Mapping Pluto
What We (Think We) Know So Far
Only a few high resolution frames have so far emerged from New Horizons' 14 July fly-by of Pluto, and they've already answered dozens of questions and raised hundreds more. A press conference featuring the science team is scheduled for 2 pm EDT on Friday, but until then I thought it'd be a good exercise to break down the terrain we've seen so far. Note: In order make everything less confusing I've rotated some of these images to keep them oriented north (up) to south (down). So they may look a bit different than how you see them displayed elsewhere.
Tombaugh Regio
Named for Pluto's discoverer, Clyde Tombaugh, Tombaugh Regio is the very bright, heart-shaped region towards the bottom of the visible disk from New Horizons' approach:
Tombaugh Regio. Credit: NASA/JHUAPL/SwRI
We've actually known about Tombaugh Regio since some of the earliest Earth-based observations of Pluto. The region stood out noticeably on photographs from the Hubble Space Telescope and even ground-based telescopes because of the extreme difference in albedo (brightness) between it and its neighboring regions. What we didn't know -- and still do not, though New Horizons data will likely answer this question -- is why it's so much brighter than the surrounding terrain. It's the focus of all of the first high resolution frames that have been returned from the spacecraft, so it will be subject to plenty of speculation and scrutiny in the coming weeks. And it's already forced us to toss out all of our previous assumptions about Pluto.
One of the first interesting things New Horizons has taught us about Tombaugh Regio is something that is invisible to our human eyes. The spacecraft's Linear Etalon Imaging Spectral Array (LEISA) instrument detects light reflected from Pluto in the infrared, a part of the electromagnetic spectrum with shorter wavelengths than the visible light we are able to see. By analyzing which wavelengths of infrared light are absorbed by a given region of Pluto's surface, LEISA can tell mission scientists what that region is likely made of. In its observations of Tombaugh Regio, LEISA detected a region with very high concentrations of carbon monoxide ice.
Map from LEISA data highlighting a region of very high carbon monoxide ice concentrations in Tombaugh Regio. Credit: NASA/JHUAPL/SwRI
Scientists are not certain what mechanism is producing this concentration yet (that's going to be a recurring theme with Pluto data for a while). It was known that carbon monoxide ice would be present on Pluto, and generally expected that it would be mixed in with larger concentrations of nitrogen ice and methane ice. It's interesting that such a pronounced concentration would occur in a narrow range of longitudes. For a world like Pluto, scientists would have expected composition to be more a function of latitude (i.e. varying from north to south) than longitude (east to west). But we're learning that there's a lot more at work in shaping Pluto's composition than we ever expected.
Norgay Montes
Norgay Montes, at the southwestern tip of Tombaugh Regio. Credit: NASA/JHUAPL/SwRI
Norgay Montes, named for famous mountain climber Tenzing Norgay, was the focus of the first high resolution frame returned from the closest approach imaging by New Horizons. As I wrote last week, perhaps the most astounding thing about the image is what's not in it. As Co-Investigator John Spencer said, "I would never have believed that the very first high res image of Pluto's surface would have zero impact craters."
As with all bodies in our solar system, Pluto has been pelted with impacts from smaller bodies as it plows through its orbit around the Sun ever since its formation some four and a half billion years ago. Any region that shows no evidence at all of this bombardment must be, in geological terms, very young. Which means some process is resurfacing Pluto -- a process that would require a great deal of energy and activity.
Norgay Montes themselves are also a surprising find. The mountains are comparable to those of the American Rockies, with some rising up to 11,000 feet high. The most common ices that comprise Pluto's surface -- nitrogren, methane, and carbon monoxide -- are not strong enough to form mountains this high. Under the kinds of pressures they would be subjected to as part of a tall mountain, at the temperatures in Tombaugh Regio, these ices would buckle and melt. The mountains must, therefore, be made of water ice, which at temperatures found anywhere on Pluto is rock solid.
It's suspected that these water ice mountains are supported by a subsurface bedrock of water ice, and erupted through the surface nitrogen, methane, and carbon monoxide ices via some undetermined geological process. This means that the surface ices must be a thin veneer on the planet's surface. These volatile surface ices are constantly sublimating (turning from solid to gas) into Pluto's atmosphere, which in turn is continually escaping the planet. This means that some process must be regularly replacing the surface ices, or else they would have long ego evaporated into space. Norgay Montes is a towering piece of evidence for ongoing geological activity on the dwarf planet. Pluto is an active, evolving world.
Sputnik Planum
Sputnik Planum, to the east and north of Norgay Montes. Credit: NASA/JHUAPL/SwRI
North and east of Norgay Montes, stretching up towards the center of Tombaugh Regio, is a smooth and bright expanse the team has dubbed Sputnik Planum, in honor of the first man-made satellite to orbit the Earth. Plain as it may look from a distance, there are a lot of features at work here that prompted Jeff Moore from the geology team to call it "not easy to explain." They are readily visible on the higher resolution excerpt that the team provided:
Detail of Sputnik Planum. Credit: NASA/JHUAPL/SwRI
First, there are the strange looking "polygon" features; smooth stretches of terrain are delineated sharply by troughs that run across the surface. These may indicate convection -- a subsurface process in which internal heating causes material to rise, carrying heat energy with it and shedding it closer to the surface, then sinking back down in a cooler state. This process could continually transport materials from different depths to just below the surface of Sputnik Planum and result in the strange, lumpy terrain.
Elsewhere, towards the bottom right of the detail image, dark raised hills are visible, surrounded by what look like lots of small pits in the surface, the origin of which has yet to be explained. Darker black traces on the left of the image, set in some of the troughs, are likely deposits of some different material than the rest of the plains, but we don't yet know what mechanism did the depositing, or what the material could be. Some of these deposits have shown possible evidence of wind streaks, which would be our first visible evidence of Pluto's atmosphere shaping surface features.
We've only known that Pluto has active internal geology for about a week now. It's likely that, as more data comes down, Sputnik Planum will have a lot to tell us about the nature of that activity.
Southwestern Edge of Tombaugh Regio
Southwestern edge of Tombaugh Regio. Norgay Montes is visible in the bottom of the image, and the western edge of Sputnik Planum is visible on the right. Credit: NASA/JHUAPL/SwRI
Though it's not a distinctly named region like the previous ones, this high resolution frame covers a large area and includes some important features that have inspired further head-scratching on the part of the New Horizons science team. Northwest of Norgay Montes, and west of Sputnik Planum, this region is the western edge of Tombaugh Regio, where it meets the dark, contrasting terrain that has variously been referred to as "the whale" and "Cthulhu" by the team.
The obvious difference in albedo immediately indicates that the Cthulhu region is of a different composition than Tombaugh Regio. The specifics of this composition are not yet known, but the dark color, which indicates a more reddish terrain, may be caused by tholins.
As I wrote about last week, Randy Gladstone, Co-Investigator focusing on Pluto's atmosphere, has speculated that incoming ultraviolet photons from the Sun could drive chemical reactions with the gaseous nitrogen, methane, and carbon monoxide in Pluto's thin atmosphere. This process would create hydrocarbons like acetylene, ethylene, and hydrogen cyanide, which would condense out of the atmosphere and transport to the surface.
Once on the surface, these molecules would further interact with one another to create tholins, extremely complex organic molecules that, in large quantities, result in a thick, tar-like mud. That may be what is coating the surface in the Cthulhu region, causing the dark, ruddy appearance. Or it may be something else entirely. The high resolution composition maps created by the complete LEISA data set will hopefully answer that question.
It's also apparent that the Cthulhu terrain must be much older than that of Tombaugh Regio, since it sports quite a lot of visible impact craters. This suggests that whatever process is resurfacing Tombaugh Regio is not occurring beneath Cthulhu. The terrain where Cthulhu and Tobaugh Regio meet, therefore, is a fascinating place. One of the most important (and likely complicated) questions the science team hopes to answer is how the dark material and the light material interact. At least from this image, it appears that the lighter material has filled in some of the craters on Cthulhu's eastern edge, and vein like extensions of the dark material stretch into the western fringe of Tombaugh Regio. As with Sputnik Planum, there is probably a lot to be learned there about what's going on underneath the surface.
There is another (yet unnamed) range of mountain and plateau features towards the right side of this image, north and west of Norgay Montes. These are not quite as high as Norgay Montes, likely more comparable to the Appalachian Mountains in height (3,000 to 4,000 feet). But, like Norgay Montes, they're probably composed of water ice, protruding from water ice bedrock up through Pluto's surface nitrogen, methane, and carbon monoxide ices.
That's a ton of puzzling features, data to be anticipated, and questions to be asked, all from just the first few high resolution frames taken by the LORRI instrument. To tie it all together, take a look at this mosaic produced by UnmannedSpaceflight user Exploitcorporations, which is an attempt to integrate all these imaged regions on the global map:
Global map integrating the high resolution frames of the regions discussed. Credit: UnmannedSpaceflight user ExploitCorporations
Another press conference for the release of more Pluto science results is scheduled for Friday 24 July at 2 pm EDT. There's a lot of data that could help us start to chip away at these mysteries still to come, like stereo imaging that shows topography, detailed color data, more compositional data from the spectroscopic instruments, and so forth. We may not get any of that on Friday, but at the very least we'll see some tantalizing new high resolution frames.