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Shifting Perspectives on Pluto’s ‘Heart’

One of the great pleasures of doing this site is watching researchers matching ideas in peer-reviewed papers. A paper can meet the highest standards for publication but still present an argument that subsequent researchers question, igniting a new round of debate. Trying to get at the heart of a scientific question requires patience, but it’s also as absorbing as a chess game, as witness the continuing debate over the history and significance of Pluto’s Sputnik Planitia. And in this case, we have a researcher working both sides of the controversy.

Resembling a polar ice cap, Sputnik Planitia is about 1000 kilometers across, and is centered on a latitude of 25 degrees north and a longitude of 175 degrees. Moreover, it is directly opposite the side of Pluto that always faces Charon, the result of tidal lock. Two weeks ago we looked at the possibility that this western lobe of Pluto’s ‘heart,’ a deep basin filled with frozen gases like nitrogen, carbon dioxide and methane, was the result of a relatively late impact, perhaps one as large as the impact that created Charon. The implications here were striking, particularly the possibility that Sputnik Planitia was a clue pointing to a subsurface ocean (see Pluto: Sputnik Planitia Gives Credence to Possible Ocean for references).

Today we look at a different possibility in a paper from Douglas Hamilton (University of Maryland), who worked with Francis Nimmo et al. on the subsurface ocean paper. The point is, there are dueling explanations here, and we need to examine them both along with any other potential candidates to explain Sputnik Planitia. After all, it’s a huge feature, and we find ice caps on only three worlds in our Solar System, making its formation a matter of interest whether or not its presence actually indicates an ocean beneath the ice.


Image: Pluto, shown here in the front of this false-color image, has a bright ice-covered ‘heart.’ The left, roughly oval lobe is the basin provisionally named Sputnik Planitia. Sputnik Planitia appears directly opposite Pluto’s moon Charon (back). Credit: NASA/JHUAPL/SWRI.

The model Hamilton studies in this paper is one in which there is no impact, and the ice cap actually formed early in Pluto’s development, providing the surface asymmetry that eventually causes the ice cap to become locked on the side of the world opposite Charon. Remember that Pluto’s spin axis is tilted to 120 degrees. Ice would form in the coldest places on the planet, which in Pluto’s case are latitudes 30 degrees north and south. Temperatures here are far colder than either pole, a zone that includes latitude 25-degree north Sputnik Planitia.

Hamilton and colleagues see a runaway albedo effect as a possibility here, a positive feedback mechanism that begins with a small ice deposit and accumulates more and more ices until a dominant ice cap emerges. It is the gradual shift in the dwarf planet’s mass, then, that came into play to ensure that Sputnik Planitia would, as Pluto slowed due to Charon’s gravity, either face Charon or emerge on the opposite side of the world from its moon. The chances are 50-50, as Hamilton notes: “It is like a Vegas slot machine with just two states, and Sputnik Planitia ended up in the latter position, centered at 175 degrees longitude.”

So there we have it, a new possibility in which Sputnik Planitia formed not through an impact, but through the weight of accumulating surface ices. The formation of a basin like this one is not so different from what we see on Earth in the Greenland Ice Sheet, where a basin forms and pushes down on the crust beneath it, although Hamilton adds that Pluto’s basin is larger than the volume of ice it currently contains, evidence that Sputnik Planitia is losing mass over time. Either formation model fits the New Horizons data, leaving us with an inconclusive result about that putative ocean. Under the right conditions, says Hamilton, either model is viable.

The paper is Hamilton et al., “The rapid formation of Sputnik Planitia early in Pluto’s history,” Nature 540 (01 December 2016), 97-99 (abstract).


Comments on this entry are closed.

  • ljk December 2, 2016, 14:37

    Pluto may have some form of exotic life in its presumed subsurface ocean, too:


  • Christian G December 2, 2016, 19:12

    “… a paper from Douglas Hamilton…”

    “…there are dueling explanations…”

    I see what you did there.

  • Phil December 4, 2016, 17:34

    “we find ice caps on only three worlds in our Solar System”



  • Paul Gilster December 4, 2016, 21:09

    Titan has no polar caps, as far as I know — this would be an ethane ice cap, I assume, and I don’t think that such exist. But someone jump in if I’m wrong.

  • ljk December 28, 2016, 19:21

    Ancient Water World: Tectonics on Pluto’s Moon Charon Point to Frozen Subsurface Ocean

    By Paul Scott Anderson

    Charon is Pluto’s largest moon and despite being so cold and remote from the Sun, has been revealed to be a fascinating and active world, just like Pluto itself. Residing in the far outskirts of the Solar System, it had been expected that Charon, and Pluto for that matter, would be little more than frozen, dead little worlds. But just like the rest of the Solar System, there were surprises waiting to be found. Thanks to the New Horizons spacecraft, we got our first close-up views of the Pluto system in July 2015. It soon became evident that not only were Pluto and Charon geologically active in the ancient past, but they perhaps still were in some ways even now. One of the most surprising findings was both Pluto and Charon likely had subsurface water oceans; while it is thought that Pluto’s is probably still liquid, Charon’s is likely completely frozen, and now additional evidence for its existence has been published by researchers.

    The evidence is presented in a new paper in Icarus titled “Charon Tectonics,” and suggests that Charon was not only tectonically active in the past, but also possessed a subsurface ocean which is now frozen. The new paper is a summary of observations from New Horizons’ Long-Range Reconnaissance Imager (LORRI, Cheng et al., 2008) and the Multi-spectral Visible Imaging Camera (MVIC) on the Ralph instrument (Reuter et al., 2008). The conclusion in brief is that Charon has undergone “global extension” which produced the tectonic features seen by New Horizons and provides evidence for the subsurface ocean. Basically, expanding ice in the slowly freezing ocean would cause the surface to stretch and fracture.

    Full article here:


  • ljk January 5, 2017, 14:36

    The telescope that Clyde Tombaugh used to discover Pluto in 1930 is going to be renovated:


    To quote:

    The Pluto Telescope is technically known as an astrograph, a telescope specifically designed for taking photographs of objects in space. In addition to Tombaugh’s 1930 discovery of Pluto, the instrument was also used by Lowell astronomers to study comets and asteroids, as well as stars with measurable proper motion (apparent angular movement). But it is the Pluto discovery that continues to generate public interest in the facility, resulting in ever-increasing visitation from guests around the world. In 2016 alone, Lowell welcomed a record 100,000 visitors.

  • ljk March 13, 2017, 13:13

    A brief history of Pluto’s 87-year planetary identity crisis

    WRITTEN BY Akshat Rathi

    March 13, 2017

    “Doctor Slipher, I have found your Planet X.” On Feb. 18, 1930, this is what astronomer Clyde Tombaugh told his boss, Vesto Slipher. For years, scientists at the Lowell Observatory in Arizona had scanned million of stars in search of a fabled planet at the far reachers of our solar system. Tombaugh was now sure he had found it.

    After taking more photos to confirm the finding, 87 years today, on March 13, 1930, the world was told that a new planet would be added to the solar system. It was named Pluto in part to honor Percival Lowell, as his initials would make up the first two letters of the name. Lowell was a wealthy businessman, mathematician, and astronomer who built observatory where Tombaugh and Slipher worked and where the search for Planet X began 24 years earlier.

    According to Lowell, Planet X had to be out there, somewhere, because of wiggles in the orbits of Uranus and Neptune. The theory was that a large planet must be lurking beyond Neptune in order to create the gravitational tug that accounted for the orbital perturbations.

    Full article here:


    To quote:

    But almost as soon as Pluto was discovered, astronomers began questioning its status as Planet X. It looked too small to create orbital perturbations. Over the next 50 years, Pluto’s mass was calculated, argued over, and then recalculated. In 1978, the discovery of Charon, Pluto’s moon, allowed a conclusive measurement of the planet’s mass, of about 1/500th of the Earth’s.

    Within a decade of that realization, based on data received from Voyager 2 as it crossed Neptune in 1989, Pluto’s status as Planet X was dealt a fatal blow. It turns out that Lowell’s original observations were slightly off, and Neptune’s mass had been overestimated by 0.5%. This seemingly tiny tweak meant that its orbital perturbations were not necessarily down to the pull of another planet in the vicinity.

    What’s more, in the early 1990s a mess of bodies were discovered orbiting the sun, roughly in the same area and of the same size as Pluto. They even had similar skewed orbits as Pluto, which doesn’t orbit the sun in the same plane as the other eight planets of the solar system.

  • ljk March 29, 2017, 15:04


    Article Updated: 28 March 2017

    by Matt Williams

    When Pluto was first discovered by Clybe Tombaugh in 1930, astronomers believed that they had found the ninth and outermost planet of the Solar System. In the decades that followed, what little we were able to learn about this distant world was the product of surveys conducted using Earth-based telescopes. Throughout this period, astronomers believed that Pluto was a dirty brown color.

    In recent years, thanks to improved observations and the New Horizons mission, we have finally managed to obtain a clear picture of what Pluto looks like. In addition to information about its surface features, composition and tenuous atmosphere, much has been learned about Pluto’s appearance. Because of this, we now know that the one-time “ninth planet” of the Solar System is rich and varied in color.

    Full article here: