Wind-Sculpted Dunes Sweeten Pluto’s Planetary Trappings

This photo shows where the icy plains of Pluto’s Sputnik Planum meets a range of mountains. Parallel ripples, which may be windblown dunes, extend to about 47 miles (75 km) from the higher terrain before fading out. The photo was taken by NASA’s New Horizons spacecraft during its July 2015 flyby. NASA / JHU APL / SWRI

Last night during a talk I gave about the night sky, a woman asked why Pluto is not a planet. She, like many other people who’ve attended my classes and talks since August 24, 2006, the day the erstwhile planet was demoted to dwarf planet, was still attached to the idea that it should remain a planet. Her reasons were based partly on sentiment and partly on the new data from the New Horizons 2015 flyby that revealed a world so active, it resembled a planet in every other way but semantically.

New Horizons captured this image of Pluto’s heart-shaped frozen nitrogen plains, including the Sputnik Planum dune region. NASA / JHU APL / SWRI

We went over the new definition, its origin and how a select group of astronomers voted to demote Pluto. And I do mean select — some astronomers don’t accept the change. But I also explained that since the early 1990s, astronomers have been discovering lots of objects orbiting near and beyond Pluto that resemble that 1,477-mile-wide globe in size, composition and orbit. Unless we want to start calling them all planets, it makes a lot of sense to view the whole lot as icy asteroids that occupy the far reaches of space beyond Neptune. They’re the reason for the push to redefine Pluto.

Transverse dunes form with the wind blowing perpendicular to the ridges, and have only one slipface, on the lee side. Po ke jung / CC BY 3.0

Well, I’m here to report that lovers of Pluto as a planet have another small reason to celebrate. Pluto has dunes! In the June 1 issue of Science, Matt Telfer (University of Plymouth, UK) and his team believe they have identified dunes in Sputnik Planitia on Pluto, a bright, craterless region of nitrogen, carbon monoxide and methane ice that forms part of the dwarf planet’s “heart.”

Their models reveal that dunes could form when sand-sized grains of methane ice are blown about by prevailing winds in Pluto’s atmosphere. They write that the methane grains “could have been lofted into the atmosphere by the melting of surrounding nitrogen ice or blown down from nearby mountains” with speeds topping out around 20 mph (10 meters/sec) to create regularly spaced, linear ridges resembling transverse dunes seen on Earth.

Learn how Pluto’s dunes form

Heat from the sun vaporizes nitrogen and methane ice in the mountains, releasing a blizzard of methane crystals. Blown by the wind they eventually fall under the force of gravity, strike the ground below and release more grains that are gathered into dunes by the dwarf planet’s bitter cold breezes.

Once again, a process familiar on Earth finds its analog on a lively little ball of ice 3.7 billion miles from the sun. Will Pluto fight its way back to planethood one day? Maybe not, but it continues to blur the distinction we make between asteroid and planet.

2 Responses

  1. Troy

    I wish I still had it, but I asked the Planetary Society’s Clark Chapman in the 1990s about Pluto possibly getting demoted. His response was to shame me more than anything. His response Pluto is Pluto and is incredibly interesting. I actually think there is very good reason for demotion into a new category of dwarf planets based on precedent in the 1800s after the big 4 asteroids were discovered and more and more were creating a volume of planets reminiscent of Harry Potter receiving letters from Hogwarts.

    1. astrobob

      Hi Troy,

      Good point. I agree that the 1800s precedent came into play here as far as Pluto’s demotion.

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