Comets and asteroids

[Gregory Pratt]

http://dsc.discovery.com/news/2008/01/24/comet-dust.html

[Balta1701]

I'll try to keep this brief. Please note that every time I say probably or a similar word, it means that this is the most popular current theory, but that things haven't yet been proven. When the solar system formed, it formed as basically a cloud that collapsed towards the center, probably due to the shockwave of a nearby supernova. There's a problem with this process though - angular momentum. Just like a figure skater who slows down by extending arms or speeds up a spin by pulling them inwards, if you try to move mass that is spinning to a central point, it will start spinning faster. If you try to collapse all the mass of a solar system in to a sun, you can't do it, because the momentum would drive the spinning velocities so high that the sun would shear itself apart. What winds up happening is that some of the material in the cloud winds up containing most of the angular momentum, so you wind up with a disk of gas, with a proto star forming at the center with most of the mass, while most of the angular momentum stays outside in the disk.

 

Because there's a star forming at the center, which is very hot, there are temperature and pressure gradients within this disk. These lead directly to differing compositions of material forming at different places in the system. For example, we don't get gas giants near our sun because it's too hot for ice to form there, and water ice winds up forming a significant chunk of the mass of the gas giants. There are probably many important variations in this disk.

 

One hope of this mission was that by sampling a comet that we thought came from the outer solar system, we'd be able to get a glimpse of the material in the outer solar system. We have lots of samples of the inner solar system, because we have chunks of earth, Mars, the Moon, and meteorites/asteroids. We don't have a lot of the outer solar system, and that's interesting to know about for a number of reasons (perhaps most notably, because a good portion of our atmosphere may have been delivered by comets coming back in from the outer solar system).

 

So, they sent this probe through the tail of a comet, hoping to collect some of the dust coming off of it to allow us to actually have samples from the outer solar system. To collect the stuff, they use a material called aerogel, a wonderful material that is basically like a frozen foam made of silicon. Small grains ram in to this stuff, slow down, and are captured. One problem they're noting though is that it seems that the grains that came in may have gotten hotter than they anticipated, maybe hot enough to melt the grains and mix them with melted aerogel.

 

If they're correct in this initial article, instead of getting material from the outer solar system, they think they may have gotten material from the inner solar system. This would be interesting and confusing, because you wouldn't expect comets from beyond Neptune to be made of the same stuff in the asteroid belt from our current theories, so there would have to be some way of explaining how you move a lot of mass from the inner solar system out beyond Neptune and then lock it into a comet. Then again, if the reaction with the aerogel was more than expected, that could possibly have obscured whatever specific grains they were looking for. Or, we could simply be totally wrong about what we think comets are made of, and there could be another source somewhere for the stuff they've sampled from the atmosphere that they think came from comets.