Closer, closer, closer. Rosetta sent back more detailed images taken on July 30 that confirm the comet’s bright neck collar and show much more surface texture than earlier photos – including two craters! The collar’s light tone hints of fresh ice vaporizing in sunlight but could also be caused by different materials on that part of the comet’s surface. Maybe the next round of photos will offer a clue.
The wider-view navigational camera sent back an even more fascinating image showing dual craters, one on each lobe, with hints of other crater or depressions in the ice. Comets, including 67P C-G, are made of mostly water ice and dry ice or frozen carbon dioxide. The ices act as glue to hold the dust, sand-sized grains and pebbly pieces of the comet together.
When a comet’s orbit takes it into the inner solar system, some of the ice vaporizes, releasing dust and grit, along with a variety of gases like carbon monoxide, methanol, ethane and ammonia. The melange forms the comet’s coma or temporary atmosphere.
Next, the pressure of sunlight pushes back coma gases and dust to form tails – one of dust and the other of gas.
As Rosetta’s camera tried to record the coma, which is much fainter than the nucleus, the brighter nucleus was overexposed, creating an artifact. Taking pictures of comets is challenging enough, but evidently even more so when you’re standing right next to one!
* UPDATE 8/1: The ‘craters’ are more likely artifacts in processing of the image according to other sources.