From Dune to Waterworld: Part I

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? Consider the science-fictional creations of Tenebra (Hal Clement) and Arrakis (Frank Herbert) – both orbit relatively close to their stars, Altair and Canopus, yet are strikingly different. Tenebra has retained its water, but as a super-critical atmosphere/ocean with an infernally hot surface temperature varying between 380-370oC. The pressure and gravity are crushing, but also the ground is unstable due to the highly reactive atmosphere – super-critical water – dissolving and recrystalising the rocks continually. Arrakis is hot, but not inhumanly so everywhere, and it is utterly dry, with only tiny ice-caps. Every scrap of water is conserved and, unless needed, stored up.

Surprisingly Arrakis/Dune isn’t absurd. A world with more land than sea – a world of, at most, disconnected lakes – is more stable against the higher levels of insolation that threaten to propel it into a super-torrid Greenhouse state like Tenebra, or Venus. Recent findings from Earth’s “Evil Twin” indicate distinct types of rock masses rather than uniform lava-plains – in otherwords, continental land-masses. Thus Venus may once have been more like Earth, with Earth-like geological processes making Earth-like land. Then it “died” in a planetary autoclave as its oceans evaporated in what’s called a “runaway greenhouse” effect – a rise in water vapour causes more heat retention, producing even more water vapour and even more heat… until it’s all steam and the planet cooks. This runaway is thought to occur when the heat from the Sun is 30-40% higher than what Earth currently receives, though that’s complicated somewhat by more reflective cloud masses forming.

But what happens when there’s not enough water to runaway? Hot, but reflective desert plains, can bounce a fair bit of heat away, and the lack of water means things never runaway like on Venus or Tenebra. According to work by Yutaka Abe and colleagues such a “Land-Planet” can remain hospitable, in part, at up to 70% more heat input from the Sun.

At the other end of the heat-scale does water or desert make a difference? see Part II

4 thoughts on “From Dune to Waterworld: Part I

  1. The habitable zone is based on more than just the distance from the primary. The thicker the atmosphere and the more water there is to the planet, the further out the habitable zone for that planet will be. If Mars was the size of Earth and had an atmospheric pressure of, say, 2-3 atmospheres, it might even be a warmer planet than our own Earth, despite being further out from the sun! Perhaps there is some calculation and a graph that shows the correlation of the habitable zone of a planet with its atmospheric pressure and quantity of water.

  2. Hi kurt9

    Indeed. That’s Part II’s discussion. I largely agree with what you say, though with a couple of provisos. Firstly, carbon dioxide clouds start forming beyond about 1.4 AU and their stability and warming/cooling effects are somewhat uncertain. Also too much water vapour can result in runaway glaciation – at Earth’s current atmospheric composition that happens not much past 1.05-1.1 AU. Something other than CO2 is needed.

    Secondly, too much CO2 and the atmosphere becomes liable to collapse into a liquid CO2 ocean. Dramatic to be sure, but possible.

    Finally, planets with different rotational regimes might not be able to sustain a greenhouse at all. A tide-locked Venus, for example, might be unstable against atmospheric collapse with the CO2 being locked up as carbonates – or clathrate on the dark side if there’s enough water.

  3. Just out of interest – how would the evaporation of water (where water molecules, if high enough in the atmosphere, are split by outside radiation, and the hydrogen escapes into open space) fit into this picture? This would work best for smaller bodies with less gravity, and may eventually reduce the runaway greenhouse effect – thus producing a colder body with little water.

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