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New data sheds light on composition of Pluto’s frozen ‘heart’

Pluto in natural color

Four images from New Horizons’ Long Range Reconnaissance Imager (LORRI) were combined with color data from the Ralph instrument to create this global view of Pluto. The images, taken when the spacecraft was 280,000 miles (450,000 kilometers) away from Pluto, show features as small as 1.4 miles (2.2 kilometers). Image & Caption Credit: NASA/JHU-APL/SwRI

Computer models based on data collected by the New Horizons mission team are shedding light on the formation and structure of Sputnik Planum, the left side of Pluto’s iconic ‘heart’ feature.

A study, published in the journal Nature by Tanguy Bertrand and Francois Forget of the University of Paris, uses a mathematical computer model to propose that a deep basin within Sputnik Planum became filled with deposits of nitrogen that condensed from Pluto’s atmosphere over time, forming the nitrogen ice glacier that covers this area.

Located in Pluto’s northern hemisphere, this glacier is 650 miles (1,046 kilometers) in diameter and 2.5 miles (4 kilometers) deep. The mathematical model used by Bertrand and Forget addresses the glacier’s formation over the last 50,000 Earth years.

Comparison between the observed and modelled ice distributions on Pluto in July 2015.

Comparison between the observed and modeled ice distributions on Pluto in July 2015. (Click to enlarge) Image Credit: T. Bertrand & F. Forget / Nature

According to their findings, atmospheric pressure at the base of Sputnik Planum is higher than it is on its surface and on the terrain surrounding that surface. This causes nitrogen to condense out of the atmosphere and be deposited as ice at the basin’s bottom. Some carbon monoxide and methane ices mingle with the nitrogen ice.

“Because of this, the crater is a cold trap,” Bertrand explained, adding that colder temperatures at the basin’s bottom increase the rate at which the ice cools, causing even more atmospheric nitrogen to condense and fall into the basin.

Sputnik Planum is essentially a permanent glacier because there is so much ice, and the glacier is so massive that even seasonal changes during Pluto’s 248-year solar orbit, at most, sublime one meter of nitrogen during the warmer seasons and condense, at most, the same amount during the colder seasons, Bertrand stated. The glacier flows outward and contracts seasonally by approximately one kilometer.

That is not the case for Pluto’s polar caps, whose methane frosts are thinner than the nitrogen ice on Sputnik Planum. Ices in these regions melt when winter gives way to spring, which will happen on Pluto’s northern hemisphere within approximately the next ten Earth years.

“It’s a bit similar to Earth, because, for example, in winter, in the northern hemisphere, you have the water ice covering the Arctic Ocean. On Pluto, it’s the same; you have methane ice covering the northern hemisphere during winter, and this frost can melt in the spring. It’s just that on Pluto, the seasons are really long; one year on Pluto is 248 Earth years, so one season is several decades,” Bertrand said.

The mathematical model used by Bertrand and Forget accurately predicted both the increase in Pluto’s atmospheric pressure over the last three decades and the development of bright, layered methane frost at each pole as it experiences winter.

A separate study of Sputnik Planum’s basin finds evidence of a subsurface ocean beneath the plain and propose that the 559-mile (900-kilometer) wide basin was created by the impact of a 124-mile (200-kilometer) or larger asteroid on Pluto’s surface.

Researchers led by Brandon Johnson of Brown University published a study detailing the dynamics and results of that impact in the journal Geophysical Research Letters. By modeling the dynamics of the impact, they hope to determine the thickness of the subsurface ocean.

“Thermal models of Pluto’s interior and tectonic evidence found on the surface suggest that an ocean may exist, but it’s not easy to infer its size or anything else about it,” Johnson said. “We’ve been able to put some constraints on its thickness and get some clues about composition.”

Based on the models they used, Johnson’s team concludes there could be up to 62 miles (100 kilometers) of liquid water in Pluto’s underground ocean and that this water contains as much salt as the Dead Sea on Earth.

Pluto and Charon are tidally locked to one another and always present one another with the same side. Located exactly on the tidal axis that connects both worlds, Sputnik Planum likely contains more mass than most other areas on Pluto’s crust.

Pluto's Sputnik Planum region

Sputnik Planum is the informal name of the smooth, light-bulb shaped region on the left of this composite of several New Horizons images of Pluto. The brilliantly white upland region to the right may be coated by nitrogen ice that has been transported through the atmosphere from the surface of Sputnik Planum, and deposited on these uplands. Image & Caption Credit: NASA/JHUAPL/SwRI

Sputnik Planum’s higher than average mass and alignment on the tidal axis result from Charon’s gravitational tug, which proportionally tugs on higher mass areas. This higher than average mass is referred to as a “positive mass anomaly”. It is also the opposite of what one would expect from an impact crater.

“An impact crater is basically a hole in the ground. You’re taking a bunch of material and blasting it out, so you expect it to have [a] negative mass anomaly, but that’s not what we see with Sputnik Planum. That got people thinking about how you could get this positive mass anomaly,” Johnson said.

Some of the higher mass comes from nitrogen ice that condensed and accumulated in the basin. However, the nitrogen ice is insufficient to create the positive mass anomaly on its own, leading scientists to suspect the increased mass comes from a subsurface ocean.

The initial asteroid impact would have created a crater followed by a rebound in which material from the planet’s interior is pushed upward. If this material has a higher density than that excavated by the impact, the crater will end up with the same mass that it had initially, a phenomenon known as “isostatic compensation”.

If the material pushed upward was water, which is denser than ice, it could have welled up in the plain and evened up the crater’s mass. Subsequent deposits of nitrogen ice could have produced the positive mass anomaly.

“This scenario requires a liquid ocean. We wanted to run computer models of the impact to see if this is something that would actually happen. What we found is that the production of a positive mass anomaly is actually quite sensitive to how thick the ocean layer is. It’s also sensitive to how salty the ocean is because the salt content affects the density of the water,” Johnson said.

In creating their model, his team assumed the impacting object was big enough to create a basin the size of Sputnik Planum. They ran the model simulating a range of conditions from no subsurface water to an ocean with a thickness of 124 miles (200 kilometers) and found the scenario that best fit Sputnik Planum’s current depth and size to be one with an ocean of 62 miles (100 kilometers) with 30 percent salinity.

“It’s pretty amazing to me that you have this body so far out in the Solar System that still may have liquid water,” Johnson marveled.



Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.

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