Pluto has been revealed to be a truly spectacular world, featuring mountains made of water ice, rivers, lakes, and perhaps even cryovolcanoes. Its most prominent geological feature, though, is definitely Sputnik Planum, the now-famous heart-shaped basin photographed so beautifully by New Horizons as it zoomed by. Sputnik Planum is curiously flat and featureless compared to the jagged landscapes surrounding it. There are no clearly defined impact craters on it at all, which means that something is continually smoothing it down.
Two separate teams of researchers have spent some time trying to solve this mystery, and rather wonderfully, they've both come to the same conclusion. As reported in a pair of independent Nature studies, polygon-shaped convection currents of flowing nitrogen ice within the basin are acting to remove any bumps or blemishes. This continual motion of ice at the surface keeps it smooth, as well as covers up any potential craters that may have formed.
Essentially, Sputnik Planum is an active alien sea – one that’s 900,000 square kilometers (347,000 square miles) in size – full of nitrogen icebergs that continually act to keep its surface young.
Significantly, in order to generate these currents, temperature differences within Sputnik Planum must exist. Pluto is too small to still contain any primordial heat, the thermal embers present from its presumably violent formation. This means that the only plausible explanation is that radioactive elements at depth are still decaying, producing enough heat to constantly drive geological processes at the surface, including all across Sputnik Planum.
“Not only is it the heart of Pluto, it’s the beating heart,” Bill McKinnon, a professor of planetary geology and geophysics from Washington University in St. Louis, and lead author of the first of the studies, told National Geographic. “There are actually things happening. If we were to come back in 100,000 years, the pattern would be markedly altered.”
Image in text: Mapping the polygonal shapes within Sputnik Planum. McKinnon et al./Nature
A side-on view of Sputnik Planum, where its nitrogen icebergs can just about be seen. NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
The nitrogen ice across Pluto is structurally quite weak, which allows it to deform. However, the extremely low surface temperatures on this ice world – which can dip as low as -233°C (-387°F) – mean that things there deform and move incredibly slowly.
Consequently, the overturn rate of the icebergs, where warmer chunks rise to the surface as colder, denser chunks descend to the depths of Sputnik Planum, will also be slow. According to the second study, led by Alex Trowbridge, a graduate student at Purdue University, this will never be faster than 2 centimeters (0.79 inches) per year.
Using this rate to calculate how fast it would take for the surface to be as smooth as it looks today, McKinnon’s study gives a surface renewal age of around 500,000 years. Trowbridge and his team calculated that the maximum age of SP is about 1 million years. Either way, it’s incredibly young – about 99 million years younger than Saturn’s rings, for example.
An enhanced color image of Pluto highlighting its wildly varying geological features, including the oddly smooth heart-shaped Sputnik Planum in the center to lower-right. NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
“Within this pool of nitrogen ice, there are mountains of water ice that have collected at the edges of the polygons,” Trowbridge said in a statement. “The way they have collected suggests they have moved or floated like icebergs with the convection current. If this is true, we can calculate how deep the pool would need to be for the icebergs to float freely without catching on the bottom.”
As it turns out, the icy nitrogen sea within Sputnik Planum is, at least, 5 kilometers (3.1 miles) from top to bottom. So, far from being old, cold and dead, Pluto turns out to have a youthful, roiling, deep radioactive heart.