Five new studies describe Pluto and its atmosphere, showing that Pluto is much more active and complex than previously thought, and still has more surprises to be discovered. Pluto’s surface exhibits a wide variety of landscapes, some significantly different from its largest moon Charon.

“The atmosphere of Pluto as observed by New Horizons,” by G.R. Gladstone et al. This material relates to a paper that appeared in the March 18, 2016, issue of Science, published by AAAS. The paper, by G.R. Gladstone at Southwest Research Institute in San Antonio, Texas, and colleagues was titled, “The atmosphere of Pluto as observed by New Horizons.”
Credit: G.R. Gladstone et al. / Science (2016)

Whether or not we agree on Pluto’s planet status, we all have to agree that Pluto is a pretty surprising place. Previously discarded as a barren, frozen wasteland, Pluto has revealed itself to be quite the active place. Deep Horizons passed by the dwarf planet, gathering 50 gigabits of data with its instruments, a trove of valuable information. So far, only a fraction of that data has reached Earth, but as more and more of it does, it is constantly being analyzed by scientists. Therefore, it shouldn’t be a surprise that so many papers are being published in such a short amount of time.

In the first of five studies, Jeffrey Moore et al. offer some of the first descriptions of the wide array of geological features on Pluto and Charon. They describe phenomena such as tectonics, glacial flow, transport of large water-ice blocks, and broad mounds on Pluto — possibly a result of cryovolcanoes. According to their interpretation, these processes are much more recent on Pluto than Charon, indicating a surprising level of geological activity on the former planet.

In the second study, Will Grundy et al. analyze the colors and chemical compositions of the icy surfaces of Pluto and Charon. Again, they report a complicated distribution of the volatile ices on Pluto, which indicate a troubled geological history.

The third study came perhaps as the biggest surprise, showing that Pluto’s atmosphere is significantly colder than expected. One explanation is that a surprisingly thick layer of smoglike haze particles acts as a coolant, absorbing and emitting solar energy that would otherwise heat up nitrogen gas molecules in the atmosphere. Another possible explanation is hydrogen cyanide, an efficient coolant that was recently detected in Pluto’s atmosphere.

This high-resolution image captured by NASA’s New Horizons spacecraft combines blue, red and infrared images taken by the Ralph/Multispectral Visual Imaging Camera (MVIC). The bright expanse is the western lobe of the “heart,” informally called Sputnik Planum, which has been found to be rich in nitrogen, carbon monoxide and methane ices.
Credits: NASA/JHUAPL/SwRI

In a fourth study, Harold Weaver et al. examine the small moons Styx, Nix, Kerberos, and Hydra, which are irregularly shaped, fast rotating and have bright surfaces. Pluto’s biggest moon is Charon, but it features other six (much smaller) moons.

Fran Bagenal et al. report how Pluto modifies its space environment, including interactions with the solar wind and a lack of dust in the system. Together, these five studies pave the way for a better understanding of Pluto. We’ve seen it up close, we know it’s more intriguing than we thought, and through it, we can get a better understanding of planetary formation and evolution.

NASA put together a list of the most interesting things Deep Horizons has taught us about Pluto:

1. The age-dating of Pluto’s surface through crater counts has revealed that Pluto has been geologically active throughout the past 4 billion years. Further, the surface of Pluto’s informally-named Sputnik Planum, a massive ice plain larger than Texas, is devoid of any detectable craters and estimated to be geologically young – no more than 10 million years old.
2. Pluto’s moon Charon has been discovered to have an ancient surface. As an example, the great equatorial expanse of smooth plains on Charon informally named Vulcan Planum (home of the “moated mountains” informally named Kubrick and Clarke Mons) is likely a vast cryovolcanic flow or flows that erupted onto Charon’s surface about 4 billion years ago. These flows are likely related to the freezing of an internal ocean that globally ruptured Charon’s crust.
3. The distribution of compositional units on Pluto’s surface – from nitrogen-rich, to methane-rich, to water-rich – has been found to be surprisingly complex, creating puzzles for understanding Pluto’s climate and geologic history. The variations in surface composition on Pluto are unprecedented elsewhere in the outer solar system.
4. Pluto’s upper atmospheric temperature has been found to be much colder (by about 70 degrees Fahrenheit) than had been thought from Earth-based studies, with important implications for its atmospheric escape rate. Why the atmosphere is colder is a mystery.
5. Composition profiles for numerous important species in Pluto’s atmosphere (including molecular nitrogen, methane, acetylene, ethylene and ethane) have been measured as a function of altitude for the first time.
6. Also for the first time, a plausible mechanism for forming Pluto’s atmospheric haze layers has been found. This mechanism involves the concentration of haze particles by atmospheric buoyancy waves (called “gravity waves” by atmospheric scientists), created by winds blowing over Pluto’s mountainous topography.
7. Before the flyby, the presence of Pluto’s four small moons raised concerns about debris hazards in the system. But the Venetia Burney Student Dust Counter only counted a single dust particle within five days of the flyby. This is similar to the density of dust particles in free space in the outer solar system — about 6 particles per cubic mile — showing that the region around Pluto is, in fact, not filled with debris.
8. New Horizons’ charged-particle instruments revealed that the interaction region between the solar wind and Pluto’s atmosphere is confined on the dayside of Pluto to within 6 Pluto radii, about 4,500 miles (7,000 kilometers). This is much smaller than expected before the flyby, and is likely due to the reduced atmospheric escape rate found from modeling of ultraviolet atmospheric occultation data.
9. The high albedos (reflectiveness) of Pluto’s small satellites – about 50 to 80 percent – are entirely different from the much lower albedos of the small bodies in the general Kuiper Belt population, which range from about 5 to 20 percent. This difference lends further support to the idea that these satellites were not captured from the general Kuiper Belt population, but instead formed by agglomeration in a disk of material produced in the aftermath of the giant collision that created the entire Pluto satellite system.

Science Papers Reveal New Aspects of Pluto and its Moons