After 10 years, Rosetta has arrived at Comet Churyumov-Gerasimenko. Its close-up views reveal an active world that harbors surprisingly complex terrain.
Although Rosetta has been taking increasingly detailed images of 67P since mid-July, its work really only started once it performed the final rendezvous burn on August 6. The biggest task for it at the moment is to survey the comet, in order to find a good landing site for the tiny Philae lander, which the ESA hopes to set down on the comet in mid-November. For Philae’s landing to be a success, the navigation team needs to know the safest and most scientifically interesting (unfortunately not always the same thing) places on the surface.
After a small flare-up regarding public funding vs. priority for the scientists who have spent a decade working on the mission, the ESA has settled on releasing select images every Monday and Thursday to the public. While the most scientifically valuable images are being held in reserve for mission scientists for up to a year, the images that have been released to the public so far add some amazing character to the comet.
Here’s an image of Churyumov-Gerasimenko taken on August 7. It shows both lobes of the comet (the top lobe is the larger of the two), as well as a portion of the bridge connecting them. I’ve highlighted a few of the areas I’m going to talk about.
The most interesting feature, in my opinion, is the big striated cliff that dominates our view of the upper lobe. Most objects in the size range of Churyumov-Gerasimenko are thought to be loose rubble piles, or dust, rock, and boulders barely held together by gravity. But here, it looks like below a thin veneer of loose dust and rock on its surface, the comet is roughly solid material. The solidity could come from the action of water ice, which can flow around under pressure. The low gravity of 67P could be enough squeeze it into open spaces inside the comet.
However, it doesn’t explain the striped appearance of the outcrop. The ESA image release refers to these stripes as “layers”, but offhand I can’t think of any known process that explains how those layers formed. The stripes could also be cracks, but then the issue is explaining why they’re so straight and spaced so evenly. I’ll be interested to see what the ESA comes up with to answer this as they get closer, more detailed views of the area.
There’s plenty of evidence for mass wasting (the scientific term for stuff moving downhill) all over the comet. The bottom lobe in this picture has numerous cliffs. The cliff bases are littered with a number of large boulders. The most likely explanation is that the comet’s activity is slowly chipping away at the cliffs, loosening rock that would slowly drift down to the surface.
Unlike rockfalls here on Earth, which are dangerous because of how quickly they can start and crush things below them, there would be plenty of time to get out of the way on Churyumov-Gerasimenko. The downward gravitational pull of the comet is only about 50 μm/s^2, so it would take more than 7 minutes for a boulder to fall 5 meters!
The bridge between the two lobes is also collecting a lot of small debris. It’s a little counter-intuitive for dust to collect here, rather than stick to one of the lobes, but this area is the furthest “downhill” (closest to the center of gravity) that debris can get. Interestingly, there are two debris fans spilling out onto the bridge from the larger lobe (at top in this image). I’ve seen the larger of these two features described as a “landslide deposit”, but I don’t think the term is accurate.
The deposit lies at the foot of a long, fairly deep valley feature, so it’s more likely to be something resembling an alluvial fan deposit. In the low gravity environment of the comet, dust and rocks that get kicked up by impacts or cometary activity can move great distances if they’re given a little bit of horizontal velocity. If that debris happens to start falling into the valley, the topography will channel its movement towards the mouth of the valley. Over time, it accumulates, forming a fan-shaped deposit.
Low Gravity Craters
One of the coolest features on the surface is the crater that appears at bottom right. Look closely – the rim is standing almost vertically! This crater is almost completely unchanged from the moment of its formation, which is something we don’t see too often in the Solar System.
Here’s why. An impact event creates a very large explosion underground. This is because the impactor usually travels well into the ground before it is stopped. That forward momentum has to go somewhere, and that somewhere is outwards. The resulting explosion lifts the overlying rock upwards and outwards to form a nearly perfect hemisphere in the surrounding rock. Usually, gravity makes the steep sides of the crater unstable, and the walls slump inwards. The end result is a crater that is wider and shallower than the initial cavity that was excavated. But thanks to Churyumov-Gerasimenko’s low gravity, the original hemispherical crater was stable enough to resist falling in on itself. Or at least almost stable enough. Some of the big boulders inside the crater are an indication that a portion of the crater wall has collapsed, but judging by how far the walls tower above the surrounding landscape, they haven’t collapsed much.
Rosetta has only just begun to explore 67P in detail, and as it settles into closer mapping orbits some of the current mysteries of the comet’s landscape will be solved, while new ones will turn up. Especially interesting to see will be how the surface changes in response to increasing solar heat as it heads towards perihelion in next August.