Volatile degradation of Pluto’s craters

This is an introduction review for what I cover in my upcoming LPSC oral presentation (2555).

examples of geology on Pluto

Anyone familiar with Pluto knows it has a diverse range of geologic features. I show just a few of these in the figure above. The range of terrains that exist on Pluto diversifies it’s surface giving it a unique beauty, even compare to other worlds like Mars or Venus (that’s a science fact). I cannot help but compare these images to the picture of Pluto I had in my mind growing up. The picture I had was define by the episode of the Magic School Bus when the Friz takes the kids on a Journey through the solar system. At the end, they visit Pluto (considered the last planet in the Solar System at that time). It was this dark desolate place similar to the moon or Mercury.

Pluto as seen on the Magic School Bus

New Horizon’s shattered that picture in my mind, and I think the difference between the two is part of why I love Pluto so much. It proved to be much more than I ever expected. Why is that? Perhaps it was a lack of imagination. The team at the Magic School Bus failed to consider how significantly volatiles might modify the surface.

More images of Pluto focusing on the role of volatiles

Much of what we see on Pluto’s surface are in fact driven by volatile processes. Anyone moderately familiar with Pluto is likely to know about Sputnik Planitia, the giant lake or sea of N2 that traps heat and produces beautiful convection cells. There are other lakes, dendritic networks, glacier flow. All of this exists because the volatiles methane, carbon monoxide and nitrogen are unstable at Pluto conditions. This range of modification can be studied and quantified on an individual basis, but on a global basis as well. That is, we can consider how it shapes Pluto’s crater population which can work as reference point for how much degradation is occurring.

The focus of this post will be on Stern et al., 2015’s work looking at how Pluto’s volatile inventory can be responsible for the loss of craters. The figure shows how erosion will decrease the crater population count (R) and erosion with relaxation. What does this mean? When we think erosion, we think slow degradation driven by the removal of material, usually through the application of a physical force. Stern et al is referring to the loss of material to the atmosphere, I think by sublimation. Pluto is thought to have loss a great deal of volatiles over time. They enter the atmosphere and get sputtered away by various means. To be clear, Pluto’s bedrock is water ice. Stern et al is referring to craters that form in thick layers of volatile ice, specifically N2. These structures will fade away, and with it, the crater itself. Similarly, craters that form in N2 (or even CH4) ice will relax on the timescales that Pluto has existed. Relaxation will flatten craters, but rims are retained, but those rims can easily be loss.

Stern et al. 2015 estimates how much the overall cater population (R ~ crater count) would be modified by varying amounts of erosion (top) and erosion with relaxation (bottom).

I want to take this discussion a bit further because it suggests we can make predictions about the type of craters we should expect to find. Obviously, craters formed in pure water ice should be retained. Craters formed in sufficiently thick N2 ice (as thick as a crater depth) may relax on the order of thousands of years (millions at the most). Craters formed in thick CH4 ice would not be as conducive to relaxation. At the temperatures observed on Pluto, we might expect this to happen on the order of billions of years. We may see some craters entirely degraded (or flattened), but there may be some that are partially relaxed. CH4 is also less volatile than N2, so the structures would not be as susceptible to loss. This implies we should expect to see craters rich in methane and partially degraded, likely because it is formed in methane ice.

The temperatures expected on Pluto (left) and the relaxation rates of ices (right).

There are other in-between states to consider as well. What about craters that form in a thick layer of N2 or CH4 but not so thick that the crater can relax. In this instance, the crater base would be made of water ice. This base would uphold the other ice, preventing the crater to relax. However, N2 ice is likely to still erode away. Therefore, we will likely see partially degraded craters, depleted in N2 and CH4 (beyond surficial deposits) because they are the remnants of a dual ice crater. With CH4, the ice will be less conducive to erosion, so we might expect to see fairly pristine craters rich in CH4 because they have a water ice base.

These predictions have been tested by New Horizons. We have elevation data and compositional data as well. The compositional data is surficial, so there are limitations to what we can say about a crater. However, it offers first order constraints on what is present and what type of ice the crater may be formed in. We have used this data to study the degradation of craters on Pluto and relate them to the volatile abundances in their region. In doing so, we can constrain the volatiles in the region and the history of volatiles on Pluto.

Check out our talk at the 52nd LPSC to see our results!

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