Processed NAVCAM image of Comet 67P/C-G taken on June 15 2015. Image credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

Jun 24, 2015 Finding water on a comet

Sen—You wouldn't think it would be so hard to find water on a comet. There's no question that they're substantially composed of water ice; you can see the stuff clearly in comet tails. But seen up close, comets are not obviously icy. In fact, theirs are among the darkest surfaces in the Solar System.

Churuyumov-Gerasimenko is among these incredibly dark comets. It doesn't look that way in Rosetta's pictures because the comet is the only thing around other than the blackness of empty space. Command your camera to expose the scene for the ambient brightness, and you can see lots of detail on the surface, as well as plumes of comet material spreading into space, as shown on the main image.

Occasionally, a very dark icy world in the Solar System will have some small patches of exposed ice. When your spacecraft camera exposes the scene properly to see the mostly dark surface of the world, the bright patches can saturate the detector, washing out any detail. That's exactly what's going on with the Dawn images of the bright spots on Ceres.

We know there has to be ice close to Churyumov-Gerasimenko's surface, but no Ceres-like bright patches have been visible. So it was news, today, to see these newly published images from OSIRIS, the high-resolution science camera on Rosetta, showing bright patches of material exposed on the comet's surface. As a bonus, they're in color!


Examples of icy bright patches cropped from OSIRIS images of Comet 67P/Churyumov-Gerasimenko during September 2014. The two left hand images were acquired on Sep. 5, 2014; the right hand images were acquired on Sep. 16, 2014. During this time the spacecraft was about 30 to 40 kilometers from the comet. The images are false colour red-green-blue composites assembled from monochrome images acquired at different times with the 882.1nm (red), 649.2nm (green) and 360.0nm (blue) channels. Each channel was stretched and slightly saturated to emphasis the contrasts of colour across the scene such that dark terrains appear redder and bright regions appear significantly bluer compared with what a human eye would normally see. Image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

The OSIRIS team reports 120 detections of bright material on the comet. These bright patches are not as bright as the ones on Ceres; the text accompanying the images states that they are "up to ten times brighter than the average surface brightness." Previously, Rosetta has reported a six per cent albedo for the comet. A 50 to 60 per cent albedo is darker than pure ice; if these patches are ice, they are still mixed with some dirty material.

Don't just look at the bright patches in these images. Look, too, at the distinctly reddish material around it. That knobbly red dark material is the dominant comet surface. It is what looks so brightly reflective in the NavCam image shown at the top of this post. What a difference a contrast adjustment makes!

The OSIRIS images show that the bright material occurs in boulders, either piles of boulders or isolated chunks. Boulder piles occur at the bases of cliffs; isolated chunks can be anywhere. All of them are in places where they are shadowed for much of the comet's day.


Example of a cluster of bright spots on Comet 67P/Churyumov-Gerasimenko found in the Khepry region (top) and an individual boulder with bright patches on its surface in the Hatmehit region (bottom). The bright patches are thought to be exposures of water-ice. Image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

These patterns give clues to how the icy material was exposed—and why it's rare on the surface. Piles at cliff bases suggest that recent erosion—the collapse of a cliff wall—has exposed fresh material from the interior of the comet. Isolated boulders are erratics, things that originated elsewhere and were transported to their current locations, likely flung out by cometary activity. Wherever the stuff fell, it was protected by shadow from sublimation. One month's observations by OSIRIS didn't show any obvious changes to any of the patches.

The activity of the comet is keeping the spacecraft at a distance for now, but it's a productive time for the Rosetta instruments that are devoted to studying the icy materials that the comet is jetting into space. MIRO, the Microwave Instrument for the Rosetta Orbiter, has produced a beautiful map of the distribution of water in the comet's coma. Each of the little squiggles represents an average of several individual spectra. Wherever MIRO sees the coma against black space, a characteristic hump in the spectra identifies water molecules emitting radiation; wherever MIRO sees the coma against the comet, a dip in the spectra identifies water molecules absorbing radiation.


Image credit: ESA / N. Biver et al. (2015)

The comet is approaching perihelion now, and is outgassing like crazy. It's likely that the patches that OSIRIS saw will have changed by the time that Rosetta can see them up close again. Rosetta will definitely have the opportunity; ESA has just announced an official extension of the mission for a year, until September 2016.