Hi All

Nick wrote a good critique, but I wasn't trying to write fantasy. Let me justify my speculations...

NICK: Your model is the reverse of this, but still based on the same fiction of sequential development of habitable worlds.

...well there was a sequence - first Venus was wet and Earth was frozen, now Venus is dry and Earth is wet. Sequence enough for me.

NICK: There is some small shred of validity in the model of Venus being almost Earth-like in its youth and rapidly changing as the sun warmed until its atmosphere experienced a catastrophic runaway greenhouse event that boiled its oceans dry and even stripped its atmosphere of hydrogen.

...Mark Bullock and David Grinspoon think the wet phase of Venus could have lasted ~ 2 Gyr thanks to clouds. Modellers have previously avoided the role of cloud cover and most acknowledge their effect would be significant, just hard to quantify. The Sun was still only 82% of its current luminosity 2 Gyr after it hit the Main Sequence, hence insolation of Venus was ~1.56 Earth and I have read cloud models that enabled a stable climate up to ~ 1.5 times Earth's insolation. After that Venus dried out but perhaps not as rapidly as usually modelled, say over 2 Gyr - 3 Gyr after formation. With Bullock and Grinspoon I think Venus has had only one Great Transition, leading up to the resurfacing event at ~ 0.8 Gya. How long would stagnant lid volcanism have to build up for the resurfacing to occur?

NICK: What is not at all probable is some fantasy of C.S. Lewis-style Cytherians evolving to a higher plane and escaping the fate of their planet.

...did I say that? They engineered their way out of it, just like we'll have to. With so much free oxygen Venus would be a very odd place. Firstly, the biosphere wouldn't need plants to make oxygen. Secondly, perhaps to dump excess oxygen creatures might make and store peroxides. Imagine creatures using hydrogen peroxide as a monopropellant for bursts of real speed...

NICK: There just wasn't enough time between Venus stabilising and boiling.

...Earth went from the first low number clusters of formerly single-celled eukaryotes to us in ~ 600 Myr. A lot of researchers into Archean and Proterozoic biology are looking at geochemical causes as to why life stayed single-celled for so long. Archaea conceivably appeared ~ 3.8 Gya, then probable prokarya ~ 3.5 Gya, and eukarya traces ~ 2.7 gya. Then nothing much for 2 Gyr. A common scientific speculation - and most of the field is just that with such meagre remains - is that geochemistry limited oxygen levels and thus limited what life could do. When the Icehouse Earth finally came about, the long climb in O2 levels had just started to reach the point where life could take off into multi-cellular complexity.

...Also point taken about temperature and I wasn't advocating ultra-extreme biochemistry - though it's a big unknown still, the limits of the possible. We haven't seen multi-cellular extremophiles perhaps because the size of the available hot habitats have declined to small ephemeral pockets. But like I said this was more fiction and free speculation, something to think about. Venus is an odd world and so is Iapetus - weaving them together into a story is fun and hopefully illuminating. We must prepare ourselves for finding alien remains on other planets as our knowledge of the planets explodes thanks to orbiter missions.

Adam