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Psychedelic Pluto
This enhanced color image of Pluto highlights the many subtle color differences between Pluto’s distinct regions. The imagery was collected by the spacecraft’s Ralph/MVIC color camera on July 14, 2015, from a range of 22,000 miles. (Credit: NASA / JHUAPL / SwRI)

Rivers and lakes of liquid nitrogen may have splashed over Pluto’s surface hundreds of millions of years ago, and could do so again, due to shifts in the dwarf planet’s orbit and the tilt of its orbit.

That hypothesis is a good fit for the evidence collected last July when NASA’s New Horizons spacecraft zoomed past the dwarf planet and its moons, scientists said today.

Today’s revelations came during a review of New Horizons’ findings at the Lunar and Planetary Science Conference in The Woodlands, Texas. Research teams shared their latest findings about the mission, including some that have yet to be published in journals such as Icarus.

One team member, MIT’s Richard Binzel, said the conditions conducive to creating a lake of liquid nitrogen might have existed 800 or 900 million years ago. During that epoch, Pluto’s axial tilt was dramatic enough to expose a wide area of the distant world’s surface to constant sunlight for long periods of time.

“If you were to have a flyby at one of those epochs, millions or billions of years ago, you might have seen, for example, that lake reflecting sunlight specularly at you, because that surface is liquid rather than frozen as it is today,” said mission principal investigator Alan Stern, a planetary scientist at the Southwest Research Institute, also known as SwRI.

The phenomenon would explain modern-day features on Pluto that look as if they’re lakes or rivers that have frozen in place.

The “lakes on Pluto” scenario emerges from computer models that run the sequence of shifts in Pluto’s orbit and the tilt of its axis backward for billions of years. Binzel said those models suggest that Pluto is at an intermediate point between climate extremes.

Today, Pluto’s axis is tilted about 120 degrees, which is far more dramatic than Earth’s 23-degree tilt. That means there’s an overlap between Pluto’s arctic zone, where the sun never sets during the planet’s summer season; and its tropical zone, where the sun is directly overhead at some point in Pluto’s orbit.

Pluto's seasons
Pluto’s “tropical arctic” experiences both direct sunlight and prolonged periods of sunlight and darkness. (Credit: Richard Binzel and Alissa Earle / MIT, adapted from submission to Icarus)

The models suggest that the axial tilt was even more extreme many millions of years ago, due to an astronomical phenomenon known as the Milankovich cycle. The same type of cycle is thought to have given rise to long-term climate shifts on Earth.

On Pluto, the climate shifts would have affected the density of Pluto’s nitrogen-rich atmosphere. Stern said the atmosphere in ancient times could have been denser than that of Mars. “This opens up the possibility that liquid nitrogen may have once or even many times flowed on Pluto’s surface,” he said.

The researchers cautioned that New Horizons’ readings were still being analyzed. The “lakes on Pluto” scenario may have to be fine-tuned or discarded when more of the data comes back.

“We are just beginning to understand the long-term climate of Pluto,” Binzel said.

Stern said about half of the data remained to be downlinked from the spacecraft, which is now more than 180 million miles beyond Pluto.

Among other findings reported at the conference today:

  • Binzel said the orbital shifts could also explain why most of Pluto’s equatorial region is covered with a reddish layer of nitrogen-containing compounds known as tholins. More of the ice may have been cooked out of that region by heightened exposure to sunlight for billions of years. The bright, icy region known informally as Sputnik Planum is an exception. That region’s ice was apparently trapped in a huge impact crater, Binzel said.
  • Pluto’s surface shows extensive evidence of glacial flow and erosion. “There are two likely scenarios for the erosion we see,” Orkan Umurhan, a researcher at NASA’s Ames Research Center, said in a statement. “It could be gradual, when much of Pluto’s nitrogen ice was lost over time. Or it could be part of a cycle in which the nitrogen ice evaporates and redeposits on the highlands before flowing back into the plains. In all likelihood, both scenarios have been and still are operating.”
  • A study of the size and the number of craters on Pluto and its moons suggests the satellite system was created in a giant impact between Pluto and another celestial object about 4 billion years ago. “This is our first proof that the giant impact that created the Pluto system must have been ancient, not recent,” SwRI researcher Kelsi Singer said in a statement. “That puts the impact on a timeline going back billions of years, rather than millions.”

Stern will deliver a public lecture titled “The Exploration of Pluto” at 5:30 p.m. PT Tuesday.  The lecture will be archived online. For the full schedule of LPSC’s live and archived Web events, see http://livestream.com/viewnow/LPSC2016.

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