There is a strong similarity between the present surge of interest in Water on Mars, and the turn-of-the-(last)-century events surrounding Schiaparelli and Lowell and the infamous canals on Mars. For those unfamiliar with with the story of canali and canals, an excellent description of the observational history of Mars is online at the University of Arizona Press. Follow this link for the Text. When Schiaparelli used the word canali, he meant channels, but this was slightly mistranslated as canals.

With the benefits of hindsight we can now say that Schiaparelli and Lowell overinterpreted the telescopic images of Mars and, although they earnestly believed that they were seeing thin lines across the face of Mars, they were mistaken. From this "observation" Lowell went on to deduce seasonal growth patterns of plants and hence life on Mars. At its most extreme, the evocative story described a doomed civilisation on a dying planet vainly trying to stave off the encroachment of the deserts. Although we now know this to be pure speculation, the idea of life on our neighbouring planets is a strong attractor for human thought. From H.G. Wells' "War of the Worlds" through Edgar Rice Burrough's Barsoom to 'Little Green Men', the concept of intelligent life on Mars has been thoroughly explored.

During the 20th Century, improved observations of Mars, culminating in spacecraft flybys revealed that Mars is distinctly un-Earthlike. The atmosphere is cold and dry, and extremely thin (less than 1% of Earth's) and composed almost entirely of CO2. Large areas of the planet are heavily cratered, like the Moon, implying that the surface has remained sterile and inactive for billions of years. At around this point, the fortunes of life on Mars reached an all-time low. Without water, how could there be life?

Then the new wild card came into play. Continued spacecraft observations revealed real channels on Mars. The scale of these channels was huge, and with the natural assumption that they were eroded by floods of liquid water, suddenly Mars became a more attractive host for life. The reasoning was simple. If Mars had water flowing in the past, then life could have, indeed should have, evolved. The present cold dry state of Mars must be some sort of long dormancy during an aeons-long planetary "winter". When springtime comes again, perhaps Mars will bloom and water will flow again?

There are two types of erosional features, of rather different ages and geometry. Firstly, in the very old Noachian (~3.8 to 3.5 Ga) terrains, there are dendritic (branching) valley networks, reminiscent of river drainage networks on Earth. Considerable work has taken place in attempting to explain these as the result of groundwater emerging into the networks and sapping (eroding by undermining and collapse) the valley heads, thus extending the networks. Other workers prefer ephemeral snow-based erosion. I have other ideas, based on the association of these networks with impact craters.

Dendritic valley networks on Mars - Image Copyright Calvin Hamilton       Click on image for enlargement

The other type of channels are the "outburst flood" channels. These emerge abruptly from collapsed terrain, extend for hundreds or thousands of km across the surface of Mars, then flow into basinal lows where palaeo lakes or small oceans are sometimes invoked by the proponents of water on Mars. These channels have a range of ages but appear to peak at around 1 to 1.8 Ga. The difficulty for proponents of water flows is to explain why Mars became fluvially active at this time and then became dormant again.

The head of the Ravi vallis system -  Image Copyright Calvin Hamilton      Click on image for enlargement

The seed planted by the idea of canals on Mars has remained dormant for many years but is now sprouting again. Unfortunately, I am convinced that we are travelling down a similar blind alley as Lowell. I don't think Mars was ever warm and wet. In fact I think it was colder and drier in the past than it is now. But how were the channels eroded, if not by water? That is the solution to all the paradoxes of Mars, and it involves CO2 - the "forgotten" volatile of Mars.

Read on...
 

      Created: May 2002
      Last modified: May 2002
      Authorised by:  Head, Earth Sciences

      Maintained by: Nick Hoffman
      Email: nhoffman@unimelb.edu.au