The paradigm of the snow line (1)

 

Saturn's Rings (photo from NASA's Cassini probe)

Saturn is very unlike the Earth. For one thing, its rings are made of almost pure water ice. Rings like that wouldn't last long in our neck of the Solar System. In daytime, the surface of the Moon can reach over 100 degrees Celsius, the boiling point of water on earth at sea level (at night, it gets below -200 Celsius). Water ice exposed to sunlight here wouldn't last very long. But out where Saturn is, water ice persists through the centuries that we've observed it. 

The Sun--with the help of atmospheric greenhouse gases keeping the average temperature of the Earth above freezing--keeps things toasty here near Earth. Exposed ice out from the Sun, past Mars, and into the Asteroid Belt does not last very long. Frozen comets traveling near the sun thaw and spew off water vapor and dust. However, once we pass a certain point in the Asteroid Belt, water ice is finally stable. This is the snow line; it is perhaps one of the simplest ideas but most profound-in-result structures in the Solar System. 

The snow line in our solar system isn't really that much different from snow lines on earth. Near the equator, the snow line is far up in elevation above the heat radiating surface and above the greenhouse gases that hold heat in. The further from the equator that you go, the lower the snow line gets until "up" in Greenland and "down" in Antarctica, the snow line is practically at sea level. This is the land of perpetual ice where the average temperature is below freezing. 

In space, the snow line seems set where it is between Mars and Jupiter. However, as the Solar System has changed from a disk of gas and dust to the aged system of today, the snow line's distance from the sun has changed through time. Even now, it slowly is moving outwards from the Sun as it slowly gets brighter (since the Cambrian explosion of life, the Sun has gotten about 10% more luminous).

Why do we care where the snow line is, that its location has changed through time, and that it exists at all? Why am I calling this blog "The paradigm of the snow line" (besides the rhyming of the words)? It seems mundane that the further that one goes from a heat source, the colder it becomes, correct?

In one way, the snow line defines the make up of our solar system. The gas giants--Jupiter, Saturn, Uranus, Neptune--are very different from the smaller terrestrial planets. The asteroid belt straddles the snow line, which is fortunate as tidal forces from the gas giants kept larger bodies from forming and melting, which preserved the very material that makes up the planets for us to study. 

In the next few posts, except when news interrupts the process, I want to explore the snow line and its ramifications. I'll get into ideas about the pre-solar nebula, how planets formed, and the role of the snow line in that process. What did the snow line have to do with where gas giants are located? When we see a gas giant planet very close to a star, does that mean that the snow line didn't affect its formation? Or does that say something about that solar system that is very different from our own, a place where huge planets can spiral inwards and probably destroy terrestrial planets that may have been there? 

This is a big topic, but understanding it is as big a thing as it is to understand that, on Earth, the mantle convects and drives plate tectonics, a mechanism that explains or affects the majority of processes that occur on Earth's surface. The snow line isn't a mechanism; it's simply a place that the defines the stability of water. But it's more than saying that "here Saturn's rings are stable" and "here they are not stable". Without the snow line, would Saturn itself exist?