To be a planet, or not a planet – that is the defining debate around Pluto’s existence. But whilst everyone decides whether to join the pro-Pluto planet party or not, there is another issue about Pluto that a consensus is yet to be reached on – what were the exact conditions of its formation?
A new study, published in Nature Geoscience, suggests that contrary to previous analysis, Pluto may have had a “hot start” in the icy Kuiper Belt, and developed its subsurface liquid ocean much earlier than thought. The US researchers reached this conclusion by comparing different thermal simulations of Pluto’s interior with observations of the dwarf planet by NASA’s New Horizon’s spacecraft.
“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,” co-author Francis Nimmo, professor of Earth and planetary sciences at UC Santa Cruz, said in a statement. “Now that we have images of Pluto's surface from NASA's New Horizons mission, we can compare what we see with the predictions of different thermal evolution models.”
Until this mission imaged Pluto in 2015 we barely knew what it looked like. But from Pluto’s heart to its icy surface volcanoes, New Horizons revealed that it was a geologically active object. Indeed, the suspected subsurface salty ocean beneath Pluto’s icy shell was a particularly intriguing feature.
Researchers have traditionally attributed the formation of Pluto’s ocean to heat generated by radioactive decay in the dwarf planet’s interior, carved out inside of the ball of frozen ice and rock. But from the team’s new simulations this cold-start scenario would not account for some of the surface features of Pluto seen by New Horizons.
“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,” first author Carver Bierson, a graduate student also at UC Santa Cruz said in a statement. “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.”
In order for Pluto to have been hot enough to have a liquid ocean in its early days, a large majority of the gravitational energy released from accreting material must have been retained as heat. For that to happen, its formation must have happened quickly as well.
“How Pluto was put together in the first place matters a lot for its thermal evolution,” Nimmo said. “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.”
From their calculations, the team shows that if the dwarf planet formed over a period of less than 30,000 years then the heat would have been kept. If, on the other hand, this period lasted for a few million years, then large impactors would have needed to bury their energy deep in Pluto’s surface to generate enough heat.
Well, Pluto, if you started off hot and fast, it only makes us love you more.