By Adam Stevens
When Mariner 4 took the first close up photographs of Mars, scientists were disappointed by a vista that looked like the Moon – cold and dry, with impact craters being the only visible feature. Later, Mariner 9 showed the surface to be much more complex, with what looked to be river valleys criss-crossing the landscape. Since then, evidence has suggested that there was a significant amount of liquid water in rivers, lakes, or possibly even oceans, across the whole planet. Now some scientists are beginning to challenge this view, and say that Mars may never been anything other than dry.
One of the more obvious pieces of evidence for a wet period in Mars’ history is the outflow channels that dot its surface. Some of these are truly massive, suggesting catastrophic floods with thousands of tonnes of water flowing very fast. There are smaller channels, many on the rims of craters, and gullies that could have been created by water flow, some of which appear to have changed in the few years that we have been looking in their direction. Some craters appear to have runoff channels flowing in and out, suggesting they might once have been home to large lakes, and some have alluvial fans equivalent to deltas on earth which imply they were not just passing flows of water. Most of these channels and gullies flow towards the great northern plains, the Vasistas Borealis. If not, they tend to point towards the giant impact basins of Argyre or Hellas, implying a hydrological cycle like ours, where water flows into large oceans to be recycled back into the system.
These features are not the only evidence for water. Orbital instruments have detected numerous examples of rocks that tend to form in the presence of water; hydrated silicates, clays, sulphates, and carbonates. There are also sedimentary rock formations visible in many places, including in areas explored directly by the Mars Exploration Rovers Spirit and Opportunity.
The evidence for the possibility of a warm, wet Mars with rivers and oceans, as described in so much science fiction is compelling, but in no way conclusive.
We know that there is water on Mars. The problem is that almost all of it is locked away beneath the surface. Measurements have shown that there is a lot of hydrogen in the subsurface of Mars. The only real candidate for a material that could hold that much hydrogen in that environment is ice. This means that there is a layer of water ice under wide areas of the planet. It could be deep, very, very deep, and more than capable of holding enough water to have filled a northern ocean sometime in the past. There is also water ice in the polar caps (although these are mostly made of carbon dioxide ice) and some floating around in the atmosphere. In fact, we have observed variation of water vapour in the atmosphere, implying that there is an active, if tenuous, water cycle going on, right before our eyes.
Many scientists believe that Mars was once capable of supporting liquid water on the surface. However, there is a growing view that this might not have been the case. Some models say that it was possible, or even probable, but some say otherwise. This shows, in a way, how little we know about climate and how it develops, that similar models can show such different things. These newer models, though, are starting to change some people’s minds.
Mars is a cold place, with temperatures just above freezing. On a good day. In a sunny spot. Most of the time the entire surface is way below freezing, and at the north and south pole it’s cold enough to freeze even the carbon dioxide out of the atmosphere. It’s possible that in the past, greenhouse warming could have created a warmer climate, even above zero degrees (though maybe only in some places). The issue, really, is pressure. Mars’ atmosphere is also very thin. Even if temperatures rose above freezing, water would find it hard to last in liquid form, as the pressure is so low. Exposed ice would go straight from solid to water vapour, sublimating away. This means that to have had a wet period, the atmosphere must have been much warmer and much thicker, and it is very difficult to find a way for this to have happened while remaining realistic and still accounting for the evidence we have of Mars’ geologic past.
There are other options. It may be that the features we see were created by short bursts of water (short in geological timescales, at least). Large flows would have been able to stay liquid on the surface for a while, but would have eventually sublimated out into the atmosphere or frozen into the ground. It would have been hard for this fast moving water to create the hydrous minerals that have been detected, but it could be that they were created deep underground and brought to the surface by some other means.
Some believe that many of the channels and gullies could have been created by the action of large sheets of ice. Thin layers of water running under the glaciers could have carved them out under hydraulic pressure from the heavy ice above. There are examples of similar channels on Earth which were created in a frozen, not wet environment, and there are many other groups of glacial features that show similarities between Earth and Mars.
The ExoMars Trace Gas Orbiter, along with NASA’s MAVEN orbiter, will give us a better view of the martian atmosphere and with it more information that will help us decipher this mystery. If we knew more about what gases are flying around, water vapour in particular, as well as when they increase or decrease and where they come from, we might be able to build up a picture of the current water cycle, which might tell us something about the past water cycle.
Curiosity (the Mars Science Laboratory) has been sent specifically to an area that shows signs of having sedimentary rock formations. In particular, it will be able to drive up an exposed face of sedimentary layers, examining each one in turn for chemical evidence of the water that (could have) deposited it. It’s possible that this latest rover could be a little more conclusive about how wet Mars’ past was, or at least provide some compelling evidence either way.
So, the current evidence indicates that Mars was once a wet planet, though there are conflicting facts becoming apparent that might preclude this, saying instead that Mars has always been cold and dead. However, a dry Mars doesn’t necessarily rule out martian life.
The most likely place for life to have started, if it ever did, is deep under the surface, where liquid water is almost certain to have been present. Our best theory at the moment is that life on Earth started at hydrothermal vents, where the confluence of liquid water and warm, mineral rich magmatic fluid gave the first organisms a source of energy and carbon to do their thing. Even if Mars never had an ocean, simple life could still have made this first evolutionary step in the crust where the bottom of the permafrost layer would melt and interact with the rock there to produce conditions almost the same as these hydrothermal vents.
A dry Mars might be a little less enticing than a wet one, and it would make our neighbour a little bit less like Earth. One of the reasons we want to go is because we can use Mars as a comparative planetology experiment, learning more about our own planet by looking at another that is slightly different. If Mars never developed oceans it would make it slightly less useful as a control, but I still think that even if it was dry, Mars will always remain somewhere that we have to explore, somewhere that we have to go.