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.

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!

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

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 N 2 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.