Pluto has been puzzling us with its weirdly smooth surface, but if it’s the first Kuiper Belt Object we’ve visited, how did we know how many craters to expect in the first place? Here’s everything we’ve figured out about collisions in this chaotic area of our Solar System.

When we got our first close-up look at Pluto’s surface, we degenerated into inarticulate cursing over its smooth surface. Of all the things we guessed about what we might see when we finally reached Pluto, finding craterless plains in Tombaugh Regio was no where on the list. In the following days, we got into how strange it is to find active processes resurfacing Pluto and even what might be driving those processes, but we neglected to explore why it is so baffling to find a virtually crater-free surface in the far reaches of the solar system. If this is our first look at the deep reaches of the Kuiper Belt, how can we be so certain it’s just as chaotic out there as it is closer to the sun?

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We met up with planetary scientist Luke Dones to explain what we know about impact rates in the Kuiper Belt. Dones is a planetary scientist at the Southwest Research Institute in Boulder who specializes in orbital dynamics, the complex of planets, asteroids, and other objects around our Solar System conducted by the strong and sometimes subtle pull of gravity.

Pluto is the first Kuiper Belt Object (or KBO) we’ve ever visited. The Kuiper Belt is a distant ring of asteroids, comets, and dwarf planets out beyond the orbit of Neptune. It’s freshly discovered; we only realized it was a distinct region in our Solar System in 1992! The innermost lip of the Kuiper Belt is the Scattered Disk, a region populated by objects with unusually eccentric orbits. The Scattered Disk is home to Eris, the dwarf planet that is slightly smaller than Pluto in diameter yet somehow more massive, and the suspected source of short-period comets that sometimes invade the inner solar system.

Where Do Impactors Come From?

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Hipparchus Crater on the moon. Image credit: NASA.

Space is big, but our solar system is busy, chaotic, and messy. Things crash into other things all the time. The nature of those impactors—comets, asteroids, and any other objects that smack into something—changes based on location within the solar system, and time since the formation of the solar system.

Here on the terrestrial planets—Mercury, Venus, Earth, and Mars—the most likely impactors are near-Earth asteroids. These near-Earth asteroids are recent arrivals from the main asteroid belt between Mars and Jupiter; once they get nudged into the inner solar system they usually last about 10 million years before crashing into something, somewhere. If they don’t hit a planet, they die a fiery death crashing into our Sun, or are unceremoniously booted from the inner Solar System by a hefty yank from our guardian gas giant, Jupiter.

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For the gas giants—Jupiter, Saturn, Uranus, Neptune, and their assorted moons—the most common impactors are comets on highly elliptical orbits. These can be Jupiter-family comets (like 67P/Churyumov-Gerasimenko, the comet Rosetta and Philae are exploring) and centaurs (half-comets/half-asteroids like Chiron and Chariklo). These planet-crossers are also visitors; Dones explains they probably originated in the Scattered Disk of the Kuiper Belt. We can’t see craters on the nebulous, hazy surfaces of the gas giants, but some of their smaller satellites hold a partial record of their violent pasts.

The Kuiper Belt is where things enter the realm of space-cannibalism. The biggest source of impactors for Kuiper Belt Objects is other Kuiper Belt Objects; this demented factoid makes a sense when you consider that the Kuiper Belt is a snapshot into the early history of our Solar System, when everything constantly slammed into everything else. For the inner part of our solar system now dominated by planets, those collisions were a slow build of accretion as individual rocks clomped together into protoplanets, grew into dwarf worlds, and eventually became the grand masters of their orbits as fully-fledged planets. For the deep space region of the Kuiper Belt, this growth and clearing hit a hiccup and left a slew of dwarf planets rocketing around in a mess of asteroids and comets playing a continuing game of smashy destruction.

What is still To Be Determined is what exactly is the dominant component of Kuiper Belt impacts. Is it dwarf planets smashing into each other, potentially generating subsurface oceans that stick around eerily long? Is it a rain of comets delivering fresh ices to the surface that will only be lost to a long trail of sublimation? Dones doubts the impactors are from the Scattered Disk, so we can’t just borrowing the history of impacts on the moons of gas giants as a proxy for likely impact rates.

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Dones points out another bit of quirkiness for characterizing impact events in the Kuiper Belt: they’re slow.

Impacts on the terrestrial planets and the moons of the giant planets are “hypervelocity” impacts, that is, the impact speeds are far greater than the speed of sound in the planet or moon, so the impacts are essentially explosions. Very roughly, hypervelocity impacts typically produce craters about 10 times bigger than the size of the impactor. Impacts on Pluto and Charon occur at typical speeds of 2 km/s, which is comparable to the speed of sound. Thus the impacts are barely hypervelocity.

Instead of being massive explosions of shattered rock, fused impact glass, and catastrophic pressure waves, these slower, gentler impacts do something different. What exactly “different” craters look like is not yet well-understood or characterized: maybe smaller, maybe less violent, but still a visible scar tracing out the violent history of impact.

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How Do We Know Impact Rates?

Fresh impact crater on Mars formed between July 2010 and May 2012. Image credit: NASA/JPL-Caltech/Univ. of Arizona.

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We puny humans have only been around and staring at rocks in space for a brief blink in the cosmic timeline, so how do we know how frequently objects in space smash into each other? Looking at current impact rates is easy. Dones explains:

Current rates can be estimated from the known number of large near-Earth asteroids and scaling relationships that relate impactor properties to the size of the crater the impactor makes.

Even getting into the recent past isn’t too tricky. For anywhere with a hard surface —the terrestrial planets or the ice and rock moons of the gas giants—we can use visible craters to build up a history of cratering rates over time. The icier moons present their own problems as our understanding of hypervelocity impacts on even water ice is a bit hazy, muchless impacts into more exotic compositions, but they at least give us a clear visible trace of an impact history. We can even tell relative ages through stratigraphic principles: older things are on the bottom, and anything that cross-cuts must be younger. Active geological processes on Earth constantly resurface our planet, but places like our moon and Mercury bear their scars like a badge of honour. Dones continues:

Even before the Apollo missions, Bill Hartmann figured out that the Moon must have experienced an early era of heavy bombardment, because the number of craters on the Moon was far in excess of what could be produced at current impact rates. [...] The ages of rocks measured by Apollo led to the idea of the Late Heavy Bombardment (LHB) or lunar cataclysm, the timing of which is still a subject of debate.

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We can pull the same trick for the outer solar system, establishing a different impact rate influenced both by different impactor sources (thus trajectories) and the huge gravity well of Jupiter.

When it comes to the outer edges of the Solar System in the Kuiper Belt, we’re even more limited in our understanding. Our observations of the Kuiper Belt so far are limited by what we can see with a telescope, which means we’re heavily biased to spotting large objects, yet logic dictates that most of the impactors are going to be smaller things crashing into those large objects. This gets messy because then calculating impact rates depends on us extrapolating how many small objects must exist.

It isn’t an entirely hopeless situation. We can use observations of the larger bodies to extrapolate how many small objects are dancing around the Kuiper Belt. Better, we can use observations of Jupiter-family comets and impacts on Jupiter’s moons to calculate how many smaller objects escape from the Kuiper Belt on a regular basis. Dones works through the math for how we use our observations of Shoemaker-Levy crashing into Jupiter to extrapolate information about Kuiper Belt Objects:

We think we know the diameter (~1.5 km) and density (0.3 to 0.4 g/cm^3, i.e., less than half the density of water ice) of Comet Shoemaker-Levy 9, which broke up in 1992 and whose fragments hit Jupiter in 1994. We estimate that a comet nucleus of this size hits Jupiter about once every 200 years, with a possible range of 100 to 400 years or so. The rate of impacts on Jupiter can be related to the number of bodies of that size in the Kuiper Belt.

The argument is that Kuiper Belt Objects escape the belt at a rate that can be calculated, if we know how many KBOs there are in different kinds of orbits. Many or most of the KBOs in the belt remain there for billions of years, but of those that do escape, about 1% eventually hit Jupiter. But there’s a complication because comet nuclei often fall apart, sometimes into smaller fragments and sometimes into dust, for no discernible reason. So the number of comets doesn’t stay constant, and the rate calculation isn’t quite right.

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We can also attempt to observe the smaller objects directly... or, rather, almost directly. The objects in the Kuiper Belt are far too faint to see directly with a telescope, so instead researchers have used the massive star surveys by telescopes like Hubble to look for any occultations—times when a Kuiper Belt Object temporarily blocked out the light from a distant star—to identify more objects. Dones elaborates:

In principle, with an occultation you can determine both the size and the distance of a KBO through the light it diffracts from the star it passes in front of. Despite many attempts, there are only two claimed detections, both with the Fine Guidance Sensor of [the Hubble Space Telescope]. This is a very hard observation, and it’s tough to know what to make of this until it’s confirmed [by other observations]. [...] TAOS II, which will be the best groundbased occultation survey to date, is supposed to begin operations in Baja California in a few years, so hopefully by the end of this decade it will find some KBOs, and we’ll have a better idea of how many small ones exist.

The original models for Solar System evolution predicted the gas giants would hit a point of instability and spark system-wide bombardment 3.9 billion years ago. However, that theoretical bombardment would produce a more intense cratering history for the moon than we see in real life, so that theory is still being revised to match observations. In the meantime, you can help researchers identify and count craters to pin down a better understanding of how these impact rates change with time since the formation of the solar system, and distance from the massive gravity well that is our sun.

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Why Does Pluto Have Crater-Free Patches?

Boundary between smooth, light terrain and dark, cratered terrain in the north of Tombaugh Regio on Pluto. Image credit: NASA/JHUAPL/SwRI.

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Even with all this uncertainty, we can tell a few things from our New Horizons observations of Pluto and Charon. Both the dwarf planet and its over-sized moon have craters, just not as many as we expected. Pluto in particular seems to have distinct regions of darker, heavily cratered terrain and lighter, smooth and virtually craterless terrain. Even without any solid constraints on impact rate models for the Kuiper Belt, that means the darker cratered terrain is substantially older than the lighter smooth terrain, probably on order of hundreds of millions to even billions of years.

As for what Dones is most excited to see on Pluto and Charon? He’s all about the craters:

Craters! It looks like we’re beginning to see more craters in the uncompressed images, so perhaps we’ll be able to say something about the ages of different regions beyond the off the cuff (I assume) “younger than 100 million years” that was said last week.

It would be great to see plumes like those Voyager saw on Triton.

The New Horizons probe has completed downlinking its First Look data, and will spend the next month sending home the earliest plasma and particle data. It’s going to take a long time (16 months!) to get the full lossless datasets back from the flyby, at which point it’ll be a lot easier to identify features without wondering if we’re being fooled by strange artifacts of image compression.

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But it gets better—we can get a second Kuiper Belt flyby out of New Horizons. The Pluto-Charon system is the very first time we’ve ever visited a Kuiper Belt Object, but the New Horizons probe is fully capable of making a flyby of a second, totally unexplored object. The extended mission to keep listening to the probe and supply the scientists with computers and coffee hasn’t yet been funded, but the team already has a plan in place.

In August, the New Horizons team will make the final decision between two smaller targets within the Kuiper Belt. Both objects are roughly the size of the tiny moons Nix and Hydra, or potentially smaller if they’re actually binary systems. The candidates are:

  • 2014 MU69, which is smaller at an estimated 30 to 45 kilometers (19 to 28 miles) wide, easier to get to (so more fuel for tweaking the flyby as the probe gets closer), but dimmer (magnitude 26.8). If we pick this target, the flyby is estimated for January 2019 when the object is 43.4 AU from the sun.
  • 2014 PN70, which is bigger at an estimated 35 to 120 kilometers (22 to 75 miles) wide (so more scientifically interesting), harder to get to (so less fuel for last-minute tweaks), but brighter (magnitude 26.4 so easier to see). If we pick this target, the flyby is estimated for March 2019 when it’d be 44 AU from the sun.

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Both are part of the classical Kuiper Belt population, as opposed to the Plutinos (including Pluto) that are in orbital resonance with Neptune. Unlike Pluto, which probably formed closer to the sun and migrated farther out over time, these objects could well have formed way out in the distant reaches of the solar system. If we can see an object that formed and evolved entirely in the Kuiper Belt, we’ll have an excellent data point to constrain what is going on with collisions in this distant, mysterious region of our Solar System.

We can only go to one more Kuiper Belt Object, but whichever one we pick, it’s going to be exciting.

Top image: Artist’s concept of a Kuiper Belt around the distant star Vega. Credit: NASA/JPL.

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Additional reading: Craters and ejecta on Pluto and Charon: Anticipated results from the New Horizons flyby, Impact and cratering rates onto Pluto, Icy Satellites of Saturn: Impact Cratering and Age Determination, Measuring the Abundance of sub-kilometer sized Kuiper Belt Objects using Stellar Occultations