From Giovanni Schiaparelli to Robert Heinlein the notion of water-filled "canals" on the surface of Mars was a popular one in science and science fiction during some of the late 19th and early 20th century. By the time I discovered science (fiction), the hypothesis of straight canals with flowing water on Mars had long since been dispelled. By the time I began to study Mars, Mariner and Viking had already sent back images from the Martian surface that meant I didn't have to imagine it. I could just look up the images. As I looked at Martian landscapes apparently devoid of any visible water, the notion of streams with flowing water on Mars appeared quaint and naive.
Now, Curiosity has sent back images from Gale Crater that contain evidence of ancient, quick-flowing water on Mars. NASA's newest rover on the Red Planet found a tilted block of rock that is part of an ancient streambed, along with gravelly rocks whose shape and size indicate they were formed by having been tumbled in water, for a long time. The gravel is too heavy to have been shaped by wind. This is the first time that water-transported gravel has been found on Mars.
Evidence of an ancient streambed on Mars (left), compared to a similar formation on Earth
The size and shape of the gravel also provides clues about flow rate and duration: The water was between ankle- and hip-deep and flowed at a rate of 3 feet (1 meter) per second. That is a pretty brisk-moving stream. When water did flow on Mars in this location, it flowed for a very long time, a very long time ago.
We are certain to find additional clues as Curiosity continues its journey towards the Glenelg area of Mt. Sharp on Mars. In the meantime, I am left with a whole new, awesome dimension of Martian geological history. While Gale Crater was picked as the rover's landing site partly because orbital data indicated the presence of an alluvial flow, I have to ask: If Curiosity practically landed in an ancient Martian streambed, how common was water on Mars back when Gale Crater sported at least one knee-deep, briskly moving stream of water? Was it common or rare? Did the flowing water environment support any life? What happened to the planet between then and now? How did it change from an environment supporting liquid water to what it is today? Where did all that water go?
We may not have reliable answers to these questions for a while but the fact that we can now ask them and look for clues in data directly from the Martian surface is a spectacular achievement of planetary science.
For my coverage of Curiosity's landing on Mars from Planetfest 2012, click here and here
For my coverage of the NASA/JPL tweetup featuring Curiosity in June 2011, click here