Pluto’s Purple Mountains’ Majesty Turns Out to Be … Methane

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Mar 6, 2016
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One of the biggest surprises coming from the New Horizons spacecraft flyby of Pluto was that the little ice ball has mountains. Big ones. Some of them soar to more than 3,000 meters, making them the outer solar system’s rival to the Rockies.

There’s a mountain range on Pluto that stabs through the center of a vast reddish region nicknamed—and how I love this—Cthulhu. The range is more than 400 kilometers long, and while the lower parts of the mountains are as red as the surrounding area, the tops are much brighter, whitish, and clearly made of a different material. A newly released observation of that area now shows that the bright material is almost certainly methane ice, covering the mountain peaks like snow does here on Earth!

This is so cool (literally, as well as more metaphorically). In the color-enhanced full-disk image at the top of this article, Cthulhu is the red region in the bottom center. The color is likely due to tholins, complex carbon-based molecules created when simpler ones like methane are exposed to ultraviolet light from the Sun. They break down and rearrange themselves into bigger molecules and tend to be red.

The rectangle marks the zoomed-in region shown just above. On the left is the enhanced color shot, and on the right (colored purple) is an infrared view taken by an instrument on New Horizons called Ralph. Methane ice strongly absorbs some colors of infrared light, making it detectable by Ralph’s Multispectral Visible Imaging Camera. As you can see, the correspondence is nearly a perfect match.

On Earth, methane is a gas. But on Pluto it’s a little more complicated. Pluto has an atmosphere, but it’s very thin, and that affects methane’s behavior. Temperatures on Pluto’s surface vary by a little bit; in some places it’s cold enough for methane to be an ice, and other places its warm enough to turn from an ice directly into a gas. In Cthulhu, apparently, the lower parts of the mountains are warm enough to keep methane as gas, but higher up it cools and methane freezes out as snow.

In this way, methane on Pluto is like water on Earth, and you get snow-capped peaks.

I live near Boulder, Colorado. I can literally look out my window and see the Rocky Mountains; the lower stony slopes brown and gray, the peaks covered in white water ice. How amazing, how astonishing, how wonderful, that 5 billion kilometers away from Boulder there lies a similar view?