Astronomy Cast: Ice in Space

How much of the Solar System is ice? Well, not a lot by mass. In fact, if you pull together all the mass in the Kuiper Belt, it’s kind of a large rocky planet’s worth of materials — not a lot, and it’s also scattered into what’s called the Scattered Disk, and beyond the Kuiper Belt, beyond the Scattered Disk, the Oort Cloud – not really sure how much mass is tied up out there, haven’t really observed it yet. It’s a lot of stuff though. It’s sort of like when you’re cleaning your house, you don’t realize how many forks you own until you realize that you have forks in every room of your house from those random snacks that have been gathered. the water line is midway through the Asteroid Belt, and then the freeze line is out there pretty much between Jupiter and Saturn, and these are basically the places where you go from completely blasted dry, potato-shaped asteroids to potatoes with water to things of varying mixtures of rock and ice, and then the, in general, pure ice stuff in the outer solar system, and what you’re seeing is, essentially, the thermal gradient of when our Solar System formed. early Solar System was this mix of molecules and atoms. You had all the iron, all of the silica, all of the stuff that we think of as heavy metals. We had all of the carbon molecules, and mixed in all of this was O2 (oxygen); mixed into all this was all kind of carbon gasses, mixed into this was the ammonia and methane, and part of this mixing process…you had different things segregated out into different places due to gravitational attraction pulling things into the center of the Solar System vs. the light pressure forcing things back out. For the most part, the nice, happy, solid icy bodies we see — these are the Centaurs, the Kuiper Belt objects, the Scattered Disk objects — while they have variation in composition, we think, we’re still figuring this out. These suckers are far away and they’re faint. While they may have differences in composition (and we guess at that based on differences in albedo), they all formed in basically the same area and then got scattered around by gravitational interactions. So it’s more like you take the snow bank on the side of the road that has some of it has been attacked by pollution from cars, some of it has random spilled coffee from someone falling on the ice, there’s unfortunately dog pee on the snow bank, and where you grab — or don’t grab — a handful of snow from the snow bank, there’s going to be variations in composition, but in general, it’s all one snow bank. And in this case, it’s all one family of icy objects.