Some scientists hope to terraform Mars, turning it into a blue world with streams, green fields and fresh breezes and ultimately providing mankind with a new home.

But first, they would need to thicken its atmosphere and heat up the planet so ice trapped in the Martian soil would melt and be used to sustain agriculture. One proposal: place a large mirror many miles in diameter in orbit above Mars to focus the Sun’s rays onto a polar icecap, melting it and releasing its frozen carbon dioxide, which would then trigger greenhouse heating.

Downside: it might wipe out indigenous lifeforms in the soil.