Nature loves to riff on existing motifs, even in the most unearthly of environments.
The most interesting worlds are those that are most varied and vibrant. Consider Earth. Lying between its churning interior and solar-driven atmosphere, its surface is constantly reworked, producing infinite diversity and beauty. Maybe I’m biased, but that winning combination of vigorous internal and atmospheric activity makes for the best of planetary exteriors.
Fortunately, we keep finding more places where the fertile interface between geology and meteorology cooks up marvels. Such features are often a beguiling mix of the familiar and the exotic, with recognizable forms assembled from available materials that are malleable in the extreme (to us) conditions found on other worlds. On Titan, for example, a hydrological cycle creates recognizable fractal rivers and lakeshores, but the working fluid is methane, not water.
Pluto, as revealed to us by the New Horizons encounter of 2015, sports precipitous water-ice mountains capped with methane snow, ringing a vast nitrogen glacier called Sputnik Planitia. Upon seeing that massive smooth plain, clear and fresh and free of craters, we were consumed by the mystery of its self-erasing surface. What is happening below to drive that sea of solid nitrogen to turn itself over? It may simply be that the radioactive decay of the rocky interior generates enough heat to churn the soft nitrogen ice.
There’s no new physics here — simply common materials found in such otherworldly scales and conditions that our pre-encounter imaginations were not quite up to the task. This is why we explore rather than simply stay home and construct models.
Yet it’s not just internally driven activity that gives Pluto its dazzling variety and complexity. Our favorite Kuiper Belt orb, we find, is another place where many of the grandest enigmas and most exquisite features arise from the interaction between surface and atmosphere.
The wonderful discovery of dunes on the western flanks of Sputnik confirms this. Among the discoverers is Jani Radebaugh, a planetary geologist at Brigham Young University, who was instrumental in gathering the international, multi-disciplinary team that produced the June 1st Science paper that made the case for Plutonian dunes.
I admit I was skeptical when I first heard about potential dunes on Pluto. I assumed its wispy atmosphere, 100,000 times thinner than Earth’s at the surface, would be far too rarefied to blow around material in the way needed to fashion dunes.
But Jani knows dunes through and through, on our planet and elsewhere. Applying her physical intuition, honed from years of hiking over, measuring, and comparing geologic features, she recognized the telltale forms of surface landscapes formed by windblown deposits of fine materials. “When I first saw the high-resolution images of those areas, I realized that these were dunes,” she told me. “I knew there had to be an atmosphere that could produce them.”
The rare Plutonian air hosts strong breezes, but what generates the initial force to lift the small particles of methane snow that seem to be forming these drifts? One possibility: As the solid nitrogen surface heats up in the Sun, it vaporizes, launching particles skyward. Alternatively, the dunes could have formed in the past when Pluto’s changeable atmosphere was thicker.
Nitrogen glaciers, ice mountains, methane snowcaps, and now — apparently — methane “sand” dunes. Wonders never cease.
This article originally appeared in print in Sky & Telescope's September 2018 issue.