Anyone for a game of interplanetary hopscotch?

The late astronomer Carl Sagan studied the planet Venus and put forward the idea that there, for all to see, was a view of a possible future for Earth. It's not a very pleasant future to look forward to.

Rainfall on Venus comes in the form of concentrated sulphuric acid. The surface pressure is 96 times higher than Earth's; almost all spacecraft sent to investigate Venus have been crushed by the pressure before they landed. It's hot too; about 462 [degrees] C. The atmosphere is mostly carbon dioxide with a little nitrogen gas thrown in.

Dr. Sagan said that Venus, being closer to the sun than Earth might have suffered from a runaway greenhouse effect. He warned that if we didn't curb our wasteful use of fossil fuels, we might trigger the same sort of catastrophe on our planet.

Science fiction writers took Dr. Sagan's idea and concocted stories around the notion that if Venus was our future, might it not also have been our past? Had distant ancestors of ours lived on Venus when it was a far more hospitable place? Had they fouled their environment so badly that they were forced to leave? And, had some of them travelled to their nearest habitable neighbour, Earth, and colonized it?

That's a pretty fanciful idea, but there is a nice symmetry to it.

Some scientists are warning us that we are fouling Earth's atmosphere so badly that it may become unlivable. They warn that we might trigger a runaway greenhouse effect here.

Isn't it interesting that probes have already been sent to our nearest neighbour, Mars? NASA has scheduled a manned mission to Mars for 2016, and there are more distant plans in the works for colonies to be established there.

Some people have even suggested bold schemes for altering the environment on Mars to make it habitable. At present, the atmosphere is thin and contains only a tiny amount of oxygen. But, Mars probably has water, the absolute essential for life as we understand it to exist. The water is all frozen now. But, what if we could establish plant life on the planet? It could gradually convert the atmosphere, which is 95% carbon dioxide, to one that has a higher oxygen content. A thicker atmosphere would mean a warmer planet. Frozen water might become liquid water. And, Mars might become the new frontier ready for future generations of colonists from Earth.