A New Horizons image shows an interesting feature:
NASA’s New Horizons spacecraft spied several features on Pluto that offer evidence of a time millions or billions of years ago when – thanks to much higher pressure in Pluto’s atmosphere and warmer conditions on the surface – liquids might have flowed across and pooled on the surface of the distant world.
“In addition to this possible former lake, we also see evidence of channels that may also have carried liquids in Pluto’s past,” said Alan Stern, Southwest Research Institute, Boulder, Colorado—principal investigator of New Horizons and lead author of a scientific paper on the topic submitted to the journal Icarus.
This feature appears to be a frozen, former lake of liquid nitrogen, located in a mountain range just north of Pluto’s informally named Sputnik Planum. Captured by the New Horizons’ Long Range Reconnaissance Imager (LORRI) as the spacecraft flew past Pluto on July 14, 2015, the image shows details as small as about 430 feet (130 meters). At its widest point the possible lake appears to be about 20 miles (30 kilometers) across.
See also a recent presentation of New Horizons results so far: New Horizons Team Presents Latest Pluto Science Results at Planetary Conference.