If you read this column yesterday, you may recall that it began with lines from Julian Lennon's song Saltwater. . That same lyric continues with:
We light the deepest ocean, send photographs of Mars;
We're so enchanted by how clever we are.
But despite our cleverness, our photographs have yet to provide the answer to a quasi-crucial question: is there water, or is there not on Mars? It's a long story.
The issue first arose in 1877, when Giovanni Schiaparelli, observing the red planet through his telescope, noticed a network of straight lines which he called canali. . More imaginative and adventurous souls decided that these were irrigation channels, dug by an intelligent race of Martians to transport water from the melting polar ice-caps to the more habitable, but arid, lower latitudes. Later observers looked at their pictures more closely, and exposed the "canals" as mere natural features transformed by tricks of Martian light. There was never any water on the planet Mars.
Then in 1972, the spacecraft Mariner 9 took photographs, and the images clearly showed what could well be giant river valleys, or canyons channelled out by floods. But since Mariner 9 also told us the average temperature on Mars was something like minus 50 degrees, it was obvious they were not of recent origin. Perhaps there may have been water on Mars about a billion years ago, when the planet was warmer than it is at present, but not now.
More photographs from Mars came earlier this year from Global Challenger, and turned all previous theories on their heads. They clearly showed deep gullies on the sides of many Martian craters, several kilometres long with piles of debris at the bottom, and closely resembling earthly gullies known to have been formed by water. Crucially, none of them had been there long enough to be scarred by craters or shrouded in wind-blown sand; they were obviously gouged out by some "relatively recent" flood. Perhaps there was water on Mars a mere million years ago?
And now some suggest that maybe Mars had water almost yesterday. Could it be, they ask, that there is a layer of liquid water, say, 100 metres underneath the Martian surface? Perhaps it is kept liquid by pressure from the rock above and when, here and there, it was exposed at a crater cliff-face caused by meteor impact, the water quickly froze to provide a kind of bung. But now and then, perhaps, the pressure for some reason rises deep inside and blows the bung, and a flash flood, maybe, carves a channel down the cliff?
Clever as we are, we need more photographs to know the answer.