Last updated June 1, 2018 at 5:01 pm
Analysis from New Horizon’s flyby suggests particles blown by Pluto’s thermal winds.
On a frozen world with the barest trace of an atmosphere, windswept dunes have been uncovered, as NASA’s New Horizons spacecraft flyby of Pluto continues to astound.
Scientists say the rippling dunes made from solid grains of methane at the bottom of a major mountain range on Pluto. They lie at the boundary of an ice plain called Sputnik Panitia.
Perhaps the most surprising part of the discovery is that the dune have formed in the familiar “windswept” pattern we see in many places on Earth, given Pluto’s thin atmosphere.
Dunes here form as grains of sand in deserts or ice at the poles are blown by consistent winds along a surface. They might meet an obstruction, a rock or plant and begin to pile up, the nature of the seed for a dune is unimportant just that it exists is enough.
Wind – the critical ingredient
As a dune grows it obstructs, and is built up by catching, evermore wind-blown material. When the crest of the dune is perpendicular to the wind direction and so they called transverse dunes (as opposed to longitudinal, or seif, dunes that run with the wind direction).
The critical ingredient to the dune formation is a wind. Yet how can a world with an atmosphere of just one hundred thousandth that of Earth, possibly sculpt dunes that can be seen from space, in images taken by NASA’s New Horizon’s flyby?
As reported in the recent Science article, more than 350 individual dunes were detected, some running up to 75 km long, and regularly spaced between one another.
Incredible as it seems, Pluto’s tenuous atmosphere can form these dunes, as the wind speeds of around 10 m/s (or 36 km/hr) are just enough to transport ice grains if they are of sufficiently small.
Researchers found that methane grains 200 – 300 microns (two or three hair widths across) can be carried in such a “wind”, even though it is of such light pressure that we would not feel it.
Why there are no nitrogen dunes?
There are other ice grains on Pluto, but nitrogen dunes would be too soft under the surface conditions and the hydrocarbons (or tholins) present elsewhere would stain the pristine white ice a darker colour.
While it seemed possible for tiny windblown methane ice grains to form these dune, the latest research has made clear that the winds were far too weak to lift the grains from the surface in the first place.
On this world at the dim, frozen edges of the Solar System an unlikely source provided the required lofting of grains into the air – the Sun.
Each afternoon in the weak sunlight of that star, forty times further from it than we are, every square metre of subsurface ices sublimate into an eruption of a thousand cubic metres of gas.
This gas is able to carry the ices high enough into the forming atmosphere to be carried by these most tenuous of winds.
The age of the dunes of Pluto can be estimated as the ice-surface Sputnik Planitia is renewed, literally turned over, in the course of 500,000 years or less.
Alien but oddly familiar
The complete lack of craters also indicates a very new surface that hosts the Dunes. Therefore they have formed on timescales of less than 100,000 years – in fact more likely to be over thousands of years given the lengthy seasons of Pluto, which enjoys a leisurely 249 Earth-year orbit around the Sun.
Finally, the direction of the transverse dunes does change across the different locations on Sputnik Planitia, reflecting the direction of the wind as local features such as the water-ice mountains deflect it.
These winds themselves are revealed by dark streaks of smog-like hydrocarbons, or tholins, chemically altered by the radiation from our distant Sun and exploding stars from interstellar space.
Despite the impossibly extreme alien nature of the world that the dunes reside in, it’s hard to tell them apart from those on Earth.
A reminder that nature’s few basic physical principles will guide the formation of the most recognisable of patterns through an astounding diversity of processes.