Ever since New Horizons zipped past Pluto in July, we've marveled over the dwarf planet's complex terrain. Among the biggest puzzles Pluto presents us with is a vast, crater-free ice field informally known as Sputnik Planum. The leading hypothesis for how this surface came to be? An epically violent collision. That's according to New Horizons Principal Investigator Alan Stern, who gave an overview of the Pluto science to date at the 227th meeting of the American Astronomical Society yesterday. That talk included the New Horizon science team's latest theories on the dwarf planet's remarkably smooth, patterned ice plains. "We believe [Sputnik Planum] is a large impact basin," Stern said, later elaborating that the depressed terrain was probably punched out by an impactor approximately 10km across.
That's freakkin' huge. We're talking an asteroid on the same order of magnitude as Manhattan. And it may have struck Pluto hundreds of millions to billions of years ago, long before Sputnik Planum migrated to its present position near the equator.
Located in the interior of Pluto's now-famous Tombaugh Regio region, Sputnik Planum's remarkable morphology has already yielded profound insights. Ringed by jagged mountain ranges, this smooth, low-lying, and blindingly bright terrain has a distinctly blocky structure not seen anywhere else on the planet. Near Sputnik Planum's "shoreline", we see evidence for glacial flows — ices that have poured off mountain ranges and pooled up. And as far as we can tell, Sputnik Planum is completely crater free. All of these observations tell the story of a very young (< 100 million year) surface that's being constantly renewed.
Pluto is a geologically active world — and no region speaks to its dynamic nature as well as Sputnik Planum does.
A high-resolution view of the "shoreline" where Sputnik Planum bumps up against the al-Idrisi mountains. This image was taken from 17,000km above Pluto's surface during the New Horizons flyby on July 14. Image Credit: NASA
But there are still a lot of big, outstanding questions about this region, including how the icy plains formed, and how exactly their surface is being replenished. On both counts, a picture is starting to emerge. As Stern explained yesterday, the blocky ice structures characteristic of Sputnik Planum are probably the result of thermal convection and density differences between different ices. "Water ice floats in nitrogen ice," Stern explained. "These blocks appear to have been removed from a subsurface layer, and they are now 'floating' in a large reservoir."
If the New Horizon science team's hunch is correct, it indicates a heat source in Pluto's interior. As for how a tiny world at the arse-end of the Solar System has remained warm for four billion years? That's another a big mystery. "We did not predict that a small planet like Pluto could still be active and would not have completely cooled off," Stern said.
Regarding Sputnik Planum's formation, the leading hypothesis is now a large impact. When that impact occurred isn't yet certain, but it's possible that Sputnik Planum didn't start out in its present location near the equator. "[Sputnik Planum] is so large and its volume so great that the negative mass anomaly caused by its impact has very likely caused this object to move to its current position near the equator," Stern explained. He likens Sputnik Planum's migration to the polar wander geologists observe on Earth and other rocky planets. According to Stern, the chaotic mountain ranges that surround Sputnik Planum may be the result of the same large impact.
Jumbled, broken "chaos" terrain is visible in the center of this image, with the northwest edge of Sputnik Planum shown to the right. This chaos terrain may be the result of the same ancient impact that produced the patterned plains in the first place. Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
Another fascinating insight: the southern part of Sputnik Planum is pockmarked with deep pits that are probably formed as ice sublimates into the atmosphere. But the material beneath these pits is very dark. "One working hypothesis is that everywhere on Pluto, the actual planet is very dark, and all the bright regions are due to volatile deposition," Stern said. It's a theory that, yet again, underscores the dynamic linkage between Pluto's complex atmosphere and its surface.
For all that we've learned about the Pluto system to date, we're still just scratching the surface. The New Horizons probe collected so much data that Earth is going to continue downlinking it until August of 2016. Meanwhile, New Horizons is zipping merrily along toward the Kuiper Belt, with another close encounter planned for 2019. (That encounter still has to be formally approved by NASA.) While most of the insights discussed here will have to be verified by additional studies, if one thing is clear, it's that New Horizons has already earned its place as one of the most remarkable missions humans have ever built.
Sputnik Planum from 77,000km away. Image credit: NASA/JHUAPL/SwRI