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SYFY WIRE Bad Astronomy

Pluto and Charon Keep Getting WEIRDER

By Phil Plait

If there’s one thing we always find when we see a solar system object up close for the first time, it’s something surprising. Or somethings. Or lots and lots of somethings.

New images and data were released today from the New Horizons probe, now well past Pluto and its system of moons. We’re still getting these sent back from 5 billion kilometers away as a trickle, but what’s being seen so far is, well, amazing. As the mission’s top dog, Alan Stern, said Friday at the press conference, “I may be a little biased, but I think the solar system saved the best for last.”

An image released Thursday of Pluto’s moon Charon has a feature so bizarre a lot of people are scratching their heads over it. Check this out:

First, mind you, the images we’re getting back from New Horizons are compressed JPEGs, “quick look” images that are promises of things to come. The probe is still busily taking measurements of Pluto and its moons, and cannot do that and send them back to Earth at the same time. The higher-resolution data take much longer to send back to Earth, so they’ll have to sit on New Horizons’ disk drive until things have settled down.

So ignore the big, blocky squares you see in the images and look instead to the bigger picture.

This Charon close-up, taken from about 79,000 kilometers away on Tuesday, shows an area about 300 kilometers long, a region of contrasts. Overall it looks smooth, like a lunar mare, but dotted with craters. Some appear to have central peaks, which are common in large impacts. Those long channels look like rilles, collapsed lava tubes like we see on the Moon. I wonder though. They may be fault lines, cracks where the surface split (we see these on Mercury where the crust shrank as the planet contracted).

But the real mystery here is the weird feature to the upper left. What the heck is THAT?

It appears to be a mountain sitting in the middle of a depression. That’s bizarre. First, how would a mountain form on Charon in the first place? And once it did, why would the surface around it do that? The New Horizons scientists rightly called it a mountain in a moat.

We saw mountains on Pluto in the first image release, but that doesn’t really help here, since we don’t know how those formed. Weirdly, there don’t seem to be any other similar features on Charon around this thing, either. A slow speed impact with another object is incredibly unlikely, but so is a mountain pushing up from below. The moat is easier to explain, actually; it’s likely subsidence, sinking, of the material around the mountain due to its weight. But did it happen simultaneously with the mountain’s formation, or after?

Mysteries upon mysteries.

Speaking of the mountains of Pluto, enough data came back to generate this animated flyover of part of Tombaugh Regio, the heart-shaped area on the surface.

Amazing! Some of the mountains are more than 3 kilometers high, and wouldn’t be out of place in the Rockies … except they’re made of rock-hard water ice instead of granite like in my hometown mountain range. Incredible. That area has been dubbed Norgay Montes, after the Nepalese Sherpa who guided Sir Edmund Hillary to the top of Mt. Everest.* That’s lovely.

The second part of the video shows the icy plains north of the mountains. The close-up of that area, called Sputnik Planum, is also just plain weird:

Again, ignore the blocky JPEG compression artifacts … but it’s a patchy ice field! The individual segments are 20–40 kilometers across, separated by shallow troughs. It’s unclear what these are; perhaps they’re contraction features as the crust shrank (like cracks in mud after rain dries up in the desert). They could also be the tops of convection cells! If the material under the surface is liquid, it could be that warmed stuff from lower down rises and cooler stuff sinks. I personally am skeptical, but there’s not a lot of info to go on here. Who knows?

The troughs are interesting; in some places it looks like darker material has piled up in some places. And there are clusters of hills that follow the troughs on the right. Is that stuff that’s built up, or tougher material that has resisted erosion? There’s also pitting in the region to the lower right, but that’s on such a small scale that JPEG compression effects muck them up. We’ll have to wait for the higher-res images to see what’s what there.

I’m interested in the context of these plains as well, the more general layout of the terrain. Here’s a quick animation showing where they are in Tombaugh Regio:

Ooooooh, interesting. It almost looks like there was a massive flooding event, flowing down from the north, reaching to the foot of those mountains. I’d bet a good deal that’s the case here. We see this sort of thing on the Moon too.

Mind you, the lack of craters indicates this area is young, perhaps less than 100 million years old. Look to that picture of Charon above again, and note the craters. If they aren’t in Sputnik Planum, it’s because there used to be craters, but some event resurfaced that area. That supports (but does not prove) that this may have been from flooding.

This area of Pluto is rich in carbon monoxide ice, too, as seen by New Horizons’ Ralph instrument. This strong a concentration isn’t seen anywhere else on Pluto. Is that correlated with the weird terrain here? Maybe. Maybe not. More mysteries!

Next up: Nix!

Nix is one of Pluto’s smaller moons, and doesn’t show much detail when seen from nearly 600,000 kilometers away! But we see some. The image shows it to be about 40 kilometers across, and from other data it appears to be elongated, perhaps 80 kilometers across its long dimension. Not much can be said from this image so far, but it’s interesting to note that back in April or so, Pluto itself looked like to this to New Horizons. We won’t get hugely better images of Nix, but it goes to show you how far we’ve come. Heck, we didn’t know this moon existed until a few years ago!

I want to leave you with this, a lovely and emotionally moving portrait of Pluto and its large moon Charon:

This is actually a mosaic of two separate images taken on Monday and Tuesday, and assembled so that the size, separation, orientation, and brightness of the two objects is approximated. You can see how much darker Charon is than Pluto, and also how close together they are. These images were taken on approach to the system … but now, as I write this, New Horizons is already 3 million kilometers past Pluto, and increasing that distance by more than 1 million kilometers per day.

The probe will be sending back data for the next year or more, and as of right now only 1–2 percent of what’s stored on the hard drive has been returned to Earth. Yet look at what we’ve found! Imagine: There is 50 times as much wonder yet to be seen.

And I quoted Stern above as calling this the “best for last.” While I like the turn of phrase, I have to disagree with my friend. Pluto is but the biggest (maybe) of the Kuiper Belt objects, and there are millions of them out there. And there are still so many asteroids, comets, and moons unexplored … and we still haven’t orbited Uranus or Neptune, just flown past them. Heck, even long-term missions like Cassini are barely scratching the surface of these amazing places.

All these worlds are ours. We should explore them.

*Correction, July 17, 2015: This post originally misspelled Sir Edmund Hillary’s last name.

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