"Plans have been proposed to change Mars and Venus, the nearest planets, into worlds like Earth - even though Venus is so hot that lead melts there, and Mars is so cold that its air freezes in winter and falls as snow. To alter whole worlds, we would employ the tiniest of living things - minute plans called algae."
"Special new kinds of algae would be bred to be resistant to the conditions on Venus and Mars. Huge quantities would then be sent to the planets. On Venus the algae would convert the atmosphere of carbon dioxide into oxygen. Water could come from ice-bearing comets diverted to Venus. The temperature would fall until it was cool enough for people to land and begin making a new home there. On Mars the temperature would need to be raised by using the planets to darken and warm the white ice caps. The ice would melt and moisten the soil, releasing oxygen into the thin atmosphere. As the air thickened, it would get even warmer."
Ardley, N., (1981) "Out into Space (World of Tomorrow)", Franklin Watts