Rosetta: Close-up photos from a low pass over the surface.

A Comet, Up Close and Very Personal. And Very, Very Weird.

A Comet, Up Close and Very Personal. And Very, Very Weird.

Bad Astronomy
The entire universe in blog form
Feb. 16 2015 12:00 PM

Rosetta Dips Low Over an Alien World

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

On Saturday, Valentine’s Day, the Rosetta spacecraft dipped down low over the comet 67P/Churyumov-Gerasimenko. How low? At one point, it was less than 9 kilometers from the surface!

Given that the comet is 4.3 kilometers long and shaped like a rubber ducky that’s been sitting in the Sun for 4 billion years, this was a pretty low and gutsy pass. It was done to get extremely high-resolution pictures of the comet, of course, but the spacecraft will also be making a series of more distant passes to sample the environment around the comet at different locations.


Around the time of closest approach, the lower resolution NAVCAM instrument was used to snap photos of the comet. One of them showed the very, very weird Imhotep region of the comet, and, well, see for yourself:

the comet 67P/Churyumov-Gerasimenko
Fantastic high-resolution image of the surface of the comet seen from less than 9 kilometers away. Click to encomaenate.

Photo by ESA/Rosetta/NAVCAM

Yeesh. What a mess! Imhotep is the name given to the broad, flattish area on the outer part of the bigger of the two lobes. The resolution on this image is staggering; it’s about 0.76 meters per pixel. A human standing on the surface would be just under three pixels long.

As you can see, the surface is ragged, littered with boulders, some the size of houses. There are two features in particular I want you to see.

comet surface
A "flooded plain" probably filled with dust.

Photo by ESA/Rosetta/NAVCAM

That’s a closer view of the flat area at the upper left of the big picture (bear in mind I am not a cometologist, so I’m speculating a bit here; hopefully we’ll hear more from the Rosetta scientists about these features). This may be where ice under the comet’s surface is turned into gas when warmed by the Sun. As the gas escapes the comet, it leaves behind dust and gravel that can flow around; this may be a low spot in the surface that has been filled. Note the smooth area stops on the left just like water at a shoreline. But it’s not like a liquid, really; note the sharp step in the middle, a scarp that may be a slight collapse feature, where the ground suddenly gave way. It looks to be a few meters high.


The cliff-like region at the upper left looks very much like it’s been eroded, but not like it happens on Earth. There’s no water flow! In this case, it seems more likely that as ice turned to gas, the material erodes back, into the cliff, leaving behind the rocky material. Also, those boulders may be chunks that have fallen and rolled into place, or been exposed as icy material around them turns into gas and blows away.

Another fascinating area is this one:

comet surface
Sedimentary layers? No, quite the opposite.

Photo by ESA/Rosetta/NAVCAM

Note the layering! On Earth or Mars (y’know, normal places) I’d wager a feature like that is from sedimentary action; deposited season after season by rains and flooding bringing sediment into a lake. But on a comet? I’d guess that this represents the exact opposite: As the comet orbits the Sun on an ellipse, it gets farther and nearer to our star. When it gets closer, the ice near the surface turning to gas will drop the surface down a bit, and that stops as the comet moves away from the Sun. Then the cycle starts up again, over and over. The plateau is probably rockier material, exposed more and more every orbit as the ice goes away.

Note also the circular crater-like features to the right. Those almost certainly aren’t impacts! More likely they are where gas is escaping the comet, the pits forming and growing over time as the area around the venting region loses ice.

Comets are really strange. They have extremely low gravity, their orbits determine their seasons, their erosive properties are backwards. That’s why I want to be clear with my caveats about not being a comet scientist! Places like this are hard enough to interpret by the experts, and my guesses might be wildly wrong. What I’m hoping to do here is to get you thinking about what you’re seeing, and to understand that we’ve never seen a comet’s surface in detail like this before. Ever.

There’s a huge amount to learn, and it’s essentially all virgin territory, all alien and bizarre. Shakespeare was right: There are more things in heaven and earth than are dreamt of in our philosophy.

I’ve always interpreted that to mean Nature is more clever than we are, and we will always be surprised by what we find when we explore the Universe … but we’re clever too, and just because we didn’t imagine something a priori doesn’t mean we can’t figure it out a posteriori.

Here are the photos of Nature’s imagination. Now we let the science get to work.