People love to speculate that Mars was once a great place for life to form, and claim that there is plenty of evidence that there used to be oceans and rivers there. But this isn’t true. People used to claim there were big Canal-like features on Mars, and used this as evidence that Mars was very wet.
It was later realized that these weren’t canals, but rather geological features caused by impact craters from astroids. But more recently, people have been claiming that images like the one above are examples of dried-up riverbeds.
But this turns out not to be the case. When we take a closer look, we find that these aren’t riverbeds, but are rather sand dunes, most likely formed by a very dry surface eroded by wind:
So it looks like there really isn’t evidence for Mars having a wet past, except for this fact: there are icecaps at the Martian Poles. Also, there are ice clouds found in Mars’ atmosphere. Take a look at the picture below:
So let’s ask the big question: even if there’s isn’t any water on Mars now, was there ever a lot of liquid water on Mars?
The quick answer: probably not very much, and if so, not for very long.
How do we know this? The atmosphere of Mars is too thin for liquid water to last for very long. We see Martian sand dunes frozen, and we see them dry (two pictures up), but we never see them wet. On Earth, the atmospheric pressure is high enough that water will be a liquid anywhere between 273 Kelvin and 373 Kelvin, and it happens that the average temperature of the Earth falls in between these two numbers (somewhere in the 280s and 290s for most places), so water is abundant. But on Mars, the atmospheric pressure is somewhere in between 0.6% and 1.0% of that on Earth, or between 100 and 150 times smaller. Mars is also much colder than Earth, so that these two factors combined means that there’s either never liquid water on Mars, and it goes from solid ice straight to water vapor when it gets warm (and back again when it gets cold), or it can be liquid water for a very narrow temperature range that only occurs very infrequently on Mars. Take a look at the phase diagram of water below.
There will never be liquid water on Mars if the atmospheric pressure is less than 0.7% that on Earth, and there’s only rarely liquid water there if the pressure is greater than that number. The phase diagram for water is shown above, with the letters M, E, and V showing where the average temperatures are for Mars, Earth, and Venus. While the temperature of Earth is pretty stable, temperatures on Mars can range from a minimum of -140 Celcius at the coldest to 20 Celcius at the warmest. While the Earth has a difference of about 20 degrees Celcius between the hottest and coldest parts of the day in any one place, the typical difference on Mars is 100 degrees Celcius.
So these two factors:
- A tiny, tiny atmosphere, and
- a temperature that is mostly very cold and varies a lot over short times,
makes it impossible for liquid water to exist on Mars for more than a few hours, maximum, at a time. While the average temperature of Mars may have been different in the past, there was never a larger atmosphere and the temperature was never stable. So while plenty of reputable-looking sources will tell you why we think there might have once been water on Mars, you now know why Mars could never have had a stable ocean or river; the best it could have had was an ice-bed that melts during the day, is lucky enough not to boil or evaporate (and if you know how that’s possible, because I don’t think it is, please leave a comment!), and then re-freezes for the night! So don’t believe the hype, Mars is dry and cold, and it doesn’t have the atmosphere to sustain liquid water! And if you liked this post, digg it!
UPDATE: After reading Briony’s comment and doing some research into Mars and the Martian surface, I must amend my conclusion — it was too strong. It is unfair to say there has never been liquid water on Mars. While the evidence is not completely conclusive as to the presence of liquid water at some point (e.g., the deposits we expect are often not found, but they could be simply covered), there is strong evidence that Mars did have a liquid past. See http://phoenix.lpl.arizona.edu/mars151.php, for example. It means, instead, that never is too strong a word, and instead I should say that, for the last three billion years, there hasn’t been liquid water on Mars. But there may have been prior to that. The how is still extremely speculative, but the evidence is too strong to say with any certainty that there was never any liquid water on Mars.
(But there hasn’t been for billions of years.)