Over the past decade (roughly) as more visual and on-site evidence has been gathered, it’s become accepted that Mars had a ‘water era,’ a time when liquid water was relatively common on the surface of the planet. Liquid water is now long-gone from the surface and what didn’t escape into the atmosphere is now trapped as ices at the poles or under layers of soil. A provocative question remains: How long did significant amounts of liquid water exist on the Martian surface? It’s provocative because the answer might be, “Long enough to harbor life.” New research carried out by Nathalie Cabrol, a member of the NASA Astrobiology Institute (NAI) and a principal investigator with the SETI Institute, proposes that Mars had lakes as recently as 300 million years ago, and that surface water may have existed at least periodically much longer than previously thought.
Cabrol studied images from the Mars Viking spacecraft, and concluded that Mars had numerous lakes located within a band of 20 degrees on either side of the equator. She was able to assign age estimates to the lakes based on the evidence of cratering; essentially, areas with more craters are older. Some of the lakebeds have so few craters; she believes them to be only 300 million years old. (Not very old in geological terms.)
Another key to Cabrol’s research is the evident existence of deltas in Martian lakebeds. A delta is the ‘fan’ of sediment typical of inflowing streams and other water movement. Such deltas are common on Earth and they typically require 10,000 to 100,000 years to form. Their presence in Martian lakes may indicate these lakes existed for at least that long.
These deltaic structures also provide clues about whether the deposits formed at the surface of a lake, under water or, as Cabrol believes occurred in Gusev Crater, under ice. “These things have been studied for a long time on Earth, for example by oil companies. So we have a lot of information about delta morphologies and what they mean in terms of hydrological processes,” says Cabrol.
If life once existed on Mars, argues Cabrol, ancient lakebeds would be a likely place to look for its signature. Lakebeds that show evidence of continuous or episodic flow over many thousands of years would be particularly good candidates.
So, we have speculation – with adduced evidence – about the long-term existence of lakes. Then further speculation that the Martian lakes were good places for life to develop.
There is some corroboration for Cabrol’s findings. One of her colleagues at the NASA Ames Laboratory, Robert Haberle a climatologist, believes that the area where the Martian lakes existed could have had conditions necessary for snow and melting in the relatively recent past. If his hypothesis can be substantiated, it would help explain the existence of young Martian lakes.
The findings by Cabrol will be scrutinized (and probably challenged) by astrogeologists. After all, researchers are still looking at images and geologists are inclined to be skeptical about ‘evidence’ garnered at the edge of a camera’s resolution. Close inspection, preferably eyeballs on-site, when combined with chemical analysis of samples is much more convincing, but unfortunately won’t be part of the Mars geology kit anytime soon.
Year by year, finding by finding, the circumstantial evidence for ‘conditions favorable to life’ are reported, not only on Mars but also some of the moons of Jupiter (Europa, Ganymede) and Saturn (Titan). There is almost enough such evidence where it could be said that if no life is found, a really good explanation for why not will be needed.