Of all the things we thought we might see while speeding past Pluto, we weren’t expecting the icy world to look quite this much like our home planet’s frozen poles. This is the science so far of those eerily familiar landscapes.
Geomorphology is the study of landforms—what they are, how they’re formed, and why they change over time. For Pluto, we’re practicing this science by trying to draw analogies to this alien landscape, looking for terrestrial features with a twist.
Within the iconic heart of Tombaugh Regio lays Sputnik Planum, vast icy plains that shocked us with their youthful, crater-free surfaces. Roughly the size of Texas, the plains jut up against steep mountain ranges or rugged cratered terrain. The closer we look at Sputnik Planum, the more we realize just how creepily at home they’d look transplanted to the Antarctic.
The smooth, bright region of Sputnik Planum dominates the western lobe of the heart-shaped Tombaugh Regio. Image credit: NASA/JHUAPL/SwRI
To demonstrate a true abundance of initial-findings cautions, everything from here on out should be loaded with qualifiers like, “glacier-like” and “apparent flow,” but that’d make for brutally awkward reading. Even more aggravating, the names on Pluto are still informal, but we’re using them as a better alternative than, “That crater, no that one, the one over there!” until the official maps tell us otherwise. Instead, let’s dive into the wild speculation and overly-aggressive interpretation on insufficient data, see what we can come up with to explain this little alien world, and use their tentative names until someone officially yells at us to knock it off.
The Nature of Ice in the Dim Light far from the Sun
The temperature change between home and the outer edges of the solar system demand compositional differences to create familiar features: normal water ice would be hard as rock, and a good candidate to make those steep, rugged mountains, while more exotic nitrogen ice has just the right softness to slowly and gradually flow with the stately grace of a glacier.
Where exactly nitrogen to create all this fresh ice is coming from is still a matter of speculation. We know that Pluto has a nitrogen-rich atmosphere, but we also know the dwarf planet is shedding this gas at a truly prodigious rate. Research in the lead-up to the flyby make it clear that a bombardment of asteroids and comets couldn’t provide enough fresh nitrogen to keep up with the loss, so what’s going on? The best working theory for the moment is that Pluto is supplying its own nitrogen, with heat driving geological processes to free nitrogen from the rocky interior.
Pluto’s delicate, layered atmosphere and unexpected ground haze are stealing nitrogen from its ice sheets and glaciers. Image credit: NASA/JHUAPL/SwRI
The ice sheets of Sputnik Planum aren’t pure nitrogen. The initial map of carbon monoxide on the dwarf planet reveal a single, disproportional bulge centred on the icy feature, and methane is everywhere to one degree or another.
On Earth, glaciers grow as they accrete and shrink as they ablate. Accretion is any new material glomming onto the glacier, usually through fresh snow accumulating, but it can also happen when basal ice freezes to the mass. Ablation is that same ice lost through evaporation, sublimation, or melting. While the mass balance works the same on Pluto, the rapidly-escaping atmosphere makes accretion far more likely to happen from below than above.
Slowly Flowing Ice Sheets and Glaciers
We got our first whisper of flowing glaciers during the initial data downlink this summer, but we’re learning far more about the complex dynamics of Pluto’s apparent ice sheets with every new image we get.
The first hint at glacial flow was along the northern margin of Sputnik Planum where we saw both the characteristic streamlines of flow texturing the ice. Intuitively, it looked like the plains were expanding, flowing into the older, rugged terrain and even starting to infill some craters.
Swirling patterns in the northern reaches of Sputnik Planum suggest ice flowing around obstacles into depressions. Image credit: NASA/JHUAPL/SwRI
But recent releases along the southwestern margin offer a more complicated story in a a tiny snapshot of the hydrological cycle on Pluto. It appears nitrogen ice evaporates from Sputnik Planum, redeposits in the eastern highlands, then flows back out into the plains.
Location of glacier close-ups within Sputnik Planum. Fresh nitrogen ice deposits may be coating the bright highlands in the eastern lobe of Tombaugh Regio. Image credit: NASA/JHUAPL/SwRI
The bright white reflections off the eastern highlands suggest fresh ice deposits, while the rugged texture makes it clear any mantle is quite thin. Zooming in reveals the secret: the faint features look an awful lot like frozen streams along the margins of ice caps in Greenland and Antarctica.
Valley-filling glaciers (red arrows) in the highlands apparently flow out into Sputnik Planum (blue arrows). Arrows indicate features, not flow directions. Image credit: NASA/JHUAPL/SwRI
In contrast to the vast ice sheets of Sputnik Planum, the eastern highlands are home to Pluto’s version of alpine valley glaciers. The broad valleys are 3 to 8 kilometers (2 to 5 miles) wide; if they’re truly analogous to Earth they’re carved far deeper. Texture differences in the icy streams look like lateral and medial moraines, long, thin piles of unsorted rock and debris pushed and dragged by the glacier’s slow flow.
This alpine glacier in Greenland on Earth has dirty moraines marking ice flow. Are we seeing the same thing on Pluto? Image credit: NASA/Earth Observatory
A fuzzier, backlit view of the same region that emphasizes the intricate flow lines. Image credit: NASA/JHUAPL/SwRI
Strange Textures of a Thermal Nature
Patterned ground in the frozen fringes of our planet are one of the geomorphic riddles that continue to puzzle researchers despite decades of study, so it’s not terribly surprising that precocious Pluto has its own twist on the landform. The interior of Sputnik Planum is textured by polygons, irregularly-shaped segments that look more like frozen mud cracks than anything else.
Odd polygon textures and ice encroaching on ancient heavily-cratered terrain characterize this segment of the souther reaches of Sputnik Planum. Image credit: NASA/JHUAPL/SwRI
When examined in detail, the polygons are roughly 20 kilometers (12 miles) across, bordered by shallow troughs. Some of the troughs are home to a darker, unidentified material, while others are bordered with tiny pinprick hills.
While mostly smooth in their interiors, some of the polygons are etched by pits, possibly a large-scale analogue to sublimation specks formed when ice transitions directly from a solid to a gas.
Troughs, hills, and pitted features within Sputnik Planum. Image credit: NASA/JHUAPL/SwRI
About all we know for certain about these landforms is that they’re very likely caused by some sort of thermal process. The question is: are they indications of internal heat, driving convention from below, or of a cooling, contracting at the surface? We haven’t learned enough to resolve the riddle except that it’s another tantalizing hint at the complex dynamics of the miniature world.
Building Mountains from Ice
The transition from Sputnik Planum to the surrounding mountain ranges doesn’t make the science simpler. Between the gentler Hillary Montes northwest of the plain to the steeper Norgay Montes along the southwestern boundary, the presence of mountain ranges on Pluto are undeniable, but how they formed is less clear-cut.
Chaotic terrain on the northwestern edge of Sputnik Planum. The smallest visible features are 0.8 kilometers (0.5 miles) across. Image credit: NASA/JHUAPL/SwRI
On Earth, mountain ranges typically form as the result of tectonic action: the rigid yet flexible sheets that create the surface of our planet deform and crunch as they move around; Pluto may have an analogous process involving plates of hard water ice buckling and bending.
A mélange of ice textures near Greenland on Earth. Can the rocky mountains and isolated blocks of ice give us insight into Pluto’s diverse terrain? Image credit: NASA/Earth Observatory
On Earth, isolated mountains can form when hot spots in the mantle spurt up volcanoes. On Pluto, the oddly isolated peaks free of any range may have an entirely different origin. Jeff Moore, leader of the New Horizons Geology, Geophysics and Imaging, speculates:
The randomly jumbled mountains might be huge blocks of hard water ice floating within a vast, denser, softer deposit of frozen nitrogen within the region informally named Sputnik Planum.
Pluto’s diverse landscapes range from dark, heavily cratered ancient surfaces to bright, young smooth ice, with far too much confusion in-between. Image credit: NASA/JHUAPL/SwRI
The origin of a field of darker, aligned ridges may be even more peculiar. Despite Pluto’s tenuous atmosphere, these features look suspiciously like dunes formed by steady winds piling up loose sediments.
Sand dunes in the massive ergs of Oman make sense, but does Pluto have enough atmosphere for the same process? Image credit: NASA/Earth Observatory
Seeing dunes on Pluto — if that is what they are — would be completely wild, because Pluto’s atmosphere today is so thin. Either Pluto had a thicker atmosphere in the past, or some process we haven’t figured out is at work. It’s a head-scratcher.
Sand dunes aren’t the only thing that can create long, linear features. These could be evidence of tectonics, compression, or some other mechanism entirely.
Evaporated strandlines from an ancient coastline in Argentina offer another intriguing solution for linear features. Image credit: NASA/Earth Observatory
The deep, bowled craters hold one more familiar feature: those striations running down the slopes into the curved valley floors look like gullies, erosional features cut by the flow of liquid.
So many mysterious landscapes to investigate! Image credit: NASA/JHUAPL/SwRI
Every time we get more data form New Horizons, it creates more questions than it answers.
This is science at its finest, when we’re grasping onto the recognizable edge pieces in an effort to assemble the puzzle, hoping that we’ll soon piece together enough context to figure out the big picture. We don’t know what any of the answers to these geomorphic mysteries are yet, but it sure is fun finding out!
The ice and banks of Taz River are loaded with interesting textures from stress cracks to soggy sediments. Can any of us help figure out what’s happening on Pluto? Image credit: NASA/Earth Observatory
After a nine year journey, the New Horizons spacecraft made its closest approach to the dwarf planet Pluto and its moons on July 14, 2015. Moving far too quickly to swing into orbit, the probe still collected enough data during the brief flyby that it will take until next autumn to downlink all the data. New Horizons will soon be making a trajectory correction to set it on a new path that will take it close to the Kuiper Belt Object MU69 in January 2019.
Update: New photos arrived from New Horizons, including the highest-definition detailed shots, the full disc in enhanced, extended colour, and a perplexing new texture.