The New Horizons team has released the first research paper based on early findings from the spacecraft's flyby of Pluto. The results, authored by the team and led by principal investigator Alan Stern, were published today in the journal Science.
Three months ago, Pluto and its moons came into focus for the very first time. Journalists, the scientific community, and space enthusiasts have since pored over each and every photo beamed back by the New Horizons spacecraft in hopes of learning new things about the icy dwarf planet. Many of the observations made along the way by NASA and the New Horizons Team are included in the study, which serves as a summary of what they've learned and what mysteries persist. It wasn't expected to be this way, but Pluto and its moons are dynamic, colorful worlds that show both signs of recent geologic activity as well as surface features that date back to the dawn of our Solar System.
The paper mostly breaks the discoveries down across the various geologic regions and features of each world in the Pluto system, starting with the dwarf planet itself. The team confirms, within a 4 kilometer margin of error, that Pluto has a mean radius of 1187 kilometers, slightly larger than had been anticipated before the flyby. The study then quickly pushes on to describe Pluto's "diverse range of landforms," including a crust made heavily of water ice.
On the left is the cratered Cthulhu Regio, and on the right is some of Sputnik Planum. Water ice deposits, which were discovered a few weeks ago, are highlighted in blue.
Impact craters have marked up much of Pluto's surface, with some measuring up to 260 kilometers in diameter. Some of these craters have broken down or filled with material over time, and others boast bright, "ice-rich deposits" on their rims or floors. The Cthulhu Regio — a very dark region just to the southwest of the infamous heart-shaped Tombaugh Regio — is the most heavily cratered, and also shows evidence of tectonic activity, with slopes and valleys that stretch as long as 600 kilometers.
Pluto's heart is broken in half
On flyby day we learned that Pluto's heart was broken in half — enhanced color images showed that the Tombaugh Regio's surface was actually split down the middle. The relatively smooth, flat plains to the west were named Sputnik Planum, and the team has yet to spot any craters in this particular region. The fact that craters vary in number depending on location has to do with the differing ages of the surfaces, the researchers suggest. The more craters, the older the surface and the more "dead" it tends to be, like we see with our Moon. But the scientists still don't know what causes the resurfacing process that erases craters in the younger, more active parts of Pluto, like Sputnik Planum. There we see long, icy expanses free of craters. This plains region is made of bright slabs of land measuring tens of kilometers wide, likely formed by a process similar to the way mud dries here on Earth.
Lastly, Pluto is sweetly colorful. Look at it through New Horizons' color camera and you'll see "spectacular diversity," the researchers write. The pictures we've all seen are no forgery — Pluto really does have a very distinct color palette. The Cthulhu Regio registers as a deep red, the western section of the heart-shaped Tombaugh Regio is a much lighter, pink-ish red, and the eastern half is lighter still. These colors come from refractory organics known as "tholins," the result of frozen nitrogen and methane in the soil (and atmosphere) being irradiated by UV rays and charged particles.
Pluto's largest moon, Charon, was even more of a mystery before the historic flyby. At half the size of Pluto, it was harder to study at a distance. What scientists have learned since then is that Charon is no boring, dead moon. Instead, it's as diverse a world as Pluto, and one that also shows evidence of geologic activity.
Detailed study of Charon has revealed a "complex geology," from cratered regions to smooth plains, massive faults and deep canyons, great changes in surface brightness (although it's nearly colorless compared to Pluto), and a giant dark spot centered around the north pole that makes it look like the Death Star. The edges of the spot's dark, inner zone — which measures about 275 kilometers across — are marked by a ridge, or maybe an exposed fault, which implies that Charon's polar spot is probably the result of a large impact or a "complex tectonic structure."
Charon has smooth, new surfaces, and features as old as our Solar System
Charon is also heavily cratered like Pluto. The New Horizons team believes some of these craters (specifically, the ones located in what's known as Vulcan Planum, which is found to the southeast of Charon's massive canyons) appear be about 4 billion years or older, which could link them to the Late Heavy Bombardment that happened in the early days of our Solar System. Charon also has a massive network of canyons. These fractures cuts across most of Charon's southern hemisphere, with the two largest — named Macross Chasma and Serenity Chasma — run for 1050 kilometers. At its widest, Serenity Chasma measures 60 kilometers across and runs, in places, 5 kilometers deep.
Charon's coloring is "much more limited than Pluto’s," but that north polar region is "distinctly red," the researchers write. This could be from a number of reasons. One is that Charon’s poles trap chemical compounds that, when exposed to sunlight and radiation during warmer seasons, become tholins like those seen on Pluto. It's also possible that the surface of Charon has a different composition than what lies underneath, and the color was churned up as a result of a massive impact.
New Horizons has so far provided the first true measurements of some of Pluto’s smaller moons, Nix and Hydra. But the measurements of Styx and Kerberos are still onboard the spacecraft. Nix is oblong, measuring just 49 kilometers long by 32 kilometers wide, with a diameter of about 40 kilometers. The reflective properties of Nix indicate that the small moon is likely covered in water ice. Hydra is also non-spherical, measuring 43 kilometers by 33 kilometers, with a diameter of about 41 kilometers. It also has a highly reflective surface, and is likely covered in water ice.
That confounded the New Horizons team. "How such bright surfaces can be maintained on Nix and Hydra over billions of years is puzzling," they write. Radiation or impacts with other objects should have darkened and reddened the surfaces of these moons over time, they say. What's also still unknown with any degree of certainty is the mass, volume, or density of these moons.
Before the flyby it was thought that Pluto might have more than the five moons, but New Horizons conducted "seven deep searches for satellites and rings" in the months leading up to the closest approach. Nothing else was spotted.
Even with all of this data and insight in hand, the New Horizons team has merely dipped a toe into the data collected by the spacecraft. Somewhere around 90 percent of it remains on the spacecraft, waiting to be beamed back to Earth over the course of the next year.
And yet, so much has already been learned. Not only have we seen the Pluto system for the first time, it is a much more geologically active than we thought it could be. That's not just exciting for fans of Pluto — the researchers say this implies that other large bodies in the Kuiper Belt (like Eris, Makemake, or Haumea) might also host complex geologies that "rival those of terrestrial planets such as Mars." The New Horizons team has already selected where they want to go next — an object found 1 billion miles beyond Pluto in the Kuiper Belt called 2014 MU69. All that stands in the way is final NASA approval.
Verge Video: The biggest discoveries from the Pluto flyby