Researchers have proposed a mechanism that could explain the surprisingly active surface of Pluto that was observed by New Horizons during its flyby in July. They think that Pluto’s crust rests on a mantle of water ice and ammonia whose convection movements are responsible for rejuvenating the dwarf planet’s exterior.
Pluto’s mantle was thought to be made of pure water ice, which would not allow for much underground activity. So the researchers created a model that mixes the ice with five percent ammonia, and they used it to deduce the Rayleigh number, a physical value which indicates the presence and strength of convection. Convection happens in a fluid when hotter particles rise, forcing colder particles to fall towards the heat source. This movement transfers heat from one location to another – in the case of Pluto, from the possibly radioactive core to the surface.
"We found that a mantle containing a small amount of ammonia – which has been seen on the surface of bodies in the outer Solar System and plausibly condensed in the planets in this region – lowers the temperature required to achieve a Rayleigh number where convection occurs," said Alex Trowbridge, lead author of the research, in a statement. "The ammonia lowers the viscosity of water-ice by a factor of 100,000. This would allow for the geologically active and vigorous Pluto seen in the New Horizons images."
The study, presented at the annual meeting of the Division for Planetary Sciences of the American Astronomical Society, is a first attempt to solve the mystery of Pluto’s active geology.
We are not sure how such a small world could remain active 4.5 billion years after its formation. However, there are clear signs of subterranean activity on Pluto. The Sputnik Planum (the western lobe of Pluto’s “heart”), for example, has no craters larger than 10 kilometers (six miles), a clear indication that something in the last few million years has smoothed out the surface.
The latest data from New Horizons has also indicated the potential presence of cryovolcanos, another clue of the dwarf planet's busy interior.
"We wanted to know why this small, icy cold dwarf planet is so active and to find a way to predict such activity for planets and other planetary bodies for which we have little information," Professor Jay Melosh, co-author of the study, noted. "The New Horizons mission has already provided an astounding amount of new information, and its surprises remind us how little we know about the far reaches of our Solar System and the depths of outer space beyond it. This is why these missions are so important."