From Dune to Waterworld: Part II

Fire and Ice, Venus and Mars, are just beyond the limits of the Habitable Zone in our solar system. How might worlds turn out differently, on the Outer Fringe….

Good question. People have noted before now that Mars and Venus seem to be in the wrong places – if Mars was in Venus’ orbit and Venus in Mars’ then they both might be habitable. Or at least able to have liquid water on their surfaces.

Would they? Venus has about 1000 tons of carbon dioxide sitting on every square metre of its terrain, much like an ocean worth of the stuff. If the planet cooled to its effective temperature, 235 K (-38^oC), then the CO₂ would become unstable against condensation. Its liquid range is between ~-79^oC and 31^oC at ~5 and 74 bars respectively. Thus it really would form an ocean if the planet was cool enough as Raymond Pierrehumbert points out…

Atmospheric collapse on Venus-like planets orbiting M-dwarfs

…thus the inspiration for this piece. Venus is, technically, an Ocean Planet already with an ocean of supercritical CO₂ below the 74 bar pressure line and some enzymes will work quite happily in supercritical CO₂. A cooler Venus, with a deep CO₂ ocean, could conceivably host some form of life, albeit without the lipid bilayer cell walls terrestrial life employs, as they require polar solvents to form. Carbon dioxide is definitely non-polar.

What of oceans more like our own? A real Ocean Planet will probably form a Snowball beyond a certain distance from its Sun without some super-strength Greenhouse gases. Why so? The trick is to do with the albedo feedback mechanism – basically what happens is that a bit of snow forms sea-ice increasing the planet’s reflectivity (albedo), thus reducing the amount of heat reaching the sea, thus forming more sea-ice… and before you know it the whole sea is locked in ice right down to the equator. It happened to Earth several times during the late Proterozoic and would happen on any Earth-ish planet just a bit further out from the Sun. Without serious greenhouse gases that is.

Carbon dioxide isn’t bad, but it forms high albedo clouds of ice crystals when the planet is creeping out to ~1.3-1.4 AU. If such clouds covered a planet 100% then they’d be a fantastic greenhouse heat-trapping layer – Pierrehumbert’s early work on them gave Mars a toasty 25^oC surface even when the Sun was at 70% of its current luminosity. Problem is they probably can’t give total coverage, thus reflecting away more than they can trap.

Methane is a better gas at trapping heat, but it has a limitation: the anti-Greenhouse effect. This is Titan’s problem – a world-girdling high-altitude haze layer of ‘polymerised’ methane by-products which stops the light/heat from getting to the ground. Simulation work by James Kasting et.al. shows that the haze kicks in when the CO₂/CH₄ concentration ratio approaches unity.

Ammonia is even better at trapping heat, but just doesn’t last under the UV bombardment from a G2 star. A cooler star, with a lower UV output, would allow the stuff to survive, but into red-dwarf territory there’s too much UV/x-rays from flares, at least around younger red-dwarfs.

So what to do? Surprisingly a decent greenhouse can be achieved with hydrogen/helium…

Ocean-bearing planets near the ice line: How far does the water’s edge go?

…in fact with more than 200 bar of H/He the surface is too hot for liquid water at all. The planet just doesn’t cool enough. So how do you get it cool enough?

Take away the Sun… Part III