The image above, taken by the New Horizons space probe in July of 2015, shows some detail of an extensive dune field (lower right) that abuts onto a major mountain range (upper left) on the surface of Pluto.
Since it has long been known that solar system objects with rocky surfaces and substantial atmospheres have dunes on them, the nature and extent of the dunes on Pluto came as a surprise to most researchers. The high-resolution images taken by New Horizons revealed the presence of a 120 km-long dune field alongside the massive Al-Idrisi Montes mountain range that appears to be comparable, if not identical to dune fields on Earth.
However, while dunes on Earth are composed of silicate grains that are transported over sometimes-vast distances by powerful winds, the dunes on Pluto are made of small methane particles, which posed somewhat of a problem to researchers.
In a study recently published in the scientific journal Science, Matt Telfer, the lead author, explained how the dunes might have formed- despite the fact that Pluto’s atmospheric density is only 0.001% that of Earth’s, while its atmospheric pressure is about 100,000 times lower, and its surface gravity12 times less than Earth’s.
To solve the problem, the researchers explain that exceedingly detailed spatial studies of the dunes revealed the presence of orthogonal winds, which like similar wind patterns on Earth, result in the deposition of material in a manner that result in the formation of dunes. Moreover, advanced spectral and numerical modelling techniques revealed two possible origins for the methane that forms the dunes.
The first possibility involves a thin covering of methane on top of the dunes that is constantly being released into Pluto’s atmosphere as the result of a deeper layer of frozen nitrogen sublimating. The second possibility is that the methane comes from a reservoir of frozen methane particles at the bottom of the mountain range. In this scenario, it is likely that slow-moving winds pick up the methane particles, which are then transported further by higher-velocity winds that flow downward from the mountain peaks.
According to co-author Eric Parteli, this mechanism is analogous to the process in which high-speed winds on Earth are able to transport relatively massive sand grains over long distances through a thick atmosphere. However, on Pluto, the surface gravity, and both the atmospheric pressure and density are exceedingly low, which means that the wind velocity required to transport the methane particles can be at least one hundred times lower than would be required to transport particles of the same mass on Earth.
Thus, the 35 km/hour winds that flow down off the mountain are perfectly capable of lifting, and transporting frozen methane particles through Pluto’s atmosphere for considerable distances before gravity causes them to settle on the surface, which is then sculpted into a dune field by the wind, just like on Earth.
Although New Horizons is now past the point where it can gather any additional useful information on Pluto or its moons, it will reach the trans-Neptunian object designated 2014 MU69 on January 1st 2019, which at a distance of 1.6 billion km beyond Pluto, will be the most distant (solar system) object to be visited by a space probe.