Pluto is thought to possess a subsurface ocean beneath its thick ice shell. It has generally been assumed that the dwarf planet formed out of cold material and then later developed its ocean due to warming from radioactive decay. By combining numerical simulations with geological observations by NASA’s New Horizons mission, a team of researchers from the University of California Santa Cruz and the Southwest Research Institute demonstrated that Pluto was instead relatively hot when it formed, with an early subsurface ocean.
This high-resolution image of Pluto was taken by New Horizons on July 14, 2015. Pluto’s surface sports a remarkable range of subtle colors, enhanced in this view to a rainbow of pale blues, yellows, oranges, and deep reds. Many landforms have their own distinct colors, telling a complex geological and climatological story that scientists have only just begun to decode. Image credit: NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute.
“For a long time people have thought about the thermal evolution of Pluto and the ability of an ocean to survive to the present day,” said Professor Francis Nimmo, a planetary scientist in the Department of Earth and Planetary Sciences at the University of California Santa Cruz.
“Now that we have images of Pluto’s surface from New Horizons, we can compare what we see with the predictions of different thermal evolution models.”
“Because water expands when it freezes and contracts when it melts, the hot-start and cold-start scenarios have different implications for the tectonics and resulting surface features of Pluto,” added Carver Bierson, also from the Department of Earth and Planetary Sciences at the University of California Santa Cruz.
“If it started cold and the ice melted internally, Pluto would have contracted and we should see compression features on its surface, whereas if it started hot it should have expanded as the ocean froze and we should see extension features on the surface.”
“We see lots of evidence of expansion, but we don’t see any evidence of compression, so the observations are more consistent with Pluto starting with a liquid ocean.”
The thermal and tectonic evolution of a cold-start Pluto is actually a bit complicated, because after an initial period of gradual melting the subsurface ocean would begin to refreeze. So compression of the surface would occur early on, followed by more recent extension.
With a hot start, extension would occur throughout Pluto’s history.
“The oldest surface features on Pluto are harder to figure out, but it looks like there was both ancient and modern extension of the surface,” Professor Nimmo said.
The next question was whether enough energy was available to give Pluto a hot start.
The two main energy sources would be heat released by the decay of radioactive elements in the rock and gravitational energy released as new material bombarded the surface of the growing protoplanet.
The team’s calculations showed that if all of the gravitational energy was retained as heat, it would inevitably create an initial liquid ocean.
In practice, however, much of that energy would radiate away from the surface, especially if the accretion of new material occurred slowly.
“How Pluto was put together in the first place matters a lot for its thermal evolution. If it builds up too slowly, the hot material at the surface radiates energy into space, but if it builds up fast enough the heat gets trapped inside,” Professor Nimmo sad.
The authors calculated that if Pluto formed over a period of less that 30,000 years, then it would have started out hot.
If, instead, accretion took place over a few million years, a hot start would only be possible if large impactors buried their energy deep beneath the surface.
The findings imply that early oceans may have been common in the interiors of large Kuiper Belt objects.
These oceans could persist to the present day in the largest objects, such as the dwarf planets Eris and Makemake.
“Even in this cold environment so far from the Sun, all these worlds might have formed fast and hot, with liquid oceans,” Bierson said.
The study was published in the journal Nature Geoscience.
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C.J. Bierson et al. Evidence for a hot start and early ocean formation on Pluto. Nat. Geosci, published online June 22, 2020; doi: 10.1038/s41561-020-0595-0
