Pluto Has Tropics, and Other New Findings by Planetary Scientists


On approach in July 2015, the cameras on NASA’s New Horizons spacecraft captured Pluto rotating over the course of a full “Pluto day.” The best available images of each side of Pluto taken during approach were combined to create this view of a full rotation. Image and caption credit: NASA/JHUAPL/SwRI

The New Horizons spacecraft is healthy and zipping along, now 185 million miles past Pluto. Last year, the spacecraft began returning data and extraordinary photographs of the ninth planet of the classical solar system, and yesterday, at the 47th Lunar and Planetary Science Conference in The Woodlands, Texas, members of the New Horizons team announced their latest findings about Pluto and its moons.

“Pluto is a very complicated place,” said Richard Binzel, a professor at MIT and a co-investigator of the New Horizons mission. “We’ve been trying to go back to basics to see how seasons and climate might be shaping Pluto.”

Scientists have worked out the location and nature of Pluto’s tropics—a concept that might seem unlikely on a frozen planet 6 billion kilometers from the Sun. To understand what “tropics” means in this context, consider the axial tilt of the Earth, which is 23.5 degrees. The tilt is the reason that our planet experiences seasons, and over the course of a year, the Sun is directly over one of any latitude between the Tropic of Cancer (23.5 degrees north) and the Tropic of Capricorn (23.5 degrees south). That’s why the tropics are known for their warm weather.

For comparison, Pluto’s axial tilt is 120 degrees. This makes the range of tropical latitudes much broader than Earth's. Over a 248-year revolution, there are times when the Sun is directly overhead Pluto's very southern latitudes. There are also other times, depending on the position of the planet in its orbit, it is over the northern latitudes. Moreover, just as the axial tilt of the Earth gives us arctic circles with their attending stretches of dark winter or midnight Sun, Pluto's extreme tilt creates arctic circles as well—circles that reach nearly to its equator. “If Earth were tilted by same amount as Pluto, we [in Texas] would be in the arctic zone on Earth," Binzel said. A result of the overlapping arctic and tropical zones is that Pluto actually has "tropical arctic" bands.


In July 2015, the New Horizons team set a new record for the planetary reflection of a radar signal. The Deep Space Network sent 80kW radar signals to Pluto, which reflected those signals back at the New Horizons spacecraft as it flew by. The previous record for such a radar signal reflection was 1 billion miles. This radar signal tripled that, reflecting at 3 billion miles. The purpose of this technique was to acquire independent, highly precise measurements of textures on Pluto's surface. The radar signal also collected independent data related to surface composition. Having proven that reflection at such a tremendous distance is possible, New Horizons intends to do it again in 2019 when the spacecraft arrives at 2014 MU69, a tiny Kuiper Belt Object composed of ice and rock.

Among the other newly announced discoveries: Scientists now know that Pluto's atmospheric pressure is atypically low, and that in the past it was 1000 to 10,000 times higher. Its atmosphere is always escaping. Currently, its pressure is 10 microbars, which is 1/100,000th of atmospheric pressure at sea level on Earth. Because of these atmospheric findings, scientists believe that in extreme cases, liquids may have existed on the surface of Pluto—not water, but free-flowing liquid nitrogen. In addition, researchers have also observed patterns consistent with erosional sculpting of landscapes, and observations of certain types of sculpted landforms dictate glacial flow and erosion from long ago. There are signs, in fact, of active flowing glaciers on Pluto today.


Pluto's moons were formed by the collision of Pluto and some giant object in that area of space. In addition to the creation of Charon, Pluto's largest moon, the impact created a disc-shaped debris field that went on to form Pluto's other moons. The moons are now known to have certain common characteristics. They all have circular orbits, they orbit the same plane, they have similar brightness, and they have similar spins. Imagery from New Horizons has allowed scientists to study craters across the surfaces of moons Nix and Hydra. The densities of these craters, and the age of the surface retaining these craters (approximately 4 billion years old), suggests that the moons all formed at the same time. This is the first proof that the giant impact was ancient and not a recent event.

The discoveries have only just begun. The New Horizons team is still examining the data returned, and half of the data from last year's flyby remain on spacecraft waiting to be downloaded to Earth. The next three weeks will prove to be especially exciting for the wider planetary science community, as the first set of Pluto data will be submitted to the NASA Planetary Data System, the open archive accessible by anyone for scientific study. Scientists new to the data will have something exciting in store, according to Jim Green, director of the planetary science division at NASA. "What the data revealed did not surprise us," he said. "It shocked us! What a beautiful system to study."