Astronomer Dean C. Hines of the Space Telescope ScIence Institute is using the Hubble Space Telescope to study Comet ISON in polarized light. This technique offers new insight into ISON’s structure and composition – factors that will play a key role in the comet’s eventual fate. We sat down with Dean to discuss his recent results.
ISONblog: Can you briefly explain polarization?
Dean Hines: Sure. Polarization is a property of light that happens most frequently when light scatters off particles like dust or reflects off a surface. We use polarization to tell us about the scattering material or the reflective surface. Different materials will scatter or reflect light differently, and will impart different polarization signatures. If you’re looking at snow with polarized sunglasses, for example, that’s a lot different than looking at the surface of water. You can put on polarized sunglasses, which filter out polarized light, and see the fish in the pond without the glare from the reflected sunlight. But putting on polarized sunglasses when you’re snow skiing reduces the brightness, but doesn’t do much else because the snow doesn’t polarize the scattered sunlight.
When you look at a comet with polarization data, what can you see?
You can determine the particle sizes and their structure. Is it made of rocky material? Are the particles big or small? Are they round? Are they fluffy like snowflakes, or hard like little hailstones?
What does polarization show for ISON specifically?
What we find is that this comet looks different in polarized light than comets that have likely come from the Kuiper Belt – than short-period comets. It looks like it’s missing significant amounts of the kind of material that’s been “cooked” by the Sun. The particles that we’re seeing in this comet are probably fresh – they haven’t been cooked by the Sun before, which makes sense if they were out there far beyond the Sun.
Comets that are periodic comets – that live in the Kuiper Belt or even closer – those things get exposed to the Sun a lot. So what we’re looking for is any differences between comets that are coming in from the Oort Cloud, which would have the material from the beginning of our solar system, versus stuff that we know has been cooked over the last 4.5 million years. That was the reason to look at this comet.
So this really supports the idea that ISON has never been to the inner solar system before?
Exactly. It also means that ISON may not have as thick of a crust. It’s like cooking a crème brûlée or even bread: the longer you cook it in the oven, the harder the crust gets.
Comets have “crusts?” How does that work?
The way to think about it is: if you put something in the freezer and make it really cold, and then you bring it out and set it on the counter and let it frost up, the frosty coating will be nice and smooth. Now let that coating melt a little bit and stick it back in your freezer. And then bring it back out again. Now it’s not as smooth, because you’ve got little drips that have frozen. Let it gather a little more water and melt a little bit, and so you get a coarser and thicker “crust” on the surface of whatever you’ve got.
When they come into the Sun, comets melt, and then they go back out again and freeze. And then they come back in again and melt, and then they go back out and freeze. You’ve got more of a slushy surface and you refreeze that, and over time you build up a thicker crust. Over time you also get rid of the volatiles and the gases, so maybe the gas bubbles aren’t as prominent on the surface.
When ISON passes close by the Sun, will a thin crust help or hurt its chances of survival?
It could help if it kept Comet ISON from having pockets of gas that explode and blow it apart. The surface of the comet is going to be in the 2,800-degree-Fahrenheit range, so if you had a thick, deep crust you might get pockets of gas that might get really hot but still hold together until they rupture. That could promote fragmentation of the comet. With a thinner crust, it might be less susceptible to this type of disruption.
On the other hand, it's going to go inside of what’s called the Roche lobe, which is where the stress from differential gravity is so strong that it can rip the comet apart. A thinner crust may not hold together as well. A harder crust might have helped that, or not – we just don’t know. What you should see is a little more activity from the comet: instead of bursts of activity it could be more constant, because with the thinner crust it’s easier for stuff to get out.
So when will we know which interpretation is right?
That’s a good question. All we’ll know is that if it survives its pass behind the Sun, people in my group will claim it’s because of what we discovered; namely that it appears that the comet may have a thin crust. If it disintegrates, we’ll claim it’s because of what we found – (that) it isn’t very strong. It’s win-win.
On a serious note, the fate of the comet, combined with our results, will inform us about the composition. This in turn will enable us to use similar observations to better predict the behavior of other comets.