As the data from New Horizons continues to come in, we are learning more about what an incredible little world Pluto really is, with tall mountains of rock-hard water ice, as well as glaciers and vast icy plains composed of nitrogen ice. In some ways, these features are visually reminiscent of similar ones on Earth, and now this week another cool discovery was revealed: methane snow on some of Pluto’s mountain peaks.
The snow-capped mountains are located in the large dark region called Cthulhu (pronounced kuh-THU-lu), which is west of the ice plains called Sputnik Planum, and is 1,850 miles (3,000 kilometers) long and 450 miles (750 kilometers) wide, extending nearly halfway around Pluto’s equator. This region is much rougher than the smooth ice plains, with mountains, craters, and fractures. The dark coloring is thought to be the result of tholins, complex molecules which form when methane is exposed to sunlight. The stark contrast of this region compared to Sputnik Planum can be easily seen in both close-up and more distant images taken by the cameras on the New Horizons spacecraft. Cthulhu is older than the ice plains, which is indicated by the much smoother and crater-free terrain of the plains, where glaciers of nitrogen ice have continued to re-shape the surface. The mountain range itself is 260 miles (420 kilometers) long.
When scientists looked closer at the images of the mountain ranges within Cthulhu, they noticed that the highest peaks were coated with a bright material which made them look like snow-capped mountains on Earth. Based on observations from New Horizons, the scientists now think that these bright areas are methane frost, ice, or snow, which has condensed onto the peaks from the thin atmosphere. Their bright white-ish appearance contrasts sharply with the surrounding terrain, which is dark red in color.
According to John Stansberry, a New Horizons science team member from the Space Telescope Science Institute in Baltimore, Md., “That this material coats only the upper slopes of the peaks suggests methane ice may act like water in Earth’s atmosphere, condensing as frost at high altitude.”
Even before the New Horizons mission, it had been theorized that Pluto’s atmosphere may partially condense onto the surface as part of a regular cycle.
The location of the bright areas on the mountain peaks was found to correlate nicely with the distribution of methane ice already known to be present, when compositional data from the Ralph/Multispectral Visible Imaging Camera (MVIC) on New Horizons was examined. The methane ice is shown as purple in the false color images. The mountain range itself is situated among a heavily cratered landscape, with the peaks separated by narrow valleys. The mountains on Pluto may look like mountains on Earth, but they are composed of solid water ice, which is as hard as rock in Pluto’s extremely cold temperatures. Along with the ices of methane and nitrogen, as well as carbon monoxide in other areas, it is a uniquely alien version of processes common on our own planet—visually similar, but with compositions very different from anything on Earth.
The many craters indicate that this region is very old geologically, while the mountains and valleys provide evidence for geological activity in Pluto’s past. The nearby glaciers are still active, however, flowing very slowly over the surface, much like glaciers on Earth. This keeps Sputnik Planum refreshed with new ice, explaining why no craters at all have been seen in this region. Much of the ice is also thought to be upwelling to the surface from below, in a very slow but regular process. There are also “icebergs” of water ice floating in the sea of nitrogen ice in Sputnik Planum. It’s almost a surreal kind of landscape, at first glance similar to polar regions on Earth, yet weirdly alien at the same time.
Also, while there is some water ice on the surface of Pluto, most of the ice consists of methane, nitrogen, and carbon monoxide. Earth is too warm for those ices to form naturally, but not Pluto. But the other, more exotic ices, are common on Pluto and its moons.
“Large expanses of Pluto don’t show exposed water ice,” said science team member Jason Cook, of SwRI, “because it’s apparently masked by other, more volatile ices across most of the planet. Understanding why water appears exactly where it does, and not in other places, is a challenge that we are digging into.”
There is water ice on Pluto’s largest moon Charon as well, but there is much more underground—in fact a frozen ocean of it, as revealed by analysis of the data from New Horizons. That ocean is thought to have been liquid at one time, much like the current subsurface oceans in other moons such as Europa and Enceladus. An earlier study had predicted seeing the evidence for an ancient ocean, if it was there:
“The New Horizons spacecraft should provide the first images of the surfaces of Pluto and Charon,” according to researchers, in a report which was published recently in the journal Icarus. “Our predictions indicate that the presence (or absence) of tidally driven fractures could be used to constrain Charon’s internal structure and orbital parameters at the time of fracture formation. Furthermore, these fractures should be distinct from those produced through orbital recession and de-spinning. If tidally driven fractures are quite prevalent, it would require that Charon experienced a high eccentricity during its orbital evolution and that the ice shell remained relatively warm during this time period. In contrast, very limited tidal fracturing would place upper limits on Charon’s eccentricity and the viscosity of the ice shell. The specific distribution of fractures, either sparse or numerous, could be used to further distinguish between a thick and thin ice shell.”
As also noted by As Dr. Alyssa Rhoden, a planetary scientist at NASA’s Goddard Space Flight Center: “Our model predicts different fracture patterns on the surface of Charon depending on the thickness of its surface ice, the structure of the moon’s interior and how easily it deforms, and how its orbit evolved. By comparing the actual New Horizons observations of Charon to the various predictions, we can see what fits best and discover if Charon could have had a subsurface ocean in its past, driven by high eccentricity.”
The findings illustrate how both Pluto and Charon have been, and in some ways still are, more geologically active than had been anticipated. Not much activity was expected on such small, cold worlds so far from the Sun, but as often is the case in planetary exploration, scientists were in for a surprise.
According to Alan Stern, New Horizons Principal Investigator: “We did not predict that a small planet like Pluto could still be active and would not have completely cooled off. The Pluto system surprised us in many ways, most notably teaching us that small planets can remain active billions of years after their formation. We were also taught important lessons by the degree of geological complexity that both Pluto and its large moon Charon display.”
The same may be true for other objects in the Kuiper Belt, where Pluto and Charon reside. New Horizons is currently on course for its next encounter, with a smaller Kuiper Belt Object called 2014 MU69, on Jan. 1, 2019. Given what has already been seen in the Pluto system, it will be very interesting to see this little world next.
There is also an intriguing new report of possible clouds seen in the Plutonian atmosphere, but at this point the finding is still tentative and has not been officially confirmed. The images are certainly interesting, and AmericaSpace will report more on this when or if more information becomes available.
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