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Geology

Pluto perhaps not so icy after all

Hammond, Noah P., Amy C. Barr, and Edgar M. Parmentier. “Recent Tectonic Activity on Pluto Driven by Phase Changes in the Ice Shell.” Geophysical Research Letters (2016). DOI: 10.1002/2016GL069220

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Figure 1 – Artist rendering of New Horizon passing Pluto. Courtesy of NASA/JHU APL/SwRI Steve Gribben

Pluto, the celestial object formerly known as a planet, got a close look from a passing NASA spacecraft last summer. New Horizons was launched in January of 2006 to study the cloud of small objects at the edge of the solar system called the Kuiper belt (Fig 1). Nine years later, it passed by Pluto, in the process capturing the most detailed observations of the plane…er…astronomical body to date.

A few months ago, scientists released a paper describing these new high-resolution images that New Horizon captured of Pluto. Their analysis suggested that many of the surface features are relatively young and, therefore, not formed by tectonic activity. The discussion left open the tantalizing possibility that the surface was shaped by glaciers and oceans.

Enter Noah Hammond, a graduate student at Brown University. Along with his advisors, he set out to try and explain how these surface features might have formed. Their analysis revealed something surprising: it’s possible there a liquid ocean on current day Pluto. Water. Approximately 5 billion kilometers from the sun. On an object whose surface temperature hovers at about 40 Kelvin (or -233°C/ -384°F).

Arriving at this conclusion took a lot of work and computer wrangling. Hammond started by simplifying Pluto’s geometry and breaking it into lots of little boxes. What is happening in each box is described by a mathematical relationship between many parameters — things like temperature, time, and density. Hammond was able to poke and prod his theoretical Pluto by varying these parameters and watching what how it’s thermal properties changed in time.

The team assumed that the dwarf planet has a solid core with a separate ice layer on top. While there is ample evidence suggesting this is the case, scientists are unsure of the exact make-up of the core. The composition of Pluto’s center is an important component of Hammond’s model since different materials transfer heat differently. To account for that uncertainty, Hammond varied the core’s thermal conductivity, a number indicating how well a material conducts heat.

As the model evolved under each set of conditions, Hammond watched for phase changes in the water. When ice melts or water freezes, the volume of the material changes. Likewise, at certain temperatures and pressures, such as those on Pluto, ice can become a denser material called ice II. At large scales, these shifts in phase would alter the volume of the whole planet: as water expands into ice, the volume will go up. As ice condenses into ice II, the object will shrink.

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Figure 2 – Graphs illustrating the Plutonian ocean evolution over time. The x-axis is in millions of years. The top graphs have a y-axis of depth from the core to the surface. The bottom ones show stress in Pascals. (a) are the end result of a warm scenario. At the end of model run, there is liquid water and the net force is extensional. (b) shows a cold scenario. At the models end, there is no water, but a lot of ice II. The force is net compressional. Adapted from Hammond et al., 2016.

Hammond’s model settled into either a “warm” or “cold” final state depending on the thermal properties of the core. In warm scenarios, a 50 km thick ocean forms beneath 250 km of ice and persists to the present day (Fig 2a). Leading up to the cold outcome, the ocean completely froze before the model time ended time. But the increased pressure from more ice resulted in the formation of ice II (Fig. 2b).

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Figure 3 – Image of Pluto’s surface from New Horizon. The areas marked (b) show flow lines associated with glacial features. The inset at bottom right shows a close up. Adapted from Moore et al., 2016.

These very different end states have similar planetary effects: dramatic shifts in Pluto’s volume. The formation of ice II would cause global contraction. The formation of ice from water would result in global expansion. Hammond argues the liquid water case is more likely. He points to the surface features observed by New Horizon, which were likely cause by extensional, rather than compressive, forces.

Is there actually water on Pluto, buried beneath miles of ice? We may never know for sure, but Hammond suggests the New Horizon data can be used to verify his findings. More detailed analysis of the images could verify that the observed geographic features are from expansion forces and verify the timing of the events. Maybe after that NASA will support an expedition to Pluto. Who knows what it might find. I am banking on space whales.

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