starslayer wrote:Sikon wrote:Although other parts of terraforming like getting rid of the bulk of the extra atmospheric mass are hard to do at all, the cooling the atmosphere part could occur in a timeframe of centuries if sunlight was mostly prevented from reaching it during that time, though even that is pretty long for a human project. You're apparently worrying more about timeframes of millions to billions of years.
The reason I'm doing so is that if we have seen fit to terraform Venus, of all places, then we would be expecting a very large return of investment, which implies these extreme time scales. After all, even a relatively god-like Clarke civilization probably simply wouldn't bother if it couldn't use what it had spent its effort on for a very, very long time. And if we really need the living space, the problem probably isn't one that will be going away anytime soon. Being able to use Venus for only a few millenia to a few million years wouldn't be worth it, IMO.
The atmospheric mass loss during the first million years is rather low, like how that on Mars was substantial over billions of years but much less over an orders-of-magnitude shorter timeframe, such as
this implies.
But if they wanted to set a magnetic field up even from the start, the manufacturing requirements are relatively small in this context anyway. Quick order-of-magnitude calculations indicate that the equivalent of less than a billion amps in a single loop of superconducting cable or rather such instead spread out over many cables with proportionally less current in each could cause a large fraction of a gauss over a volume around the planet. Such would be more or less comparable to earth's magnetic field strength. With the various cables carrying up to at least a significant fraction of a MA/cm^2 current density, much less than a billion tons of superconducting cable mass could suffice.
Such superconducting cables would be many orders of magnitude more efficient usage of resources than setting up a natural dynamo of molten iron convection in a planetary core, since the former involves a fraction of a billion tons of cables, instead of dealing with a substantial portion of the planet's 500 trillion billion ton mass.
However, in any case, considering astronomically long timeframes would introduce a whole new can of worms.
Over the eons, the likelihood of terraforming still focused towards creating an environment for a facsimile of current planetary human civilization could decrease. I'm not a big fan of how sci-fi will often show a species as having advanced greatly in some technological regards yet somehow be assumed to have their own bodies be the same after a million or a billion years as pre-civilization. All sorts of assumptions can be thrown into question.
As just one of many possible examples, is near 1 atm atmospheric pressure really needed? Deep sea divers even now can temporarily breathe at least tens of atmospheres pressure in the case of a limited partial pressure of O2 combined with primarily helium, and surely there would be advancement in genetic engineering of plants and animals before millions of years from now, aside from more likely progression to postbiological bodies anyway.
Meanwhile, in the bigger picture, although an interesting curiosity, terraforming is probably more of a doubtful temporary diversion if it ever occurs than the overall future. In early centuries, it is more difficult than building space habitats, but partial measures might be considered a substantial increase to living space (although with a better chance of being practical or done on Mars than Venus by far). However, over really long timeframes such as ones with self-replicating factories, there's relatively quite limited benefit. A planetary surface is small compared to disassembly of asteroids, small moons, larger moons, and so on allowing up to literally millions of times more area of artificial constructions. Having 5 billion trillion tons locked up in planetary mass seems increasingly inefficient to utilize mostly just that within tens of meters of its surface, under 1/100000th of its total mass.
In timeframes of millions to billions of years, a civilization could even colonize billions of stars throughout the 0.1-million light-year diameter galaxy. If anything, it's too easy to underestimate change over the long term, like over a timeframe orders of magnitude longer than that over which the change from living in caves to New York City occurred.
