Pluto and Charon have captured our hearts and imaginations. But how did these adorably strange worlds form, and what consequences could that have on what we see now? Researcher Amy Barr Mlinar chatted with us about catastrophic collisions, subsurface oceans, and Pluto’s lack of craters.

The New Horizons spacecraft is bombarding us with a steady stream of discoveries and realizations about Pluto, Charon, and all the little moons. By far the most startling was when our first close-up look at Pluto contained no craters. The oddly-smooth surface is the hallmark of a freshly resurfaced world, one where the plains of Tombaugh Regio are less than 100 million years old. But how could that happen?


When looking at Pluto at high enough resolution to spot features just 1 kilometer (0.5 miles) in size, we’re seeing a shocking lack of craters. Image credit: NASA/JHUAPL/SwRI.

One of the ideas is that a catastrophic impact between what are now Pluto and Charon could have created a subsurface ocean. That ocean could be the driving force behind icy tectonics, an idea explored by planetary geophysicists Amy Barr Mlinar and Geoff Collins in a paper published last year. In it, they explore how an impact between two equal-sized bodies might have spawned Pluto with Charon starting a piece of shrapnel caught in orbit, and how their tidal push-pull may have formed a subsurface ocean while they were in the process of settling into tidal equilibrium.


I caught up with Barr Mlinar to explain how the new results coming home from the Pluto flyby by the New Horizons probe fit in with her theories.

How did Pluto and Charon Form?

The most popular theory for how Pluto ended up with a moon nearly 1/10th of its mass is through a catastrophic collision between two equal-sized bodies. The collision must have been a glancing blow, knocking the surface off what is now Pluto while more substantially decimating the impactor until Charon was the largest of the intact shards. Some theories look at if the smaller moons—Nix, Hydra, Styx, and Kerberos—could be remnants of debris from the same collision.


A glancing blow between two similarly-sized rocky bodies with ice shells could produce Pluto, Charon, and a debris disk possible of accreting into tiny icy moons with only minimal heating. Image credit: Robin M. Canup.

Barr Mlinar points out this didn’t have to be a huge, high-velocity collision between the proto-worlds to create the system we see today:

The current best model of the collision involves two relatively similar-sized objects hitting at near escape-velocity, which is less than 1 km/s. It is a gentle collision and is thought to heat the young Pluto by a few tens of degrees Celsius.

The object that becomes Charon is essentially a piece of the impactor that remains intact during the impact. This is consistent with the fact that Charon’s mean density is different from that of Pluto.


The timing of this collision is the first place things get interesting. If the two bodies were made of the normal mix of solar system stuff, they’d have silicate rocks with a specific quantity of radioactive material. If they collided long enough ago that they were still heated by radioactive decay at the time of collision, then it’s plausible that being heated by only a few tens of degrees would be sufficient to push the icy worlds into warm enough to create a subsurface ocean.

What Happens After the Collision?

Even if those few degrees aren’t quite enough, the next stage of evolution for the miniature system involve tweaking both dwarf planet and moon as they settled into their new orbital dance.


After the collision, both world would have been tugging on each other, their asynchronous rotations and orbits exerting force slowing the other down to match. They first stage is to become synchronous, where the smaller Charon is yanked enough to always keep its same face towards Pluto. This is equivalent to what we have right now on Earth: while the Earth turns freely so all of us on its surface experience moonrise once a day, the same side of the moon is always facing us. The near side of the moon saw just one Earthrise a long time ago. The second stage, dual synchronous state, is when the larger body falls into the same lock.

Although Barr Mlinar and her co-author Geoff Collins consider this transition from chaos to synchronous as near-instantaneous, the transition to from synchronous to dual synchronous took much longer. According to Barr Mlinar, it probably took Charon about fifty years to reach a synchronous state, but took the much longer timespan of millions of years to reach its current dual synchronous state where both Pluto and Charon are in lockstep with each other. This is part of how we know that the system-forming collision couldn’t have taken place in the recent past: it takes a long time for a system to reach the stability of circular orbits and a full tidal-lock of dual synchronicity.

How Could a Collision Create an Ocean?

One of the things we’ve seen with icy moons around the solar system is that the tidal pull of their massive gas giant parents can be enough to drive geological activity. The same can’t be true on Pluto due to the total lack of nearby giants, but there might be another way. This window of a few million years after Charon settled into gazing steadily at Pluto but when the dwarf planet had not yet mirrored the situation is when Barr Mlinar thinks an ocean could’ve been melted.


To understand why, we’re going to need to talk about mathematics on at least a conceptual level. The tidal impact of one body on another is going to depend on four things:

  1. The ratio of mass between parent and moon
  2. The distance between parent and moon
  3. The eccentricity of the moon’s orbit around its parent
  4. The difference between the spin period of the parent and the moon’s orbital period


The larger the mass ratio is, the more the parent can throw around any children-moons with impunity. Tidal effects drop off rapidly with distance incredibly quickly: for every unit of distance a moon is farther from its parent, the tides are just one-sixth as strong. The higher the eccentricity of the orbit, the more change in distance between parent and moon takes place over the course of an orbit, then the greater the tidal stress. Finally, as long as the spin period and orbital period are mismatched, then the disequilibrium state of asymmetrical tidal bulge will be enough to keep creative massive tidal pulls deforming the smaller world.

For gas giants with orbiting tiny moons, the mass ratio is 10,000:1 with the huge parent 10,000 times more massive than the insubstantial moons. That means the gas giant is going to be nearly completely unaffected by the moon, while the moon is going to be dramatically impacted every time it gets anywhere near its overwhelming parent. This means the prime component of tidal massage on the moon is going to be the orbital eccentricity of the moon. Barr Mlinar says:

For a moon orbiting a gas giant, the most important effect is its orbital eccentricity. If the moon is in an eccentric orbit, the distance between the planet and moon changes with time, and so the height of the tidal bulge changes with time.


The momentum transfer between Pluto and Charon over millions of years eventually resulted in a tidally-locked system in a dual synchronous state. Image credit: Barr and Collins.

The isn’t the situation for the Pluto-Charon system, where the mass ratio between parent and moon is far more notable. Charon is the largest moon in the solar system compared to its parent. The result is that Pluto is just ten times more massive than Charon, resulting in a nearly unbelievable low mass ratio of just 10:1.


If we look at the time after Pluto and Charon settle into a synchronous state but have not yet reached a dual synchronous state, then the distance between the worlds is small, the mass ratios nearly on-par, and the mismatch between Pluto’s spin period and Charon’s orbital period is going to be severe. The result is a perfect combination to not just subject Charon to a tidal massage, but also Pluto. In theory, this could be enough to push an initially warm and mushy Pluto over the edge into generating a subsurface ocean in a brief bloom of activity.

One possible scenario for heat flux from tidal dissipation in Pluto, with residual viscoelastic tensile stress after orbital evolution and its likelihood of driving tectonics. Image credit: Barr and Collins.


How Long Can a Subsurface Ocean Last?

Once you have a subsurface ocean, you aren’t going back to a boring no-ocean world. Barr Mlinar explains that icy worlds might have a feedback loop when it comes to oceans:

If we find tectonics that could indicate that Pluto had an ocean in the past. It is my personal opinion that if an icy satellite has an ocean ever, it is very difficult to get rid of it. As the ice freezes, all of the anti-freeze materials (salt and ammonia) are excluded. So as the ocean freezes it gets salty and richer in anti-freeze stuff, until eventually you’re left with a thin, deep layer of briny/ammonia-rich liquid.


In other words, as part of the ocean freezes into ice, the salts and ammonias are left behind, reaching higher concentrations in the remaining ocean. Because they act as anti-freeze, they inhibit further freezing. The more water freezes into ice, the more challenging it is for the remaining water to freeze.

The result is that even though the Pluto-Charon collision must’ve happened millions of years ago for it to be in its current tidally-locked dual synchronous state, any subsurface ocean that formed during the collision could potentially still be clinging to liquidity today.

How Could We Tell if Pluto Has a Subsurface Ocean?

If Pluto did have a subsurface ocean, that could be enough to drive icy plate tectonics. If it did, Barr Mlinar is hoping that we might see similar landforms to what we see on Europa, the icy moon of Jupiter with a subsurface ocean and an icy crust. Europa is coated in distinctive linear features and double ridges; although we haven’t seen anything like that on Pluto yet, we could still see a less dramatic version as more photos come back from New Horizons. She’s also hoping that we see any of the usual array of faults and wrinkle ridges we see on any other solid planet with a tectonically active past.


Will we find linear features, double ridges, and faults on Pluto like we see on Europa? Image credit: NASA/JPL.

Finding linear features on Pluto wouldn’t necessary indicate a currently active world with a subsurface ocean; they could be geologic relics from the past, if we can just find a chunk of Pluto’s surface old enough to preserve it.


It’s also possible that we can put some constraints on Pluto’s interior structure through carefully observing its global shape.

Charon is also turning out to be under-cratered for what we’d expect from an ancient, dead moon, leading scientists to speculating if it, too, could have geological activity to some extent. We’ve seen no indication of linear features on Pluto’s largest moon yet, but we’re going to keep looking.

How Does What We’re Seeing at Pluto Impact Our Theories on Icy Worlds?

While Pluto is a dwarf planet, not an icy moon, one of the reasons its smooth surface and implied geological activity are causing so much fuss is because it breaks our current theories of how thermodynamics applies to small, icy worlds. When I asked Barr Mlinar if she could think of other startling discoveries that had a similar impact on trashing the theories, she immediately leapt to how surprising we found Enceladus when we got our first up-close look:

Enceladus was a huge surprise and a big shock. It south polar region has a heat flow something like 10x that of the Earth, yet it’s a moon that is 250 km in radius, the size of an asteroid. A “traditional” tidal model underestimates the amount of heat coming out of Enceladus by a factor of 100, and it also predicts that the heat should be coming out uniformly, so a uniform heat flux across the surface. No tidal model has yet been able to explain either the magnitude or distribution of tidal heat coming out of Enceladus.


Enceladus has a ridiculously hot pole, and we still can’t explain why. Image credit: NASA

Similar statements are true about Jupiter’s moon Io.

So I think it would be safer to say that we don’t understand tidal dissipation at all, either in rocky or icy bodies. We always underestimate.


So yes, Pluto has once again broken our theories for thermodynamics of icy worlds, but it’s not the first time those theories have been royally stomped on by new observations. This time, we’re getting clarification in that Pluto is so far away from anything that could be exerting a substantial, ongoing tidal pull, so any abnormalities cannot be attributed to tidal stress from a gas giant. That means we can’t keep leaning on blaming gas giants as a handy crunch to explain the unexpected. Instead, we need to fundamentally rethink how we’re approaching thermodynamics in these icy worlds.

What Happens Next?

We’ve gotten back the last of the First Look data from the New Horizons probe, the immediate failsafe data downlinked in case something catastrophic happened to the spacecraft. That data is still being released to the public, potentially with all sorts of new surprises including the possibility of finding some of Barr Mlinar’s linear features. Even after that, the probe will be returning data for another 16 months, including lossless, uncompressed versions of existing images so we can crater-hunt with more confidence.


Before we closed out our conversation, I asked Barr Mlinar if she had any final thoughts on the New Horizons mission:

I want to pass along my congratulations to the New Horizons team. The team has done a phenomenal job of planning and executing a very tricky encounter. I am also thrilled for them that Pluto didn’t turn out to be a lifeless cratered ball of ice. We really had no idea what to expect, and I’m so glad that the team’s unwavering interest in Pluto was rewarded with these magnificent images.

[T]here was a time when it looked like budget issues and politics would cause the mission to be cancelled. Alan Stern and the team did a heroic job of fighting to save the mission. I think the message here is, “never give up, never surrender.” If you really want something to happen, you have to go out and fight for it, and not listen to naysayers.


Getting this mission was no sure thing — at one point it involved a postage stamp campaign advertising that the then-ninth planet still hadn’t been explored. Now the spacecraft is out exploring the outer solar system, the extended mission to send New Horizons out to explore a second Kuiper Belt Object is still awaiting funding confirmation. And yet, we’re learning so much already.

The surprises in the Pluto-Charon system are just barely starting to be defined. Scientists on the New Horizons team and those outside it like Barr Mlinar are going to be kept busy sorting how how all these discoveries warp and change their theories of how our solar system functions.

Top image: Pluto and Charon in real colour and appropriate scale, composited from data from the New Horizons probe. Credit: NASA/JHUAPL/SwRI.


Read Tectonic Activity on Pluto After the Charon-Forming Impact here.

Barr Mlinar and I share a peculiar history: she was the scientist who first taught me about asteroids when I was but a wee proto-scientist attending the Summer Science Program. We both highly recommend the program to high school students with a lively interest in science and a desire to be truly pushed to their intellectual limits.