Pluto: 'Active heat source' revealed under Sputnik Planitia
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When NASA’s New Horizons spacecraft flew over the distant world in 2015, the probe captured stunning views of one of Pluto’s biggest craters, Sputnik Planitia. The crisp images revealed features “smaller than half a city block” and exposed an unusual pattern of geometric shapes, seemingly carved into the surface. According to NASA, the “polygonal or cellular” features were thought to be the result of heat exchange in the layer of nitrogen ice covering the dwarf planet.
The US space agency said: “The surface of Sputnik Planum appears darker toward the shore (at top), possibly implying a change in composition or surface texture.
“The occasional raised, darker blocks at the cell edges are probably dirty water ‘icebergs’ that are floating in denser solid nitrogen.”
A team of international researchers, including experts at the University of Exeter, have explained how these unusual structures took their shape.
Sputnik Planitia is a colossal impact crater in the planet’s northern hemisphere, that consists of a plain slightly bigger than France.
The crater is filled with nitrogen ice, which makes up more than 90 percent of the planet’s surface, with trace amounts of other gases, such as methane and carbon dioxide.
Using cutting-edge modelling techniques, scientists found the ice can form rough polygonal shapes through the process of sublimation.
When ice sublimates, it transitions from a solid into a gas without going through a liquid state.
According to the research team, the sublimating ice powers convection – the transfer of heat – in the ice layer by cooling down at its surface.
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The findings were published in the journal Nature.
Dr Adrien Morison, a Research Fellow from Exeter’s Physics and Astronomy department said: “When the space probe New Horizons performed the only, to date, fly-by of Pluto in 2015, the collected data was enough to drastically change our understanding of this remote world.
“In particular, it showed that Pluto is still geologically active despite being far away from the Sun and having limited internal energy sources.
“This included at Sputnik Planitia, where the surface conditions allow the gaseous nitrogen in its atmosphere to coexist with solid nitrogen.
“We know that the surface of the ice exhibits remarkable polygonal features – formed by thermal convection in the nitrogen ice, constantly organizing and renewing the surface of the ice.
“However, there remained questions behind just how this process could occur.”
Dr Morison and his colleagues ran a series of numerical simulations to prove that the cooling from sublimation can power the convective process.
And the data was consistent with the discoveries made by New Horizons – the size of the polygons, surface velocities and amplitude of topography.
The data was also consistent with the timescale that predict sublimation within Sputnik Planitia, starting between one and two million years ago.
According to the research team, the dynamics on the nitrogen ice layer are reminiscent of those found on Earth’s oceans, driven by the climate.
They believe similar, climate-driven processes can occur on the surface of other bodies in the solar system, such Neptune’s Moon Triton or Eris and Eris and Makemake in the Kuiper Belt.
Sputnik Planitia was named after Russia’ Sputnik, the world’s first artificial satellite to be put into orbit.
NASA’s New Horizons has been soaring through the Solar System for nearly 16 years and in 2019 made a flyby of the Kuiper belt object 486958 Arrokoth (Ultima Thule).
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