There’s been a lot of wonderment and rethinking going on about how Pluto and Charon appear to have far fewer impact craters than one might expect. Early hypothesis say this suggests their surfaces are young.
But I don’t see anyone advancing the argument that maybe they’re less cratered because they live in a very low-traffic area, and have for a long time. Want to decrease your chances of getting your car dinged up? Never drive it around other cars.
Is this a reasonable hypothesis as well?
I don’t think so.
Something like Earth’s moon will show virtually every impact it has ever had in its 4 billion (or whatever) years of existence. With no geologic or weather activity, the only thing that will erase one crater is a new crater. Even if impacts are relatively rare, you expect them to be happening at least a few times every billion years.
Secondly, Pluto was demoted to dwarf planet status because it had not cleared out its orbital zone. As a result, there are more things out there to collide with, not less. I suppose the absolute density of those things is pretty low, and Pluto’s orbit is skewed relative to the ecliptic but it still isn’t a zone that we’d consider low risk for impacts.
Given that, a small number of visible impacts means there’s some mechanism for resurfacing.
I had always envisioned Pluto’s surface to be a lot like Triton, Europaor Enceladaus, which are all relatively small, ice-covered, rocky bodies. When you look at those, you also see relatively few craters. Of course, as moons of gas giants, all of these have the potential for gravitational forces to generate internal heat, so I suppose it wasn’t a given that Pluto would be this active.
No, because there are a crapload of flying rocks out there in the Kuipier Belt - that’s where all the comets come from, for instance, as well as a lot of meteors and such. It’s not really a low traffic area.
I think re-surfacing is a long shot, Pluto is pretty small and so it’s crust would be much thicker. There’s speculation this occurred on Venus “recently”, but here we have (presumably) a rather thin crust, thin enough to break up and sink into her mantle zones.
We have the Late Heavy Bombardment theory which relies on planets changing orbits. If we can make a speculative extension of what is already speculation, we perhaps could have Pluto in a far more distant orbit. Extending even further perhaps Pluto was formed in a distant solar system and has spent much of the past several billion years in the emptiness of interstellar space.
It’s a small target in an area with less density of random rocks. I think I’d be more surprised if Pluto was as beat up as our Moon, things get crowded this close to the Sun and in the ecliptic.
You give this explanation, but think resurfacing is the long shot?
If it ever participated in a truly huge collision, it’d get entirely resurfaced at that point. If that happened fairly recently on the relevant timescale, we’d expect to see a couple of newish craters and that’s about it.
Could Charon & Pluto be the surviving chunks of a once larger planet? Or even if not, Pluto still could have been whacked pretty good.
I’m not suggesting this *did *happen, but merely that it *could *have happened.
Pluto has a relatively large moon, Charon, which is about 12% of the mass of Pluto and orbits closely enough that the two are actually tidally locked and orbit a common barycenter that lies outside the radius of Pluto’s surface. Given the composition of the surface of Pluto as frozen methane and nitrogen, it isn’t surprising that tidal forces would smooth out blemishes over a relatively short period. Although the average distance between objects in the Kuiper belt is pretty vast, the objects are often moving well out of the plane of the elliptic, and impacts would produce fragments that no doubt bombard Pluto and other Kuiper objects frequently (in astronomical terms).
It is certainly possible that Pluto could have been ejected from another slowly passing system and captured by Sol, which would go to explaining its odd orbit, but even if so it must have evolved within a system and been impacted there. It is very unlikely to have been ejected from a system at great distance and just happened to have been captured here owing to the great differences in velocity between distant star systems and the extremely low probability of passing close enough to Sol to be captured. However, it is far more likely that Pluto was formed as many Kuiper objects were by coalescing somewhere else in our star system and being thrown out by gravitational interaction of the gas giant planets. A mission to land upon and sample the subsurface of Pluto would probably be required to confirm or falsify that, but such a mission is well beyond practicality at this point.
Stranger
From the press conference today it sounded like geological processes responsible for creating relatively young surfaces was taken as given, the only question being what those processes were. Possibilities mentioned later included volcanism, migrating ice, latent heat from a subsurface ocean, or perhaps internal radioactivity. Apparently Pluto is believed to contain silicate rock under the ice surface which, as on earth, will have various radioactive components which generate heat, and that could be another possibility.
In any case the full-disc Pluto image from Monday does have noticeable cratering, it’s just a lot less than expected. There were no craters at all in the roughly 150-mile square hi-res closeup image released today, just huge ice mountains suggesting significant geologic activity of some kind. Charon might have even less, but it has vast geological formations, too.
One of the interesting sidelights from today’s press conference is that there is so much brand new information that most questions that the observations raise don’t have answers yet, just educated guesses.
My understanding from today’s press event is that because both Pluto and Charon are both tidally locked with each other, tidal forces would not exist as heat-generating and surface-reforming agents as they do on the icy moons of the gas giants. I’m not sure but I also think it was said that they expected that they became tidally locked this way relatively early in the history of their formation.
They won’t experience fluctuating tides as Earth does due to relative motion of our moon, but the bodies still experience differnential centrifugal stresses due to roatation just as water in a bucket swung at the end of a rope will move outward (and if you have a deep enough bucket, will show a measurable gradient in pressure). It is also possible that the bodies have an oscillating axial component of momentum, and of course they’ll experience minor fluctuations from interations with the other moons of Pluto (Styx, Kerebos, Hydra, and Nix). While these forces are all small, the surface ice should really be considered a very viscous liquid with irregularities causing stress risers that naturally smooth out.
Stranger
The Plutonians are E-x-c-e-l-l-e-n-t landscapers.
But this should already be baked into their shape. The surfaces of both will be (approximately) at equipotentials for the effective potential, and the forces causing them to take that shape should be approximately the same as the forces causing a stationary isolated object to be a sphere.
That doesn’t explain the 11,000ft high water-ice mountains though, I think. Any thoughts on the process behind them, Stranger? Is it possible that that the Pluto system has resulted from, in the formation-of-the-solar-system timescale, a fairly recent collision? The orbit around the sun is kind of odd after all.
Here’s some 2012 speculation on radioactive potassium keeping a liquid ocean going under the outer ice layers. We know there’s a rocky core down there based on overall density and that’s only going to form when a planet(oid) is hot enough for a sufficient period of time to let components sort themselves by density. Radioactive heating seems to be the default assumption for anything that has layers.
But it’s probably a little premature to be answering this kind of question with any certainty.
Very true. I loved the glee with which the project team addressed similar questions - “We don’t know, but let’s find out…” 
Pluto’s orbit is only odd when judged by the standards of planets. Kuiper belt objects can have all sorts of screwed-up orbits, most of them much more screwed-up than Pluto’s.
Yes, and everytime it experiences a major impact it creates a dislocation that will be resolved by the methane or nitrogenous ice flowing. Think of a bag filled with gelatin swung at the end of a tether. Any void of significant size, especially on the surface, will tend to flow together (or on the front side, flow away but then be filled in so as to equalize surrounding presure). I’m not surprised there are some surface processes going on, but the lack of relative motion probably limits subsurface heating. Unless Pluto has a radioactive core, it probably isn’t doing much below the surface.
No idea on that one, and frankly you’d expect a crest of that high to flow back down in short order. The fact that Pluto and Charon appear to be in a stable tidal lock with each other, and have collected other moons with stable orbits about their barycenter argues against a recent collision or expulsion (e.g. in the past few million years) but we really know so little about both the early evolution of the solar system and the composition and behavior of Kuiper Belt objects that I don’t think any marginally plausible hypothesis can be ruled out.
The data from New Horizons is likely to leave us with more questions than answers, and it is unfortunate that another, more sophisticated mission to Pluto-Charon will likely not happen in our lifetimes, but that’s planetology for you. I’m still pinning for a Neptune-Triton Orbiter mission that is unlikely to ever be funded.
Stranger
I think you may have this backwards. The Earth (and other planets) cleared their orbital zones by colliding with all that stuff. So then, wouldn’t it stand to reason that something which hasn’t cleared its orbital zone would have been in fewer collisions than something which has?
Does that mean objects could get trapped at the barycenter, like in an L5 point, so there could be a bunch of stuff hanging around there? That’d be cool.
Also, they seem to be comparing the number of impact craters to the Moon. Is there enough relative difference in speed between comets & other things at 1 AU distance from the sun compared to 1 Pluto-unit distance? If they’re going slower way out there, the craters might be smaller.
The barycenter of a doublet (or larger system) is just the fixed point about which the planets rotate, and only fixed in the rotating reference frame coincident with the barycenter; in other words, if you had a string connecting the two planets, the barycenter would be the point along the string at which no moment from rotation would be experienced. The barycenter is determined strictly as a function of the instantaneous mass distribution of the system (ignoring relativistic effects) and is not typically completely stable–it gyrates as masses move through their not quite circular orbits and thus become closer or further away–and is not a point of gravitational equilibrium unless there happens to be perfect symmetry between the two bodies a circular orbit.
The libration points are points of gravitational quasistatic equilibrium (or quasiequilibrium) that occur at saddle points between the Hill spheres of two masses with M[SUB]1[/SUB]/M[SUB]2[/SUB] > 25 and thus depend on the relative distribution of the gradient field. The inline points (L[SUB]1[/SUB], L[SUB]2[/SUB], and L[SUB]3[/SUB]) are unstable equilibrium (an object or spacecraft will tend to fall back to one of the masses because of radical gradients about the points) while the L[SUB]4[/SUB] and L[SUB]5[/SUB] points are relatively stable (large regions of essentially flat gradients). However, the Pluto-Charon system doesn’t meet the criteria for stable libration points.
Pluto-Charon at 4.7 km/s (average) is moving much slower than Earth-Moon (~30 km/s) but has both an eccentric and inclined orbit, which means it is much more likely to be impacted by debris in more regular orbits as well as other KBOs at may have opposite inclinations with additive relative velocity. Although impacts are probably infrequent they could still pack a wallop.
Stranger