New research presented today at the 2016 Lunar and Planetary Sciences Conference reveals tropic and arctic regions on Pluto, and a dynamic climate cycle that's causing its atmosphere to fluctuate in size over time — possibly allowing for lakes and rivers of liquid nitrogen to form at the surface. New Horizons made its historic flyby of Pluto eight months ago, yet less than half of its data has been received by scientists. As this information trickles in, new findings about the dwarf planet are steadily emerging. This week alone, some 40 technical papers will be presented at the 2016 Lunar and Planetary Sciences Conference being held in The Woodlands, Texas.
In the past, atmospheric pressure was much greater on Pluto, allowing for liquid nitrogen at the surface. The features above are all indicative of liquid on the surface, including the smooth, pond-like feature at far right. Image: NASA/New Horizons
The big revelations today came from Richard Binzel of MIT and Alan Stern from Southwest Research Institute. Respectively, their research shows that Pluto features both tropic and arctic regions, and an exaggerated axial tilt that alters the dwarf planet's atmosphere over time. Together, the research suggests that liquid nitrogen may have once — or even repeatedly — flowed on Pluto's surface, forming lakes, ponds, and intricate networks of terrain.
The word "tropic" doesn't come to mind when we think about Pluto, a frigid dwarf planet located 5.91 billion kilometres from our sun. But like Earth, Pluto spins on an axis, producing predictable seasons and tropical and arctic climate zones. In a tropical region, the sun sometimes appears directly overhead, whereas in arctic regions, the sun is lower to the horizon. Here on Earth, this gives rise to the arctic circle and its "midnight sun," which occurs during the prolonged winter.
Image: NASA/New Horizons
Because of its tilt, Pluto also has arctic circles. But unlike Earth and its 23 degree tilt, Pluto is angled on a whopping 120 degree axis compared to its orbital plane. This means that Pluto's arctic zone is very active, sometimes extending near the equator depending on the time of year (a year on Pluto is equal to 249 Earth years). The big tilt also means that most of Pluto is tropical. As Binzel pointed out during the conference, "There's no analogue to that here on Earth." It's important to point out that Pluto's tropic regions are anything but balmy; it's still damn cold in the outer reaches of the solar system.
And Pluto's axial tilts shift gradually over time. Pluto, like Earth, is subject to Milankovitch cycles, which cause dramatic shifts in climate over periods of hundreds of thousands to millions of years. On Pluto, these cycles cause the boundaries of tropical and arctic regions to advance and recede over time. Binzel said that Pluto is currently in an intermediate stage of this cycle, which is continuing to have a pronounced effect on the planet's surface features. Pluto has a dark stripe across the equator, which appears to be a global feature. This area, it so happens, is a band within Pluto's tropics that never experiences arctic winter or arctic summer. Bizel speculated that this distinct feature exists because ice and other volatile materials don't tend to accumulate there.
In a separate but related paper, Alan Stern showed that these multi-million year cycles can also produce atmospheric effects. Data from New Horizons suggests that atmospheric pressure has changed dramatically over the course of Pluto's history. At some points, it was as low as 10 microbars (which is about 1/100,000th the pressure found on Earth at sea level), but at other times as high as 200 millibars. Pluto has even exceeded Mars's atmospheric pressure in the past, which Stern said, "really changes your view of how this little planet operates." For comparison, the average air pressure at the surface of Mars is six millibars, compared to 1013 millibars on Earth.
These radical changes to atmospheric pressure suggest dramatic changes to circulation in Pluto's atmosphere. They could explain how haze works on Pluto, and the rate at which the atmosphere is escaping. What's more, high pressure is what allows certain elements, like nitrogen (which is common on Pluto), to take on a liquid state. This finding thus strengthens the suggestion that liquid nitrogen may have once existed on the surface.
During the press briefing, Orkan Umurhan, a researcher from NASA Ames Research Center, presented slides showing a number of glacial landforms, glacial flows and glacial erosion across Pluto. He noted two possible explanations: it's either a slow, gradual process, or it's erosion caused by flowing liquid nitrogen — a scenario supported by the work of his colleagues.
So more incredible findings. And the researchers assured the press that there's still plenty to come.
[New Horizons Team at the 2016 Lunar and Planetary Sciences Conference]
Image: NASA/New Horizons