>That's the reason Earth has kept its atmosphere while Mars did not
How would you explain Venus having an atmosphere while not generating a magnetic field?
How would you explain Venus having an atmosphere while not generating a magnetic field?
Even so, Venus has had most of its hydrogen stripped away over time. Thanks to its high temperature, that also includes hydrogens formerly bound in heavier molecules, like water. The process continues, very slowly. Venus is still losing its hydrogen and helium, more quickly than Earth.
One terraforming plan for Venus involves shipping magnesium hydride there, decomposing it, and using both the H2 and the Mg to react with all the CO2 in the atmosphere to make soot, chalk, and water, reducing the surface pressure to about 3 atm of mostly N2. Nothing works without shipping hydrogen to Venus. But once it's there, it will take many millions of years to get stripped away again, because that happens so slowly, over geologic time scales.
And honestly, if you have the resources to mine global amounts of magnesium hydride from the belt and ship them all the way to Venus you have the technology to build a nearly unlimited number of space colonies that would be much much nicer to live on than even a terraformed Venus.
However, I do agree that this colonize-Mars idea seems a little silly. We have a large body (the Moon) that's only a few days away, instead of 6-18 months; why not start there? Once we get pretty good at mining on the Moon and nearby asteroids, we should get better at building some rotating space stations.
What I feel would be missing for me is the scale and freedom of living on an earth like planet. It might be different if one is born and raised on a space station.
After 1000 years of dumb robots throwing foil-wrapped-dust-balloons at each other, and at Venus, a ton at a time, Earth throws some algae and extremophile vent bacteria at Venus. Then, after another 1000 years, you maybe have a planet ready to be landed on without immediately being crushed and burned by the hellish sky.
This is not mutually exclusive with building in-space colonies, or with terraforming other celestial bodies.
And we can build vacuum-breathing space robots now. Building a space habitat that can support an Earth-life ecosystem indefinitely is currently beyond our means, if not our abilities. Space robots playing catch is easier.
To terraform Venus's atmosphere with MgH2 in 1000 years, the robots in the belt and near Jupiter would have to process roughly 3.6e17 g per day, which is hundreds of billions of tons, hundreds of millions of times more than the entire Earth mines in a day.
You would need a lot of robots.
The project would have to be an AI capable of the following:
- Run for 1000 years without failure.
- Build robots that can build robot-building robots.
- Build robots that can build robots that mine S-type asteroids.
- Build robots that can build robots that mine hydrogen from Jupiter or its moons to make MgH2 and CaH2.
- Build robots that can build robots that patiently move bulk cargo around the solar system.
- Build robots that can build robots that electrolyze water in Venus orbit.
- Build robots that can build robots that defend and repair the project infrastructure.
- Build robots that can build robots that build and operate orbital shades, lenses, and mirrors.
- Fairly allocate the surplus by-products to human bidders.
The project would be a significant source of cheap aluminum, iron, nickel, silicon, sodium, potassium, and oxygen, as well as as much magnesium and calcium as is needed for ecosystem life support in space. Enough Ca and Mg to make all the bones of a billion animals is an insignificant fraction, compared to the amount required for terraforming.
The attention span of a human is already too short to spend 100 years moving tons of boring old rock to a rendezvous orbit. The goal has to be to set up a system that benefits humans to allow it to continue without interference, rather than one that requires cooperation.
In the asteroid belt there are some so-called Kirkwood gaps [1]. The main one is at 2.5 AU from the Sun; an asteroid orbiting at that distance is in a certain resonance with Jupiter (and with the Earth too) that makes the orbit unstable (on large time scales). The orbit becomes more and more eccentric, until it crosses Mars's orbit, and in time it gets close to Mars and gets sling-shot somewhere. There is a good chance you can nudge an asteroid close to this or another Kirkwood gap, and do it in such a way as to obtain the exact Mars slingshot that the asteroid ends up crashing into Venus. How can you nudge an asteroid? By ablation, e.g. [2]. You can use either a large parabolic mirror that orbits the asteroid, or a powerful laser. What timescales are we talking about? Asteroids in the Alinda group [3] have an orbital period of 4 years; if we manage to perturb one's orbit and execute the Martian slingshot in 10 periods, that's roughly half a century, which is in line with the mission length of Voyager.
[1] https://en.wikipedia.org/wiki/Kirkwood_gap [2] https://en.wikipedia.org/wiki/Asteroid_laser_ablation [3] https://en.wikipedia.org/wiki/Alinda_asteroid
And if the lost hydrogen is never replaced, the terraforming-added hydrogen will still likely last longer than human civilization. Venus has been losing H2 and He for billions of years, and still has some left to lose. If we add enough H2 to precipitate all the CO2 out as mountains of C and oceans of H2O, and cool off the planet enough that the hydrogen stays in the water, the atmosphere will be fine for the foreseeable future.
One basic Venus atmospheric-terraforming equation is "2 CO2 + MgH2 -> C + MgCO3 + H2O", by multiple reactions at different temperatures and pressures. By mass, 4.6e23 g CO2(g) + 1.4e23 g MgH2(s) -> 6.3e22 C(s) + 4.4e23 g MgCO3(s) + 9.4e22 g H2O(l).
That produces an ocean about 12% the mass of Earth's, but requires roughly 4% the mass of the asteroid belt in MgH2, or a sphere of solid MgH2 with diameter 570 km--about the same volume as Pallas, Vesta, or Hygeia (but half the mass). Magnesium is the 12th-most abundant element in the solar system, and could be mined from olivine-plagioclase asteroids. (Calcium can also be used to transport H2.) About 17% of asteroids are S-type. Seems doable.
Alternatively, decompose the MgH2 to Mg and H2 in Venus orbit, and only send down the H2, sending the Mg back out to store more hydrogen. That's "CO2 + 2 H2 -> C + 2 H2O". By mass, that's 4.6e23 g CO2(g) + 4.2e22 g H2(g) -> 1.3e23 C(s) + 3.8e23 g H2O(l), which would make an ocean about 28% as massive as Earth's, but requires that some magnesium (or calcium) make several round trips just to carry H2, which is relatively light.
If you're going to do that, you could also just ship H2O to Venus, electrolyze it in orbit with solar power, drop the H2 down the gravity well, and ship the O2 back out to the comet wranglers for breathing gas. You'd need to move exactly as much water into orbit as you're eventually creating on the surface, so 3.8e23 g H2O. That's more mass to ship than the MgH2 answer, but at least we know that the water is easily found in comets, and on Europa and Callisto, and easily processed.
There's no reason why any of these should be mutually exclusive.
Also, you have to mine and refine the rocks, so that the pure metals can be turned into hydrides. Ceres wouldn't have enough hydrogen just as itself.
It could still work as olivine to carbonate weathering. Cut Ceres up into pea-sized gravel, and drop it onto Venus, then use a solar shade to cool off Venus enough that metal carbonates don't immediately calcine into metal oxides. The exposed rock would suck up some of that CO2. No ocean, though.