In this post, I will be reviewing and summarizing the paper, Impact Craters on Pluto and Charon and Terrain Age Estimates (Singer et al., 2021). It is a part of the University of Arizona Press book, The Pluto System After New Horizons (reference at the end of the post). If you’re interested in reading more accessible papers on Pluto’s craters, you can check out this review of the population, this review of their depths and morphology, or this review of cratering rates. The paper I am reviewing is a part of a book of papers written to summarize recent findings in the Pluto-Charon community. Therefore, everything that’s discussed here is really just pulling from papers like those I’ve linked to. Note, I will be primarily focusing on the Pluto results. There is naturally more about Pluto in most of these papers, but it is also what I am most interested in. Feel free to check out the paper/papers if you want more details on Charon.
Perhaps the most significant thing we gained from the study of Pluto’s craters was a better understanding of its geologic history. Prior to New Horizon’s, it was unclear if we would find any craters on Pluto because of the types of processes that were predicted to possibly be occurring. What they found was more of an in-between state. Pluto’s surface has a diversity of crater patterns that highlight the vast range of geologic processes that occur across its surface. It leads to a world with valuable information on the origin of the solar system and the various impactors that have collided with it, but superimposed in places are unique processes that reveal valuable information about Pluto’s history. That is one of the things that makes it particularly interesting to me because I am use to thinking of crater processes on a global scale. Titan has a range of crater morphologies due to varying amounts of fluvial and aeolian processes, but even this is a fairly ubiquitous on Titan. Then on worlds like Europa and Ganymede, where no atmosphere exists to modify the surface, viscous relaxation is the primary erasure and modifier of craters. Pluto on the other hand, has regions distinctly unique from the rest, as if moving from one to the other is moving between different worlds (or at least in my perspective).
The basic shape craters make is a bowl, but they collapse due to material strength, gravity, and impactor energy. This happens in ice because it is much weaker than rock, even at these temperatures (10s to 100 K). Therefore, craters on Pluto are similar to those of other icy worlds. They transition from bowls, to flatter complex craters with central peaks and potentially pits (though the latter is less abundant than expected). There are also two large basins on Pluto, one being the well known Sputnik Planitia and the other being the smaller (~1/3 the size of SP) Burney Basin. After formation, geologic processes can modify the crater shape. The primary form is through, erosion, relaxation, and infill (you can see my discussion of a paper on this here). However, these are largely volatile driven (methane, nitrogen and carbon monoxide ices), and Charon, less abundant in volatiles, has a more pristine crater population. One area this is particularly noticeable is in the lack of ejecta blankets on Pluto.
Cthulhu is characterized by its dark (tholin layered) albedo, and it has areas that appear very ancient (heavily cratered) and smooth. However, its abundance of large craters (see the figure in Terrain Ages) suggests it is very old overall. The midlatitudes are suspected to be middle aged to old. There is a noticeable lack of large craters suggesting very early resurfacing (or mantling) as that is when newer craters would have formed (they could also just have not happened as frequently, by chance). There is some evidence of erosion (sublimation/ re-deposition). The fretted terrain shows signs of eroded valleys, with some navigating around craters suggesting the valley is young and the crust (ice base) is old. To the east lies the Burney basin. Apart from its rim, it is heavily cratered with a unique texture that is suggested to be very old (which is consistent with its high crater rate in the figure below. In the North, significant deposition and sublimation cycles are ongoing, modifying the surface, and the albedo of the craters are though to relate to volatile processes. Then there are several regions with minimal to no craters (see part b of figure in Terrain Ages). There are a couple possible cases in Eastern Tombaugh Regio (E_TR) and near Wright Mons (WM), but the authors seem less keen on marking a crater than I am (perhaps naively, on my part). Craters are fairly abundant on Charon, even the smaller ones (to an extent) that are lacking on Pluto, with the exception of a smoother darker terrain in the north. Overall, slope (i.e. the knee) of the R-plots suggest this says something about the impactor population and not just the geologic processes on Pluto.
Cratering Population and Rates
Pluto’s varying terrain types helps differentiate between the impactor population and the geologic history. However, calculating cratering rates is complex and difficult to constrain. First, it depends on the structure and history of the Kuiper belt. Currently, the prevailing theory is the Nice model of Planet Migration + Jupiter capture which in turn effects predictions for impacts early in the solar system. Even still, much of the predictions rely on the current knowledge about the structure of the Kuiper belt (believed to be the same for the last ~4 G.y.). Then, there is uncertainty in the distribution of sizes in the Kuiper belt that leads to uncertainty in the rates they measure. On average, an impact speed of 2 Km/s is used, but the range of populations in the Kuiper built can create a range of impact speeds as well. From there, a impact rates are estimated. Despite all the sources of uncertainty, the predictions seem to agree within a factor of 3 or 4 (translating to age differences on the order of 1-4 G.y.). Rates are calculating by scaling the impact rates and considering the target and impactor properties. However, it was unclear in the text whether they use a consistent water ice bedrock. I can’t help but wonder what would change if we consider thick mantles of volatile ices that would have significant differences in material properties. The predictions made leading up to New Horizon’s are shown in the figure above.
Terrain ages are estimated by using the measured craters and predictions for crater density of a terrain. It reflects the adding or removal of craters through resurfacing. It assumes all the craters observed are primary and not secondary (as there is no evidence of these despite the expectation that they would exist). Given all the uncertainties, the ages are intended as order-of-magnitude estimates. Furthermore, surfaces with no craters are aged with an upper limit given the resolution limitation (i.e. there could be smaller craters on an older surface). This gives a surface like SP an age of 30 – 50 m.y. despite modeling of the convection cells that suggests an age of ~500 k.y.. Other young terrains includes the E_TR and WM with estimated ages of ~250 m.y. and 1.5 G.y. respectively, but if some depressions are actually craters, like on WM, the age would be closer to 3 – 4 G.y.. The older surfaces are discussed more thoroughly in the crater frequency section. The heavy volatile processes in the north give it some of the youngest surfaces.
The bigger connection between terrain ages and geologic terrains is more complex. White et al. (2021) discuss the geology of Pluto (in this same book) discuss the geologic processes, and one of the oldest surfaces is assumed to be the north, despite the lower crater count. The intersection between these ages and the geologic processes is a larger conversation I hope to have as I study crater modification in my upcoming paper. This will look beyond crater count and study active modification of craters that are present.
Singer K. N., Greenstreet S., Schenk P. M., Robbins S. J., and Bray V. J. (2021) Impact craters on Pluto and Charon and terrain age estimates. In The Pluto System After New Horizons (S. A. Stern, J. M. Moore, W. M. Grundy, L. A. Young, and R. P. Binzel, eds.), pp. 121–145. Univ. of Arizona, Tucson, DOI: 10.2458/azu_uapress_9780816540945-ch007.