Charon Poses a Geomorphic Riddle: How do You Make a Mountain in a Moat?

Hi, Charon! What’s that? You’re jealous that Pluto broke the brains of geophysicists worldwide yesterday, and want attention? Fine, let’s see what you look like up close! Beautiful craters, lovely smooth plains, a few cool linear features... Wait a minute, is that a mountain in a moat? How did you create that?!

After impressing us yesterday with a gorgeous full-disk view of Pluto’s largest moon Charon, the New Horizons spacecraft sent home an amazing high-resolution frame looking at a small segment of the surface in glorious 380 meters per pixel (1,200 feet per pixel) resolution. Taken just 79,000 kilometers (49,000 miles) about 1.5 hours before the closest approach on July 14th, the swath covers an area approximately 390 kilometers (240 miles) long. This is the highest-resolution look we’ll get at the moon’s surface during initial downlinks.


And it’s making my head hurt in the best possible way.

How to See Like a Geomorphologist

Geoscience is at its heart a very straightforward science. The first stage of exploring a new world is simple: look at it. Look at it from far away to get the big picture, and in as much detail as you can manage to spot all its little quirks. Later, we’ll add in spectrographs to check out chemical distributions in the surface and the air, temperature, pressure, and even exotic things like how plasma and high energy particles are interacting with the worlds. But for now, we use what we can see in the optical spectrum to practice the fine art of geomorphology, trying to understand how the features and landforms were created on this massive moon.

Charon at approximately 380 meters per pixel resolution, rotated so that north is to the left. Image credit: NASA/JHUAPL/SwRI


When I see a photograph like this, the first thing I do is try to match up the landforms I see with other, similar landforms I’ve observed in the past. Sometimes it’s easy — as a lifelong resident of the west coast, I can spot a fault trace without hesitation. Other times it’s like a geomorphic game of Concentration as I struggle to recall where I once saw a similar feature alien to my local terrain. Gazing at this first close look at Charon, I’m flipping through my inventory of every other moon I’ve seen photos of, trying to remember the features carved in ancient rock and mobile ice.

For years, Charon was a fuzzy lump blurring Pluto’s spherical lines in any observation we made. Last week, we got our first look at the surface, and I described it as probably fairly dull and boring. A few days later, we saw our first traces of geology. Yesterday, I gleefully apologized for being wrong while oohing over colossal canyons. And today, I’m reassured by familiar craters and totally confused by a mountain ringed by a moat.


So, what’re we looking at?

First, we need to get all the non-geology out of the way:

Processing Artifacts: The beautiful, lossless image is aboard New Horizons, we only got the compressed version back here on Earth (for now!). The result is a strangely pixelated terrain, where what are otherwise smooth patches of the plain have a faint blocky overlay. Ignore it.


Lighting: This photograph was snapped along the terminator, the boundary between day and night on the moon’s surface. That’s excellent because the low angle of incidence enhances shadows and throws the surface into high relief, but it also means the distinct change in lighting across the scene is due to shading, not changes in albedo. (Changes in albedo would mean very different surface materials: shiny ice is more reflective than dull rock.)

Now, on to the fun bit: geomorphology! Notice the extremely technical annotations. It looks silly, but the first step of understanding what’s going on is to describe what’s actually there. After that comes interpretation, and if I’ve learned anything from the New Horizons flyby so far is that whatever I interpretations I make will probably be proven wrong in the next few days.


Extremely preliminary descriptive annotations for the geomorphology on Charon.

Relative ages: In geology, we use a series of stratographic principals to establish a relative timeline of events. Some of those rules are that the oldest things are on the bottom (material exposed in crater walls is older than the surface), and that a thing needs to exist before anything can happen to it (the plains are older than the craters, even if parts of it have been subsequently resurfaced). One particularly interesting bit is the superposition of crater and crack in the northwest: which came first, the crack or the crater? For now the imaging artifacts are obscuring things, but that tiny puzzle should be solved as the uncompressed images are returned later.


Plains: The most basic feature is a relatively smooth plain, broken with occasional craters and hills. It’s fairly uniform with no immediately distinguishable regions of drastically different terrains, although things are looking decidedly more lumpy to at the northern edge of the shot.

Lumps: Lumpy things can be either erosional features like hills of carved bedrock (bed ice) or depositional features like piles of sediments. Material can be eroded or deposited by liquids (could Charon have a history of rivers of nitrogen or neon?) or gases (could Charon have an atmosphere, at least part-time?), or this could be the well-preserved relic of some ancient surface disruption. TBD.


Craters: Unlike the shockingly-smooth first peek at Pluto yesterday, Charon’s surface is reassuringly cratered. The crater counts still look a bit low compared to our own moon, but that’s actually not too surprising. Because Charon and Pluto are tidally locked, the same sides face each other all the time and are a bit more protected from impact events.

The craters are very crisp with unbroken rims, suggesting that they might be relatively young. Paired with such beautifully smoothed plains and the relatively low number, I’m wondering if we might find evidence of relatively recent resurfacing on Charon.


I’m not seeing ejecta — material thrown out of the craters at the time of impact. One of the craters looks like it might be complex with a central peak, a relic from a particularly high-impact event.


Left: The distinctive bullseye crater from the 2.3 kilometer/pixel resolution image is a good example of a complex crater with ejecta rays.

Cracks: Long, snaky things are always fun. These aren’t particularly linear, so they’re probably not tectonic in origin. They could still be faults from smaller, non-global activities. At least one set follows the distinctive Y-shape of things flowing downhill, so could be old river beds, or collapsed tubes from cryovolcanism. I can almost imagine some of them form pentagons, the classic cooling-crack pattern. If it is, that could reflect some form of thermal contraction.


Over on the Planetary Society, Emily Lakdawalla is letting her inner geologist show by pointing out that the fissures reminded her of the Rima Hyginus region on our own moon, where a flood of lava cooled, shrank, and fractured. It even has a crater-crack superposition pair of its own! It’s possible these features on Charon formed the same way, but with liquid ice from instead of lava.


Left: Lakdawalla points out that Rima Hyginus on Earth’s moon is looking suspiciously similar; did the features on Charon form the same way?

???: Here’s the bit where I admit I have no idea what’s going on. We have some sort of mountain that’s on the order of 20 kilometers wide sitting in a ditch that’s roughly twice as wide.


A single spiky mountain sitting by itself normally screams volcano to me, and if Pluto’s going to be a geologically active world through yet-to0-be-determined ways, it’s plausible Charon could theoretically be rocking some form of cryovolcanism. It doesn’t look all that volcano-y to me, but with all the shadow and imaging artifacts, it could be.

But that doesn’t explain why it’s sitting in a very clear indent in the surface, with a distinct moat around it. It almost looks like the whole thing sunk, except I’m not (yet) seeing anything that suggested the plains were ever gooey enough to be that ductile. It’s a challenge to come up with a process that will heat an icy surface enough to bend without actually melting it entirely — it’ll be fascinating to find out what type of chemical mix is going on.


At least I’m in good company with my delighted confusion — the Geology and Geophysics lead Jeff Moore is quoted in the related image feature saying:

The most intriguing feature is a large mountain sitting in a moat. This is a feature that has geologists stunned and stumped.


Whatever this feature is, it’s weird, it’s wonderful, and I just know it’s going to get an awesome name. The only reason I’m not flailing harder is that geomorphology is used to being confused about how even common landforms happen, so seeing something new and weird is almost par for the course.

But what is it?

I have no idea.

Top image: Charon seen from 79,000 kilometers away on July 14, 2015. Credit: NASA/JHUAPL/SwRI


Share This Story

Get our newsletter