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Headshot of Emily Lakdawalla

First look at New Horizons' Pluto and Charon images: "baffling in a very interesting and wonderful way"

Posted By Emily Lakdawalla

15-07-2015 16:42 CDT

Topics: trans-neptunian objects, New Horizons, pretty pictures, Pluto, Charon, Triton, explaining science, dwarf planets beyond Neptune, Pluto's small moons

The New Horizons spacecraft has only had time to downlink seven LORRI images since its flyby of Pluto yesterday. Today's press briefing at the Applied Physics Laboratory in Maryland was preceded by hours of New Horizons team members cryptically dropping hints on Twitter at astonishing details in those few images. And the images are astonishing, as well as beautiful, surprising, and puzzling. Team member John Spencer aptly summed them up when he described them as "baffling in a very interesting and wonderful way."

Charon’s surprising, youthful, and varied terrain

NASA / JHUAPL / SwRI

Charon’s surprising, youthful, and varied terrain
Remarkable new details of Pluto’s largest moon Charon are revealed in this image from New Horizons’ Long Range Reconnaissance Imager (LORRI), taken late on July 13, 2015 from a distance of 466,000 kilometers. The LORRI image has been combined with color information obtained by New Horizons’ Ralph instrument on July 13.

Here's how Cathy Olkin described it (I did my best to transcribe as she talked, but did so imperfectly, so consider this a paraphrase, and I also convert Imperial units to metric):

Look at the north pole (informally referred to as Mordor). The red coloring extends beyond just the deepest darkest part of that polar region. Dark coloring could perhaps be a thin veneer, because you can see craters poking through. The darker area is polygon-shaped, while wider red area is more diffuse. Farther down on the disk, from northeast to southwest, is a series of troughs and cliffs. Striking, amazing. That could be due to internal processing. Just below that is a region where it's relatively smooth; there's less craters. Perhaps there's recent resurfacing in that area, so that's very exciting to see as well.

Near the top, at about 2:00, you can see a long, linear feature, and a notch where you're looking through to space on the other side. That canyon is really quite deep, like [6 to 10 kilometers] deep. I find that fascinating. It's a small world with deep canyons, troughs, cliffs, and dark regions that are still mysterious to us. Another canyon at about 10 or 11:00 is [5 kilometers] deep. So much science in this one image alone. Soon we'll get a higher resolution image that won't get all of Charon that will have a factor of 5 better resolution. Pluto did not disappoint; I can add that Charon did not disappoint, either.

Why are these features so surprising? Look at the worlds that are similar in size to Charon, such as Dione and Tethys. Charon has way too few craters for a body of its size. That implies a very youthful surface. That broad, smooth area near the bottom, in particular, is geologically quite young. And there is a diverse array of terrains visible in this image. I never expected Charon's story to be so complicated. I can't wait for higher-resolution images.

There are other bodies in the solar system similar in size to Charon: Ariel, Umbriel, and Oberon. They, too, have some craters, as well as chasms. Yesterday, people were saying Charon looked more heavily cratered, hence, older. Seen at higher resolution, Charon is looking younger than we thought. Now I really, really wonder, if we could look more closely at Uranus' moons, would we see more youthful surfaces than we thought?

The other most amazing image of the press briefing was Pluto, seen at high resolution. It is in the bright area that we have been informally calling the "heart" but which now has a still informal but slightly more official name: Tombaugh regio, named for the discoverer of Pluto. It is located near the southern end of Tombaugh Regio, near the terminator, where the Sun glances across the landscape at a low angle, highlighting topography. And oh my goodness, what topography.

The icy mountains of Pluto

NASA / JHUAPL / SwRI

The icy mountains of Pluto

This close-up image of a region near Pluto’s equator reveals a giant surprise: a range of youthful mountains rising as high as 3,500 meters above the surface of the icy body. The mountains likely formed no more than 100 million years ago—mere youngsters relative to the 4.56-billion-year age of the solar system—and may still be in the process of building, says Jeff Moore of New Horizons’ Geology, Geophysics and Imaging Team (GGI). That suggests this region, which covers less than one percent of Pluto’s surface, may still be geologically active today. Moore and his colleagues base the youthful age estimate on the lack of craters in this scene.

The image was taken about 1.5 hours before New Horizons closest approach to Pluto, when the craft was 77,000 kilometers from the surface of the planet. The image easily resolves structures smaller than a mile across.

I see a minimum of four distinct terrain types in this image, and not one of those four is an impact crater. Plus really interesting albedo variations (though those could be textural effects rather than albedo, given the low angle of the sunlight). Here's what John Spencer had to say about the image (again, paraphrased, combining in some responses to later questions from media, and also metric-converted):

We haven't found a single impact crater on this image. Pluto is being bombarded by other objects in the Kuiper belt; craters happen. Eyeballing it, we think it has to be under 100 million years old; it might even be active right now. Mountains are up to [3500 meters] high. We know the surface of Pluto is covered with nitrogen and methane and other volatile ices; you can't make mountains out of that stuff. We are seeing the bed-ice of Pluto. Water ice is strong enough to hold up big mountains, and that's what we think we are seeing here.

What's particularly exciting to me about this is that this is the first time we've seen an icy moon that isn't orbiting a giant planet. We usually attribute strange features on icy worlds to tidal heating. That can't happen on Pluto. There is no giant body that can be deforming Pluto on a regular basis; Charon is too small to do that. This is telling me that you do not need ongoing deformation from a giant planet to power deformation on an icy body. That's a really important discovery that we just made this morning.

We will have more of this mosaic to show you on Friday.

We have no idea at this point how mountains formed. Triton doesn't have this kind of rugged terrain. It has a lot of strange materials, but it doesn't look at all like this.

The terrain to the lower right looks really strange. It's like piles of stuff with grooves on it. It's baffling in a very interesting and wonderful way. I don't think it looks like the surface of a lava flow, but perhaps a similar process happening on a much larger scale.

[In response to a question about how Pluto and Charon could retain heat for so long]: We have a couple of options. We know there's radioactive material inside Pluto and Charon; radioactive heat is powering geology inside the Earth. It may be telling us that even small bodies, if they're icy, can store heat. Maybe they can store heat for a long period of time.

Those mountains are something else. They don't line up like impact crater rims. There are kind of similar mountains on Io, but this weird dense patch isn't a perfect match to that. Earth has pointy mountains, but Earth also has water erosion. So how's this for a going-out-on-a-limb, crazy idea: Are Pluto's mountains like Earth's or Titan's? Have they been eroded into their present shapes by fluid flow: water on Earth, methane on Titan, and, I don't know, nitrogen or neon on Pluto?

With both Charon and Pluto appearing so youthful, my first question about this was: is it time to consider the idea that the Charon-forming impact happened a lot more recently than we thought? I asked the question at the press briefing, but as it was my second question they didn't answer it. I've been polling scientists since, and while geologists like the idea, dynamicists say that the odds of such an impact happening late are "infinitesimal" (that's a quote from Bill McKinnon). I asked him whether a late impact is less likely than retaining primordial heat to the present day, and he -- a geophysicist -- seems to prefer rethinking his geophysics work to considering a late impact.

One really important point about this image, to me, is that it does not look anything like Triton seen up close. As a reminder, this is Voyager 2's highest-resolution observation of Triton:

High-resolution view of Triton's surface from Voyager 2

NASA / JPL / Ted Stryk

High-resolution view of Triton's surface from Voyager 2
On its closest approach to Triton on August 25, 1989, Voyager snapped several high-resolution mosaics. Triton is considered to be a Kuiper belt object that was captured into Neptune orbit, an event that would have heated it and altered its surface through cryovolcanism. As a result, there are few large craters; a few small ones are visible in this mosaic.

From a distance, Pluto and Triton have many similarities, but seen up close, it's clear that the two have very different geologic histories. I think this demonstrates how important the kind of resolution you can only get from spacecraft encounters is to understanding the history of a world. We can't write the story of Pluto's history yet -- it'll take years of work on New Horizons' data to do that -- but it's safe to say that its geologic history is quite different from Triton's.

A very recently-published open-access paper by Amy Barr and Geoff Collins, "Tectonic activity on Pluto after the Charon-forming impact," hints at what might have driven tectonic activity on these bodies. I asked Amy to say a few words about this work that would be relevant to the just-released images:

The pictures are showing a potentially younger and evidence of some surface activity. The question is what is driving the activity.

A paper by myself and Geoff Collins published earlier this year in Icarus suggests that activity may be kick-started by the Pluto/Charon impact if Pluto is warm (ice layer warmer than, say 200 K) before the impact.

If that is the case, we would expect that the bodies could have significant tidal dissipation in them, perhaps enough to drive a brief bloom of geological activity on both bodies.

The basic physical principle is that a body that is hot before the impact will be mushy enough to experience tidal heating in the subsequent orbital evolution to get hotter. (The hot get hotter and the cold stay cold, if that makes sense.)

Every time we have looked at a body that has experienced tidal heating, such as Io, Europa, Enceladus, we find that the body has been more active, or is putting out 10 to 100 times the amount of heat predicted by models.

Enceladus taught us that our models of tidal heating for icy bodies were not right, and I think the Pluto/Charon system could be telling us something similar.

Food for thought. Thanks, Amy, for responding to my call so fast.

They released two other images at the briefing. Here is the first well-resolved image of Hydra, which settles the question of its size and shape. The best previous estimate, based on Hubble data, was 58 x 34 kilometers, quite elongated. This image shows it's even smaller than that -- 42 by 33 -- but one reason for the smaller big dimension is that the moon appears to be bent!

Hydra viewed by New Horizons

NASA / JHUAPL / SwRI

Hydra viewed by New Horizons

Since its discovery in 2005, Pluto's moon Hydra has been known only as a fuzzy dot of uncertain shape, size, and reflectivity. Imaging during New Horizons' historic transit of the Pluto-Charon system definitively resolved these fundamental properties of Pluto's outermost moon. Long Range Reconnaissance Imager (LORRI) observations from a distance of approximately 640,000 kilometers revealed an irregularly shaped body characterized by significant brightness variations over the surface. With a resolution of 3 kilometers per pixel, the LORRI image shows the tiny potato-shaped moon measures 43 kilometers by 33 kilometers.

Like that of Pluto's largest moon Charon, Hydra's surface is probably covered with water ice. Observed within Hydra's bright regions is a darker circular structure approximately 10 kilometers in diameter. Hydra's reflectivity is intermediate between that of Pluto and Charon.

This is a tantalizing image -- it tells us the size and shape of Hydra, but it contains variations in brightness that speak of either albedo variations across the surface or a double- or even multiple-lobed shape, or both albedo variations and lobe-y shape. The phase angle of this image is only 16 degrees -- very close to "full" -- so its crescent shape is not due to lighting; it's due to the actual shape of the moon. Is it like Churyumov-Gerasimenko? Hydra is a lot bigger than Rosetta's comet. Remember, the eventual extended-mission target of New Horizons will be a body of comparable size to Hydra.

The other image released today included spectra of the surface from the Ralph LEISA instrument. They windowed the data, returning only a portion of the spectrum, including the portion where methane is strongly absorbing.

Methane on Pluto

NASA / JHUAPL / SwRI

Methane on Pluto
Spectra from the New Horizons Ralph instrument acquired on July 12, 2015 reveal an abundance of methane ice, but with striking differences from place to place across the frozen surface of Pluto. This is the first detailed image of Pluto from the Linear Etalon Imaging Spectral Array, part of the Ralph instrument. The observations were made at three infrared wavelengths. In this picture, blue corresponds to light of wavelengths 1.62 to 1.70 micrometers, a channel covering a medium-strong absorption band of methane ice, green (1.97 to 2.05 micrometers) represents a channel where methane ice does not absorb light, and red (2.30 to 2.33 micrometers) is a channel where the light is very heavily absorbed by methane ice. The two areas outlined on Pluto show where Ralph observations obtained the spectral traces at the right. Note that the methane absorptions (notable dips) in the spectrum from the northern region are much deeper than the dips in the spectrum from the dark patch.

Pluto's polar cap shows a very deep methane absorption. There is also methane in the dark region, but the absorption is not as deep, suggesting that there is something in there that is scattering more light. (If you had a photo of Pluto in a methane absorption band, the polar cap would look nearly black, while the dark regions would be bright! This is just the bare minimum of information that can be wrung from this image; it serves as a hint of the science to come.

The next press briefing is scheduled for Friday afternoon at either 1:00 or 2:00 and will be held at NASA Headquarters; I will go down to Washington, D.C. to attend in person. The next briefing after that is a week from Friday. There are not currently any stated plans for image releases tomorrow but I am hopeful that they will throw us Pluto fans a bone with at least one image release, if not more; they should be able to show us Nix, at least. I've been updating my "what to expect" post as plans change, but here's the relevant stuff for the rest of this week, fodder for the Friday briefing:

Thursday, July 16 at 04:23 UT / 00:23 ET / Wednesday, July 15 at 21:23 PT: 1.9hr downlink: First Look C

Thursday, July 16 at 07:23 UT / 03:23 ET / 00:23 PT: 1.9hr  downlink: First Look D

Thursday, July 16 at 13:22 UT / 09:22 ET / 06:22 PT: 4.3hr downlink: First Look E

Stay tuned! They are kicking us out of the media center shortly, so that's all I'll have time to post today.

 
See other posts from July 2015

 

Read more blog entries about: trans-neptunian objects, New Horizons, pretty pictures, Pluto, Charon, Triton, explaining science, dwarf planets beyond Neptune, Pluto's small moons

Comments:

Tony Fisk: 07/15/2015 05:33 CDT

re: your late Charon impact theory. If it were the cause of these features, Is 100my enough time for the system to settle into the current stable orbital configuration (tidal locking, etc.)? I suspect not but, whatever. Interesting times are not always a curse.

Lepton: 07/15/2015 05:55 CDT

Maybe the reason why Charon is so different than Triton is because Triton suffers from being in the neighborhood of a huge planet?

rickray777: 07/15/2015 05:59 CDT

"Oh, my goodness, what topography" is right! By Jove, I not only see mountains; but I also see a lot of gullies and grooves within these mountains! What in heaven's name could have created them? A bit like the Appalachians on Earth, perhaps? Nope; once again, distinctly Plutonian! -- probably unique in all the Solar System. In any event, I'm impressed! I mean, REALLY, REALLY impressed! WOW! Truly my life's dream come true (at 54). And Congratulations to NASA and the Planetary Society! Keep those images coming...

karle: 07/15/2015 06:05 CDT

Did you ever get an answer to the second part of your question (impact theory I think it was)?

Phenocryst: 07/15/2015 06:12 CDT

Superb blog to follow a superb news conference. (I encourage all who didn't see it check "Vidio File" on NASA TV). Easy to loose yourself in those images as myself and Emily did. Emily forgot for a minute she was in Maryland and not at JPL. I love these Emily, Thanks!

John Walker: 07/15/2015 06:18 CDT

Why 100My ? That is only an estimate of the max surface age. The proto Pluto/X Body collision could have been significantly earlier perhaps 1Gy. For small body heat retention much easier than 4.5 Gy. But, if the planetary dynamicists say the chances of late collision are infinitesimal then I would tend to believe them. The impactor was huge. If the impact was late what changed the orbital dynamics after so long?

MikeQ: 07/15/2015 07:07 CDT

Looking at these mountains a number of them don't come to a sharp peak but have a flat top or a dot right at the peak. Could we be looking at volcanic caldera?

Rock Howard: 07/15/2015 07:36 CDT

1) I can't abide 'infinitesimal' given the reality of Haumea. KBOs seem to be more prone to weird orbits than inner system objects. 2) I see the two sections of the 'heart' having different ages given the extra mottling and perhaps cratering on the right side lobe. Define the processes that formed those lobes and you may have a candidate for mountain erosion.

dissembly: 07/15/2015 08:04 CDT

As usual you have the best coverage on this Emily, I'm sharing your posts on facebook. Thank-you for providing such a good resource.

Bob Ware: 07/15/2015 08:29 CDT

Congratulations to all involved making this mission possible. Pluto is more intriguing than I imagined! It is not going to be a boring body in any manner! The same for Charon!

Dominique: 07/16/2015 01:06 CDT

The mountains remind me some of the ice rafts of europa. Could be remnants from the collision with charon. I bet the collision is (relatively) recent, that would explain why the two bodies are essentially craterless.

Dominique: 07/16/2015 02:07 CDT

Craters on hydra and nix could also tell if the systèm is Young or if only pluto and chaton surfaces are.

Mewo: 07/16/2015 02:34 CDT

The biggest surprise for me is the scarcity of craters on both Pluto and Charon. I did not expect that some process would be resurfacing both worlds.

Harvey: 07/16/2015 03:05 CDT

Utterly fascinating! Yet another body comes under closer scrutiny and reveals itself to be different from anything else yet observed. Perhaps we shouldn't be surprised at the diversity of all these worlds given the diversity of conditions under which they formed and have since existed. What a privilege to have been alive to witness all this from Apollo onwards!

Vishnu Dutta: 07/16/2015 03:47 CDT

Looks like a planet to me :) BTW Emily, you are doing a great job of consolidating all the info at one place with amazing narration. Whenever a new event happens i dont go to NASA website, i come here because i know you would update it with great explanations. You are doing a great service to science. Keep it up Vishnu

MartyvH: 07/16/2015 06:29 CDT

"Planning your July around New Horizons' Pluto Pictures" I'm an IT student but I feel like I am doing that! Guys, make sure you spend time taking in that phrase: "The mountains of Pluto" Also spend time taking in that photograph of the mountains. Compared to what we had before, it is a miracle.

NearCharon: 07/16/2015 07:07 CDT

There is a very small body off the edge of CHARON at 1 o'clock position in the photo. May be one of the smaller moons of Pluto or a star or what ??

Ray: 07/16/2015 07:52 CDT

At least on Pluto, could a significant amount of internal heat come from isostatic rebound from the large (and presumably heavy) nitrogen snow deposits that move around with the seasons?

Rittmann: 07/16/2015 08:15 CDT

Looking at the image of the mountains, I see a long fault going from approx. 10 to 4 in diagonal in the rougher and darker mountain terrain. The fault disappears in Tombaugh region, which appears to be covering the older terrain. But if we look at the division between the mountains and the filling, there appears to be some sort of cliff. Here are my thoughts on it: a pre-existing underground reservoir of liquid or fluffy material erupts due to some event that generates the fault (I see two more smaller faults parallel to the big one). The filling covers Tombaugh region, and as it cools down it shrinks, causing the cliff terminator we see in the image. With the crust hardened after cooling down and with increased pressure due to the shrinking, the subsurface reservoir erupts through cracks into the mountains we see, which feature brighter material than the rough terrain around it, and looks different as well. The mountain in the middle of Tombaugh has no cliffs surrounding it.

LocalFluff: 07/16/2015 08:46 CDT

Pluto has a great flat ice hockey rink up there to the left in the image. But they might not be trained for the small NHL standard. Could the high mountains be icebergs turning over, or sheets of ice in some arctic semi-tectonic kind of process? They seem to have such sharp ridges, like broken glass.

rickray777: 07/16/2015 09:56 CDT

You know, I would say that in many ways, this Pluto encounter is to us (baby-boomers) what the Apollo 11 Moonwalk (July, 1969) was to our parents' generation. Back then (as now!) there was a lot of stuff going on: the Cold War, Vietnam, drug busts, pollution, etc., etc.! Sometimes, it IS good to get one's mind off of worldly things, and into Something Grander! Even though I was only eight at the time, it wasn't until I grew a lot older that I truly began to understand. There IS one key difference, however. While the Apollo astronauts had a pretty good idea of what to expect (having seen many close-ups of the Moon), Pluto (even in the world's largest telescopes) was just a tiny speck of light (set against the much-brighter background stars!). And even with the newly-refurbished Hubble Telescope, there was still very little to be seen, really. Until just two days ago (and ongoing!); when the New Horizons spacecraft transformed that erstwhile speck of light into an actual Place, as real as the Earth! Hey, my deepest heartfelt congratulations to ALL of us for making it possible!

Paul F: 07/16/2015 10:43 CDT

It was quoted above: "We usually attribute strange features on icy worlds to tidal heating. That can't happen on Pluto. There is no giant body deforming Pluto....Charon is too small to do that" I wonder. Tidal heating isn't just about mass. It must also consider distance between the bodies, the materials being heated and the physical extent of the tidal effect. One wouldn't need to heat the crust very much to turn something like pressurized methane from a solid to a liquid or gas which might push up mountains, or perform resurfacing if it can reach the surface. The "lava" doesn't have to be silicate or sulfur like Earth or Io. Also, if Pluto and Charon are still settling into a tidally locked state and are librating a bit, one could imagine that the heating would be localized to just "under" the opposing body. That might drive the asymmetry in the surface features that we are seeing.

Rob: 07/16/2015 10:45 CDT

Great job Emily. "From a distance, Pluto and Triton have many similarities, but seen up close, it's clear that the two have very different geologic histories." It was the other side of Pluto during approach that looked like Triton to me. Less than half of Triton itself has been imaged, so it would be interesting if the other side of Pluto looked like the part of Triton that has been imaged, while the unknown part of Triton looked like the surface of Pluto that we know up close.

drjonoverton: 07/16/2015 11:49 CDT

Hi Emily, re: methane distribution, just spotted a small error in your otherwise excellent article: You wrote: "Pluto's polar cap shows a very deep methane absorption." It's actually the opposite way around - the green colour indicates low absorption of IR by methane, due to the strong dilution of methane with frozen nitrogen at the pole. It's that nitrogen that gives strong absorption in IR at the pole. From the legend to the photo: "... blue corresponds to light of wavelengths 1.62 to 1.70 micrometers, a channel covering a medium-strong absorption band of methane ice, green (1.97 to 2.05 micrometers) represents a channel where methane ice does not absorb light, and red (2.30 to 2.33 micrometers) is a channel where the light is very heavily absorbed by methane ice. " And from NH mission pages: "“We just learned that in the north polar cap, methane ice is diluted in a thick, transparent slab of nitrogen ice resulting in strong absorption of infrared light,” said New Horizons co-investigator Will Grundy, Lowell Observatory, Flagstaff, Arizona. One of the dark patches along the equator show that the methane ice has shallower absorptions, suggestive of a very different texture in that region. “The spectrum appears as if the ice is less diluted in nitrogen,” Grundy speculated “or that it has a different texture in that area.”"

rickray777: 07/16/2015 02:22 CDT

Hey, did I forget to mention: we didn't even have the slightest notion of what to expect at Pluto (there being so little to see!). Heck, we didn't even know the (now-dwarf) planet's precise orbit! Indeed, by the time New Horizons got there, Pluto had only made it about one-third of the way around the Sun since its discovery (back in 1930!). And so it was literally more unknown to us, than it was to our Apollo astronauts back at that time. Hey, I remember all too well reading a 1985 caption underneath the then-best photos of the Pluto-Charon system: will we ever know what Pluto and Charon really look like close-up? Well, Voyager 2 was just about to make close flybys of Uranus and Neptune (and their major moons); but, alas, no mission to Pluto was even planned (then)! Hey, maybe in another century (or two), our descendants would undertake this (then) modern-day impossible dream. And now we're already naming actual features? One thing I enjoy seeing is Pluto before-and-after; hey, it sure feels good being able to complete the family portrait of the classical Solar System. Yes, artist conceptions are great, and all that; but nothing beats the first actual close-up photos of previously-unexplored worlds. Congratulations, and medals all around!

Steven: 07/16/2015 04:15 CDT

I think a question to be asked is are Pluto or Charon truly spherical - if they are distorted that might be a clue to something going on inside - or if not that that isn't related to the young surface. Now that they measurements are more precise is there a chance of a pole vs equator (and perpendicular to Charon-Pluto vs along C-P) differences...

brossa: 07/16/2015 04:34 CDT

Is the lack of cratering not somewhat explainable by the loss of surface ices over time? If hundreds of meters or kilometers of ice have sublimated over time, or sublimated and snowed out over and over...

wizard2399: 07/16/2015 08:41 CDT

10,000 foot ice H2O mountains on Pluto. What an amazing discovery! 100 million year old resurface, another unlikely discovery. What other secrets does Pluto have for us. Much more data to be downloaded and examined. What a toy factory for the scientists involved in this program! Could it be that geologists will finally gain some more respect? Lol.

Johan: 07/17/2015 12:18 CDT

@brossa I was going to suggest the same thing on my comment. I remember hearing that Pluto had a substantially thicker atmosphere earlier in the solar systems history during the press briefing. After reading your comment it struck me that the further back you go in time the hotter the sun was so the rate of loss could have been a lot more than we imagine. Now was it enough to expose some topography or is this more a product of the impact that created the system. Congrats New Horizons team!! As with any good NASA mission, more mysteries unearthed.

ChiMing: 07/17/2015 04:33 CDT

when we talk about impact craters, we usually mean those formed by rock (or ice) impactors on rock surface, but for an icy world like Pluto, shouldn't we be looking at said impactors on icy surface? And wouldn't this change things quite a bit due to the much lower melting/sublimating point of ice? e.g. would most such impacts even leave craters, since the icy surface is likely to melt easily and smooth over any craters?

metingun: 07/18/2015 05:05 CDT

Paul F , very good suggestions indeed `, one could imagine that the heating would be localized to just "under" the opposing body. That might drive the asymmetry in the surface features that we are seeing. ` Gravitational attraction in between Pluto and Charon(nearest moon) logically should be the driving -tidal force- no matter how small it is can easily explain `Water Ice mountain formation up to 3500 meters high` -particularly certain regions of Pluto effected by Tidal Gravitational Forces more than other regions -as we see clearly- in other words rather than symmetrical circular pattern of mountain formation in random distribution these mountainous terrains are limited to -probably opposing exactly to the gravitational force `pull` regions of Pluto by it`s moon Charon , The proof of this hypothesis ; rests with the calculation of `Orbits` and `balanced alignments` of Pluto and Charon . If the `Mutual Tidal pulling force vector ` periodically crossing repeatedly` over and over -like sawing machine- at the same surface regions of Pluto where Water Ice mountainous located as we observe - than it should be the `absolute proof of the origin of mountains that is caused by the tidal pull no matter how insignificant and small it might be ` . If the Tidal vector calculated by mutual orbital movement has no correlation whatsoever with the surface regions where the mountains located , than there should be some other reason behind it . Bottom line is ` gravitationally and tidally locked ` statement has be `defined` more specifically by the calculation of orbital movements and vector forces on the surface regions of both bodies Pluto and its Moon Charon . The answer clearly lies at that calculation .

dissembly: 07/20/2015 07:00 CDT

metingun - my understanding is that there should not be any tidal force between Pluto and Charon, as they have reached the end-point of their tidal evolution, being fully tidally locked. There is no tidal bulge travelling over Pluto's surface due to Charon, because Pluto rotates perfectly in sync with Charon's orbit. That's what's so amazing about this discovery - it's the only example of an icy body where we can actually rule out - 100% for sure - a (modern) tidal origin for the presumed internal heating!

Michael Paine: 08/05/2015 03:19 CDT

This is a very usually binary (planet!) system - both objects are tidally locked and the baeycentre is between them (not inside Pluto). One possible source of "heat" might be the loss of angular momentum during the spin-down process of tidal locking. That would mean, however, that tidal locking has occured relatively recently (100s of millions of years.)

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