Transport in Pluto's Atmosphere, Step By Step
If you've been following my coverage on Twitter, you know a lot of information has trickled out from the science team members over the course of the day. Some of it has been speculation, some of it simple conclusions that were inferred from the early returns. We've still only been hit with a few drops of what will be a deluge of data tomorrow and over the course of the coming months, but a major theme, prominent perhaps because it unites two teams' fields of interest, is the transport of material in both directions between Pluto's atmosphere and surface.
This shapes so much of what makes Pluto unique, and so much of what has surprised the science team in the last few days. It is likely the dominant factor in making Pluto's surface much more than just an ancient, crater-pocked sheet of rock and ice. So, sitting here and waiting for the "phone home" signal that will tell us New Horizons survived the fly-by, I thought I would break it down step-by-step.
Pluto's atmosphere, Fran Bagenal told us this morning, is perhaps the "purest nitrogen atmosphere we've encountered" in the solar system, but it's not 100% nitrogen. There are trace amounts of methane and carbon dioxide, and both play an outsized role relative to their abundance. In Pluto's atmosphere, these molecules of nitrogen, methane, and carbon monoxide are exposed to energetic ultraviolet photons emanated by the Sun. The interaction between these two can set in motion a chemical reaction that breaks up the molecules and forms other hydrocarbons like ethylene, acetylene, and others. These molecules then condense, and can "scavenge" other gas molecules as they begin to descend from the atmosphere toward the surface.
Unofficial "best guess" attempt at mapping color data from the MVIC instrument to the most recently received LORRI image from this morning. Credit: User ZLD on UnmannedSpaceflight.com
Now in solid form, these hydrocarbons can interact with one another once on the surface to create tholins. Tholins are extremely complex organic (carbon-based) molecules that have a remarkably diverse set of properties. When they collect, they form a thick, tarry substance that is typically dark in color. It's now suspected that this tholin-rich substance is what's causing the dark, reddish band dubbed the "whale" that stretches across Pluto's equator. A similar substance is expected to coat the surface of Saturn's moon Titan, one of three known bodies in the solar system to have a predominantly nitrogen atmosphere (the other two are Earth and Pluto). What happens with these tholins on the surface and whether they transport elsewhere will depend on compositional maps from spectrographic instruments like ALICE and LEISA. It's also not readily apparent why the tholins would settle on a distinct region of Pluto's surface and not in other places, like the very bright "heart" terrain. That will have to be determined from thorough interpretation of data that will come down over the next few weeks.
Just as material is transported down to the surface from the atmosphere, it also moves in the opposite direction. Methane and nitrogen ices, which likely comprise a lot of the bright portions of the surface, sublimate when heated by the Sun -- that is, they make a direct transition from solid to gas. They then rise up into the atmosphere, either to be transported elsewhere by the troposphere (the region of the atmosphere where lateral winds occur), or escape the system altogether, via the separate process of Pluto's atmospheric escape; this escape is also a complex interaction under continued study by mission scientists.
So that's the speculation right now. Our knowledge of this process will be steadily refined as more data comes back, and, as in all things Pluto, it will likely turn out to be more complicated than we thought.