We now suspect Pluto's atmosphere is collapsing and Cthulhu Regio has glaciers.
NASA's New Horizons spacecraft is the gift that keeps on giving. After completing a historic flyby of Pluto on July 14, the spacecraft continues to provide scientists with never-before-seen views of Pluto and its five moons as well as incredible data packets. Last Friday, the spacecraft left us with some amazing science data to digest and a hauntingly beautiful view of Pluto backlit by the Sun.
Transmitting data's slow in part due to the vast distance between the Earth and New Horizons, but the probe also has to share its "talking time" with NASA's Deep Space Network (DSN)—a special worldwide network of large antennas designed to support interplanetary missions. As a result, scientists cannot download all the data on board New Horizons at one time. Over the next few weeks, mostly engineering data will be beamed back, so this is the last of the new images we'll see for a while.
Scientists have known for years that Pluto has an atmosphere but have never been able to directly observe it. The first detection of Pluto's atmosphere came in 1988 during a stellar occultation—meaning Pluto passed between the Earth and a distant star, blocking out the star's light and allowing scientists to gather data. If Pluto had an atmosphere, the light from the star would gradually fade versus being suddenly blocked out.
By analyzing this image, we can discern a great deal about Pluto's atmosphere, which will help us understand what is happening on the surface. We can see that Pluto's atmosphere has two distinct layers of haze extending 130 kilometers above the planet's surface—that's five times farther than previously predicted. This came as quite a shock to the science team, as they estimated that atmospheric temperatures would be too warm to form haze above the 30 kilometer mark.
The hazes form when simple atmospheric hydrocarbons, like methane gas, are blasted by ultraviolet light from the Sun. The breakdown of methane leads to the buildup of more complex hydrocarbons such as ethylene and acetylene, both of which are present in Pluto's atmosphere. As the particles fall through Pluto's frigid atmosphere, they condense, forming the haze. When the haze particles become very large, they fall to the surface, forming a dark, reddish brown gunk. The gunk is a complex mix of chemicals collectively termed tholins, and they are likely the cause for Pluto's reddish surface color. "The hazes detected in this image are a key element in creating the complex hydrocarbon compounds that give Pluto's surface its reddish hue," said Michael Summers, a New Horizons co-investigator from George Mason University, Fairfax, Virginia.
Pluto orbits the Sun once every 248 years. Ever since they had their first glimpse of Pluto's atmosphere, scientists hypothesized that following perihelion, as the icy world retreated from the Sun, temperatures would drop. That would allow the nitrogen-rich atmosphere to freeze and disappear. However, this hasn't been the case. Evidence that the atmosphere doubled puzzled scientists—until now. Data beamed back from New Horizons indicates we may be witnessing Pluto's atmosphere changing right before our eyes. Measurements taken by the REX instrument show that Pluto's atmosphere has a much lower surface pressure than expected.
Along with fascinating atmospheric data, New Horizons beamed back close-up images of Sputnik Planum—an area within the western lobe of Pluto's heart-shaped region unofficially called Tombaugh Regio. In this view we see evidence of nitrogen ice flows, along with carbon monoxide and methane ices. "At Pluto's temperatures of minus-390 degrees Fahrenheit, these ices can flow like a glacier," said Bill McKinnon, of Washington University in St. Louis, deputy leader of the New Horizons Geology, Geophysics and Imaging team. In the southernmost region of the heart, adjacent to the dark equatorial region, it appears that ancient, heavily-cratered terrain (informally named "Cthulhu Region") has been invaded by much newer icy deposits.
One of the first major discoveries on Pluto was a mountain range that resembled the Rockies found here on Earth. Just to the west of that range, dubbed Norgay Montes, there is a second mountain range that is similar in height to the Appalachians in the United States. Towering 1.6 kilometers above the surrounding plains, the newly-discovered mountain range has been unofficially named Hilary Montes. Alan Stern said, "For many years we referred to Pluto as the Everest of planetary exploration. It's fitting that the two climbers who first summited Earth's highest mountain have their names on this new Everest."
A week ago, we got our first pixelated view of two of Pluto's smaller moons—Nix and Hydra. This week we see them in color for the first time. These two small moons may be similar in size, but the similarities end there. Nix is an oblong, jelly bean-shaped body with an intriguing red area on the otherwise grey surface. Scientists suspect this region is a crater but cannot confirm this theory until compositional data is sent back to Earth. New images of Hydra reveal it is shaped like the state of Michigan, and has at least two craters on its surface. Hydra may vary in composition as the one part of the moon appears darker than the rest. Hydra is estimated to be 55 kilometers long by 40 kilometers wide, while Nix is 42 kilometers long by 36 kilometers across.
Images of Pluto's tiniest moons, Styx and Kerberos are not expected before October. And who knows what surprises wait on Pluto's surface as data continue to stream back to Earth.