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Pluto: Sputnik Planitia Gives Credence to Possible Ocean

We’ve been looking at the idea of an ocean beneath Pluto’s icy surface for some time, including interesting work on the thermal evolution of the dwarf planet’s ice shell from Guillaume Robuchon and Francis Nimmo (University of California at Santa Cruz). Back in 2011, The Case for Pluto’s Ocean looked at their view that the stretching of Pluto’s surface would have clear implications for an ocean kept warm by radioactive decay in the interior. Now Nimmo is back with a post-New Horizons analysis that also points to an ocean.

The key here is Sputnik Planitia, forming part of the heart-shaped feature that was so distinctive during the flyby — think of Sputnik Planitia as the heart’s ‘left ventricle.’ The impact basin here is aligned almost exactly opposite from Charon. We learn in Nimmo’s paper in Nature that there is only a 5 percent chance that the feature’s alignment with Pluto’s tidal axis is by coincidence. To Nimmo and colleagues, the alignment is a dead giveaway that extra mass in the location is indicated. This would cause tidal interactions between Pluto and Charon that oriented Sputnik Planitia opposite the Charon side.

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Image: In this image of Pluto taken by NASA’s New Horizons spacecraft, different colors represent different compositions of surface ices, revealing a surprisingly active body. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

Thus we’re looking at forces that led to a re-alignment of the two small worlds, and enough mass to make that kind of shift possible. A deep basin doesn’t provide the heft, but an ocean could explain the result. In this way of thinking, Sputnik Planitia is the result of a major impact, which would have been followed by an infusion of water pushing up from below. The ice shell at the top hardens, leaving a deep basin gradually filling with nitrogen ice. Both the nitrogen ice and the water from below are necessary to explain the mass needed.

“It’s a big, elliptical hole in the ground,” says Nimmo, “so the extra weight must be hiding somewhere beneath the surface. And an ocean is a natural way to get that.”

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Image: These schematic diagrams show how the gravity anomaly at Sputnik Planitia is affected by an uplifted ocean and the thickness of the nitrogen layer. Either a nitrogen layer more than 40 km thick (panel b) or an uplifted ocean (panel c) could result in a present-day positive gravity anomaly at Sputnik Planitia; otherwise, the gravity anomaly will be strongly negative (panel a). Credit: Nimmo et al., Nature, 2016).

Researchers also believe that the frozen nitrogen ice in the Sputnik Planitia basin may be convecting thanks to a weak spot at the bottom which would let heat rise from the interior. That would be an indication of a thin crust in this area, which would have allowed material from below to push upward to create, along with the nitrogen ice, the extra mass observed.

“Pluto is small enough that it’s just about almost cooled off but still has a little heat, and it’s about 2 percent the heat budget of the Earth, in terms of how much energy is coming out,” says New Horizons co-investigator Richard Binzel (MIT), a co-author of the Nimmo paper. “So we calculated Pluto’s size with its interior heat flow, and found that underneath Sputnik Planitia, at those temperatures and pressures, you could have a zone of water-ice that could be at least viscous. It’s not a liquid, flowing ocean, but maybe slushy. And we found this explanation was the only way to put the puzzle together that seems to make any sense.”

Nimmo believes the subsurface ocean is mostly water mixing with ammonia. This natural ‘anti-freeze’ slows the refreezing of the ocean, but as it continues to freeze, the ocean stresses the icy shell enough to cause the fractures we see in New Horizons imagery. We have images from the flyby that appear to be nitrogen glaciers flowing out of mountainous terrain around Sputnik Planitia, adding credence to the idea that the impact basin filled with nitrogen that froze out in the higher elevations and gradually migrated downward.

Looking into nitrogen flows on Pluto for this post, I came across a separate study, submitted to Icarus and led by Orkan Umurhan (NASA Ames), that supports the movement of this material near Sputnik Planitia. Note this:

We find that the wavy transverse dark features found along the northern shoreline of Sputnik Planum may be a transitory imprint of shallow topography just beneath the ice surface suggesting the possibility that a major shoreward flow event happened relatively recently within the last few hundred years. Model results also support the interpretation that the prominent darkened features resembling flow lobes observed along the eastern shoreline of the Sputnik Planum basin may be a result of wet nitrogen glacial ice flowing into the basin from the pitted highlands of eastern Tombaugh Regio.

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Image: Tip o’ the heart: This animation shows how Pluto reoriented in response to volatile ices filling Sputnik Planitia (the left lobe of Pluto’s “heart”). Sputnik Planitia started northwest of its present position, and as it filled with ices, the tides from Charon (Pluto’s largest moon) caused the entire dwarf planet to reorient. If Sputnik Planitia is still accumulating ice, then Pluto may still be reorienting. Credit: James Keane/NASA/JHUAPL/SWRI).

The same issue of Nature that features the Nimmo work also includes a paper by James Keane and Isamu Matsuyama (University of Arizona), who back the re-orientation idea. In their view, a pileup of frozen nitrogen is what caused Pluto’s ‘polar wander,’ re-orienting it to place extra mass close to the equator. Keane believes Sputnik Planitia continues to accumulate nitrogen as frozen gases sublimate and re-condense on the other side of the dwarf planet, causing seasonal ‘snowfall’ there.

To understand this pattern, remember that Pluto spins on its side, so that the planet’s poles get the most sunlight during the course of a year, while the equatorial region stays permanently cold. Frozen polar gases that sublimate as they warm up during the Plutonian year can play a major role not just in the world’s weather but also in its orientation:

“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.”

The duo’s computer models, moving Sputnik Planitia and observing the effect on Pluto’s spin axis, bear this out. Thus while Nimmo and team see a subsurface ocean as necessary to explain the position of Sputnik Planitia, Keane and Matsuyama argue for re-orientation keyed to the cycling of volatiles, an adjustment of Pluto’s position that grows out of its weather. Even so, both teams agree that tectonic faults on Pluto are clues to the existence of an ocean.

The Umurhan paper is “Modeling glacial flow on and onto Pluto’s Sputnik Planum,” accepted at Icarus (preprint). The Keane paper is “Reorientation and faulting of Pluto due to volatile loading within Sputnik Planitia,” published online by Nature 16 November 2016 (abstract). The Nimmo paper is “Reorientation of Sputnik Planitia implies a subsurface ocean on Pluto,” published online by Nature 16 November 2016 (abstract).

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Comments on this entry are closed.

  • john walker November 16, 2016, 15:05

    On the theme of subsurface oceans, I was disappointed to hear that a deeper analysis of the INMS data from Cassini’s last Enceladus flyby(E21) will in contrast to previous statements apparently not be presented at the AGU meeting in December. The question of native H2 in the plumes remains murky due to likely instrument based anomalies (ie. titanium wall contamination) .

    • Harry R Ray November 17, 2016, 11:32

      NOT SO FAST! According to Mark Kaufman, a paper has been submitted to the journal, Nature, so the whole issue is PROBABLY UNDER EMBARGO! For details, go to his website, http://www.manyworlds.space, and check out his latest post “Waiting for Enceladus.”

      • Harry R Ray November 17, 2016, 11:34

        Sorry, I meant “Waiting on Enceladus.”

      • john walker November 17, 2016, 19:20

        The embargo policies of most scientific journals(Nature included) don’t extend to presentations at technical conferences.

  • Thomas Goodey November 16, 2016, 15:07

    “… there is only a 5 percent chance that the feature’s alignment with Pluto’s tidal axis is by coincidence…”

    I sympathize with this argument, but such arguments are not always accepted in science. For example, it could be argued that the almost-equality (from our point of view) of the Moon’s angular size with the Sun’s (which explains total and annular eclipses) is of very low probability, certainly less than 5%. Yet science utterly ignores this argument, which is only left to creationist cranks. Similarly, four of the planets of the solar system (Earth, Mars, Saturn, and Neptune) have axial inclinations to the ecliptic (axial tilts) that are very similar, which must be of very low “probability”, yet this is ignored by all right-minded people.

    • EricSECT November 18, 2016, 6:09

      This hypothesis has at it’s basis that a more dense, heavier spot (an ocean) has found a stable configuration 180 degrees away from where the two bodies are tidally locked together. A pencil balanced on it’s tip.

      That does NOT seem like it would be as stable as having the dense clot rotated so that it aligns at the tidal lock zone, under Charon.

      • Thomas Goodey November 18, 2016, 12:45

        Not so. For example, on the Earth there are two tides, (roughly) one toward the Moon and one away from the Moon. In the Pluto-Charon case, the heavy clot on Pluto could either be toward Charon or away from Charon; both would be stable.

  • Michael Spencer November 17, 2016, 7:01

    As far as I know the similar inclinations of four planets is not understood, probability or not; nor is the odd inclination of Uranus or Pluto.

    • ljk November 17, 2016, 10:48

      Ancient impacts? The effect of various moons tugging on them over time? Planetary cores that are off-balance, not in the center where they would be expected to dwell?

      • Thomas Goodey November 17, 2016, 19:05

        None of those possibilities can explain why the axial inclinations of those four planets are all around 25 degrees.

    • Thomas Goodey November 17, 2016, 19:03

      Pluto? You probably mean Venus.

      Yes, both Venus and Uranus have strange inclinations. It is surmised that this is due to collisions in the early life of the solar system. But this cannot explain the similar inclinations of Earth, Mars, Saturn, and Neptune.

  • Michael November 17, 2016, 9:57

    Could this technique potential aid tidally locked planets, if a large amount of ice accumulated on the dark side of these planets there would be a torque that would tend to slowly move the planet around.

  • Harry R Ray November 17, 2016, 10:38

    Remarkably similar to the situation at Enceladus’s south pole; a global ocean with a pertruding dome bringing liquid water closer to the surface in a certain area! Could the analogy CONTINUE with water guysers erupting along the creases of Sputnik Planum? Like Europa, all of the water would fall back to the surface instead of some escaping the Hill Sphere, like at Enceladus. Keep in mind ,FLYBYS(Pioneer 11 and the two Voyagers did NOT detect guysers at Enceladus!

  • ljk November 23, 2016, 11:52

    ‘Cracked and Tipped Over’ Pluto Has a Subsurface Ocean: New Evidence From New Horizons

    By Paul Scott Anderson

    Pluto, a tiny frigid world in the distant outskirts of the Solar System, has been full of surprises, as first revealed by the New Horizons spacecraft back in July 2015. Expected to be mostly a cold, geologically dead place, it has instead been shown to be quite the opposite. Yes, it’s bitterly cold of course, but New Horizons found ample evidence that it has also been geologically active in the past and in some ways still is. With tall mountains of solid water ice, ancient riverbeds carved by nitrogen rivers, vast plains and still-flowing glaciers of nitrogen ice and possible ice volcanoes, Pluto is a wondrous world indeed.

    Another new finding makes it even more remarkable – evidence for a subsurface ocean of water. This had also been reported on previously by AmericaSpace, but the new update strengthens the case.

    Full article here, with nice non-CGI artwork and an interview with New Horizons PI Alan Stern:

    http://www.americaspace.com/?p=96597

  • ljk November 29, 2016, 15:55

    https://apod.nasa.gov/apod/ap161122.html

    Pluto’s Sputnik Planum

    Image Credit: NASA, Johns Hopkins U./APL, Southwest Research Inst.

    Explanation: Is there an ocean below Sputnik Planum on Pluto? The unusually smooth 1000-km wide golden expanse, visible in the featured image from New Horizons, appears segmented into convection cells. But how was this region created?

    One hypothesis now holds the answer to be a great impact that stirred up an underground ocean of salt water roughly 100-kilometers thick. The featured image of Sputnik Planum, part of the larger heart-shaped Tombaugh Regio, was taken last July and shows true details in exaggerated colors.

    Although the robotic New Horizons spacecraft is off on a new adventure, continued computer-modeling of this surprising surface feature on Pluto is likely to lead to more refined speculations about what lies beneath.

  • ljk December 1, 2016, 9:50

    New theory proposes weather, not impact, produced Pluto’s Sputnik Planitia

    by Laurel Kornfeld

    December 1, 2016

    An impacting object might not be the cause of the formation of Sputnik Planitia – the left side of Pluto’s now famous heart feature – as previously thought. This is according to a new theory proposed by Douglas Hamilton of the University of Maryland. He argues the basin could have formed as a result of the dwarf planet’s spin axis and unusual climate.

    Hamilton, the lead author of one of several Pluto studies published in the Dec. 1 issue of the journal Nature, bases his theory on a computer model.

    Full article here:

    http://www.spaceflightinsider.com/missions/solar-system/new-theory-proposes-weather-not-impact-produced-plutos-sputnik-planitia/

  • ljk December 2, 2016, 10:05

    Could Pluto have life as well? It’s surprised us so far…

    https://source.wustl.edu/2016/12/life-plutos-ocean/

    To quote:

    “In fact, New Horizons has detected ammonia as a compound on Pluto’s big moon, Charon, and on one of Pluto’s small moons. So it’s almost certainly inside Pluto,” McKinnon said. “What I think is down there in the ocean is rather noxious, very cold, salty and very ammonia-rich — almost a syrup.

    “It’s no place for germs, much less fish or squid, or any life as we know it,” he added. “But as with the methane seas on Titan — Saturn’s main moon — it raises the question of whether some truly novel life forms could exist in these exotic, cold liquids.”

    As humankind explores deeper into the Kuiper Belt and farther from Earth, this means to McKinnon the possible discovery of more such subsurface seas and more potential for exotic life.

    “The idea that bodies of Pluto’s scale, of which there are more than one out there in the Kuiper Belt, they could all have these kinds of oceans. But they’d be very exotic compared to what we think of as an ocean,” McKinnon said.

    “Life can tolerate a lot of stuff: It can tolerate a lot of salt, extreme cold, extreme heat, etc. But I don’t think it can tolerate the amount of ammonia Pluto needs to prevent its ocean from freezing — ammonia is a superb antifreeze. Not that ammonia is all bad. On Earth, microorganisms in the soil fix nitrogen to ammonia, which is important for making DNA and proteins and such.

    “If you’re going to talk about life in an ocean that’s completely covered with an ice shell, it seems most likely that the best you could hope for is some extremely primitive kind of organism. It might even be pre-cellular, like we think the earliest life on Earth was.”

    and…

    “All of these ideas about an ocean inside Pluto are credible, but they are inferences, not direct detections,” McKinnon said, sounding the call. “If we want to confirm that such an ocean exists, we will need gravity measurements or subsurface radar sounding, all of which could be accomplished by a future orbiter mission to Pluto. It’s up to the next generation to pick up where New Horizons left off!”