Data collected by NASA's New Horizons spacecraft shows that tiny, distant Pluto not only likely has an ocean beneath its frozen face, but also may have had lakes on its surface in the recent past and likely will have them again in the future.
Even on its warmest days, Pluto is far too cold for surface lakes made of water, but they could contain liquid nitrogen during periods of time when the planet's atmosphere bulks up.
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Pluto, which takes 248 years to circle the sun, turns out to have large swaths of real estate with direct overhead sunlight due to an extreme axial tilt of 120 degrees, relative to its orbital plane. Earth, by comparison, is tilted 23 degrees.
That gives Pluto a much broader range of tropical latitudes than Earth, noted New Horizons scientist Richard Binzel, with Massachusetts Institute of Technology.
Because arctic and tropical zones alternatively extend over such wide swaths of the dwarf planet's surface, Pluto has regions where both extremes occur, though not at the same time.
Pluto also has a wobble, which causes its axis to tilt up an additional 20 degrees from its present orientation, triggering long-term climate cycles that far exceed anything experienced on Earth, Binzel said.
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Pluto is now in an intermediate phase between its climate extremes, with the last peak occurring less than 1 million years ago. Temperatures today measure about minus-400 degrees Fahrenheit.
Variations in the amount of sunlight falling on Pluto have direct impacts on Pluto's atmosphere, noted New Horizon's lead scientist Alan Stern, with the Southwest Research Institute.
"We find that Pluto's atmospheric pressure today is atypically low and that in the past it could have been 1,000 to 10,000 times higher," exceeding the pressure of Mars by up to 40 times, Stern said.
Computer models show that when Pluto's temperature and atmospheric pressure are high, conditions could be suitable for liquid nitrogen.
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Additional evidence comes from New Horizons itself. High-resolution images taken during the July 14 flyby reveal features that look like they could have been carved by liquids. The pictures also show what appears to be a frozen lake, measuring about 20 miles across at its widest point, that is located just north of Sputnik Planum, the western lobe of Pluto's smooth, heart-shaped region.
"Pluto is so dynamic that different cases may apply in different epics," Stern said. "We found this little planet where everything is coupled together."
The research was unveiled at the Lunar and Planetary Science Conference last week in Texas and has been submitted for publication in the journal Icarus.