The New Horizons spacecraft is now almost 300 million kilometers past Pluto, but the probe is still providing us with valuable information about the dwarf planet. The newest discovery: Pluto may have once been home to lakes and rivers filled with liquid nitrogen. Scientists say that large flat areas on the dwarf planet's surface are likely the result of bodies of still liquid, while networks of grooves were possibly cut by rivers of nitrogen, released when changes in Pluto's climate thawed some of the nitrogen ice that still covers its crust.

"We see what for all the world looks to a lot of our team like a former lake," said New Horizons principal investigator Alan Stern at the Lunar and Planetary Science Conference yesterday, pointing to images taken by the New Horizons probe. "It's very smooth, as if a liquid has frozen across one height," Stern told New Scientist. "It's hard to come up with an alternate model that would explain that morphology." But how does ice on the surface of a celestial body 3.67 billion miles from the sun ever get warm enough to melt? Scientists think it's because of Pluto's odd rotation in space.

"Most of Pluto is tropical," MIT's Richard Binzel explains. Just as the Earth's tilted axis gives us changing seasons, freezing poles, and hot tropical regions, so too does Pluto's pronounced lean. But where the Earth tilts just 23 degrees on its axis around the sun, Pluto is almost upside down, its poles tilted 120 degrees. That means the planet's tropics can stretch far to its north, just as its arctic regions can almost reach the equator, depending on its position against the sun. That tilt has changed over time, as Pluto's almost oval orbit has also taken it closer and further from the sun, giving it temperature extremes that could explain the scars left on its surface.

New Scientist says the planet would have reached a particularly hot (a relative term in the far reaches of the solar system) climate 800,000 years ago, as its axial tilt hit 103 degrees. With an atmospheric pressure greater than that found on Mars, frozen nitrogen could have melted, carving its way across the rocky surface, and pooling in flatter areas. While liquid nitrogen is not so obvious now, with Pluto in its current position, scientists say the mini-planet may still harbor some wetness — Pluto's glacier-esque flows perhaps put enough pressure on nitrogen ice beneath the surface to turn it back into its liquid form.