9 Discoveries About Pluto’s System Revealed in New Horizons Papers

Enhanced color image of Pluto’s surprising surface
Credits: NASA/JHUAPL/SwRI

It will still be quite some time before scientists receive all the 50 gigabits of data gathered by the New Horizons probe as it sped past Pluto’s system last July, but the first comprehensive set of papers to be published from the history making mission have come out in the journal Science.

According to New Horizons Principal Investigator Alan Stern of the Southwest Research Institute (SwRI) in Boulder, Colorado, “These five detailed papers completely transform our view of Pluto – revealing the former ‘astronomer’s planet’ to be a real world with diverse and active geology, exotic surface chemistry, a complex atmosphere, puzzling interaction with the sun and an intriguing system of small moons.” The list below, compiled by NASA provides a summary of what has been learned so far.

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.