IT NEVER rained on the Martian plains, or anywhere else on the Red Planet. In
fact, the latest high-resolution images from the Mars Global Surveyor spacecraft
suggest that the early Martian climate was a lot less similar to Earth鈥檚 than
researchers had thought.
Since 1972, when NASA鈥檚 Mariner 9 probe sent back pictures of what appeared
to be dried-up river beds, planetary scientists have believed that water once
flowed on the surface of Mars. And if Mars was once warm and wet, there might
have been a cycle of evaporation and condensation that caused rain.
But images from the Mars Global Surveyor seem to rule this out, says Mike
Malin of Malin Space Science Systems in San Diego. Rivers fed by rainfall have a
distinctive pattern of root-like tributaries that are easily visible from space.
鈥淲e just don鈥檛 see this in the high-resolution images,鈥� Malin told the AAAS.
Advertisement
Malin concedes that rain-fed tributaries could be hidden by drifting sand,
but thinks this unlikely. Instead, he suggests that the channels on Mars were
carved by water that welled up from inside the planet in one-off floods.
Other evidence supports this idea. Over the centuries, the course of a river
tends to change, Malin explains, and it is often possible to see where recent
channels cross older ones. But nothing like this is apparent on Mars. 鈥淭his
suggests that the features were carved by one-shot floods,鈥� he says.