Astronomer Carl Sagan, in the 1960s and 70s, detailed how this might be done both with Venus and Mars. For Venus, a planet with a greenhouse problem far worse than the Earth’s, Sagan suggested seeding its atmosphere with algae, which would lock water, nitrogen and carbon dioxide into various organic compounds. As carbon dioxide was removed from the atmosphere, temperatures would drop to “comfortable” levels. Unfortunately, later findings showed that the clouds on Venus are far thicker than previously believed, and they contain such high levels of sulfuric acid that algae would not be likely to survive.
The problem on Mars is the opposite: terraforming the red planet would require building up a suitable atmosphere and warming it. NASA was sufficiently intrigued with the notion of making Mars habitable that it held several conferences following up on Sagan’s ideas. The general idea for Mars is to unlock the carbon dioxide in the planet’s ice cap in order to increase its atmospheric pressure, and then to use phytoplankton to create a balanced atmosphere. As the planet warmed up, its frozen reserves of water would become accessible, allowing for the introduction of plants.
While re-engineering Mars to suit human purposes is theoretically more feasible than dealing with Venus, the effort involved would be enormous, since vast quantities of materials would have to be brought to the planet. Depending on the methodology, it would require the mining of ammonia on asteroids, methane or other hydrocarbons on Saturn’s moon Titan, or the mining of fluorine on Mars itself. There are also suggestions for space-based mirrors and the diversion of small asteroids to crash into the Martian surface.
