Wednesday, January 4, 2017

Penitentes on Pluto


Though not posted until after the embargo clears, I'm writing this post over the winter holidays out in Gander, NL. It's plenty cold, snowy and icy, but far too windy and humid for Penitentes to form here! The image above was captured by the Ralph/Multispectral Visual Imaging Camera on the New Horizons Spacecraft in July of 2015. The image shows the Tartarus Dorsa region of Pluto which has been sculpted by the sun into a regular pattern of icy blades.

This post is intended to offer up a few details about my recent article that appeared online in the January 4th edition of Nature. You can navigate to the paper itself using this link and the York and NASA/APL press releases are also linked.  Because Nature articles are very information rich, I was thinking that an unpacking of my recent article might be helpful to some of my readers out there. Note that how this article came to be may be the subject of a separate but related post which I'll post once things calm down a bit. Instead, this post focuses on the content of the paper and you can read more down underneath the cut.



We're all familiar with the melting of snow in the spring, where the solid changes into liquid and runs away. But snow can also evaporate directly, moving from solid directly into a gas - water vapor - under the right conditions in a process called sublimation. You can see sublimation at work in the evaporation of carbon dioxide or "dry" ice, which appears to steam if left out on a table and will bubble if placed into a warm (from the perspective of the dry ice!) bath of water. For sublimation to happen in snow and ice made from water, the air must be dry and it must be too cold for snow or ice to melt. These are conditions that often occur on Earth in equatorial mountain environments, but can also periodically occur further north or south.


This sublimation process is very sensitive to how much sunlight the snow surface receives, which means that the sun can sculpt complex patterns. Because the snow is reflective, hollows in the surface tend to act like a lens, concentrating the sunlight at their bottoms and making those hollows even deeper in a process called 'self-illumination.' Eventually, the edges of the deepening hollows meet and form steep fluted sides called 'penitentes' because they resemble a monk's hands, pushed together in prayer.
Penitentes imaged by wikipedia user Arvaki. Original caption: "Field of Penitentes on the Upper Rio Blanco, Central Andes of Argentina. The blades are between 1.5 and 2m in height, slightly tilted northwards, or more exactly about 11°, the approximate position of the sun at noon at this latitude and time of the year."

These penitentes are not random, but instead form a regular pattern under the influence of the sun's heat conducting through the snow pack and a layer of vapor-rich air which forms just over the surface of the sublimating snow when winds are calm. When the sun is directly overhead, as it is near the equator, the penitentes occur as a regular grid, but when the sun is primarily low in the sky, more common in winter further north or south, lines of ridges can be observed instead.

Penitentes near the European Southern Observatory as photographed by B. Tafreshi in 2015. Image credit: ESO/B. Tafreshi (twanight.org). This location is far enough from the equator that the penitentes form ridges instead of a grid of cells.


It was in the context of these earthly penitentes my postdoctoral fellow Christina Smith and I sought to understand the dramatic images captured by NASA's New Horizons spacecraft of a region of Pluto called Tartarus Dorsa (you can see the specific image at the top of this post, above the cut). Here, we see a regular pattern of ridges and valleys. However, where penitentes on Earth are typically separated by tens of centimeters and can be up to a few meters high, the ridges and valleys of Pluto are enormous - spaced between 3 and 5 km apart and half a km in height! Nevertheless, by combining together a computer model of the physics of penitente formation with a sophisticated atmospheric simulation of Pluto, based upon weather forecasting models provided by Anthony Toigo of Johns Hopkins Applied Physics Laboratory and Scott Guzewich of NASA's Goddard Space Flight Center, we were able to show that the ridges had precisely the correct spacing, orientation and age to be penitentes.


How can such large penitentes form on Pluto as compared to the Earth? It is because these penitentes do not form in water ice, but in methane ice which evaporates more easily. Furthermore, the atmosphere into which the sublimating methane vapor mixes is much less dense (about 15,000 times less dense that on Earth), allowing that vapor rich layer to be thicker. This has the effect of increasing the spacing between one penitente and the next. Predicted and inferred wind speeds on Pluto are light which implies that this thick vapor rich layer does, in fact, exist.  Furthermore, we were able to show that these formations are restricted to Tartarus Dorsa because penitentes wouldn't be able to form in the Nitrogen ice that exists elsewhere on Pluto.


What about the orientation of those ridges? By looking at the times of year when the penitentes could tolerate the highest winds without the vapor-rich layer being blown away, and correlating these times of year with the location of the sun in the sky, we were able to predict what directions the penitente ridges would assume.


This analysis is complicated by Pluto's orbit, which is much more elliptical than that of the Earth and currently ranges from 30 times as far from the sun as is the Earth to more than 48 times as far. At its most distant point, the Plutonian atmosphere completely freezes out on the surface and penitentes cannot form. Pluto's orbit also changes over long time periods in the same cycles that give rise to the Earth's ice ages. By examining all of these situations and looking for the the right combination of atmospheric conditions, we were able to find four different times of year (two in the present era and two in the past) when the conditions were right and penitentes should form. These four formation times lead to three different orientations (two overlap) which are exactly the orientations seen on Pluto.

As you would expect, no matter whether you are on Earth or on Pluto, the same physics applies. But exotic differences in the environment give rise to features with very different scales. We can extend these results to other environments as well. This test of our terrestrial models for penitentes suggests that we may find these features elsewhere in the solar system, and in other solar systems, where the conditions are right.

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