Since August 2014, the Rosetta spacecraft has been orbiting Comet 67P/Churyumov-Gerasimenko, providing an unprecedented look at an active comet as it moves closer to the Sun in its orbit. As expected, the level of activity increased the closer the comet was to the Sun, with jets of water vapor, gas, and dust becoming bigger and more prominent. The comet reached perihelion, the closest point to the Sun on its orbit, on Aug. 13, 2015. For the first time ever, a spacecraft is observing this activity close-up, as it happens. But now, scientists have been noticing other dramatic and rapid changes on the comet’s surface as well, which haven’t been explained yet.
The new changes were first noticed in June 2015, two months before perihelion, on the surface of the comet’s nucleus. The significant changes were seen in one region in particular, called Imhotep, which features smooth terrain covered by fine-grained material and large boulders. Imhotep is located on the largest “lobe” of the peanut-shaped cometary nucleus.
According to Olivier Groussin, an astronomer at the Laboratoire d’Astrophysique de Marseille, France, OSIRIS Co-Investigator and lead author of the new study: “We had been closely monitoring the Imhotep region since August 2014, and as late as May 2015, we had detected no changes down to scales of a tenth of a meter. Then one morning we noticed that something new had happened: the surface of Imhotep had started to change dramatically. The changes kept going on for quite a while.”
So what was happening? On June 3, Rosetta first noticed a new, roundish feature starting to form in Imhotep, using its OSIRIS narrow-angle camera. Later in June, the feature was observed to be growing in size, and then was joined by a second similar round feature. They kept growing in size and by July 2 had reached diameters of about 721 feet (220 meters) and 459 feet (140 meters) respectively. Then a third similar feature began to appear as well; shortly after that, all three of the circular features merged together, followed by the appearance of yet two more smaller but again similar features.
“These spectacular changes are proceeding extremely rapidly, with the rims of the features expanding by a few tens of centimeters per hour. This highlights the complexity of the physical processes involved,” added Groussin.
The changes were fascinating, but what was causing them? One simple explanation is that the surface material is weak, which could make rapid erosion possible. Another more involved process could be the crystallisation of amorphous ice, or the destabilization of so-called “clathrates” (a lattice of one kind of molecule containing other molecules), which could liberate energy and drive the expansion of the features at faster speeds than would normally occur. In this process, there would also be increased rates of gas outflow, including H2O, CO2, or CO.
Whatever the specific process, the sublimation of volatiles must be involved, according to the scientists, driven by sunlight hitting the comet. Newly exposed ice could be seen on the rims of the growing circular features, in color images taken by the spacecraft.
However, the rate of expansion of the features is still unexplained; they were observed to expand by a few tens of centimeters per hour, but computer models of sunlight-driven sublimation predict erosion rates of just a few centimeters per hour. Therefore, there must be some other mechanism(s) involved as well. Another clue is that the Imhotep region is close to the comet’s equator, and so receives maximum amounts of sunlight.
The OSIRIS images were also examined for signs of increased dust from the regions where the changes were observed, but none was seen. It is considered unlikely that many larger micron-sized dust particles were released, but it is possible that a smaller number of larger (millimeter-sized) particles were released, which would reflect light less and be harder to detect. Also, much of the dust released could have immediately fallen back to the surface.
Rosetta was the first spacecraft to ever closely observe a comet during perihelion. The exact moment came at 02:03 GMT when Comet 67P passed within 115 million miles (186 million kilometers) of the Sun. It is during closest approach to the Sun that comets are most active, with jets of gas and dust erupting from their surfaces like celestial volcanoes. Perihelion occurred nine months after the Philae lander was deployed, and after a bit of a bumpy ride, landed on the comet’s surface. Philae actually landed three times, landing and then bouncing, before finally coming to rest.
Last June, Philae awakened from a period of hibernation, resuming its science activities on the surface. Also, as Comet 67P became more active, Rosetta had to be moved to an orbit farther away from the comet for safety reasons. As explained by Sylvain Lodiot, ESA’s spacecraft operations manager:
“In recent days, we have been forced to move even further away from the comet. We’re currently at a distance of between 201 miles (325 km) and 211 miles (340 km) this week, in a region where Rosetta’s star trackers can operate without being confused by excessive dust levels – without them working properly, Rosetta can’t position itself in space.”
Activity is expected to remain high over the next several weeks, allowing Rosetta to make further observations of a comet at its “prime.” Nicolas Altobelli, acting Rosetta project scientist, emphasized, “Activity will remain high like this for many weeks, and we’re certainly looking forward to seeing how many more jets and outburst events we catch in the act, as we have already witnessed in the last few weeks.”
As the comet now moves farther from the Sun again and activity decreases, Rosetta will again be moved back to an orbit closer to the comet.
On July 31, the first science results from the Philae lander were released, relating to the composition of the comet, surface features and hardness, temperature, and magnetism. Philae found the surface to be “littered with coarse debris” as well as discovering “a suite of 16 organic compounds comprising numerous carbon and nitrogen-rich compounds, including four compounds – methyl isocyanate, acetone, propionaldehyde and acetamide – that have never before been detected in comets.” Philae also found the surface to be harder than expected, and did not detect any measurable magnetic field around the comet.
It has also been announced that the science mission for Rosetta will be extended through September 2016—excellent news for everyone who is interested in this mission.
Meanwhile, the scientists will continue to work to solve this interesting puzzle of how and why the comet’s surface changes the way it does.
“We are looking forward to combining our OSIRIS observations with data from other instruments on Rosetta, to piece together the origin of these curious features,” noted Grousing.
The new paper, “Temporal morphological changes in the Imhotep region of comet 67P/Churyumov-Gerasimenko,” by O. Groussin et al., will be published in Astronomy & Astrophysics. More information about the Rosetta mission is available here.
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