The ice spike forests of Pluto: What the heck are penitentes?

Astronomy websites are spreading the news these days that an icy structure called 'nieve penitentes' has been confirmed for Pluto, of a kind known from the high mountains of South America, and that a computer simulation had shown where it came from. Alas, many commenters were not too familiar with the concept and got the details wrong. The following contribution tries to explain what kind of a phenomenon is really behind the nieve penitentes, or snow penitents.

Fig. 1: Tartarus Dorsa, originally
described as a snake-skin structureNASA/JHU-APL/SwRI

First: The finding as such is not quite so new any more. The supposition that the region of Tartarus Dorsa on Pluto (fig. 1) might constitute an extraterrestrial equivalent of nieve penitentes has already been mentioned in our book Pluto & Charon, June, 2016. The news about it is the digital simulation by Moores et al. that has confirmed in December 2016 what had been but mere speculation in spring.

In the Alps or in the Pyrenees, you would not find any nieve penitentes because it is restricted to high mountains close to the equator, such as the Andes, Mount Kilimanjaro, the Elbruz or the Himalayas. The air must be very dry and very cold - maybe also very low-pressure? - to reduce layers of ice sublimating in sunlight to structures that grow most frequently up to 0.5 to 1.5 metres and to a few decimeters in width, though occasionally they may tower as high as six metres. They are arranged in regular spaces following the prevailing direction of irradiation. Mountaineers know this phenomenon also as ice spikes or ice blades. Despite its outward beauty, particularly when the moon is out, it is little popular because it is difficult to tread on and complicates making headway.

Fig. 2: Nazarenos as a model for snow penitents
By Pedro J Pacheco (Own work) [CC BY-SA 4.0],
via Wikimedia Commons

Ice blades have not been named by painter Rudolf Reschreiter, as you often may read these days, but by mountaineer Dr Paul Güssfeldt who first discovered them on Mount Aconcagua in 1883. In the Deutsche Rundschau journal, vol. 42, 1885, Güssfeldt recalled that he was first tempted to refer to the head-high ice spikes, 'which deserve to be introduced to science by a special term' as 'Kerzenfelder', candle fields,'until [mountain guide Lorenzo] Zamorano gave the better word nieve de los penitentes, or nieve penitente, "snow penitents", into my hand.' Lorenzo Zamorano was obviously reminded of the white pointed hoods of the nazarenos, the penitents of processions of the Spanish Catholic church (fig. 2). The term, nieve penitentes, was plainly claimed as the 'international name' of the phenomenon by the Popular Science journal in December 1917.

Concerning visuals, however, it was indeed Rudolf Reschreiter (* 1868; † 1939) who popularised snow penitents when he travelled the Cordilleras in 1903, in the wake of Prof Hans Meyer. On 12 July, Reschreiter discovered ice blades on the west slope of Mount Chimborazo, took photos (image 3) and converted them later into naturalistic paintings that Prof Meyer used to illustrate his 1907 book In den Hoch-Anden von Ecuador: Chimborazo, Cotopaxi, etc. One of these paintings is now on display in the Alpenvereinsmuseum Innsbruck.

Fig. 3 Ice blades rendered by
R. Reschreiter in 1907

Neither Güssfeldt nor Meyer were able to explain the origin of snow penitents beyond the obvious, namely that wind and sun had to be involved in some way or another. In 1917, Popular Science even smugly fell back on precipitated grains of 'meteor' dust to explain why ice blades should stay behind when a glacier was melting away. It was not before a hundred years later, in 2007, when a team led by M. D. Betterton finally managed to generate small-scale snow penitents in the lab:

'In nature, these spikes develop every year anew when tiny bumps on a surface of fresh snow scatter sunlight inside. More light causes more snow to evaporate, or sublimate - owing to the dryness of the air, the solid-state snow transforms into vapour. The process is accumulative: the depressions become deeper and deeper and, hence, can "trap" more and more sunlight, so that at the end there are deep troughs between which those ice spikes have remained. In winter, when the sun is less powerful, fresh snowfall fills in the troughs, so that in spring, another level surface will welcome the sun. The spikes protect the glacier surface in two ways: On the one hand, they cast shadows so that less surface is exposed to the sun. On the other hand, their three-dimensional structure multiplies the available surface of the glacier - providing more surface for heat exchange, and cold mountain winds can cool the ice more effectively.

To accelerate the process, Betterton's team had spread toner particles from a photocopier on fresh blocks of snow. If the particles were spread thin enough to let light pass to the ice, troughs developed there as well. The toner particles then protected the ice spikes against irradiation and you had to wait only half an hour, rather than three hours, for "ice blade forests" to develop.' (Was this approach inspired by the 'meteor' dust hypothesis?)

(Translated from Welt der Physik)

This model had a drawback, though: it still failed to explain why the ice blades would be set in regular spaces and turned parallel to each other. A study published by Philippe Claudin in 2015 helped explain this:

'Regarding the characteristic spacing of these troughs, Claudin and his colleagues found that vapour diffusion at the surface is essential. It" see only when there ares significant local variations in the vapour contents of the air precisely above the surface that one part of the surface can sublimate quicker than another. But lateral diffusion [by wind] suppresses look thus if this diffusion is almost, discrete penitentes can only grow a long distance striking. In other Word, the separation of the penitentes increases with the diffusion advises.'

(http://physics.aps.org/articles/v8/92)

However, the structures in Tartarus Dorsa on Pluto are very different from those on Earth. Firstly, they are arranged not in one but in three different directions, interpreted by Moores et al. as a sign of high age, while ice blades on Earth do not survives even a full year: Pluto must have changed its orbital parameters several times while they were growing. Secondly, they are overwhelmingly gigantic: Made of frozen methane rather than water ice, they are not restricted to Reschreiter's hip-high spikes but grow to pillars that loom up to 500 metres and stand so far apart that you could squeeze a colony in between!

A viewer on the surface of Pluto must consider Tartarus Dorsa a wonder of nature that only the most daring SF authors might have imagined. Maybe this description comes closest that Stanislaw Lem gave of the fictive Wood of Birnam on Saturn's moon Titan in his novel Fiasco (1985): 'The furious play of chemical radicals … created a crusty porcelain jungle that atteined heights of a quarter of a mile; the weak gravitation assisted its growth, so that there were treelike formations and thickets of glassy white laid upon each other in successive layers. … The enormous bulk was actually a solidified cloud formed of spiderweb capillaries in every shade of white, from pearly opalescent to dazzling milky.' It would be worth knowing whether snow penitents may also occur under the conditions prevailing on Titan - may this explain the remarkably bright features of the landscape of Xanadu?

Now here is a challenge for you artist illustrators: What would the ice spike forests of Tartarus Dorsa look like for astronauts landed on the ground?

Codex Regius, in January 2017