Only in recent years, since the historic New Horizons probe flyby in 2015, have we been able to understand Pluto in great depth or detail. We have learned a lot about the small outlier of our Solar System, but one of the biggest surprises was a series of indications that liquid oceans still spill out below Pluto’s icy surface.
At an average distance of 5.9 billion km (3.7 billion miles) from the Sun, in the cold confines of the Kuiper Belt, scientists thought the dwarf planet must have been frozen, and exactly how liquid water could exist in a such a cold object. It was a mystery.
Now astronomers have come up with a new scenario, detailed in a new article: If Pluto formed quickly, the heat generated by this process might have been enough to keep the underground oceans liquid for billions of years.
“For a long time, people have thought about Pluto’s thermal evolution and the ability of an ocean to survive to this day,” said Earth scientist and planetarium Francis Nimmo of the University of California at Santa Cruz.
“Now that we have images of Pluto’s surface from NASA’s New Horizons mission, we can compare what we see with predictions from different models of thermal evolution.”
Pluto, which formed about 4.5 billion years ago with the rest of the Solar System, could have accumulated more slowly, from cold material. Under this model, different mechanisms could explain liquid groundwater, such as the decomposition of radioactive elements in Pluto’s nucleus.
However, while this cold start model is a plausible way for liquid water to persist on a Kuiper Belt object, it is inconsistent with some of the features discovered on Pluto’s surface through New Horizons observations .
“If it started to cool and the ice melted internally, Pluto would have contracted and we should see compression characteristics on its surface, whereas if it started to heat up it should have expanded as the ocean froze and we should see extension characteristics in the surface, “he said. UC San Diego planetary and earth scientist Carver Bierson, lead author of the article.
“We see a lot of evidence of expansion, but we don’t see evidence of compression, so the observations are more consistent with Pluto starting with a liquid ocean.”
Extensional faults on the surface of Pluto. (NASA / JHUAPL / SwRI / Alex Parker)
You see, the presence of extension lines alone is not a smoking gun for a hot start scenario. If Pluto were to start hot, the dwarf planet would undergo an early and rapid expansion phase for approximately one billion years, followed by a longer and slower expansion phase of approximately 3.5 billion years.
But in a cold start scenario, the second phase would also be extensional; The difference is that the previous phase would be compressive. Therefore, to discover which story fits, it is important to discover the characteristics of the initial phase, which is what the team has done, to identify a system of ridges and valleys that they believe are indicative of an early extensional phase.
“The older surface features on Pluto are more difficult to understand, but it appears there was an ancient and modern extension of the surface,” said Nimmo.
The next step was to model how Pluto could have started from the beginning. A source of such thermal energy would be the accretion process: material that rains on Pluto to increase its increasing volume. As this material impacts, it imparts gravitational energy, which is then released as heat.
But the time scales in which this occurs makes a big difference.
“How Pluto formed in the first place is very important to its thermal evolution,” said Nimmo. “If it accumulates very slowly, the hot material on the surface radiates energy into space, but if it accumulates quickly enough, the heat is trapped inside.”
Traditional models for Kuiper Belt objects would see that this process would take hundreds of millions of years to produce an object the size of Pluto, 2,376 kilometers (1,476 miles) in diameter. That is too slow; Pluto would be cold before he could even start cooking.
But recent research has suggested a new formation model: a multi-stage process in which a planetesimal grows relatively slowly to about 300 kilometers in diameter, and the final accretion stage occurs quickly.
Under this scenario, Pluto could form in about 30,000 years, the time the team calculated it would take for the hot start model. And, the researchers note, their results imply that other large Kuiper Belt objects may have begun to heat up, and also have early oceans.
It is only hypothetical at this stage, but there are characteristics that could confirm the team’s ideas.
“An important distinction between cold start and hot start models is that the former, but not the latter, is likely to retain an undifferentiated rock-rich shell on the near surface … clear evidence of a rock-rich shell , like the one inferred at Ceres, would rule out a hot start to Pluto, “the researchers wrote in their article.
“Similarly, widespread evidence of compression characteristics such as wrinkles would be very difficult to reconcile with a Pluto hot start … The main prerequisite for any of these tests is a stratigraphic column for Pluto; now that the characteristics Crater basics have been established, such an undertaking can be attempted. “
The research has been published in Nature Geoscience.
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