Landslides of Serenity Chasma

This week’s guess the planet image comes from Charon, Pluto’s largest moon. It shows a series of landslides in a valley which has been informally named Serenity Chasma. Charon is quite an unusual object. Most moons are much smaller than the body which they orbit, but Charon has a diameter of more than half that of Pluto. Charon and Pluto are gravitationally locked, so that the same faces of the two worlds are always facing each other. This means that Pluto doesn’t move in the Charonian sky. As with Pluto, it was only fairly recently that we got our first good look at Charon. This week’s image comes from the New Horizons spacecraft, which visited the system in 2015 and credit goes to their team.

The version of the image below was annotated by the New Horizons team. The arrows indicate where several landslides have separated from the scarp, and produced small fans of debris further down the hill slope. These are quite subtle features, especially given that the images we have of Charon are much lower resolution than those which are available for Earth and Mars. However NASA have also provided a 3D view, annotated to show one of the features. As you can see from the scale information in the images, this scene covers a very wide area. These landslide features are by no means small features, but extend for kilometres down the slope. They are significant as they are the first such features to be found on Charon, or any body in the Kuiper Belt. 

Landslides occur when a hill slope becomes unstable. This instability can gradually increase over time, as the rocks become weaker, or the slope steeper.  At some point a “trigger” will occur, causing the slope to collapse and a “mass movement” to occur. A huge volume of material will detach from the slope and travel downhill until it comes to rest on flatter ground below. In the case of large landslides, like these ones, the material can be deposited over a very large area, often producing a wide fan of deposited material. The events which trigger a landslide can vary. Some are triggered by earthquakes, which provide a large amount of energy to dislodge unstable slopes. Volcanic processes can also shake the ground and trigger collapse. 

In some cases less dramatic processes trigger the landslide. Some slopes are held together by icy material within a soil. When this thaws the slope becomes less stable and collapses. In other cases gradual erosion of a hillside, whether by wind, water or other processes can destabilise a slope. On Earth biological weathering or the activity of humans can be responsible for triggering a landslide, although this probably isn’t the case on Charon. Charon also doesn’t have much of an atmosphere, although it might “share” some gasses with Pluto. This means that wind erosion isn’t going to be a factor on the moon. Add to this the extremely cold conditions that far from the sun and many of the likely sources of erosion are unlikely to be active in the present day. 

Assessing the cause of a landslide from remote sensing images may seem like a daunting prospect, but in some cases there can be clues as to how the event occurred. Wet material will flow downhill very differently to a mass of dry rocks and boulders, and will leave a different deposit behind once the dust settles. We can thus look at the shape of deposits like these.  Where we have good enough data the shape and composition of the material that makes up the fans can be assessed. With a bit of detective work we can sometimes determine how long ago the landslide occurred and what factors led to its occurrence. 

The material which makes up these landslides is expected to be ice, or the sort which makes up most of the crust of the small moon. However we don’t have a high enough resolution image of the terrain here to see whether it is in large blocks or not.  We can’t know for sure with the information we have, but given the lack of other trigger processes on Charon, the most likely explanation is that they were triggered by either an Earthquake of some sort, or by a meteorite impact. This potentially occurred a long time ago, as there wouldn’t have been much geological activity on Charon to erode the resulting deposits. Without getting a closer look at these features, and seeing whether they have any impact craters on them we can’t know for sure. Dating using crater counts is the only way we can estimate ages on planets which we haven’t collected samples from.  

 

Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute https://photojournal.jpl.nasa.gov/catalog/PIA21128

https://www.jpl.nasa.gov/spaceimages/details.php?id=PIA21129