Thanks to the Rosetta spacecraft, researchers can follow the cycle of water escaping from the nucleus of Comet 67P/Churyumov-Gerasimenko.
The bizarrely shaped periodic comet 67P/Churyumov-Gerasimenko reached perihelion on August 13th, coming just 1.24 astronomical units (186 million km) from the Sun. All the while European Space Agency scientists have used the Rosetta spacecraft to provide a fascinating look at what happens on the surface of a comet during its most active phase — and to peek into what's going on down below as well.
Since reaching Comet 67P in August 2014, Rosetta has documented the comet's evolving activity and the outgassing of volatiles such as carbon dioxide and carbon monoxide. But the source and cycling of its water has remained a major mystery. The comet's nucleus has been releasing plenty of water vapor, yet very little ice lies exposed on its surface.
As the comet approached the Sun, key changes became evident on the surface, especially along Imhotep, a flat region at the larger end of the double-lobed nucleus. Large pits appeared in sequences shot using Rosetta's OSIRIS narrow-angle camera in early June and July, apparently caused by collapse after underlying ice sublimated due to intense sunlight and escaped.
But these aren't your run-of-the-mill Floridian sinkholes, as water vapor and other gases appear to be percolating up from the layers below.
Mission scientists spent months using Rosetta's Visible Infrared and Thermal Imaging Spectrometer (VIRTIS) to note how outgassing is driven by sunlight and the comet's day-night cycle. Eventually they identified a surface region where water ice appears and disappears in sync with its 12.4-hour rotation period. "This keeps the comet 'alive'," explains Maria Cristina De Sanctis (INAF-IAPS) in an ESA press release. She is lead author of results published in September 24th's Nature and presented this week at the European Planetary Science Congress.
De Sanctis and her colleagues believe that a layer of ice just below the porous surface sublimates and rises to the top and escapes during midday heat. At nightfall, the surface cools rapidly — but water vapor continues to rise from the still-warm layer below. It freezes onto the surface, creating a temporary veneer of ice that dissipates in the morning light. This sequence is shown schematically below.
To support this notion, the team used Rosetta to monitor a 1-km-square patch of the Hapi region, located along the "neck" connecting the two enormous lobes. Hapi was the first-noted source of escaping water vapor, seen by Rosetta while the comet was still 500 million kilometers (310 million miles) from the Sun (out beyond the orbit of Mars. The signature of water ice appeared in near-infrared spectra of the region, but only when it was in partial shadow. When seen in full sunlight, the ice was gone.
The team finds that water ice accounts for 10% to 15% of the material near the surface and appears to be well mixed with dust and other constituents. Moreover, the studied section of Hapi accounts for only 3% of all the water vapor escaping from the nucleus, so many such sources must be scattered over the nucleus.
"We've now been able to 'clock' the water activity of the comet versus its daily rotation better than ever before," says Carey M. Lisse (Johns Hopkins University Applied Physics Laboratory), who wasn't part of the study. "We already knew from Deep Impact's studies of 9P/Tempel 1 and 103P/Hartley 2 that cometary nuclei's activity near the Sun vary on diurnal cycles, and this told us that the actively emitting regions must have been 1 to 5 cm down. The new Rosetta results tell us much more precisely where these actively emitting regions must lie."
It's fascinating to consider just what the alien world on the surface of a comet might look like and whether such outgassing would appear dramatic or subtle. Along with the dearth of water ice, 67P/Churyumov-Gerasimenko is a relatively dark object, with a surface albedo (reflectivity) near 6% — about that of fresh asphalt. This is darker than the Moon's average albedo of 14%.
Advanced amateur astronomers and ground-based observatories have kept an eye on Comet 67P as well, as it approached 11th magnitude near perihelion.
Will Rosetta see the comet split in half? How typical is Comet 67P among short period comets? Future plans call for closer in surface operations for Rosetta, including a thorough hunt for the Philae lander and — just perhaps — bringing Rosetta in for a landing on the comet as well. Or ESA managers might opt to keep Rosetta about where it is now, hovering about 200 km above the nucleus. Doing that would avoid problems with close-in dust and allow the spacecraft to ride along with the comet for years — conceivably through an entire 6½-year-long orbit.