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