Was it Ever Warm(er) and Wet on Mars ?
Mars is getting very close attention right now because of that planet’s extraordinary proximity to Earth, plus the unprecedented information which space missions have been sending back. This is the third article I’ve written in the past twelve months about water on Mars, because liquid water is essential to life as we know it, the starting point for astrobiology. The earlier columns (May ’03 & July ’02) summarized data from spacecraft suggesting that a sort of permafrost (roughly 50% ice by volume) may lie just beneath the dry surface soil of much of the frigid planet today.
Since the Viking missions in the ‘70s, images from orbiting cameras have shown Martian surface features that would be interpreted as evidence of erosion by flowing water if they were found on Earth. Debate continues as to whether these were carved by fluvial runoff (in a rainy climate) or by a process, called sapping, in which spring water eats away at the rock face where it emerges from the ground (in a cold-dry climate).
Mars Global Surveyor, which arrived at Mars in 1997, carries a laser altimeter (MOLA) that has bounced light off almost 700 million spots on the planet’s surface and generated topographic maps accurate to within a few meters of altitude. Scientists at the Lunar and Planetary Institute in Houston fed the MOLA data into a model which “drizzles computational rain on a terrain and watches the virtual run-off.” Surfaces previously eroded by running water, such as unglaciated terrains on Earth, show patterns quite unlike lunar surfaces, never touched by water. It looks like Martian topography lies between terrestrial and lunar landscapes. “Maybe there was some rain on Mars, to a small degree; it’s a mix of fluvial erosion and cratering, but there was never enough sustained rainfall to dominate the heavy cratering of 3.9 billion years ago.”
Planetary scientists at Cal Tech and MIT find that “the shapes of the main channels of Martian drainage basins are flatter and less concave than rain-fed channels on Earth and have frequent abrupt drops that indicate flowing water did not have a chance to thoroughly reshape the initial form of the land.” However, they are still inclined to interpret the data as supporting a limited amount of precipitation, rather than just erosion by sapping. Others report that some Martian valley networks appear to originate near ridge crests and crater rims, where it’s unlikely that enough underground water could accumulate to create springs. However, there seems to be little support for any prolonged warm-wet period.
The news report in Science (June 6, pp. 1496-7), from which I composed this synopsis, includes speculations about how intervals of precipitation and liquid water-flow might have been initiated in the course of Martian history by asteroid impacts, by planetary tilting, and/or other events. I’d be glad to provide anyone interested with a copy, and the journal Science is also available at the Bainbridge library.
Bill O’Neill (firstname.lastname@example.org)