Among its other geologic oddities, Pluto has clusters of hills floating in a frozen "sea" dominated by nitrogen ice. These bobbing bumps might hold clues to the plain's depth and evolution.

Within days of New Horizons' historic flyby of Pluto last July 14th, mission scientists released snapshots showing unexpectedly tall mountains partially rimming a vast and very flat plain. The plain, informally named Sputnik Planum, is dominated by frozen nitrogen (and some frozen carbon monoxide), whereas the surrounding uplands are mostly frozen water.

"Floating" hills on Pluto
A close-up of Pluto, roughly 500 by 340 km in size, shows numerous, isolated hills inside the plain called Sputnik Planum that might be fragments of water ice from the surrounding uplands.

Sputnik Planum is a fascinating expanse of Plutonian real estate. Comparable in size to Hudson Bay, it's criss-crossed with shallow fractures that carve it up into crude polygons. And it's moving, slowly, pushing outward at its margins very much like the slow inexorable downhill movement of glaciers here on Earth.

But recently the team unveiled an image of Sputnik Planum that reveals something new, quite strange, and perhaps very telling: clusters of hills that stick up through the plain's surface. Up to a few kilometers across, they appear to be bobbing along in the icy floes and become concentrated where the polygonal slabs meet. The New Horizons team suggests that the mysterious hills might be fragments of water ice from the uplands that partially surround Sputnik Planum.

Importantly, these water-ice "islands" appear to be analogous with ocean-going icebergs here on Earth — and, as such, they might offer a hint of the depth of Sputnik Planum's frozen nitrogen "sea." So how deep might that be? Let's do the math!

Floating hills on Pluto
These clusters and chains of hills on Pluto appear to be blocks of water ice "floating" in a higher-density "sea" dominated by frozen nitrogen, which has a higher density.

Assuming that these hills are truly free floating and in what geologists call isostatic equilibrium, the fractional mass of each hill below the surface is proportional to the ratio of its density divided by that of its surroundings. Very cold pure water ice has a density of 0.934 g/cm3, and that of frozen nitrogen is 1.027 g/cm3. Now, some caveats are in order here: the water ice is probably a frozen brine of some sort, and the nitrogen ice probably isn't pure, but those are second-order details in this admittedly back-of-the-envelope calculation

Pluto water-ice map
Water ice serves as the "bedrock” of Pluto's crust, and this false-color image shows where the spectral features of water ice are abundant on its surface. Click here for a fuller explanation of the map and how it was obtained.

The ratio of those two values, 0.934/1.027, is 0.91. So something like 91% of the mass (and thus volume) of each floating hill lies beneath the surface of Sputnik Planum. If each hill were a perfect cube, then 91% of its height would likewise lie hidden below the surface. If the hills are 100 m tall (a guess on my part — NASA's press release doesn't say), then their "roots" should extend downward for at least 1 km below.

But these hills are not likely solid ice throughout. "If you take a reasonable value of 15% porosity," explains Jeff Moore (NASA Ames Research Center), "the blocks will be significantly more buoyant." Even so, he continues, "We suspect that the N2 deposit of Sputnik Planum is several kilometers deep — maybe on order 10 km in places."

Interestingly, one large cluster, nicknamed Challenger Colles (honoring those lost aboard the Space Shuttle Challenger in 1986), measures 60 by 35 km. This grouping isn't out in the middle of Sputnik Planum but rather located near the eastern margin, near the uplands of central Tombaugh Regio (another informal name), so perhaps these hills became "beached" once the nitrogen ice got too shallow.

It's truly fascinating that Pluto exhibits to much geology — and ongoing geology at that. All this activity might not strengthen the case to reclassify Pluto as a "major planet." But it's surely making the (convoluted) decision to build and launch New Horizons a very sound one.


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February 16, 2016 at 2:18 pm

Hi Kelly,
I cannot quite follow your arguments of the floating icebergs. Solid ice would sit on the top of a solid nitrogen surface without sinking in, similar to rocks on a flowing glacier. This should be especially possible considering the low surface gravity on Pluto, about 1/15th of the earth gravity. The interesting point seems to me, how these hills moved to their present positions after they fell probably from the side of the neighboring mountains (similar to center moraines on alpine glaciers). Another question is the actual density of nitrogen and water ice on Pluto, after they condensed from a thin atmosphere on the cold surface. The density may be quite a bit lower than that of the solids. Maybe we learn more about this.
Regards, Martin

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Peter Wilson

February 16, 2016 at 4:24 pm

Solid ice would sit on the top of a solid nitrogen surface without sinking in because water ice is less dense than frozen nitrogen at those temperatures.

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February 20, 2016 at 10:31 am

It seems to me that the conclusion that (about) 91% of the hills' mass was below the surface would only apply if the hills were solid objects and the surface was liquid at the time they formed.

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Stub Mandrel

February 21, 2016 at 3:21 pm

The 91% height calculation only follows if the 'cube' floats with a square face flat on top.

If the 'spacebergs' have a similar shape to earthly icebergs, and take a similar orientation to maintain stability, then they will taper at the top. The pointed peak will be much smaller in cross section than the submerged parts, and this means the hidden part may only be 2-3 times as deep as the part that sticks up.

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