The planet Mars has been in the news recently, because it is going to pass very close to us soon. So this might be a good time to talk about the Red Planet. The possibility of there being life on Mars has been a topic of speculation for more than a hundred and fifty years——ever since its “canals” were mapped by an Italian astronomer, Giovanni Schiparelli, back in 1877. He drew the first reasonably realistic map of Mars, and it included a system of “canali” across its surface. In Italian, “canali” just means “channels”——it doesn’t imply artificial structures at all——but the idea caught on, and it was gradually developed, with a lot of help from fertile imaginations, into the concept of a complex, planet-wide irrigation system. Although most serious astronomers did not buy into this, the idea of an Earth-like planet endured right up to the beginning of the Space Age, when Mars was still thought to have polar ice caps and a reasonable atmosphere. It also showed seasonal color changes that some thought could be some kind of primitive plant life blooming.

But in the 1960s, NASA’s Mariner missions sent back images of something very different, of a cratered, moon-like Mars. Both the polar caps and the atmosphere turned out to be almost pure CO2, and the density of its atmosphere was only one-hundredth of Earth’s. And the “blooming plant life” turned out to be only a lot of dust, blown around by strong seasonal winds. In some ways, though, Mars became more interesting. It had giant volcanos. It had a vast maze of canyons. And it showed evidence of having had flowing water on its surface sometime in its distant past.