April 15, 2014 report
Research pair offer three possible models of Pluto ahead of New Horizons visit
(Phys.org) —Two space researchers, Amy Barr, with Brown University and Geoffrey Collins with Wheaton College, have published a paper in the journal Icarus in which they describe three possible interior models of the former planet Pluto. They suggest the possibilities include: an undifferentiated rock/ice mixture, a differentiated rock/ice mixture, and an ocean covered with ice. The third possibility suggests the likelihood, they claim, of tectonic action on the dwarf planet. A close up view of the planet by space probe New Horizons due to arrive next year, should help clarify which scenario is most likely.
Scientists believe that Pluto came to exist as it does today, in part due to a collision billions of years ago that led also to the formation of its moon Charon. When celestial bodies collide, not only do they knock each other around, they produce heat—heat, the researchers suggest that could still be evident today. Barr and Collins are leading towards a theory that suggests that shortly after impact, Pluto and Charon were much closer together—the gravity attraction between them would have caused both to be egg shaped. As time passed, melted ice from the impact would have created an icy crust on top of an ocean on Pluto, and then, as Charon moved farther away, the attractive pull would have diminished, causing ice plates to form and crack against one another, a form of tectonics. If that were the case, the two add, then in all likelihood, when New Horizons begins sending back images, they should see evidence of such tectonic action—plate edges thrust into the air, for example.
There's just one catch—Pluto circles the sun in an elliptical orbit, thus sometimes it's much closer to the sun than other times. When near, it has a defined atmosphere, when far away however, its atmosphere actually freezes to its surface—something that could hide ridges in the ice and thus evidence of both tectonic activity and an ocean beneath the crust of ice. Since New Horizons will arrive during a time when its atmosphere is frozen to the surface, it might be difficult to determine which of the three proposed models actually describes the relationship between its exterior and interior. Barr and Collins are optimistic that even in such a scenario, ridges should be apparent, proving that beneath Pluto's icy surface, lies an ocean—one that future researchers might one day sample.
The Pluto-Charon system, likely formed from an impact, has reached the endpoint of its tidal evolution. During its evolution into the dual-synchronous state, the equilibrium tidal figures of Pluto and Charon would have also evolved as angular momentum was transferred from Pluto's spin to Charon's orbit. The rate of tidal evolution is controlled by Pluto's interior physical and thermal state. We examine three interior models for Pluto: an undifferentiated rock/ice mixture, differentiated with ice above rock, and differentiated with an ocean. For the undifferentiated case without an ocean, the Pluto-Charon binary does not evolve to its current state unless its internal temperature Ti>200 K, which would likely lead to strong tidal heating, melting, and differentiation. Without an ocean, Pluto's interior temperature must be higher than 240 K for Charon to evolve on a time scale less than the age of the solar system. Further tidal heating would likely create an ocean. If New Horizons finds evidence of ancient tidally-driven tectonic activity on either body, the most likely explanation is that Pluto had an internal ocean during Charon's orbital evolution.
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