A Primer on Planetary Geology

It’s great news that Pluto is geologically active, based on the fantastic images from New Horizons. I was hoping it would be, but the level of activity just simply implied in these images is amazing. The New Horizons team has its collective hands full with the flood of data, and the ultimate story of Pluto’s activity will probably be even more complex and cool than we think right now.

What Does it Mean to be Geologically Active?

When we talk about “geologically active” as it relates to Earth, we know what that means: mountain-building processes, volcanic flows, earthquakes, canyon-creating processes, plate motions, erosion (by wind and water), and so on. These are processes that geologists study. They also use an understanding of chemistry and physics to explain the complex details of how rocks form, and interact with each other and the atmosphere.

I studied geology for several semesters when I was in school, and one learns quickly that it is the basis for understanding how our planet’s surface has changed over the billions of years since it formed. Here are just a few examples of what I mean:

• the Rocky Mountains (where I live), formed hundreds of millions of years ago, pushed up by the action of tectonic plates sliding under the North American plate — which carried much of the North American continent. Before these mountains formed, the area was covered with an ocean which deposited many layers of sandstone, limestone, and shale over what is called Precambrian bedrock. When the tectonic plates began their action, they forced the bedrock up through the layers, creating the jagged mountains we see today.
• Tectonic plate motions also spur volcanic activity in the Pacific Northwest of North America, where plates subduct (dive under) others or spread apart from each other. Tectonic plate motions also cause the earthquakes that countries around the Pacific Rim experience each day.
• The Hawaiian Island chain was built by volcanoes that formed as a result of plate motions over a hotspot (or a plume) in Earth’s mantle (the layer below the surface). As the plate moves, the spot creates new volcanoes in a sort of “arc” across the mid-Pacific.
• In addition to those activities, there are other spreading zones — the most prominent being in the middle of the Atlantic. There the spreading zone splits the crust apart, which allows the upwelling of new mantle material to the surface. In this case, they’re under the Atlantic Ocean. The action is pushing Europe and Africa apart from North and South America.

Among other things, geology examines the rocks that are “built” through these processes, and the surface formations that are created. By looking at rocks and landscapes, geologists can get a good idea of what happened to create the various surface units we see on Earth — from continents and mountains to deep-sea canyons and impact craters.

How Does Geology Help us Understand Other Planets?

There’s not room here to go into all the many details of how geology helps us understand other worlds. It’s enough to know at this point that the same principles of geological processes that help us understand Earth’s physical history also explain features we see on other worlds: volcanoes on Venus, Mars, and Jupiter’s moon Io, for example. There’s very clear evidence of volcanism on our own Moon, as well as the planet Mercury. Tectonic motions of rock most certainly helped form the giant Valles Marineris canyon on Mars.

But, geological principles don’t just apply to rocky worlds. They can be applied to icy worlds, as well. For example, the concept of “cryovolcanism” is relatively new, but perfectly explains the plumes of material we see on the Neptunian moon Triton, as well as plumes emanating from Enceladus at Saturn and what look like flow features on other icy moons. In the outer solar system, ice acts as the “lava” that flows from volcanoes driven by internal action on the icy moons.

So, when planetary scientists talk about “geologically active” at Pluto, they are referring to some kind of activity being driven from within that is affecting and changing the surface of Pluto (and probably Charon, too). As on Earth and the other worlds with “geological activity”, you need some kind of heat to drive the processes of volcanism and tectonism. Pluto clearly has had its surface “repaved” in places. Some physical process inside the planet is driving that action. We’re not sure what it is, but the evidence is laid out there in ice before us. I expect that we’ll hear about cryovolcanism on Pluto once more images and data come down from the spacecraft. I hope we’ll learn that there’s a heat source in the planet. It could be driven by the decay of radioactive materials that provides heat. Or it could be something else.

Whatever it is, Pluto has experienced mountain-building processes (just look at the mountains in the video below!) and what looks like volcanism (albeit with ice as the “repaving material”). This little world  promises a fascinating time of discovery for all of us, and I’ve no doubt the New Horizons scientists will be delivering surprises for us for years!