Pluto may once have harbored liquid nitrogen lakes
NASA’s New Horizons spacecraft has returned images of branching channels, along with a feature that appears to be a frozen lake, suggesting that liquids may have once flowed on the surface of a much ‘warmer’ Pluto millions or billions of years ago when it had a higher atmospheric pressure than now.
The channels, including what appear to be riverbeds and gullies, known in geology as dendritic features, appear to have been carved by liquids.
A region measuring about 30 km (20 miles) across, just north of Sputnik Planum – the smooth, bright terrain that comprises the left side of Pluto’s “heart” – looks very much like a frozen lake, according to New Horizons mission scientists.
“We see for all the world what looks to a lot of our team like a former lake, a frozen lake,” said New Horizons Principal Investigator Alan Stern.
Images of the frozen lake were captured by New Horizons’ Long Range Reconnaissance Imager (LORRI) as it flew by Pluto on July 14, 2015. The high-resolution photos show features as small as 430 feet (130 meters).
Because of its extreme 120-degree axial tilt, Pluto has experienced a wide range of temperatures over millions and even billions of years.
While the current average surface temperature of Pluto is about –400 Fahrenheit (–240 Celsius), its high axial tilt causes extreme seasonal changes during its 248-year orbit around the Sun, with some areas alternately experiencing 50 years of sunlight and 50 years of darkness.
Along with dramatic seasonal changes, Pluto’s atmosphere has thickened and thinned significantly over time, causing major changes in its atmospheric pressure.
Alan Stern also noted that the dwarf planet’s current atmospheric pressure is “atypically low”. In the distant past, that pressure has been as much as 20,000 times greater.
Under high enough temperature and pressure, nitrogen can melt and exist as a liquid. The last time both were high enough for liquid nitrogen to exist on Pluto’s surface was approximately 800,000 years ago, Richard Binzel of MIT said.
Nitrogen is the most abundant but not the only substance that can exist in liquid form on a warmer Pluto. Molecular oxygen, molecular helium, and neon could have been liquids as well, although the dwarf planet contains much smaller amounts of these.
While the only ice on Earth is water ice, on Pluto, nitrogen, methane, and carbon monoxide can also freeze.
More dense than water ice, nitrogen ice could have created Pluto’s water ice mountains by flowing into cracks in the water-ice bedrock.
Another possibility is the nitrogen ice had flowed deep enough into the planet’s surface to be heated and subsequently erupted back onto that surface as ice volcanoes.
“In addition to this possible former lake, we also see evidence of channels that may have also carried liquids in Pluto’s past,” Stern said.
Using computer simulations, New Horizons scientists illustrated how liquid could have once flowed on Pluto’s surface.
“Now we have the maps and a better and a better understanding of the surface composition, we can turn sophisticated models to bear on our understanding of how Pluto has changed with time,” Stern said.
Pluto’s largest moon, Charon, is covered only in water ice. Because the system’s four small moons Styx, Nix, Kerberos, and Hydra are brighter than expected, scientists surmise that they too have a water ice coating.
Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.