Cracked, Frozen and Tipped Over: New Clues From Pluto's Past
Published in the Nov. 17 issue of Nature, these are the conclusions of research by James Keane, a doctoral student at the University of Arizona’s Lunar and Planetary Laboratory, and his adviser, assistant professor Isamu Matsuyama. They propose evidence of frozen nitrogen pileup throwing the entire planet off kilter, much like a spinning top with a wad of gum stuck to it, in a process called true polar wander.
Sputnik Planitia (the left lobe of Pluto’s “heart”) is believed to be the outer solar system equivalent of a lunar “mascon” (mass concentration). Like mascons on the Moon, Sputnik Planitia is believed to be an impact basin, filled in with lavas (on Pluto, cryogenic ices take the place of lavas).
Credit: NASA/JHUAPL/SWRI
Planets like to spin in such a way that minimizes energy. In short, this means that planets like to reorient to place any extra mass closer to the equator — and any mass deficits closer to the pole. For example, if a giant volcano were to grow on Los Angeles, the Earth would reorient itself to place L.A. on the equator.
To understand polar wander on Pluto, one first has to realize that unlike Earth, whose spin axis is only slightly tilted so that the regions around the equator receive the most sunlight, Pluto is like a spinning top lying on its side. Therefore, the planet’s poles get the most sunlight. Depending on the season, it’s either one or the other, while Pluto’s equatorial regions are extremely cold, all the time.
Pluto reorienting: Sputnik Planitia (the left lobe of Pluto’s “heart”) likely formed in the aftermath of comet impact into Pluto. Sputnik Planitia formed northwest of its present location, and reoriented to its present location as the basin filled with volatile ices.
Because Pluto is almost 40 times farther from the sun than we are, it takes the little ball of rock and ice 248 Earth-years to complete one of its own years. At Pluto’s lower latitudes near the equator, temperatures are almost as cold as minus 400 degrees Fahrenheit — cold enough to turn nitrogen into a frozen solid.
Over the course of a Pluto year, nitrogen and other exotic gases condense on the permanently shadowed regions, and eventually, as Pluto goes around the sun, those frozen gases heat up, become gaseous again and re-condense on the other side of the planet, resulting in seasonal “snowfall” on Sputnik Planitia.
“Each time Pluto goes around the sun, a bit of nitrogen accumulates in the heart,” Keane said. “And once enough ice has piled up, maybe a hundred meters thick, it starts to overwhelm the planet’s shape, which dictates the planet’s orientation. And if you have an excess of mass in one spot on the planet, it wants to go to the equator. Eventually, over millions of years, it will drag the whole planet over.”
In a sense, Pluto is a (dwarf) planet whose shape and position in space are controlled by its weather.
“I think this idea of a whole planet being dragged around by the cycling of volatiles is not something many people had really thought about before,” Keane said.
The geologic structure beneath the surface of Sputnik Planitia, which is believed to be an ancient impact basin that has since filled in with volatile ices. On Pluto, it is possible that the thinned crust is overlying a liquid water
If Sputnik Planitia were a large positive mass anomaly — perhaps due to loading of nitrogen ice — it would naturally migrate to Pluto’s tidal axis with regard to Charon, Pluto’s largest moon, as it approaches a minimum energy state, according to Keane and Matsuyama. In other words, the massive accumulation of ice would end up where it causes the least wobble in Pluto’s spin axis.
“On Pluto, those processes are currently active,” Keane said. “Its entire geology — glaciers, mountains, valleys — seems to be linked to volatile processes. That’s different from most other planets and moons in our solar system.”
And not only that, the simulations and calculations also predicted that the accumulation of frozen volatiles in Pluto’s heart would cause cracks and faults in the planet’s surface in the exact same locations where New Horizons saw them.
The presence of tectonic faults on Pluto hint at the existence of a subsurface ocean at some point in Pluto’s history, Keane explained.
“It’s like freezing ice cubes,” he said. “As the water turns to ice, it expands. On a planetary scale, this process breaks the surface around the planet and creates the faults we see today.”
The paper is published alongside a report by Francis Nimmo of the University of California, Santa Cruz, and colleagues, who also consider the implications for Pluto’s apparent reorientation. The authors of that paper agree with the idea that tidal forces could explain the current location of Sputnik Planitia, but in order for their model to work, a subsurface ocean would have to be present on Pluto today.
Both publications underscore the notion of a surprisingly active Pluto.
“Before New Horizons, people usually only thought of volatiles in terms of a thin frost veneer, a surface effect that might change the color, or affect local or regional geology,” Keane said. “That the movement of volatiles and shifting ice around a planet could have a dramatic, planet-moving effect is not something anyone would have predicted.”
Co-authors on the research paper (http://dx.doi.org/10.1038/nature20120) are Shunichi Kamata at the Creative Research Institution, Hokkaido University, Sapporo, Japan, and Jordan Steckloff of Purdue University in West Lafayette, Indiana, and the Planetary Science Institute in Tucson, Arizona.
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