Space snow

The off-and-on snow we’ve been getting in Denver has actually been pleasant. It’s like Mother Nature is fighting with itself: “I want it to be spring, but it’s really supposed to be winter…but I want it to be spring…but…” I’ll take what I can of the non-sub-zero temperatures.

And those temperatures I am referring to are typically in Fahrenheit, or °F. In astronomy (and physics in general), we commonly use temperature units of Kelvin. The Kelvin scale is significant because 0 K is “absolute zero”, or the point where all thermal motion has ceased. The scale itself seems a little strange at first (70 °F = 294 K; 32 °F = 273 K; and 0 °F = 255 K), but again, it is a very useful tool. It makes temperatures of objects “easier” to understand and reference.

Astronomy provides (probably) the greatest temperature extremes that are to be found. The center of the sun is about 15,000,000 K; the “surface” of the sun is “only” about 5,800 K; interstellar space sits at about 2.5 K. What is amazing is thinking about how our solar system was able to form from a massive cloud of cold gas and dust, that created a hot star in the center (the sun), that helped create a disk of material that accumulated into planets.

Anyway…where does the snow concept come into play in astronomy? An important feature of planet formation is where the planets formed. Since the temperatures around the central star, or in this case, our sun, are too hot for gasses to form ice (or liquid, then ice), then there must be a point that the heat from the sun is low enough for ices to form. The point at which this occurs is referred to as the “snow line”, or sometimes, the “frost line”. This is the point where gas and ice are able to “survive”, as shown in the figures.

From Sean Raymond's PlanetPlanet blog.

Subaru telescope press release in 2009.

The sun is so hot that as the solar system was forming, it pushed the gas and ice out towards the outer part of the solar system. It is there where we find the “gas” and “ice giants” (Jupiter, Saturn, Uranus, and Neptune). The inner planets are made up of mostly rocky material, or solid material that was not pushed out by the sun’s intense radiation. It is a parallel concept to think about the snow line on a mountain range, like this image of Mt. Sophris in western Colorado. You can see where the temperature/weather is suitable for snow formation.

Keith Cooper image of Mt. Sophris.

Understanding the distribution of material in the early solar system provides answers to how the solar system formed: frozen moisture is vital in helping dust stick together (to then form planets). Inga Kamp, an astronomer studying protoplanetary disks at the Kapteyn Institute, at the University of Groningen (Netherlands) explains it fairly succinctly here. Various molecules condense from a vapor to a solid at different temperatures, but water ice, for example, condenses at about 180 K; methane, on the other hand, condenses at about 40 K (Lodders 2003). Dust grains also have condensation temperatures and this is important because silicate rocks need to form in order to make rocky planets (condensation temperature for a silicate ~ 1400 K). Even in my disk research—where the disk I am studying is quite warm (around 550-1150 K at the edge)—it is important to understand these condensation temperatures so as to better comprehend the make-up of these astronomical objects.