Pluto and its moon Charon have occupied the same neighborhood at the far edge of our solar system for 4 billion years, but their surfaces are completely different.
โThey donโt look a thing like each other,โ said Alan Stern, who headed up NASAโs New Horizons mission, which grabbed the first close-up photos of Pluto and its moons on July 14.
Five peer-reviewed papers in the journal Science and dozens of presentations being made at a conference Monday explain many of the reasons for those differences โ elaborating on the New Horizonsโ science teamโs initial conclusion that Pluto continues to be geologically active.
Most of the high-value imaging from that single July flyby has since made its way to Earth, though half of the scientific information is still being dribbled out bit-by-bit as the spacecraft speeds toward new targets in the vast Kuiper Belt, said Stern.
The contrast between Pluto and Charon, often considered a sister planet because of its proximity and size, could not be greater, Stern said, and is unique in planetary exploration.
โThe two have been orbiting each other for billions of years. They live in the same radiation environment and the same thermal and collision environment and they donโt look a thing like each other.โ
Plutoโs surface has been reshaped by internal and atmospheric forces, while most of Charonโs surface hasnโt been reshaped for 4 billion years, according to the new reports.
โCharon is a relic of a long time ago,โ Stern said.
Veronica Bray, of the University of Arizona Lunar and Planetary Laboratory, helped the team constrain the age estimates of the surfaces, basically by identifying and counting impact craters on the different regions of the two bodies.
โItโs like driving a car,โ Bray said. โThe longer you drive, the more bugs will hit your windshield. Use your wipers, and it will set that bug number back to zero.
With a planet, youโre going through space, getting hit by comets, which cause impact craters. But new geologic activity will wipe that slate clean.
The geological activity on Pluto has removed the craters more recently than on Charon,โ Bray said.
Pluto does have some cratered regions, but its most striking feature from day one has been a vast white, flat, plain that forms the western lobe of the heart-shaped region informally named for the planetโs discoverer, Clyde Tombaugh of Lowell Observatory.
Offset by the reddish cast of the features that surround it, crater-free Sputnik Planum, as it is informally called, is less than 10 million years old, the team concludes.
โThere is, in fact, this remarkable diversity of compositional units across Pluto, telling us that there are processes going on that are sorting things out,โ said Will Grundy of Lowell Observatory, who heads the surface composition team at New Horizons and was first author of the Science paper on it.
โSputnik Planum has all the volatile ices in it,โ Grundy said.
It is a sea of frozen nitrogen, carbon monoxide and methane, capable of convective movement and able to float glaciers of hardened water ice at geologically slow speeds. It is changing and shaping the surface of the planet.
โTo the East is another region where material from Sputnik Planum is being deposited, accumulating and forming glaciers that are flowing back down hill into Sputnik,โ said Grundy.
The planetโs composition also shifts with altitude and season, said Grundy, who helped prepare for his teamโs analysis of Plutoโs surface by creating exotic ices with a NASA-funded team at Northern Arizona University.
Pluto features mostly frozen nitrogen in lower regions and methane frost atop its water-ice peaks.
Charon, meanwhile, is mostly water ice and heavily cratered. It looks the way scientists expected it, and Pluto, to look โ with a few notable exceptions, including a reddish pole.
Grundy said most of the volatile ices that continually reshape Pluto would easily escape from its less massive moon. โThere is no way to keep volatiles on Charon,โ he said.
Some, however, get seasonally trapped at Charonโs north pole during its decade-long winter and are converted by energetic radiation into heavier molecules โ the precursors of the same tholins that give much of Pluto its reddish tint.
โThe other pole would be similarly redโ if that scenario is correct, he said, but it wasnโt lit by the sun as New Horizons flew by.
Pluto traps most of its gases in a colder-than-expected upper zone, according to a paper devoted to its intriguing atmosphere, with its layers of haze.
As it whizzed by at 30,800 mph, New Horizons kept all its eyes on the twin prizes of Pluto and Charon, but it did capture images of Plutoโs four smaller moons on approach.
Manipulation of those images produced accurate measurements of size and albedo, despite the fact that some of the images are still unclear, said Tod Lauer, of the National Optical Astronomy Observatory, who helped with the image processing.
โStyx, for example, the smallest satellite, we really didnโt get close to at all, unfortunately. We had enough pixels to say its yay-by-yay, but in terms of resolution, itโs fuzzy,โ said Lauer.
The paper concludes that the four smaller moons โ Styx, Nix, Kerberos, and Hydra โ are highly reflective and assumed to be water ice. They are also pock-marked with craters.
That led scientists to conclude that the moons formed around 4 billion years ago in the same collision that created Pluto and Charon.
Among the other findings cited in the Science paper on the smaller moons: โrapid rotation rates and unusual pole orientations; bright, icy surfaces with albedos and colors distinctly different from those of Pluto and Charon; evidence of merged bodies; and surface ages of at least 4 billion years.โ
Mission planners had worried upon approach to Pluto that it would encounter additional small moons or dust rings that could injure the spacecraft. They were relieved to find nothing in the way and they now have a measurement of how clear the path was.
The Venetia Burney Student Dust Counter, named for the little girl who suggested Plutoโs name back in 1930, recorded one speck of dust during the flyby.
