Cryovolcanism on Charon and Beyond…

LPSC 38 2007 Cryovolcanism on Charon.

(With thanks to Paul Gilster for asking Cryovolcanism on Charon? over at Centauri Dreams )

Who would’ve expected liquid water out around Neptune and beyond? Not on the surface of any object, of course. That’d be stretching physics just a bit too much. Underground. Deep under an insulating blanket of ices and dust. But how deep? Just how big does an object have to be to keep a liquid mantle of (probably) ammonia-water sloshing around? According to the study above as low as 600 km in radius is enough. That means a multitude of Dark Abyssal Oceans exist in the swarm of Dwarf Planets astronomers estimate to exist in the Edgeworth-Kuiper Belt, the Scattered Disk and probably the Oort Cloud too.

Is surface water in liquid form really so crazy though? A deep hydrogen/helium atmosphere probably got captured by anything bigger than Mars in the Outer System as the Gas Giants were forming. In theory such an atmosphere could keep the heat in quite efficiently. But just how much geothermal heat would the planet need to be putting out? Too cold and the planet’s atmosphere will start condensing in its outer reaches, causing runaway collapse of the hydrogen into vast, COLD seas of the stuff. That’s a plus if the deuterium freezes out, but the protium doesn’t – but it’s not the sort of sea I want. So, I’d expect, the planet will need to be hotter than hydrogen’s critical point, 33 K. That means the internal heat-flow outwards is comparable to Earth’s roughly 80 mW/m² (a temperature of 34.46 K in absence of solar heating) which may be feasible if it’s mostly rock. Ice would dilute things too much. A binary object with a high eccentricity might also be warmed to sufficient levels by tidal heating too.

The lesson is to not rule anything out. We don’t know that much yet.