient guidepost for aiming your telescope in the comet’s direction.

Orient yourself on the comet's location by using this map, which shows the sky facing southeast around 11-11:30 p.m. local daylight time in mid-May. Mars and Saturn are excellent guides to help you find Kappa Oph, located very near the comet. Diagram: Bob King , source: Stellarium
Get oriented on where to look for the comet by first using this map, which shows the sky facing southeast around 11-11:30 p.m. local daylight time in mid-May. Mars and Saturn make excellent guides to help you find Kappa Oph, located very near the comet. Diagram: Bob King , source: Stellarium

While you probably won’t see any jets in amateur telescopes, they’re there all the same and helped created this comet’s distinctive and large, fuzzy coma. Happy hunting!

The full sequence of images of the spinning jet in 252P/LINEAR seen by Hubble. Credit: NASA, ESA, and Z. Levay (STScI)
The full sequence of images of the spinning jet in 252P/LINEAR seen by Hubble. Credit: NASA, ESA, and Z. Levay (STScI)

 

Enceladus’ Jets Selectively Power-Up Farther From Saturn

A crowning achievement of the Cassini mission to Saturn is the discovery of water vapor jets spraying out from Enceladus‘ southern pole. First witnessed by the spacecraft in 2005, these icy geysers propelled the little 515-kilometer-wide moon into the scientific spotlight and literally rewrote the mission’s objectives. After 22 flybys of Enceladus during its nearly twelve years in orbit around Saturn, Cassini has gathered enough data to determine that there is a global subsurface ocean of salty liquid water beneath Enceladus’ frozen crust—an ocean that gets sprayed into space from long “tiger stripe” fissures running across the moon’s southern pole.  Now, new research has shown that at least some of the vapor jets get a boost in activity when Enceladus is farther from Saturn.

The gravitational pull of Saturn changes the amount of particles spraying from Enceladus at different points in its orbit. When it's farther from Saturn (left) the plume contains more icy particles and thus appears brighter. Credit: NASA/JPL-Caltech/University of Arizona/Cornell/SSI.
The gravitational pull of Saturn changes the amount of particles spraying from Enceladus at different points in its orbit. When it’s farther from Saturn (left) the plume contains more icy particles and thus appears brighter. Credit: NASA/JPL-Caltech/University of Arizona/Cornell/SSI.

By measuring the changes in brightness of a distant background star as Enceladus’ plumes passed in front of it in March 2016, Cassini observed a significant increase in the amount of icy particles being ejected by one particular jet source.

"Baghdad Sulcus," one of Enceladus' plume sources, imaged by Cassini during a close pass in Nov. 2011. (NASA/JPL/SSI)
“Baghdad Sulcus,” one of Enceladus’ plume sources, imaged by Cassini during a close pass in Nov. 2011. (NASA/JPL/SSI)

Named “Baghdad 1,” the jet went from contributing 2% of the total vapor content of the entire plume area to 8% when Enceladus was at the farthest point in its slightly-eccentric orbit around Saturn. This small yet significant discovery indicates that, although Enceladus’ plumes are reacting to morphological changes to the moon’s crust due to tidal flexing, it’s select small-scale jets that are exhibiting the most variation in output (rather than a simple, general increase in outgassing across the full plumes.)

“How do the tiger stripe fissures respond to the push and pull of tidal forces as Enceladus goes around its orbit to explain this difference? We now have new clues!” said Candice Hansen, senior scientist at the Planetary Science Institute and lead planner of the study. “It may be that the individual jet sources along the tiger stripes have a particular shape or width that responds most strongly to the tidal forcing each orbit to boost more ice grains at this orbital longitude.”

The confirmation that Enceladus shows an increase in overall plume output at farther points from Saturn was first made in 2013.

Whether this new finding means that the internal structure of the fissures is different than what scientists have suspected or some other process is at work either within Enceladus or in its orbit around Saturn still remains to be determined.

“Since we can only see what’s going on above the surface, at the end of the day, it’s up to the modelers to take this data and figure out what’s going on underground,” said Hansen.

Sources: Planetary Science Institute and NASA/JPL

Enceladus' water ice plumes were first observed by Cassini in 2005. (NASA/JPL/SSI)
Enceladus’ now-famous water ice plumes were first observed by Cassini in 2005. (NASA/JPL/SSI)

How to Find Rosetta’s Comet In Your Telescope

How would you like to see one of the most famous comets with your own eyes? Comet 67P/Churyumov-Gerasimenko plies the morning sky, a little blot of fuzzy light toting an amazing visitor along for the ride — the Rosetta spacecraft. When you look at the coma and realize a human-made machine is buzzing around inside, it seems unbelievable. 

Comet 67P/Churyumov-Gerasimenko plows through a rich star field in Gemini on the morning of August 19, 2015. Photos show a short, faint tail to the west not visible to the eye in most amateur telescopes. Credit: Efrain Morales
Comet 67P/Churyumov-Gerasimenko plows through a rich star field in Gemini on the morning of August 20, 2015. Photos show a short, faint tail to the west not visible to the eye in most amateur telescopes. Credit: Efrain Morales

If you have a 10-inch or larger telescope, or you’re an experienced amateur with an 8-inch and pristine skies, 67P is within your grasp. The comet glows right around magnitude +12, about as bright as it will get this apparition. Periodic comets generally appear brightest around and shortly after perihelion or closest approach to the Sun, which for 67P/C-G occurred back on August 13.

The surface of Comet 67P/C-G is extensively fractured likely related to the intense freeze-thaw cycle that occurs during the heat of perihelion vs. the chill experienced in the outer part of its orbit. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
The surface of Comet 67P/C-G is extensively fractured due to loss of volatile ices, the expansion and contraction of the comet from solar heating and bitter cold and possibly even tectonic forces. The smaller polygonal shapes outlined by fractures in the lower right photo are just 6-16 feet (2-5 meters) across. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

You’ll be looking for a small, 1-arc-minute-diameter, compact, circular patch of nebulous light shortly before dawn when it’s highest in the east. Rosetta’s Comet will spend the remainder of August slicing across Gemini the Twins north of an nearly parallel to the ecliptic. I spotted 67P/C-G for the first time this go-round about a week ago in my 15-inch (37 cm) reflector. While it appears like a typical faint comet, thanks to Rosetta, we know this particular rough and tumble mountain of ice better than any previous comet. Photographs show rugged cliffs, numerous cracks due to the expansion and contraction of ice, blowholes that serve as sources for jets and smooth plains blanketed in fallen dust.

Geysers of dust and gas shooting off the comet's nucleus are called jets. The material they deliver outside the nucleus builds the comet's coma. Credit: ESA/Rostta/NAVCAM
Geysers of dust and gas shooting off the comet’s nucleus are called jets. The material they deliver outside the nucleus builds the comet’s coma. Credit: ESA/Rostta/NAVCAM

The jets are geyser-like sprays of dust and gas that loft grit and rocks from the comet’s interior and surface into space to create a coma or temporary atmosphere. This is what you’ll see in your telescope. And if you’re patient, you’ll even be able to catch this glowing tadpole on the move. I was surprised at its speed. After just 20 minutes, thanks to numerous field stars that acted as references, I could easily spot the comet’s eastward movement using a magnification of 245x.

Facing east around 4 a.m. local time in late August, you'll see the winter constellations Gemini and Orion. 67P/C-G's path is shown through
Facing east around 4 a.m. local time in late August, you’ll see the winter constellations Gemini and Orion. 67P/C-G’s path is shown through early September. Brighter stars near the path are labeled. Time shown is 4 a.m. CDT. Use this map to get oriented and then switch to the one below for telescope use. Source: Chris Marriott’s SkyMap

Tomorrow morning, 67P/C-G passes very close to the magnitude +5 star Omega Geminorum. While this will make it easy to locate, the glare may swamp the comet. Set your alarm for an hour before dawn’s start to allow time to set up a telescope, dark-adapt your eyes and track down the field where the comet will be that morning using low magnification.

Once you’ve centered 67P/C-G’s position, increase the power to around 100x-150x and use averted vision to look for a soft, fuzzy patch of light. If you see nothing, take it to the next level (around 200-250x) and carefully search the area. The higher the magnification, the darker the field of view and easier it will be to spot it.

Detailed map showing the comet's path through central Gemini daily August 21-28, 2015 around 4 a.m. CDT. Brighter stars are marked with Greek letters and numbers. "48" = 48 Geminorum. Source: Chris Marriott's SkyMap
Detailed map showing the comet’s path through central Gemini daily August 21-28, 2015 around 4 a.m. CDT. Brighter stars are marked with Greek letters and numbers. “57”= 57 Geminorum. North is up, east to the left and stars to magnitude +13.5. Click for a larger version you can print out. Source: Chris Marriott’s SkyMap

Besides being relatively faint, the comet doesn’t get very high in the east before the onset of twilight. Low altitude means the atmosphere absorbs a share of the comet’s light, making it appear even fainter. Not that I want to dissuade you from looking! There’s nothing like seeing real 67P photons not to mention the adventure and sense of accomplishment that come from finding the object on your own.

As we advance into late summer and early fall, 67P/C-G will appear higher up but also be fading. Now through about August 27 and again from September 10-24 will be your best viewing times. That’s when the Moon’s absent from the sky.

Given the comet’s current distance from Earth of 165 million miles and apparent visual size of just shy of 1 arc minute, the coma measures very approximately 30,000 miles across. Rosetta orbits the comet’s 2.5-mile-long icy nucleus at a distance of about 115 miles (186 km), meaning it’s snug up against the nuclear center from our point of view on the ground.

If you do find and follow 67P/C-G, consider sharing your observations with the Pro-Amateur Collaborative Astronomy (PACA) campaign to help increase our knowledge of its behavior. Interested? Sign up HERE.

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

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

Rosetta’s Comet Keeps On Jetting Even After the Sun Goes Down

67P/Churyumov-Gerasimenko certainly isn’t a comet that dreads sundown. Images acquired by the OSIRIS instrument aboard ESA’s Rosetta spacecraft in April 2015 reveal that some of the comet’s dust jets keep on firing even after the Sun has “set” across those regions. This shows that, as the comet continues to approach its August perihelion date, it’s now receiving enough solar radiation to warm deeper subsurface materials.

“Only recently have we begun to observe dust jets persisting even after sunset,” said OSIRIS Principal Investigator Holger Sierks from the Max Planck Institute for Solar System Research.

The image above was captured by OSIRIS on April 25 and shows active jets near the center, originating from shadowed areas on the comet’s smaller “head” lobe. The region is called Ma’at – see maps of 67P’s regions here and here.

(Also it looks kind of like an overexposed image of a giant angry lemming. But that’s pareidolia for you.)

Detail of the active jets. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
Detail of the active jets. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

It’s thought that the comet has now come close enough to the Sun – 220.8 million kilometers, at the time of this writing – that it can store heat below its surface… enough to keep the sublimation process going within buried volatiles well after it rotates out of direct solar illumination.

Read more: What Are Comets Made Of?

Comet 67P and Rosetta (and Philae too!) will come within 185.9 million km of the Sun during perihelion on Aug. 13, 2015 before heading back out into the Solar System. Find out where they are now.

Source: ESA’s Rosetta blog

Hubble Captures a Collision in a Black Hole’s “Death Star” Beam

Even the Empire’s planet-blasting battle station has nothing compared to the immense energy being fired from the heart of NGC 3862, a supermassive black hole-harboring elliptical galaxy located 300 million light-years away.

And while jets of high-energy plasma coming from active galactic nuclei have been imaged before, for the first time activity within a jet has been observed in optical wavelengths, revealing a quite “forceful” collision of ejected material at near light speeds.

Using archived image data acquired by Hubble in 1994, 1996, and 2002 combined with new high-resolution images acquired in 2014, Eileen Meyer at the Space Telescope Science Institute (STScI) in Baltimore, Maryland identified movement in visible clumps of plasma within the jet emitted from the nucleus of NGC 3862 (aka 3C 264). One of the outwardly-moving larger clumps could be seen gaining on a slower, smaller one in front of it and the two eventually collide, creating a shockwave that brightens the resulting merged mass dramatically.

Such a collision has never been witnessed before, and certainly not thousands of light-years out from the central supermassive black hole.

Close-up image of the jet as seen in 2014. Credit:  NASA, ESA, and E. Meyer (STScI).
Close-up image of the jet as seen in 2014. Credit: NASA, ESA, and E. Meyer (STScI).

“Something like this has never been seen before in an extragalactic jet,” Meyer said. “This will allow us a very rare opportunity to see how the kinetic energy of the collision is dissipated into radiation.”

Jets like this are created when infalling material around an active (that is, “feeding”) supermassive black hole gets caught up in its powerful spinning and twisting magnetic fields. This accelerates the material even further and, rather than permitting it to descend down past the black hole’s event horizon, results in it getting shot out into space at velocities close to the speed of light.

Read more: Black Hole Jets May Be Molded by Magnetism

When material approaches the black hole in even amounts the jets are fairly consistent. But if the inflow is uneven, the jets can consist of clumps or knots traveling outward at different speeds.

Because of the motion of the galaxy itself related to our own, the speed of the clumps can appear to actually move faster than the speed of light, especially when – as seen in NGC 3862 – a large clump has already paved the way within the jet. In reality the light speed limit has not been broken, but the apparent superluminal motion so far from the SMBH indicates that the material was ejected extremely energetically.

It’s expected that the combined clusters of material will continue to brighten over the next several decades.

You can see a video of the observations below, and watch a Google+ Hangout with Hubble team members about these observations here.

Source: Hubble news center

Dust Whirls, Swirls and Twirls at Rosetta’s Comet

Tell me this montage shouldn’t be hanging in the Lourve Museum. Every time I think I’ve seen the “best image” of Rosetta’s comet, another one takes its place. Or in this case four! When you and I look at a comet in our telescopes or binoculars, we’re seeing mostly the coma, the bright, fluffy head of the comet composed of dust and gas ejected by the tiny, completely invisible, icy nucleus.

As we examine this beautiful set of photos, we’re  privileged to see  the individual fountains of gas and dust that leave the comet to create the coma. Much of the outgassing comes from the narrow neck region between the two lobes. 

This photo taken on Feb. 27 shows the comet with peacock-like display of dusty jets. Below center is a streak that may be a dust particle that traveled during the exposure. Credits:
This photo taken on Feb. 27 shows the comet with peacock-like display of dusty jets. Below center is a streak that may be a dust particle that traveled during the exposure. Other small white spots are also likely dust or bits of comet that have broken off. Credits: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

All were taken between February 25-27 at distances around 50-62 miles (80 to 100 km) from the center of Comet 67P/Churyumov-Gerasimenko. Looking more closely, the comet nucleus appears to be “glowing” with a thin layer of dust and gas suspended above the surface. In the lower left Feb. 27 image, a prominent streak is visible. While this might be a cosmic ray zap, its texture hints that it could also be a dust particle captured during the time exposure. Because it moved a significant distance across the frame, the possible comet chunk may be relatively close to the spacecraft. Just a hunch.

Another close-up individual image from Rosetta's NAVCAM. Credit:
Another close-up individual image from Rosetta’s NAVCAM. Credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

While most of Rosetta’s NAVCAM images are taken for navigation purposes, these images were obtained to provide context in support of observations performed at the same time with the Alice ultraviolet (UV) imaging spectrograph on Rosetta. Observing in ultraviolet light, Alice determines the composition of material in coma, the nucleus and where they interface. Alice will also monitor the production rates of familiar molecules like H2O, CO (carbon monoxide) and CO2 as they leave the nucleus and enter 67P’s coma and tail.

Alice makes its observations in UV light through a long, narrow slit seen here superimposed on a graphic of comet 67P/ C-G. Credit: ESA/NASA
Alice makes its observations in UV light through a long, narrow slit seen here superimposed on a graphic of comet 67P/ C-G. Credit: ESA/NASA

From data collected so far, the Alice team has discovered that the comet is unusually dark in the ultraviolet, and that its surface shows no large water-ice patches. Water however has been detected as vapor leaving the comet as it’s warmed by the Sun. The amount varies as the nucleus rotates, but the last published measurements put the average loss rate at 1 liter (34 ounces) per second with a maximum of 5 liters per second. Vapors from sublimating carbon monoxide and carbon dioxide ice have also been detected. Sometimes one or another will dominate over water, but overall, water remains the key volatile material outgassed in the greatest quantity.

Particularly striking and collimated jets emerge from the comet's Hathor region in the neck between the two lobes. Credit:
Particularly striking and collimated jets emerge from the comet’s shadowed Hathor region between the two lobes. Credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
A separate image taken on Feb. 28. According to ESA, The curved shape of the outflowing material likely results from a combination of several factors, including the rotation of the comet, differential flows of near-surface gas, and gravitational effects arising due to the uneven shape of the comet. The viewing perspective of the image might also distort the true shape of the outflowing material. Credit:
Look at those spirals! In this separate image, taken Feb. 28, ESA suggests the curved shape of the outflowing material likely results from a combination of several factors, including the rotation of the comet, differential flows of near-surface gas, and gravitational effects arising due to the uneven shape of the comet. The viewing perspective of the image might also distort the true shape of the outflowing material. Credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

That and dust. In fact, 67P is giving off about twice as much dust as gas. We see the comet’s dual emissions by reflected sunlight, but because there’s so much less material in the jets than what makes up the nucleus, they’re fainter and require longer exposures and special processing to bring out without seriously overexposing the comet’s core.

67P’s coma will only grow thicker and more intense as it approaches perihelion on August 13.

Rosetta’s Comet Really “Blows Up” in Latest Images

First off: no, comet 67P/Churyumov-Gerasimenko is not about to explode or disintegrate. But as it steadily gets nearer to the Sun the comet’s jets are getting more and more active and they’re putting on quite a show for the orbiting Rosetta spacecraft! Click the image for a jeterrific hi-res version.

The images above were captured by Rosetta’s NavCam on Jan. 31 and Feb. 3 from a distance of about 28 km (17 miles). Each is a mosaic of four separate NavCam acquisitions and they have been adjusted and tinted in Photoshop by yours truly to further enhance the jets’ visibility. (You can view the original image mosaics and source frames here and here.)

These dramatic views are just a hint at what’s in store; 67P’s activity will only be increasing in the coming weeks and months and, this weekend, Rosetta will be swooping down for an extreme close pass over its surface!

Detail of 67P from the Feb. 3 NavCam image
Detail of 67P from the Feb. 3 NavCam image

This Saturday, Feb. 14, Rosetta will be performing a very close pass of the comet’s nucleus, soaring over the Imhotep region at an altitude of only 6 km (3.7 miles) at 12:41 UTC. This will allow the spacecraft to closely image the comet’s surface, as well as investigate the behavior of its jets and how they interact with its developing coma.

“The upcoming close flyby will allow unique scientific observations, providing us with high-resolution measurements of the surface over a range of wavelengths and giving us the opportunity to sample – taste or sniff – the very innermost parts of the comet’s atmosphere,” said Rosetta project scientist Matt Taylor.

Read more about Rosetta’s Valentine’s Day close pass here and watch an animation of how it will be executed below.

Source: ESA

UPDATE: Here’s an image of 67P captured by Rosetta on Feb. 6 from a distance of 124 km (77 miles) as it moved into a higher orbit in preparation of its upcoming close pass. It’s the first single-frame image of the comet since leaving bound orbits.

The image has been processed to add a contrasting tint and enhance jet activity. See the original image here.

Single-frame NavCam image of comet 67P/C-G imaged on Feb. 6, 2015. Credits: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0. Edited by Jason Major.
Single-frame NavCam image of comet 67P/C-G imaged on Feb. 6, 2015. Credits: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0. Edited by Jason Major.

Rosetta Sees Fascinating Changes in Comet 67P

It only makes sense. Sunlight heats a comet and causes ice to vaporize. This leads to changes in the appearance of surface features. For instance, the Sun’s heat can gnaw away at the ice on sunward-facing cliffs, hollowing them out and eventually causing them to collapse in icy rubble. Solar heating can also warm the ice that’s beneath the surface.

When it becomes a vapor, pressure can build up, cracking the ice above and releasing sprays of gas and dust as jets. New images compared to old suggest the comet’s surface is changing as it approaches the Sun.

Take a look at this photo taken on December 9 of a part of the neck of the comet called Hapi. I've labeled a boulder and three prominent cracks. Sunlight is coming from top and behind in this image. Compare to the photo below shot on Jan. 8. Credit: ESA/Rosetta/Navcam
Take a look at this photo taken on December 9 of a part of the neck of the comet called Hapi. I’ve labeled a boulder and three prominent cracks. Sunlight is coming from top and behind in this image. Compare to the photo below shot on Jan. 8. Credit: ESA/Rosetta/Navcam

Recent photos taken by the Rosetta spacecraft reveal possible changes on the surface of 67P/Churyumov-Gerasimenko that are fascinating to see and contemplate. In a recent entry of the Rosetta blog, the writer makes mention of horseshoe-shaped features in the smooth neck region of the comet called “Hapi”. An earlier image from Jan. 8 may show subtle changes in the region compared to a more recent image from Jan. 22. We’ll get to those in a minute, but there may be examples of more vivid changes.

Although the viewing angle and lighting geometry has changed some between this photo, taken Jan. 8, and the one above, it certainly appears that the three cracks have virtually disappeared in a month's time. The same boulder is flagged in both photos. Credit: ESA/Rosetta/Navcam
Although the viewing angle and lighting geometry has changed some between this photo, taken Jan. 8, and the one above, it certainly appears that the three cracks have virtually disappeared in a month’s time. The same boulder is flagged in both photos. Credit: ESA/Rosetta/Navcam

I did some digging around and found what appears to be variations in terrain between photos of the same Hapi region on Dec. 9 and Jan.8. Just as the other writer took care to mention, viewing angle and lighting are not identical in the images. That has to be taken into account when deciding whether a change in a feature is real or due to change in lighting or perspective.

Side by side comparison of the two image from Dec. 9, 2014 (left) and Jan. 8, 2015. Credit: ESA/Rosetta/Navcam
Side by side comparison of the two image from Dec. 9, 2014 (left) and Jan. 8, 2015. Credit: ESA/Rosetta/Navcam

But take a look at those cracks in the December image that appear to be missing in January’s. The change, if real, is dramatic. If they did disappear, how? Are they buried in dust released by jets that later drifted back down to the surface?

Comparison of Jan. 22 and Jan. 9 photos of the "horseshoes" or depressions in 67P's Hapi region. Outside of differences in lighting, do you see any changes? Credit: ESA/Rosetta/Navcam
Comparison of Jan. 22 and Jan. 9 photos of the “horseshoes” or depressions in 67P’s Hapi region. Outside of differences in lighting, do you see any changes? Credit: ESA/Rosetta/Navcam

Now back to those horseshoe features. Again, the viewing angles are somewhat different, but I can’t see any notable changes in the scene. Perhaps you can. While comets are expected to change, it’s exciting when it seems to be happening right before your eyes.

Four-image mosaic shows the overall view of the comet on January 22 photographed 17.4 miles (28 km) from its center. The larger of the two lobes is at left; Hapi is the smooth region at the transition between the lobes. Credit: ESA/Rosetta/Navcam
Four-image mosaic shows the comet overall on January 22 from a distance of 17.4 miles (28 km) from its center. The larger of the two lobes is at left; Hapi is the smooth region at the transition between the lobes. Credit: ESA/Rosetta/Navcam

Rosetta’s Comet in Thrilling 3-D

She’s gonna blow! Rosetta’s navigation camera recently grabbed our best view yet of the geyser-like jets spraying from the nucleus of Comet 67P/Churyumov-Gerasimenko. They were taken on September 26 as the spacecraft orbited the comet at a distance of just 16 miles (26 km) and show jets of water vapor and dust erupting from several discrete locations beneath the surface along the neck region of the comet’s nucleus.  Mattias Malmer, a 3D technical director, created the spectacular 3D views by draping the navigation camera images over a 3D model of the comet and then photographing it from two slightly different perspectives.

Jets of gas and dust are seen escaping comet 67P/C-G on September 26 in this four-image mosaic. Click to enlarge. Credit: ESA/Rosetta/NAVCAM
Jets of gas and dust are seen escaping comet 67P/C-G on September 26 in this four-image mosaic. Click to enlarge. Credit: ESA/Rosetta/NAVCAM

Jets form when the sun warms the comet’s coal-black surface, causing ices beneath to sublimate or change directly from solid to gas without becoming liquid. This is possible because of the near-zero atmospheric pressure at the comet. Pressure builds in the pockets of gas until they find escape through cracks or pores as plume-like jets. Comet dust along with the gas fashions the coma and tail over time. Something similar happens when you shake up a bottle of champagne and then loosen the cork. Trapped carbon dioxide (what makes the “fizz”) blasts the cork across the room.


Comet Churyumov-Gerasimenko rotating from darkness into light. (Mattias Malmer) 

If you liked the still images, check out these videos by Malmer. He used the same draping technique and then animated the stills. Be sure to stop by his Cascade of Light blog for more images and videos when you get a chance.


Comet Churyumov-Gerasimenko rotating in 3D (Mattias Malmer)

I saved the best for last. What majesty!


3D rotation of Comet 67P/C-G with jets (Mattias Malmer)