Pluto and Charon imaged separately by New Horizons during approach on July 13 and July 14, 2015. The size, separation, orientations and true color are approximated in this composite image. Image Credit: NASA/JHUAPL/SWRI

Jul 21, 2015 Pluto and Charon: Who ordered that?

Sen—Last week Pluto went from a point of light to a resolved world with mountains and plains in our collective minds. New Horizons historic Pluto flyby has come and passed, and even with just a tiny fraction of the data aboard the spacecraft beamed down to Earth, it has become a case of 'who ordered that?' What has been the most thrilling, from what we have just started to learn, is that Pluto and Charon don't look like anything we've seen before. The analogs we thought would be best matches fail us, making Pluto, Charon, and the other dwarf planets of the Kuiper belt even more exciting worlds. There is so much one could write about Pluto, but too little time. So I'll start this blog focusing on a few of things that piqued my interest from last week.

A few days before the closest part of New Horizons encounter with the Pluto system, we finally have it settled who is bigger: Eris or Pluto. Pluto turns out to a bit larger than we had originally thought, coming in just a smidge larger than Eris. You might be wondering why this is a New Horizons result, and not something we should have been able to resolve from Earth. We've had a decent measurement for Eris' size from a stellar occultation in November 2010 where Eris passed in front of a distant background star and blocked out the star's light momentarily. Eris currently lacks an atmosphere, so the event duration gives very precisely the size of the body. Many previous stellar occultations of Pluto have been observed, but the measurement is more difficult to make because starlight can also be blocked partly or fully by atmospheres. It is more fuzzy to tell the boundary in the occultation measurements for where the body begins and the atmosphere starts, so the error margins are larger. To get a better size for Pluto we needed New Horizons global imaging.

Pluto now comes in at 2,370 km +/- 20 km across compared to Eris at 2,336 km +/- 12 km. Pluto might regain the title King of the Kuiper belt in terms of size, but it depends on your metric since Eris weighs in larger containing a whopping 25 per cent more mass than Pluto. What's important is that with Eris and Pluto at practically the same size, the fact that Eris is 25 per cent more massive means it is denser. Therefore there is more rock than ice making up Eris compared to Pluto's interior. I would naively expect that on average roughly the same types of objects hit Eris and Pluto. What's more puzzling to me then is how do you get two objects to be so vastly different in internal composition like this, if they were in relatively similar collisional environments. There is evidence to suggest that both Pluto and Eris have suffered past collisions, but if they were differentiated (the different components internally separating and sorting into layers) when these occurred, one might expect you would chip off ice making them denser. Maybe they did have different collisional histories? Eris has only one tiny moon, so there may not be not evidence of a huge impact that could have stripped a lot of icy mantle off to change the body's density. Previously modeling has found it hard to significantly change the density of a large Kuiper Belt Object (KBO) with impact, so this scenario might even be feasible. Does this mean they started off compositionally different from different regions in the Solar System? Is it that we don't really understand the physics of icy collisions?

For me, Charon has been the most surprising. Good old Charon was supposed to be the boring object. The first high detail images of the ghostly white body in the images was not what I was expecting. Smaller than Pluto, it doesn't have its own atmosphere. We thought that the surface would be set shortly after the collision that put Charon in orbit and the tides with Pluto that evolved its interior and its orbit. The thought was that on Pluto, there would be likely transport of nitrogen and methane ice from the summer pole snowing out onto the winter pole. So on Pluto there would be some resurfacing that would cover the cratering record, but that wouldn't be the case for Charon. We thought that Charon should have sat there being bombarded. Just like our Moon, planetary scientists expected a heavily cratered surface. With no volatile ices present from ground based observations, on Charon there should have been no active processes other than micrometeorite impacts and some contamination from Pluto's atmosphere. Nothing that should have erased craters. It was originally thought that counting craters on Charon would produce an accurate picture of the small end of the Kuiper belt's size distribution where the bodies are too small to detect directly with even the largest ground-based telescopes or the Hubble Space Telescope.

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A fairly craterless Charon as imaged by RALPH on New Horizons. Image credit: NASA-JHUAPL-SWRI

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A Voyager image of the heavily cratered surface of Jupiter's icy moon Callisto for comparison Image credit: NASA/Voyager Team

What I love is that Charon looks nothing like a heavily cratered world. In the global images, Charon is pretty craterless. Charon has some crater regions in the southern hemisphere but for the most part, it's a pretty smooth surface. In the Solar System, cratered means old and the less craters the younger the surface. Where are Charon's craters? It means that something has removed the craters. Is it that the Charon impact was earlier? I'm not sure. It's a much younger surface, gigayears younger. Which means some geologic process actively removed most of the craters. Perhaps we don't fully understand how the energy released by the radioactive Aluminum-26 is dissipated in icy bodies. Could that be enough to keep Charon warm enough to recoat its water ice surface? Now Charon is tidally locked to Pluto, so the tidal forces were stronger earlier on in the Solar System. Maybe we don't really understand how tides work in icy bodies? Many of the icy satellites of Jupiter and Saturn have relatively young surfaces. The 'out' was always blamed on the giant planet. It's been suggested that Jupiter, Saturn, Uranus, and Neptune might generate stronger tidal heating inside the icy bodies that might power the internal engines to resurface the Jovian and Saturnian moons. Charon doesn't have that out. This might strongly suggest that there is missing physics going on with tides and internal dissipation in icy bodies.

Like Charon, Pluto's relatively craterless surface suggests recent removal of craters. We expected this more, but I still would have expected more craters. The surface is surprisingly way more varied than anyone was predicitng. There are mountains as well as plains with polygonal-like troughs! For me the identification of possible wind streaks peaked my interest as I'm working on studying wind streaks on Mars' South Pole. Neptune's large moon Triton, our former analog for what Pluto would like look, has dark streaks caught in images taken by the Voyager spacecraft. The thought is this might be material coming out in geysers of carbon dioxide and other volatile gases to the surface and the material being blown by wind. That would be an active process occuring on the surface, but right now the trouble is that the dark streaks in the New Horizons images are about the same size as the compression artifacts that can be generated when compressing to jpeg format. So although there are at least two or three fan-like shapes spotted, it's going to take the full resolution image to truly confirm they are there. If they are real, then we could get wind directions and speeds at a few locations on Pluto's surface!

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Annotated highest resolution images to date of the plains of Pluto with troughs and hills Image credit: NASA/JHUAPL/SWRI

New Horizons has only beamed back a tiny fraction of the data stored on its recorders during the past few days after the encounter. Despite that, these remarkable images leave much to be explained, and the New Horizons team and the planetary community are waiting with bated breath for the full resolution images. It will take 16 months for all of the encounter data from the entire instrument suite aboard New Horizons to be downloaded to the ground in lossless uncompressed high resolution format. Right now the images we see have jpeg compression artifacts, but even so it shows that Pluto and Charon are strange and incrediably interesting worlds. Right now we are left with more questions than answers. This is just the first taste of the science yet to come.

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