Nearly a decade after Nasa’s New Horizons spacecraft made humanity’s first flyby of Pluto, scientists have uncovered fresh evidence that the dwarf planet may be far more dynamic than previously believed.
A new analysis of images captured during the historic 2015 encounter suggests that liquid nitrogen may be rising to Pluto’s surface through cracks in a giant glacier, marking the first evidence of recently flowing liquid on the icy world.
The findings come from a study published in The Planetary Science Journal, based on high-resolution imagery collected by New Horizons as it swept past Pluto in July 2015.
The research focuses on Sputnik Planitia, the enormous heart-shaped nitrogen glacier that dominates Pluto’s surface. Larger than the combined area of Texas and Oklahoma, the frozen basin has long intrigued planetary scientists because of its unusual geology.
Researchers discovered that dark linear streaks and diffuse patches along the glacier’s northern edge resemble patterns seen on Earth’s glaciers after they have been wetted by rain or by water emerging from beneath the ice.
But Pluto is far too cold for rainfall.
Instead, scientists believe the only plausible explanation is that liquid nitrogen is periodically welling up from beneath the glacier, reaching the surface through fractures in the ice in a process known as basal melting.
“Pluto never stops surprising us,” said Alan Stern, principal investigator of Nasa’s New Horizons mission and lead author of the study.
“In addition to suggesting that liquids have recently expressed themselves on Pluto’s surface, this result also suggests a new kind of time-variable feature on Pluto.”
During the 2015 flyby, New Horizons also revealed giant convection cells—city-sized polygons formed as frozen nitrogen slowly churns beneath the surface.
The newly identified dark patches appear to occur along the boundaries of these cells, indicating that Pluto’s icy shell may still be active.
Unlike Earth, where liquid water shapes glaciers, Pluto’s extremely low temperatures make liquid nitrogen the only substance capable of briefly existing in liquid form under the right conditions.
Researchers say nitrogen rain is physically impossible in Pluto’s atmosphere, strengthening the case that the liquid originates beneath the glacier itself.
The discovery adds to growing evidence that Pluto is not a frozen, geologically dead world, but an active planet-sized body where ice behaves much like rock does on Earth.
Scientists believe continued analysis of the New Horizons data could reveal even more signs of ongoing geological activity, despite Pluto orbiting nearly six billion kilometres from the Sun.
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