THE giant canyons on Mars were carved not by water but by an icy equivalent of the rivers of ash that helped to destroy Pompeii, claims an Australian geologist.
If he's right, this would mean that Mars has been cold and dry-and probably lifeless-for the past 3.5 billion years, far longer than most scientists believe. "Mars has always been a place of people's imaginations," says Nick Hoffman of La Trobe University in Melbourne. "People want Mars to be an abode for Earth-like life forms."
Hoffman thinks "density flows", similar to the so-called pyroclastic flows of gas, ash and rubble that sometimes come from erupting volcanoes on Earth, eroded the surface of Mars. Such flows can travel at tremendous speeds and cover vast distances (New Scientist, 17 July 1999, p 36).
According to Hoffman, the collapse of unstable Martian terrain, such as steep-sided craters, released liquid carbon dioxide that had been locked away underground. The rapid pressure drop vaporised some of this liquid-just like releasing the valve on a CO2 fire extinguisher-forming clouds of gas and supercooled dry ice, as well as water ice, dust and rubble. These clouds swept downhill, carving immense channels without water.
"It's intriguing and fascinating, but I'm very sceptical," says planetary scientist Jay Melosh of the University of Arizona in Tucson, though he adds that the idea is worth testing in the lab. One problem, says Melosh, is that as gas bubbled off, the flows would run out of steam and be unable to carve channels as long as those on Mars.
But Hoffman disagrees. As the "cryoclastic" flow sped along, the supercooled CO2 particles would continue to vaporise, fuelling the process, he says.
Aaron Zent at NASA's Ames Research Center in Moffett Field, California, raises another concern. The channels on Mars end in wide flat expanses, with traces of a shoreline-just as if water had flooded a plain. "It's the flattest place in the Universe," says Zent. "You would have had to have done that with water."
Hoffman counters that density currents can create the same features. "On Earth, large density flows create characteristic wide, flat plains, just like the northern plains on Mars," he says.
What's more, says Hoffman, cryoclastic flows provide a simpler explanation for the giant channels on Mars's surface than water. "You'd need sufficient water to create walls 200 kilometres wide and at least 300 metres deep, and there isn't enough space below the surface of Mars to put it all. But 1 litre of liquid CO2 gives you 250 litres of gaseous CO2 to support a flow."
Author: Rachel Nowak, Melbourne
Source: Icarus (vol 146, page 326)
New Scientist issue: 5th August 2000
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