Craters on Pluto

Craters on Pluto – Growth Rings of Solar System

Pluto Craters vs Solar System Formation? Yeah, they are relevant!

Solar System is a huge factory of rocks in various sizes. If you take the small ones into account (e.g. as small as few meters long), the population counts up to billions.

Despite the enormous size of Solar System, that dramatically increases the possibility of collision of two rocks considering the past 4.5 billion years.

The formation on a bigger celestial body is called a crater when a substantially smaller body hits to the surface of a bigger. As a result of that impact, a basin of cavity is than formed.

Those cavities has a lot to say about celestial bodies’ past. On this article we will rake up with the craters on Pluto and try to relay what they have to say about Pluto’s violent history based on recent researches.
In the specific case of Pluto and Kuiper Belt, those craters mean even more: They help researchers to estimate not only the area of Kuiper Belt and the celestial objects flying all over, but also to shed a light on early Solar System and tell us what has happened back then.

Hellas Planitia Crater
Hellas Planitia on Mars: Biggest impact basin in Solar System. Credit: NASA/JPL/USGS

Craters on Pluto Mapped By New Horizons

Analysis of craters on a celestial object surely requires a look under the hood. For such a purpose especially on far worlds, flyby and orbiter missions are necessary.

Luckily, an important part of New Horizons Mission goals was to create detailed maps of Pluto surface and Charon surface. As a part of those plans, New Horizons Spacecraft was equipped with extremely sensitive and enhanced cameras. In addition to global maps, interesting details were planned to be captured with incredible resolutions down to dozens of meters.

Everything went well and Pluto flyby of New Horizons was successfully performed. As promised, high resolution images and maps were created with extreme details. See seperate articles for details and names of major surface features: Pluto Map and Charon Map.

On this article we will refer to those data regarding Pluto craters from New Horizons visit and the recent researches build upon that.

Craters of Pluto

New Horizons Team did not only map major craters: They counted them all!

Craters on Pluto
Craters on Pluto surface mapped. Credit: NASA/JHUAPL/SWRI

The team counted a total number of 5287pcs craters which are -mostly- uniformly distributedon Pluto surface. The number also includes potential impact craters.
Number of craters on the moons are: 2287pcs on Charon, 35pcs on Nix and 6pcs on Hydra. Since the smaller moons Styx and Kerberos could not be imaged with high resolution, no crater counting data is available for those.

Pluto craters, 500 meters to 300 kilometers wide, were measured with a 10% uncertainty where the major ones were captured in 100 meters/pixel resolution. They are formed by the impactors roughly sized between 300 meters to 40 kilometers.

Although studies before New Horizons suggested the opposite, recent researches based on New Horizons data reveal that both Pluto and Pluto Moons did not face a disastrous impact in last 4 billion years. The biggest and youngest crater on Pluto surface (neglecting Sputnik Planitia) is estimated to be about 1 billion years old and is anticipated to be sourced by a meteorite 10-40km wide. Well, that could still be bigger than one of the tiny moons of Pluto!

Sounds rational to estimate the size of an impactor from the cavity it created on a surface. But how can researchers derive other properties like age, surface composition or crust thickness?

Craters on Pluto and Mars
A comparison for highlighting the similarity of the craters with central pits on (A) Mars and (B) Pluto. Credit: NASA/JHUAPL/SwRI

How to Derive Information From A Crater?

The answer is simple: By comparing with known craters on other objects and with mathematics! Let’s demonstrate with few examples, than we can dive deep into Pluto craters.

For instance, the existence of central pits inside such craters indicates a less dense icy surface and an icy crust, most likely involving water ice. That’s one of the best ways to evaluate the water ice on Pluto considering most of the water is hidden beneath the surface.

Basically density, size and speed of a meteorite together with the surface-near crustal features of the impacted body are directly effecting the dimensions of the harm we see on the surface.

Scientists look at the depths and diameter of the crater. As an example, the smaller ratio of depth/diameter on a younger crater generally means the impacted surface is warmer or weaker. And the same number is also effected by the age of the crater since the craters get smaller and smaller over years. That’s a method used to calculate approximate ages.

Another example could be the rim heights of craters. On two craters with similar size and formations, the higher rims reflect a denser crust where lower ones do the opposite and cause the rims to fall apart over time.

Craters on Pluto – How Can They Be Still Visible?

Before the New Horizons Mission, scientists believed that older craters on Pluto might have either been papered over or covered with layers of surface material depending on the thickness and structure. Than after the mission, the team mapped all craters on encountered hemispheres of Pluto and Charon and shed a light to the truth.

Although Charon was exposed to less surface variation in the past and serves more critical data about earlier meteorite bombardment days in comparison to Pluto, Pluto‘s geological complexity and younger formation hints more tips in various conclusions about planetary structure.

The reason is, researches show that luckily there are no direct signs that the big craters were covered with other materials and disappeared in time. On the other hand, there are some areas (especially the Lowell Regio) where the cavities were partially filled with ice over time. Although this could mean that smaller craters are already out of sight, it also means the surface drifting could not make the bigger ones disappear.

Another factor that could bury the craters is the cryovolcanism on Pluto. The dragged ice can also flow through the low elevation zones and can have an effect in same manner. Together with that; the convection and sublimation erosions are anticipated to have a similar effect.

Pluto surface
A great example to Pluto’s surface diversity, which also includes craters filled with snow an not. Source here, Credit: NASA/JHUAPL/SWRI

Major Craters on Pluto – Top 5

Although not yet confirmed as a crater, Sputnik Planitia is the widest basin on Pluto surface. Being a 600 to 1000 km wide basin, the area is completely covered with nitrogen ice accompanied by methane and carbon monoxide. Considering its side effects like the signs of the ejecta, researchers believe the feature was formed by a huge impact during the early years of Pluto.

Burney Crater, located on northwest of Sputnik is the biggest confirmed multi-crater basin and approximately 250 km wide.

Another crater similar in size is the Simonelli Crater with a diameter of 250 km. It’s located on east part of global Pluto map. We do not have a clear image of this area since it was on the far side of Pluto while New Horizons Spacecraft was closer to Pluto. However we can clearly see that’s a big crater.

Edgeworth and Oort Craters are located in the region informally named Cthulhu and their approximate diameters are 145 km and 120 km respectively. This is the most dense area in terms of craters and for that reason it is considered to be the oldest part of Pluto surface.

Craters on Pluto
Major Craters of Pluto. Credit: NASA/JHUAPL/SWRI

Crater Distribution

As mentioned above, Cthulhu Regio is the most dense area in terms of craters. Than comes the Vega Terra and the north and west parts of Sputnik Planitia with higher population. Hayabusa Terra and Venera Terra follows them with lower population. The area named Tombaugh Regio, which also covers Sputnik Planitia -a crater free zone-, owns very few craters.

Pluto Craters Distribution Map
A crater distribution and confidence map. Highlighted areas are Vega Terra (left), north-west part of Sputnik Planitia (right) and unofficially named Cthulhu Regio (bottom). Credit: NASA/JHUAPL/SWRI/USGS

Dating a cratered surface area (not like that!) is directly related with a) primarily the count of craters and b) their formation. Now we know who is older!
And according to detailed analysis of researchers, the interval is more than 4 billion years to less than few million years.

Crater Formation

Seeing details of altitude inside crater surfaces is also important for the scientists, but that mostly depends on the angle of the sunlight. If it is angled, you can observe the shadows and obtain more data; easier than a less angled one.

Since New Horizons was only an instant flyby, it was not possible to observe all craters in different shadowing. However, the craters on equatorial part for example were having the sunlight with bigger angle and it was possible to collect more information. But the team was not lucky with the north side craters.
We do not count the south part as there was less chance for high resolution imaging with spacecraft’s encounter position.

Although above is the case, there are many crater bases and rims on Pluto covered with volatile ices which enables better analysis with the higher albedo.

Quite a number of those craters are observed to be degraded and fallen apart over time. That can both happen as a result of geological activities like volatile flows and sublimation or the erosion on the rims and scarps itself.

Elliott Crater on Pluto
Elliott Crater and its analysis. See this link for details of this study. Credit: NASA/JHUAPL/SWRI

Craters on several surfaces, as mentioned earlier, also directly and indirectly implies existence of water ice and maintained detailed information about the layers below the surface.

Craters of Pluto – The Opportunity to Understand Kuiper Belt

What Pluto’s craters tell us is not limited with Pluto and Pluto System. Providing key details about many facts about Pluto, they broaden our look to the Kuiper Belt as well.

How is that possible? Why don’t we directly look at the Kuiper Belt Objects (KBO) instead?
First of all, they are extremely far from us and this disables extensive observations. What’s more, scientists find the population of smaller KBO‘s more valuable since they are believed to be holding the key information about earlier years of Kuiper Belt and Solar System. And smaller means more difficulty in Earth based observations.

Luckily, a group of craters on Pluto surface are older than 4 billion years, which corresponds to those early days enabling such study. And the variety of crater ages increase possibility of better evaluation.

Burney Crater
Burney Crater Basin including many small craters. Credit: NASA/JHUAPL/SWRI

KBO Conclusions of Researches on Pluto’s Craters

Recent studies show that frequency of bigger and older impact craters on Pluto, despite smaller ones might have disappeared over time, is remarkably higher than the others. That refers to a greater population of bigger KBO‘s in comparison during early years.

On the other hand, further analysis reflects a deficiency in current smaller KBO population (especially the ones smaller than 1-2 kilometers in diameter) than previously predicted. While the estimation for bigger KBO’s was correct, New Horizons Mission based studies do not match with earlier calculations on tiny ones.

Although that was not foreseen, there are some explanations about that situation. Researchers believe the ones that are structurally weak enough might have been totally destroyed, or Neptune might have cleared the strong ones off. But one way or another, this shows that the Kuiper Belt has its own dynamics and has slightly different properties comparing to the Asteroid Belt.

And what above heralds is that, as a KBO is shrinks, it becomes more intact than thought; even impact free. And that truely increases their importance to another level. That’s also proved via New Horizons Spacecraft’s close flyby to Ultima Thule, a two-lobed KBO (smaller than 100kilometers long) which has only one impact crater.

Ultima Thule
Ultima Thule as seen by New Horizons Spacecraft. Source here, Credit: NASA/JHUAPL/SWRI/NOAO

Conclusion

From those points, researchers concluded that the frequent collisions in early days of Solar System quickly enlarged the average sizes of KBO’s to 100km levels, reduced the amount of small ones and provided a kind of equilibrium state.

From another perspective, scientists believe that those smaller KBO’s might also be the residues of secondary, tertiary.. remains of other smaller collisions. Although that sounds hard to track, science finds a way: Like it did to understand a ring of millions of small rocks, via examining the few impact on one of its big residents in last +4 billion years: Pluto!

Craters on Pluto and a close look to them served us great amount of knowledge for both Pluto and Kuiper Belt. Scientists are working on further flyby, orbiter and landing missions for KBO’s, which can increase our knowledge about Solar System formation to higher levels. Can’t wait to see those days to come!

References

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