Pluto鈥檚 heart looks surprisingly fresh, and flowing nitrogen ice may be acting as its fountain of youth.
When NASA鈥檚 New Horizons spacecraft flew past Pluto in 2015, it took the best pictures we鈥檝e ever had of the dwarf planet. They showed that Sputnik Planitia, part of a bright, heart-shaped plain splayed across Pluto鈥檚 surface, has no craters.
That was a surprise to planetary scientists, because most bodies in the solar system are thoroughly pock-marked from smaller rocks smashing into them. The rest of the surface of Pluto has craters large and small, so there鈥檚 no reason to believe Sputnik Planitia would have been spared.
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For this vast plain to have no craters at all, something must be covering them up or filling them in. Qiang Wei at Peking University in Beijing, China and his colleagues calculated that the because nitrogen ice on Pluto is relatively soft, it may be flowing like molasses to fill in the craters as they form.
Ice, ice baby
Previous work suggested that the craters must be erased over a period of 10 million years or less, but it could happen much faster.
鈥淲hen you see no craters, you don鈥檛 know if they were erased yesterday or ten thousand years ago,鈥� says at the Massachusetts Institute of Technology. 鈥淚t鈥檚 like when you see a small horse from far away: it could be a colt, or it could be full-grown pony.鈥�
Wei and his team found that the maximum speed of the ice鈥檚 crawl into a crater could be about 0.6 kilometres per hour, far faster than glaciers can move on Earth. Previous studies have found that the nitrogen ice layer in these plains is likely around 10 kilometres thick, but it could vary from place to place within Sputnik Planitia.
If it is only 4 kilometres thick and its viscosity is close to what other groups have measured in Pluto-like conditions in the lab, a 2-kilometre-deep crater could be filled in as few as 10 months.
If the nitrogen ice layer is thicker, the craters could聽be erased even faster. If it鈥檚 stiffer than measured in the lab or mixed with other compounds, the process might take up to tens of thousands of years. Either way, it could explain how Sputnik Planitia looked so fresh-faced when New Horizons flew by.
Astrophysical Journal Letters
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