Odd surface patterns on Pluto hinted to astronomers that, sometime in the pastâquite possibly not that long agoâliquid existed on its surface. A team of planetary scientists believe they’ve found it.

A new analysis of New Horizon images of the Sputnik Planitia, a section of Plutoâs massive, heart-shaped glacier, suggests that liquid nitrogen once existed on the dwarf planet’s surface. The study, published late last month in The Planetary Science Journal, presents the first evidence of recent liquid flow on Pluto. That is, recent on astronomical scales; Sputnik Planitia is presumably less than one million years old, so if the findings are true, it would mean that liquid nitrogen was present sometime in that period.

âPluto never stops surprising us, and this new result certainly does that,â Alan Stern, the studyâs lead author and New Horizonsâs principal investigator, said in a statement. âIn addition to suggesting that liquids have recently expressed themselves on Plutoâs surface, it also suggests a new kind of time-variable feature on Pluto.â

The right spot

According to the paper, the area that came to be called the Sputnik Planitia caught astronomersâ interest even before New Horizons. Hubbleâs observations and other data indicated that one specific section of Pluto had an unusually bright terrain. This was one important reason that, when New Horizons made its Pluto flyby in 2015, Sputnik Planitia was deemed a primary target for high-resolution observations.

Astronomersâ assumptions were on the mark. New Horizonsâs images of the region showed unique patterns of dark linear patterns, whose albedo, or fraction of diffusely reflected sunlight, was âremarkably close to that of the Earthâs polar ice sheets,â the researchers explained in the paper. However, Plutoâs thin atmosphere, as we observe it today, makes liquid nitrogen rain physically impossible, the researchers explained in the statement.

Inklings of nitrogen

As a result, prior analysis on Sputnik Planitia, including those Stern was involved in, assumed that there were some ancient liquid flows. The latest paper, on the other hand, evaluates the hypothesis that there is currently or recently liquid nitrogen below the glacier. For the study, the team ran computer simulations to confirm it was feasible for liquid nitrogen to push upward from beneath the glacier and form the distinctive patterns we see on Plutoâs surface today.

The researchers found that it was possible for liquid nitrogen to travel up small conduits, like lava or geyser tubes, and remain liquid long enough to create dark patterns on the nitrogen glacier. That said, Pluto, due to its sheer distance, remains an enigma to astronomers, and future observations may or may not further this hypothesis.

âPluto has many unique terrains seen nowhere else in the solar system, and this area of Sputnik Planitia is one of them,â added Kelsi Singer, a study co-author and a research scientist at the Southwest Research Institute. “Its surface provides a different set of conditions compared to what we are used to on Earth, and exploring that allows us to better understand how materials behave in environments that are difficult to produce on Earth.â

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