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World of Awe

by Perry Pezzolanella

A small, frozen ball of ice covered with craters, maybe a few valleys, but mainly flat and dull - that was everyone’s perception of how Pluto would look when New Horizons was to fly past it on July 14, 2015. Pluto was known to have a thin atmosphere, surface features of contrasting brightness, and five moons, but it was thought that Pluto would be a geologically dead world. The discovery of nitrogen geysers on Neptune’s moon, Triton, by Voyager 2 in August 1989 gave hope that maybe Pluto might likewise possess them, but there was no proof. Pluto turned out to be a geologically living world of mountains, glaciers, a frozen lake, canyons, craters, dunes, and organic snowfall, all wrapped in a haze-layered atmosphere!

One of the biggest discoveries made by New Horizons is new kinds of landforms. The biggest is the vast, 400,000 square-mile nitrogen glacier called Sputnik Planitia that forms the western lobe of Pluto’s icy heart feature. There is nothing like it elsewhere in the Solar System. Several features seen on this glacier make it exotic: the cellular structure on its surface, which indicates convective motion in the ice; recharge zones along its edges; hundreds of mile-wide subduction pits caused by nitrogen ice turning directly into gas; and obvious evidence of glacial flow along the edges and adjacent surrounding mountains. There is also the lack of craters indicating that this massive glacier is constantly renewing itself in spite of it being one of the coldest spots on Pluto at nearly -400Fº! Clusters of hills up to a mile across dot the surface of Sputnik Planitia and appear as if they were bobbing on the glacier. These are most like chunks of water ice that flow along and collect near the edges where the polygon slabs meet. High resolution images from New Horizons show that the nitrogen ice is indeed flowing into Sputnik Planitia from higher elevations. They pass through narrow canyons and carry along dark ribbons of material that look like debris streams and moraines created by terrestrial glaciers. Dunes of solid methane ice also cover Sputnik Planitia, which are probably icy grains deposited by winds descending from the mountains! This vast glacier looks like a lava lamp in slow motion where nitrogen ice from beneath is warmed, rises to the surface, cools, and eventually flows back into the cracks. The polygons are the tops of these bubbles percolating from below and are probably up to three miles deep. There is evidence of a vast reservoir of water ice beneath the glacier and, if confirmed, this would classify Pluto as an Ocean World similar to Europa, Ganymede, Callisto, Enceladus, Titan, Mimas, Triton, and Ceres.

Another unique feature is the bladed terrain within the eastern lobe of the heart. These are very long, 1000-foot high linear ridges made of methane ice that are not seen elsewhere in the Solar System. These extend beyond and far into the region of Pluto that was only imaged in low resolution. This may be the dominant landform on Pluto and may be similar to the blades of ice that form in the high deserts under direct sunlight on Earth. They may also be created by wind sculpting or glaciations.

Pluto has dramatic icy mountains up to 11,000 feet high near Sputnik Planitia, but a pair of even taller mountains have a strange texture that appear to have been built up in layers and have deep central pits. These are most likely giant cryovolcanoes that were built up by a mixture of water ice, methane, and nitrogen like a thick slurry of toothpaste. These cryovolcanoes are 2½ and 3½ miles high! Pluto is so frigid that it is dominated by nitrogen and methane ices, and even carbon monoxide ice which is found mainly on the surface of Sputnik Planitia. There are several ranges of dark mountains that have peaks capped in bright methane ice. Pluto also revealed a frozen nitrogen lake to add to the fascination of this awesome world.

Cthulhu Regio is a very dark band along Pluto’s equator and wraps almost completely around it. It is not the actual rocky surface but, but most likely a thick deposit of reddish-brown hydrocarbons called tholins created by solar radiation interacting with methane. This ice-free belt corresponds to a region along the equator that receives a lot of sunlight and never experiences the continuous darkness of the long polar winters. The large contrast between light and dark on Pluto was well known since the 1980s but the extremeness and closeness is dramatic with a reflectivity of only 8% (nearly black) at Cthulhu to nearly 100% (perfectly white) at Sputnik Planitia.

New Horizons also discovered an extensive haze above Pluto extending up to 120 miles above the surface. The atmosphere is very thin, about 60,000 times less than Earth, and lacks oxygen, consisting of 98% nitrogen and traces of methane, carbon monoxide, and cyanide for a very poisonous atmosphere indeed! When New Horizons departed Pluto, it saw up to 20 haze layers, most ½ - 1 mile thick, which were probably produced when the wind blows over the mountains creating waves. Color imaging shows that Pluto’s atmosphere is blue, but from the surface the sky is still black with perhaps a band of fine haze along the horizon. This haze must create beautiful sunrises and sunsets in twilight; New Horizons saw distant mountains casting long shadows into the haze. Pluto’s orbit ranges from 2.7 to 4.6 billion miles from the Sun and the atmosphere will most likely collapse and freeze onto the surface when Pluto moves farther away from the Sun. Even though it is currently moving away, there is no sign yet that the atmosphere is collapsing.

Pluto’s largest moon, Charon, also has unique terrain. Even though it is nearly the same size as the other icy moons of the outer planets with similar composition and density, it has two types of surface features not seen elsewhere. One is the dark reddish polar stain that may be formed when gases escaping from Pluto condense on Charon’s cold poles, and then chemically altered by solar radiation. The other unique feature is the moated mountains that are surrounded by a trench as if something had flowed and surrounded them, but the exact cause remains a mystery.

The theory is that Charon was created by a collision between Pluto and another small planet that launched material into orbit around Pluto and accumulated to form Charon. One clue is the large size of Charon compared to Pluto. Another was the discovery of the Kuiper Belt in the 1990s, which could have been the source of the impactor. The discovery of four small moons in the same orbital plane as Charon by the Hubble Space Telescope strengthened this theory. New Horizons added even more evidence by discovering that Nix and Hydra are both covered in water ice, which is what would happen during a giant impact, plus both moons are the same age as Charon.

Pluto begs for another mission, preferably an orbiter. While another New Horizons type of spacecraft could be launched in late 2028 and arrive at Pluto in early 2039, it is doubtful it will ever fly. An orbiter is the next logical mission and is being drafted, but the current images of Pluto are most likely the best we will have for decades to come. Who could ever believe that Pluto would be such a rich and varied world with so much activity on the surface and in the atmosphere. Pluto is a complex living world of awe and wonder. We will return someday!