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What Do We Know About Liquids on Pluto?

Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.

Sputnik Planum

Early on in the data dump of New Horizon's Pluto flyby was evidence of ice flows on the surface of Pluto. Located in Sputnik Planum (which is over 350,000 square miles in area), the image showed nitrogen ice and the possible migration it makes through the soft ice, like glaciers on Earth.

It is another sign of a geologically active world despite the -390°F temperature found there. In fact, images of the lower portion of the Tombaugh Regio possibly show ice moving over into the dark area known as the Cthulhu Regio. It seems to be a large place where not much activity occurs and, combining that with the large craters seen, points to an old age (maybe 4 billion years old) (NASA "New Horizons," Thompson, Stern "The Pluto" 27, Stern "Hot" 32).

Sputnik Planum

Sputnik Planum

Scientists suspect those nitrogen ice flows act like a giant "lava lamp." In a June 2016 press release, William B. McKinnon (Washington University in St. Louis) and colleagues laid out their theory, which was based off imagery and computer simulations.

Whatever is heating Pluto internally (likely a radioactive material or special chemistry) gets things hot enough to melt the nitrogen (which normally would be at 35 K) into a liquid, which rises because of buoyancy. As it nears the surface, it cools off until becoming solid again and sinking back down. In addition, this ice easily changes shape because of the lack of strong bonds between nitrogen atoms. All of this creates flow patterns like an uprising convection cell, which matches what images we have of up to 50 meter height differences between cells.

But according to the simulations, the flow is slow, about a few centimeters a year. That means it would take over 500,000 years to fully circulate the surface and explains its youthful appearance. But as for why most of the nitrogen on Pluto exists solely in this region, the simulations offer no answers (John Hopkins, Timmer). Simply amazing.

The nitrogen flows.

The nitrogen flows.

What Could Be Flowing?

Alright, so what is the deal with all this nitrogen? Where does it come from? Comets are unlikely to collide enough to constantly replenish the 100s of tons lost hourly to surface sublimation. It seems as though the geologically active dwarf planet viewpoint has gained more ground.

Somehow, an internal mechanism is bringing nitrogen to the surface to make those ice flows. Alan Stern, one of the principal researchers for New Horizons, thinks that cryovolcanoes could be a possible delivery system. He also pointed out that for Pluto to be pumping as much nitrogen as it currently is suggests that it is much younger than anyone thought it was, possibly denting Kuiper Belt theories (Lewin).

The potential cryovolcanoes.

The potential cryovolcanoes.

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The talk of cyrovolcanoes in the Pluto system heated up when Wright Mons, Kubrick Mons and Piccard Mons on Pluto were identified as being similar to shield volcano-like structures on Mars and Earth. The 100 kilometer wide rim around shallow but gentle depression slopes points to a collapse of the surface once material from below vacated it. That material could be driven by convection of some sort, like we mentioned earlier.

The likely material of the cryovolcano based on those convection models would be a mix of nitrogen (as theorized before), ammonia, methane, or even water ice. And with a lack of craters in Sputnik Planum and a general lack of old craters anywhere on Pluto hints at both an active surface but also at a lack of small objects colliding with the dwarf planet. In fact, the Tombaugh Regio seems to be 1 billion years old while Sputnik Planum is 10-30 million years old! It hints at a shocking picture: that KBO's did not form through accumulation of smaller objects but rather formed as large objects near their current orbit. Solar system formation theories will need potential revising if further data confirms this (NASA "Pluto May Have," Berger, Talcott, Stern "Hot" 32, Purdue, Stern "Puzzled" 26).

More Signs

December of 2015 saw more information being released as well as new images. Amongst the findings released as continuing evidence for past and present geological activity on Pluto and with this the formation of interesting valleys that result from erosion. Sputnik Planum had its temperature readings released also, and they paint a picture for the nitrogen ices seen there. The way the heat flows in the 620-mile wide region causes the interesting polygon formations seen, and also how certain features travel around. Based on the heat, the plain is likely a few miles deep (NASA "New Findings," Stern "The Pluto" 28).

An Ocean World?

And in a rather interesting trend, Pluto joined the ranks of possible ocean worlds in our solar system. Yes, our distant dwarf planet may have a subsurface liquid ocean separate from the nitrogen cycle spotted, based off New Horizon's tectonics data. The shape of Sputnik Planitia indicates that a roughly 124 mile long object impacted Pluto there. But this presents a problem: The Sputnik Planitia is the heaviest part of the dwarf planet's surface and is opposite Charon, something that a crater should not be as its mass should be less than the surrounding region. But if Pluto has a free floating surface, then it would rotate separately from its interior.

Density readings indicate that water is on Pluto, but any that is below the surface should have cooled and become ice, causing surface expansions and contractions. Yet we cannot see this, indicating that if water is present then it is liquid and likely more than 62 miles deep. Plus if such a system were present, then the crater from the impact would have been filled with the salty water and help explain the lopsided distribution of Pluto's surface. The question now becomes what could heat the water still after so many years. The signs point to radioactive decay. But another theory that takes the tectonics and density into account is a thinner crust, which leads to different forms of ice that could exist under seen conditions and eliminate the liquid ocean theory (Wenz "Pluto Becomes", Klotz "Pluto Has," Wenz "Pluto's Icy," Wenz "Pluto's Slushy Heart").

And it may not be an ocean nor tectonics. Under the runaway albedo effect, small pockets of ice reflected light and allowed more ice to accumulate as time went on. Eventually, enough was present to apply sufficient pressure to the surface and a basin formed, much like we see today (Klotz "Pluto's Heavy").

Virgil Fossae in purple.

Virgil Fossae in purple.

But then again, if multiple signs do point to a conclusion, it gives it some weight. When scientists looked at ammonia distributions on the surface, one area was particularly interesting: Virgil Fossae, a trench-like feature in the Cthulhu region. In fact, the levels seen pointed to a replenishment source, for ammonia is easily broken up both UV and cosmic rays. This coupled with the young age of the surrounding material (because of a general lack of impact craters) and the fast that ammonia would let water remain liquid below its traditional freezing point allows for hope of a liquid ocean in fact being present on Pluto (Haynes).

And if there is an ocean, could there be life?

Works Cited

  • Berger, Eric. "Volcanoes on Pluto Look a Lot Like Those On Earth and Mars." arstechnia.com. Conte Nast, 09 Nov. 2015. Web. 19 Dec. 2015.
  • Haynes, Korey. "Ammonia on Pluto's surface points to liquid water underground." Astronomy.com. 31 May 2019. Web. 21 Sept. 2020.
  • John Hopkins. "Pluto's Heart Like a Cosmic Lava Lamp. Astronomy.com. Kalmbach Publishing Co., 02 Jun. 2016. Web. 14 Jul. 2016.
  • Klotz, Irene. "Pluto Has an Underground Ocean of Water and Slush." Seekers.com Discovery Communications, 16 Nov. 2016. Web. 24 Jan. 2017.
  • ---. "Pluto's Heavy Heart, Not a Massive Impact, Caused Its Depression." Seeker.com. Discovery Communications, 30 Nov. 2016. Web. 24 Jan. 2017.
  • Lewin, Sarah. "If Pluto Keeps Spewing Nitrogen, Why Is It Still Full of It?" Space.com. Space.com, 17 Aug. 2015. Web. 09 Sept. 2015.
  • NASA. "New Findings From New Horizons Shape Understanding of Pluto and its Moons." Astronomy.com. Kalmbach Publishing Co., 21 Dec 2015. Web. 10 Mar. 2016.
  • ---. "New Horizons team finds haze, flowing ice on Pluto." Astronomy.com. Kalmbach Publishing Co., 01 Oct. 2015. Web. 04 Nov. 2015.
  • ---. "Pluto May Have Ammonia Fueled Ice Volcanoes." Astronomy.com. Kalmbach Publishing Co., 09 Nov. 2015. Web. 19 Dec. 2015.
  • Purdue. "An Ammonia-Water Slurry May Swirl Below Pluto's Icy Surface." Astronomy.com. Kalmbach Publishing Co., 10 Nov. 2015. Web. 11 Oct. 2017.
  • Stern, Alan. "Hot Results from a Cool Planet." Astronomy May 2016: 32. Print.
  • ---. "Puzzled by Pluto." Astronomy Sept. 2017. Print. 26.
  • ---. "The Pluto System Explored." Astronomy Nov. 2015: 27-8. Print.
  • Talcott, Richard. "New Horizons Unleashes Torrent of Pluto Science." Astronomy Mar. 2016: 15. Print.
  • Thompson, Amy. "New Horizons data shows Pluto’s atmosphere, surface features." ars technica. Conte Nast, 27 Jul. 2015. Web. 19 Aug. 2015.
  • Timmer, John. "Pluto's Sputnik Planum is an Ocean of Slowly Shifting Nitrogen Ice." ars technica. Conte Nast, 02 Jun. 2016. Web. 14 Jul. 2016.
  • Wenz, John. "Pluto Becomes the Latest (and Weirdest) Possible Ocean World." Astronomy Oct. 2016: 13. Print.
  • ---. "Pluto's Icy Heart Could Hide a Warm Ocean." Astronomy Jan. 2017: 11. Print.
  • ---. "Pluto's Slushy Heart Contains A Large Ocean." Astronomy Mar. 2017: 13. Print.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2021 Leonard Kelley

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