[ExI] Double-Earth (Was: kepler study says 8.8e9 earthlike planets)

Sun Nov 17 16:59:24 UTC 2013

This deep vents life we know on Earth, needs oxygen. It gets it from green
plants in our case. Worth to remember.

On Sun, Nov 17, 2013 at 5:33 PM, Anders Sandberg <anders at aleph.se> wrote:

> OK, now I have the time to run my code, based on what I have gathered from
> the exoplanet literature. Welcome to dry and wet Double-Earth:
>
> A lot hinges on whether we assume Double-Earth started out beyond the
> iceline and moved inwards, in which case it will be really wet, or started
> out close to the sun and never got much volatiles. In the first case the
> mass will be about 3 Earths and the average density 37% of Earth, with a
> surface gravity of 0.73 g and an escape velocity of 13.6 km/s. This will
> have an ocean hundreds of kilometres deep, surrounding a rocky core covered
> with high pressure warm ices. In the second case the mass will be 15
> Earths, the density will be 167%, gravity 3.4 g and escape velocity 30 km/s.
>
> Note that if the component rock contributes water like on Earth, a planet
> with 15 times the mass but only 4 times the area will have 3.75 times
> deeper hydrosphere, assuming everything equal.  That means 16 km deep
> oceans - "dry" Double-Earth might still be a waterworld.
>
> Now we need to make some guesses at atmosphere and temperatures. The basic
> temperature for a greybody with Earthlike albedo at this orbital distance
> (1 AU around a sun-like star) is 250 K, if we add the 36 K greenhouse
> correction of Earth this becomes 13 degrees C average.
>
> In the wet case the scale height is 11.3 km - air pressure will be 36%
> less at this altitude. The temperature needed for a molecular species to
> escape is 1.49 times on Earth: in this case hydrogen certainly escapes and
> I think helium will escape (it depends on the exosphere temperature,
> something that is hard to calculate). Methane and ammonia could be
> retained, but if there is life and oxygen they will have been turned into
> carbon dioxide and nitrogen.
>
> In the dry case scale height is 2.4 km: clouds will be very low. The
> retention temperature is 7.5 times Earth - dry Double-Earth could
> potentially retain hydrogen gas. This means that potentially it could have
> gathered a much denser atmosphere from the beginning, potentially turning
> into a gas giant. Note however that by assumption we had it form in the
> "dry" zone near the star, so it might not have accumulated that much. We
> should still expect it to have a denser atmosphere than the wet case.
>
> If we make the assumption that the surface pressure is proportional to
> surface gravity (might make sense in this particular case) wet Double-Earth
> has surface pressure 0.73 and dry Double-Earth surface pressure 3.4
> atmospheres. Let's also assume the mean wind speed is a terrestrial 10 m/s
> - again, this is hard to evaluate without running a full model. Finally,
> most doubtfully, let's assume the rotation period is 24 hours.
>
> In this case wet Double-Earth gets air density 0.9 Earths. The radiative
> timescale of 1.8 days and advection timescale of 1.4 days - this means that
> the weather is complex like on Earth, and responds rather quickly to
> seasons (ah, I implicitly assumed an Earth-like axial tilt: things will get
> really strange if it is more extreme). Wet will have about 9-10 jet-streams
> (Earth has about 7). Dry Double-Earth instead has surface air density is
> 4.3 times Earth, fast timescales and 10 jet streams. Not too alien.
>
> Weather is partially driven by buoyancy. On wet Double-Earth this is
> weaker: clouds will be taller and move more ponderously, while on dry
> Double-Earth the higher gravity will make small density differences
> generate more force: flatter, more intense convection. The strength of
> hurricanes depends on the temperature difference between the ocean and the
> stratosphere; I do not know how to calculate this, but I note that in the
> absence of land they can run much longer before drifting too far towards
> the poles that they dissipate. I am a bit uncertain about whether
> latitudinal mixing is strong enough to keep the poles too warm to form ice
> sheets or not. I suspect the lack of land will destabilize ice.
>
> If we assume 20% oxygen, then dry Double-Earth will have 537 mmHg partial
> pressure oxygen - toxic to humans. Even worse, the partial pressure of CO2
> will be 10.4 mmHg - causing hypercapnia in humans. Still, local life could
> likely evolve to handle that with little problem. Wet Double-Earth looks
> pretty OK for humans.
>
> The radiogenic heating (assuming an Earthlike composition) of dry
> Double-Earth is 3.34 times higher than on Earth, 0.29 W/m^2. Still not
> enough to melt the crust into an Io-like volcanic mess, but it is far more
> active. Wet Double-Earth will have less radiogenic heating than Earth,
> although this is complicated by the very different composition. I still get
> continental drift (hence churning the ice crust), but it is not as vigorous
> and it will stop earlier (reducing convection in the deep ocean, likely
> strongly reducing the available minerals to life).
>
> Mountains on wet Double-Earth will tend to be 1.37 times taller than on
> Earth, but they will all be on the bottom of the super-deep ocean. On dry
> Double-Earth they will be just 0.29 times the height - the local Mount
> Everest will be just 2.4 km. Given my guess at mean ocean depths, this
> means that it will indeed be a waterworld.
>
> If one buys the idea that Coriolis-Lorenz dynamos in the core scales as
> sqrt(density/period) the magnetic field of wet Double-Earth will be 60% of
> Earths, while dry Double-Earth 130% - not an enormous difference, although
> wet Double-Earth will be less protected. Note that it has an enormous store
> of volatiles to bleed off, though. The higher cosmic radiation might be
> nasty for beings on the surface, but the water absorbs it fine.
>
> The optical depth of the atmospheres on the Double-Earths will be the same
> as on Earth (because of my assumption of pressure = surface gravity), so
> you can see the same distance. The vertical optical depth is 1.37 times
> more than Earth on wet Double-Earth: the sky is more milky, but not too
> alien. On dry Double-Earth it is just 1.1: almost normal. If you were to
> fly a plane, it would however turn dark blue at a much lower altitude.
>
> On the oceans, waves would be moving differently. On wet Double-Earth they
> would move at 85% of Earth speed, while on dry Double-Earth 184%. The
> height would of course scale inversely with gravity: 136% on wet
> Double-Earth, but just 29% on dry Double-Earth. So the seas would be
> choppier but slower in the wet case (but the waves will have more energy
> per square meter), while the dry case would have fast low swells.
>
> To sum up: both Double-Earths are waterworlds, but one is *deep*. Neither
> has any land. Both might have interesting deep sea vent ecologies, the wet
> case around vents in the high pressure ice and the dry case more
> terrestrial-style vents (which are more common than on Earth). On the
> surface the ocean has weather like on Earth, either strangely tall or
> fiercely squat clouds. Life could probably thrive on both worlds, but would
> be limited by minerals: no land, no surface weathering, and hence less
> minerals added to the oceans. Getting into space from wet Double-Earth is
> about as tough as on Earth, while dry Double-Earth is pretty tough to get
> away from.
>
> --
> Dr Anders Sandberg
> Future of Humanity Institute
> Oxford Martin School
> Oxford University
>
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