Pluto Has Active Geology
Convection apparently forms the polygonal cells in Sputnik Planum, a large active region on Pluto’s surface.
Feel the burn in this exclamation from the New Horizons team posted by NASA Astrobiology Magazine:
“Sputnik Planum is one of the most amazing geological discoveries in 50-plus years of planetary exploration, and the finding by McKinnon and others on our science team that this vast area—bigger than Texas and Oklahoma combined – is created by current day ice convection is among the most spectacular of the New Horizons mission,” said New Horizons Principal Investigator Alan Stern, of the Southwest Research Institute, Boulder, Colorado.
He’s talking about the heart-shaped region in Pluto’s southern hemisphere—a vast smooth area rippled with indentations and organized into polygonal cells (see high-resolution image in the article). Not exactly a “beating heart” as the BBC News and National Geographic describe it, but a piece of real estate in motion even today.
The cells are created by a “lava lamp” action, where internal heat causes nitrogen ice to rise and fall repeatedly. Scientists explained the theory in two papers in Nature this week, one by Trowbridge et al. about “vigorous convection” and another by MacKinnon et al. about the convection driving “Pluto’s geological vigour.”
To appreciate the differences in terrain on this amazing body, fly over a strip of terrain in a high-res video posted on Space.com and National Geographic. After passing over large ice-block mountains, the video flies across the smooth plains of Sputnik Planum, revealing the dimples and convective cell boundaries.
How long has this “vigorous convection” been going on? The papers don’t say, but they estimate the current surface cannot be over 500,000 to a million years old. “In addition, both groups report that the convective flow speeds are in the range of centimetres per year, meaning that the surface turns over in about 500,000 to 1 million years,” according to the News & Views summary on Nature. “This rapid resurfacing explains the lack of impact craters on the ice sheet.” Those numbers depend on the cratering rate and the depth of the material (estimates range from 3 to 20 km deep). Why the nitrogen collected in this one region is disputed between the two papers.
The NG article emphasizes the contrast between expectation and reality:
Based on the new work, the teams calculate that Sputnik Planum’s face could be completely repaved every 500,000 to 1 million years or so, meaning the region looked completely different when saber-toothed cats prowled the Earth. It’s a geologically rapid process that scientists didn’t exactly expect to see on a small, freezing world that lives, on average, 40 times farther from the sun than Earth.
Trowbridge et al.‘s paper indicates that the new upper limit is much lower than previously thought. “This is consistent with the lack of significant cratering, and further constrains the existing age estimates of a few hundred million years by two orders of magnitude.” MacKinnon’s paper halves the estimate to 500,000 years max. As an upper limit, though, it could be much younger.
What heats the nitrogen to produce convection? MacKinnon et al.’s paper suggests that radiogenic heat from Pluto’s interior has been sufficient for the planet’s history to lead to the current heat output of 3 milliwatts per square meter. That depends on models assuming quantities of radiogenic material available. But since 1 million years is 1/4,500th the assumed age of the solar system, could the same nitrogen have turned over that many times? Think of leaving your oatmeal bubbling on the stove for half a million convection cycles. Science Daily, incidentally, compares the convection to oatmeal, describing a different hypothesis from Purdue University that compares the polygons to icebergs floating in a sea of nitrogen.
“Many people expected Pluto to be a cold, dead world,” Melosh said. “What we’ve discovered through this mission is that cold worlds like Pluto have a different kind of activity that involves materials we think of as gases. This understanding offers a new perspective that cold worlds can be just as active and interesting as our own.“
It’s not just Pluto scientists have to wrestle with. “Convective renewal of volatile ice surfaces, as in a basin or basins similar to SP, may be one way in which the dwarf planets of the Kuiper belt maintain their youthful appearance.”
Space.com also posted a “gorgeous ‘twilight zone’ photo” of the dark side of Pluto, showing the atmosphere with a possible cloud. And a picture of Charon, Pluto’s large moon, appears in another Space.com piece describing a new theory about how cracks form on icy moons. It doesn’t require an impact. A passing body could create fissures as seen on Charon, Dione, and Tethys. Perhaps even Valles Marineris, the solar system’s largest canyon, formed on Mars by a “near collision” instead of by geological processes on the surface.
Pluto has sure been fun. To be alive to watch this dramatic exploration unfold is good enough, but it’s been doubly fun to watch scientists scrambling to explain young things within their billions-of-years ideological framework. Maybe Pluto looks young because it is young; would they ever think of that? Like we said before, we’re not asking for just thousands of years. We’ll compromise for a few hundred thousand or a million; how about that? What? No deal?
You know why, of course. Darwin needs things to be billions of years old. So they either come up with implausible ad hoc scenarios, or else totally ignore the implications of the hard, cold facts staring them in the face.