June 4, 2018 | David F. Coppedge

What’s Pluto Been Dune? Making Young Sand Dunes

Sand dunes were surprising enough on Titan, but et tu, Pluto? and young, recent dunes? Scientists couldn’t believe their eyes.

What do you picture when thinking of sand dunes? Dry deserts, with wind whipping piles of sand into ripples and hills shallow on one side and steep on the other— that kind of thing. You probably don’t think of Pluto. The outermost planet (or Plutoid, Dwarf Planet, or Trans-Neptunian Object, depending on your taste) is supposed to be an icy ball frozen for billions of years. The latest analysis of data from the 2015 New Horizons mission, however, shows that Pluto has been busy recently constructing sand dunes out of particles of icy methane.

Pluto sand dunes on polygonal blocks of nitrogen ice thought to be convection cells. Notice wind streaks perpendicular to the dunes. Credit: NASA/JHUAPL/SwRI

Let’s start with Nature‘s brief announcement: “Hundreds of methane dunes nestle at the base of Pluto’s mountains: The formations offer a surprise on a body with such a thin atmosphere.

Dramatic dunes of methane sweep across part of Pluto. The discovery shows that the dwarf planet’s atmosphere, although thin, can still generate winds powerful enough to blow particles across the surface….

Until now, some researchers thought that Pluto’s atmosphere, which is at 1/100,000th the pressure of Earth’s, could not support winds strong enough to sculpt dunes. But the team’s analysis shows otherwise.

The dunes probably formed in the past 500,000 years — which means Pluto is a geologically active world.

“A geologically active world”— we remember the surprise at the images that suggested ice volcanoes, glaciers, and now sand dunes. Doing the math, we see that 500,000 years (just their theory-laden estimate, not a fact of observation) represents just a 10th of one percent of the assumed age of the solar system. What could have happened so recently to form sand dunes in the fraction of an epoch when human beings can observe them?

You can see the dunes in the photos. Scientists estimate they are up to a kilometer apart. In such a thin atmosphere, the winds must be strong enough to loft the particles and carry them quite a distance over Sputnik Planitia, the heart-shaped flat region that was so surprising to planetary scientists during the 2015 encounter. Alexander Hayes says in his review in Science that the surface pressure on Pluto is 100,000 times weaker than on earth. Even so, winds are not strong enough to lift particles; they must be lofted into the atmosphere by sublimation, the scientists infer.

The BBC News reveals scientists’ unpredicted, after-the-fact explanations:

To be able to form, dunes need an atmosphere dense enough to make wind transport possible, a supply of dry particles, and a mechanism that lifts particles off the ground.

At first sight, none of those conditions seem to be met on Pluto.

But Dr Telfer and his colleagues calculate that the dunes may be in one of the windiest areas of the Pluto with wind speeds reaching up to 10m/sec – enough to keep particles moving.

And yet if these winds have been blowing for 4.6 billion years, shouldn’t the supply have been used up and the process stopped? The paper in Science Magazine is where to look for the answer. First, a summary report by Keith Smith in Science:

The methane grains could have been lofted into the atmosphere by the melting of surrounding nitrogen ice or blown down from nearby mountains. Understanding how dunes form under Pluto conditions will help with interpreting similar features found elsewhere in the solar system.

No clue yet why these young dunes have formed only recently. The official paper in Science is by Telfer et al. Watch for any clues that they expected young features on this body:

The surface of Pluto is more geologically diverse and dynamic than had been expected, but the role of its tenuous atmosphere in shaping the landscape remains unclear. We describe observations from the New Horizons spacecraft of regularly spaced, linear ridges whose morphology, distribution, and orientation are consistent with being transverse dunes. These are located close to mountainous regions and are orthogonal to nearby wind streaks. We demonstrate that the wavelength of the dunes (~0.4 to 1 kilometer) is best explained by the deposition of sand-sized (~200 to ~300 micrometer) particles of methane ice in moderate winds (<10 meters per second). The undisturbed morphology of the dunes, and relationships with the underlying convective glacial ice, imply that the dunes have formed in the very recent geological past.

Small sand dunes at Salton Sea, California (David Coppedge)

Let’s read their paragraph about the age of the dunes:

An upper limit on the age of the dunes, which sit atop the ice of the western margins of SP [Sputnik Planitia, the flat plain on Pluto], is imposed by the recycling rate of the upper surface of the convectional cells within the ice (i.e., <500 ka). This overturning of the substrate, inferred from the complete absence of identified craters on SP, provides an age constraint for superficial landforms that is not available for dunes on other solar system bodies and implies a geologically and/or geomorphologically active surface. Surface features, undistorted by the convectional overturning within the ice, must be much younger than the time scales of convection, and therefore closer to the time scales of Pluto’s strong seasons (i.e., terrestrial decades to centuries). Further evidence that the dunes form on a time scale substantially shorter than that of the convection is suggested by the superposition of the dunes over the depressions at the cell margins.

One must keep in mind that convection is theory, but dunes are eyewitness observations in the present. This is seen by their words “interpreted as” and “presumably” when talking about the supposed convective cells. All they can say empirically is that “The presence of these dunes indicates an active atmosphere that produces geologically young landforms.” Even if the dunes represent features younger than the latest cycle of convection, it would imply that this material in Sputnik Planitia has been being overturned, and overturned again, and overturned again thousands of times during the assumed lifetime of Pluto.

Although reporters mention the dunes as having formed only 500,000 years ago, “and possibly much more recently,” none of the news articles or papers so far have addressed the age conundrum for the planet itself. Monica Grady doesn’t talk about it on The Conversation. Mike Wall ignores the question on Space.com. Nadia Drake fails to deal with it at National Geographic. NASA skips over it at Astrobiology Magazine. Alan Williams doesn’t address it in a press release from the University of Plymouth, nor does lead author Matt Telfer discuss it in a video clip in that article, even though he adds that scientists believe Pluto’s atmosphere is escaping. Convection and atmospheric escape wouldn’t be issues if Pluto is far younger than scientists believe.

The second video clip in the above press release shows an animation of the model of dune formation on Pluto. An opportunity to compare similar worlds is coming. Telfer mentions that the New Horizons spacecraft is set to reach another smaller Kuiper Belt Object in January, 2019 in an even closer encounter. One thing seems predictable: expect surprises. And space fans: watch for surprises from Ceres in June as the Dawn spacecraft maneuvers down to a low point 10 times closer than ever before, just 30 miles above the surface of the dwarf planet (Phys.org).

How Did Pluto Form?

A week before the dunes announcement, a paper appeared in Icarus arguing that “Primordial N2 provides a cosmochemical explanation for the existence of Sputnik Planitia, Pluto.” The authors state that the inventory of molecular nitrogen (N2) is crucial to understanding the many diverse features on Pluto’s surface. If so, where did it come from? Christopher Glein and J. Hunter Waite of Southwest Research Institute (SwRI) say in the paper that the nitrogen accreted during Pluto’s formation from primordial nitrogen or from comets, and it has not been significantly lost to space since then. But to get their model to work, they have to solve two problems: (1) how did the ratio of nitrogen to water ice become established, and (2) what happened to the carbon monoxide? They offer two suggestions for the latter question: “burial of CO in surface ices, or its destruction from exposure to liquid water.”

The SwRI press release explains this new “cosmochemical model” for Pluto in lay terms. It contains a surprise starting with a b that strains imagination:

At the heart of the research is the nitrogen-rich ice in Sputnik Planitia, a large glacier that forms the left lobe of the bright Tombaugh Regio feature on Pluto’s surface. “We found an intriguing consistency between the estimated amount of nitrogen inside the glacier and the amount that would be expected if Pluto was formed by the agglomeration of roughly a billion comets or other Kuiper Belt objects similar in chemical composition to 67P, the comet explored by Rosetta.”

If you’re not ready to swallow the billion-comets idea, they offer an alternative: “In addition to the comet model, scientists also investigated a solar model, with Pluto forming from very cold ices that would have had a chemical composition that more closely matches that of the Sun.” Are they just covering the bases? Is either model plausible?

Scientists needed to understand not only the nitrogen present at Pluto now — in its atmosphere and in glaciers — but also how much of the volatile element potentially could have leaked out of the atmosphere and into space over the eons. They then needed to reconcile the proportion of carbon monoxide to nitrogen to get a more complete picture. Ultimately, the low abundance of carbon monoxide at Pluto points to burial in surface ices or to destruction from liquid water.

How does CO get buried? The answers to these and other puzzles will be found in futureware, they promise. “Now is the time to begin data-driven investigations into the origin of Pluto’s N2,” the paper begins.

But what if both models are unworkable? And what if there were no eons?


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