THE WOODLANDS, Texas—Liquids may have pooled and flowed on Pluto’s surface within the last million years – and they may do so again, scientists reported March 21 at the Lunar and Planetary Science Conference.

Though not a given, the presence of liquids on Pluto at any point is puzzling, given that average temperatures on the frozen world hover around -400 Fahrenheit. But, as NASA’s New Horizons team is learning, Pluto is anything but dead—or predictable.

“What the data revealed did not surprise us,” says NASA’s Jim Green. “It shocked us.”

Two lines of evidence suggest the dwarf world’s surface may occasionally be just a little bit wetter than it is now. One is based on how the planet’s atmospheric pressure changes during Pluto’s 248-year orbit, and the other comes from recent images sent home by the New Horizons spacecraft, which flew by Pluto in July 2015.

For starters, Pluto’s axis is tilted by about 120 degrees—it’s tipped so far over that its north pole actually points downward (as a comparison, Earth is tipped 23 degrees). As Pluto orbits the sun, it experiences some of the most extreme seasonal shifts in the solar system, with parts of it swinging between a half-century of nearly complete sunlight and a half-century of perpetual night.

When scientists simulated these seasonal changes over millions of years, taking into account how Pluto’s tilt can wobble just a bit, they realized that Pluto’s nitrogen atmosphere becomes dramatically thicker and thinner over millions of years.

View Images A patch of crisscrossing gullies that could have been carved by liquids. (NASA/JHUAPL/SwRI)

“The pressure changes radically,” says New Horizons principal investigator Alan Stern. Today, he says, Pluto’s atmospheric pressure is “atypically low,” noting that at maximum it can be more than 20,000 times the current reading.

That means surface temperatures must be fluctuating enough to mess with the nitrogen on Pluto’s surface, driving it from a frozen solid into a gas. And sometimes, the temperature and pressure occasionally rise high enough for liquid nitrogen to flow on the surface.

The last time temperatures were sufficiently high to melt nitrogen was around 800,000 years ago, when Pluto’s orbital alignment led to its most extreme warm climate, says MIT’s Richard Binzel.

“The current Pluto is in an intermediate phase between its climate extremes,” Binzel says.

Next, as the New Horizons team studied the images coming back from the spacecraft, scientists started to spot surface features that looked as though they’d been carved by liquid. “We see for all the world what looks to a lot of our team like a former lake, a frozen lake,” Stern says. That lake, located just north of the smooth, bright icefield known as Sputnik Planum, measures about 20 miles from one end to the other. But there are also forking riverbeds and crisscrossing gullies that could have been sculpted by a similarly liquidy hand.

View Images These branching features could be riverbeds sculpted by liquids on Pluto. (NASA/JHUAPL/SwRI)

Though it’s not entirely clear what kind of liquid may occasionally trickle across the surface of a world billions of miles from the sun, nitrogen is a decent guess. Other possibilities include neon, molecular oxygen, or molecular helium, though it’s not likely those species are present in sufficient quantities to craft the observed features.

“This story, like the planet, is evolving,” Stern says.