Pluto’s heart as a key to its reorientation together with an underground ocean

The possible origin of Sputnik Planitia and Pluto's reorientation (Image courtesy James Keane)
The possible origin of Sputnik Planitia and Pluto’s reorientation (Image courtesy James Keane)

Two articles published in the journal “Nature” offer an explanation to the reorientation of the dwarf planet Pluto’s spin axis. The heart of the matter is in the great basin of Sputnik Planitia, formerly known as Sputnik Planum, and it’s appropriate to say it because it’s in the heart-shaped region. According to one of two studies the explanation provides additional clues about the presence of an underground ocean.

James Keane of the University of Arizona, lead author of one of the research, explained that there are two ways to change the spin of a planet: the first and most familiar is a change in the inclination of its axis with respect to the rest of the solar system, the second is through what is called true polar wander, in which the spin axis remains fixed with respect to the rest of the solar system but the planet reorients due to an internal force.

The key to understanding this phenomenon on Pluto is in the fact that this dwarf planet is like a spinning top lying on its side and the consequence is that its polar regions receive most of the sunlight. Over the course of the 238 Earth years that form a Pluto year nitrogen and other gases condense on a the areas in permanent shadow and then heat up and return to their gaseous state when the Sun hits those regions.

In the region of Sputnik Planitia, nitrogen ice kept on accumulating year after year. Once the ice reached a certain thickness, which might be around 100 meters, the weight became enough as to affect the whole Pluto. The excess mass tends to move toward the equator and with the passage of millions of years caused the displacement of the whole dwarf planet.

The tidal forces with respect to Charon, Pluto’s largest moon, brought the great basin in the current position and also caused the fractures in the crust photographed by NASA’s New Horizons space probe during its July 14, 2015 flyby. The team led by James Keane conducted computer simulations that produced results very similar to dwarf planet photos.

The possible tidal forces between Pluto and Charon have also been studied by Francis Nimmo of the University of California at Santa Cruz and his team, who conducted the second research. The focus of the research described is an underground ocean, that may have pushed up Pluto’s crust in Sputnik matching Planitia when the basin was created, probably from the impact of a giant asteroid or a comet.

According to Francis Nimmo’s team, the ice that moved up towards the surface cooled and strengthened and the basin got filled with nitrogen ice. According to this research too, that provided the excess mass that determined the reorientation of Pluto’s axis with the difference that the presence of an underground ocean is seen as likely because it would made a 7 kilometer thick layer of nitrogen enough to cause the axis reorientation.

The idea of the possible existence of an underground ocean on Pluto is far from new. In June 2016 a research was published that added more clues and it was only the latest about this subject. Those are always inferences that explain some geological formations or events such as Pluto’s reorienting.

In this case, it’s a theory that puts together Sputnik Planitia and the underground ocean. NASA’s New Horizons space probe recently finished sending the data collected so it’s possible that there are also useful information to better understand the issue, which is far from solved.

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