Penitentes found on Pluto!

“Don’t tell Mars that my new favorite planet became Pluto!” said John Moores, assistant professor in the department of earth and space science and engineering at York University, whose findings appeared in the journal Nature in early January. But what caused Moores’ sudden change of heart?

                Interview of John Moores by York University

With help from NASA and Johns Hopkins University, Moores and a team of scientists discovered evidence of penitentes on Pluto. As Moores et al. explain in their article, “Penitentes are snow and ice features formed by erosion that, on Earth, are characterized by bowl-shaped depressions several tens of centimetres across, whose edges grade into spires up to several metres tall.”

Snow penitentes on Earth (Source: Alex Schwab/Flickr).
Snow penitentes on Earth (Source: Alex Schwab/Flickr).
Though these penitentes on Pluto are composed of frozen methane and nitrogen, not frozen water, the finding still means that snow and ice features previously only seen on Earth have been spotted elsewhere within our solar system. This suggests that these features may also exist on other similar planets.

“No matter whether we are on Earth or Pluto, the same physics applies. We can extend these results to other environments as well,” writes Moores on his blog.

Surprised by nature, they discovered snakeskin-like parallel ridges in the Tartarus Dorsa area on Pluto. These ridges resembled penitentes seen on Earth. There have been other examples of similar features on other planets, but these were often caused by processes different from the ones on Earth. Therefore, Moores et al. at first did not believe the features could actually be penitentes.

The bladed terrain of Pluto’s Tartarus Dorsa region, photographed by NASA’s New Horizons spacecraft in July 2015 (Source: NASA).
The bladed terrain of Pluto’s Tartarus Dorsa region, photographed by NASA’s New Horizons spacecraft in July 2015 (Source: NASA).
“Pluto was nothing like what we expected,” Moores notes on his blog. In order to determine that the features were true penitentes, Moores et al. applied a terrestrial model called the Claudin Model to Pluto. The model was originally developed to describe a mechanism to control the spacing of penitentes on Earth. When Moores et al. applied the model to Pluto, something strange happened: “The model, which was modified appropriately for Pluto, actually predicted penitentes consistent with what we saw on Pluto when using parameters consistent with Pluto’s extremely thin, yet extremely stable atmosphere,” Moores said.  “The theory fits the available facts quite well.” Keeping with these observations, the model also predicted that penitentes would not form at all in the more volatile nitrogen ices elsewhere on the dwarf planet, according to Moores.

First reported in the Chilean Andes by Darwin in the 1830s, penitentes form in areas of strong sunlight. In certain conditions, initial random irregularities in a snow surface can be deepened as curved depressions focus sunlight, accelerating sublimation (the transition of water molecules directly from a solid state to a gas state). As the depressions deepen, the higher points remain, shading the parts behind them, and thus slowing down sublimation. The result is a collection of spiky forms, all oriented toward the sun. Vapor processes within the depressions also contribute to the process of formation of penitentes.

How can such large penitentes form on Pluto, when Pluto’s environment is so different from the Earth? “It’s because these penitentes do not form in water ice but in methane ice, which evaporates more easily,” Moores explained to GlacierHub. “Furthermore, the atmosphere into which the sublimating methane vapor mixes is much less dense (about 15,000 times less dense than on Earth), allowing the vapor-rich layer to be thicker.”

The aligned ridges on Pluto resemble high-latitude terrestrial penitentes (source: Moores et al. / Nature).
The aligned ridges on Pluto resemble high-latitude terrestrial penitentes (source: Moores et al. / Nature).
Moores is excited about his findings. “Those 1,750 words are the most challenging I’ve ever written in my professional life,” he said, referring to his study published in Nature. “It has been an honor to be able to contribute to the science of Pluto, and I will be following the progress of the science results from New Horizons closely in the years to come.”

When asked about his future plans, Moores mentioned to GlacierHub that he hopes to continue his research on other planets. “We’re already looking at possibilities on Mars,” he said. “We’re also thinking about how we might use our simulation chambers to get a better idea of the rheology of methane at these temperatures.”


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