Sinuous valley networks like these imaged by Viking 1 led many
planetary scientists to think Mars’ climate once was warm and
wet, but some researchers aren’t so sure. NASA/JPL
Today marked the 114th consecutive day in Orlando in which the temperature never dipped below 70°. I'm currently sweating in Orlando, attending the annual meeting of the American Astronomical Society's Division for Planetary Sciences (DPS). Florida's warm, humid weather serves as a perfect backdrop for an ongoing debate among planetary scientists: Was Mars' climate once warm and wet, or cold and dry?
Ask planetary scientists what they think, and a firm majority would answer: "warm and wet." But like the cold front now barreling toward central Florida, a change seems to be in the air.
Two key lines of evidence point to a warm and wet climate in Mars' distant past. First, most ancient martian impact craters appear degraded far more than their fresh counterparts. Erosion by some source stronger than the planet's ubiquitous winds seems needed. Second, the ancient martian highlands are full of sinuous valley networks that look for all the world like they were carved by flowing water. The standard interpretation: Mars once possessed an atmosphere so warm and dense that liquid water could flow on the surface for up to hundreds of miles.
But the pendulum may be swinging the other way. Recent observations of gullies made with the Mars Reconnaissance Orbiter spacecraft seem to show liquid water being less important in the past than thought. And at the DPS meeting, planetary scientist Pascal Lee of the SETI Institute showed many features in Earth's polar regions that look similar to those on Mars.
Lee and his colleagues have surveyed the 40-million-year-old Haughton Crater and its surrounding terrain on Devon Island in Canada's far north. They find craters degrade far faster than previously thought, reducing the importance ofwater erosion in Mars' past. And they also see valley networks on Devon Island that look just like those on Mars, but which formed by localized melting beneath the island's glaciers, not on the surface.
Now don't get too depressed. Water certainly exists as ice on Mars today, and just as certainly existed as a liquid in the past — the Mars Exploration Rovers proved that to everyone's satisfaction. The key question now is whether Mars' climate was warm and wet enough to support flowing water on the surface, or if only the subsurface was warm and wet for relatively brief periods.
And as for the possibility of life? Even Lee thinks the Red Planet remains a nice place to search for it.