Although Pluto's surface temperature averages around 400 below zero degrees Fahrenheit, new findings point to the possibility that liquids pooled into lakes and rivers on Pluto's surface within the last million years. The findings, although not conclusive, point to nitrogen as the likely liquid and continue to remind us just how little we know about the icy planet.
"What the data revealed did not surprise us," said NASA's Jim Green. "It shocked us."
One of the main pieces of evidence for Pluto's past liquid pools lies in the atmospheric changes that have taken place during the planet's 248-year orbit. After taking into account the fact that Pluto's axis is tilted by about 120 degrees, meaning it experiences extreme seasonal shifts that are some of the most intense in the solar system, the team simulated these changes over millions of years. The results showed that Pluto's nitrogen atmosphere gets thicker and thinner over time.
"The pressure changes radically," said Alan Stern, New Horizons principal investigator, adding that Pluto's atmosphere is "atypically low" today, much lower than its maximum, which is more than 20,000 times the current reading.
These unusual changes mean that surface temperatures fluctuate enough to alter the atmosphere on Pluto's surface and have the ability to make frozen solids turn into gas. In fact, around 800,000 years ago, temperatures were high enough to melt nitrogen due to the extremely warm climate stemming from Pluto's orbital alignment at the time.
"The current Pluto is in an intermediate phase between its climate extremes," said Richard Binzel of MIT.
A second piece of evidence for Pluto's ancient lakes stems from the surface features that the New Horizons team observed in the pictures that came back from the spacecraft.
"We see for all the world what looks to a lot of our team like a former lake, a frozen lake," Stern said.
The potential frozen lake is located just north of Pluto's icy Sputnik Planum and measures approximately 20 miles across. Although the team does not yet known what was flowing in the lake, nitrogen is a likely candidate, along with neon, molecular oxygen or molecular helium.
"It's very smooth, as if a liquid has frozen across one height," Stern said. "It's hard to come up with an alternate model that would explain that morphology."
As of now, our understanding of nitrogen's behavior under extreme conditions is limited, as laboratory experiments have yet to examine the issue. Nevertheless, the more we learn about Pluto, the more questions there seem to be.
"This story, like the planet, is evolving," Stern said.
The findings were reported March 21 at the Lunar and Planetary Science Conference.
