Water: Why Mars Doesn’t have it

When I turn a globe such that the Pacific Ocean is facing me, it’s possible to adjust it such that no land is visible. The opposite isn’t possible though: there is no orientation of the globe where I can see only land. That’s a perfect visualization for Philip Ball’s line from his biography of water, H2O:

“We call our home Earth – but Water would be more apt.”

 

Why does Earth have (so much) water, whereas Mars doesn’t? After all, our theories say that when the planets were being formed, ice-laden comets were bombarding planets all the time, so why shouldn’t Mars still have water? The answer has two parts. And both parts are based on the same root cause:

“The key is size.”

 

The first consequence of the smaller size?

“Being smaller than Earth, Mars cooled off sooner from its fiery youth.”

As it cooled, its volcanic activity seized up. And:

“Without the churning of a hot mantle, Mars developed no plate tectonics.”

The (initial) water on Mars eroded its rocks, which then combined with the CO2 in the Martian atmosphere to form carbonates. As the CO2 content in the air decreased, the greenhouse effect decreased, and the planet couldn’t retain much of the sun’s heat. And so parts of the water on Mars just froze. That’s one half of the story. (In case you’re wondering, this decline was averted on Earth because plate tectonics drags the carbonates deep down, and the CO2 is then “spewed out through the mouths of volcanoes”, thereby keeping an equilibrium).

 

The other consequence of Mars’ smaller size was that its gravity was weak, which made it easier for gases in its atmosphere to escape. That in turn meant Mars’ atmospheric pressure is very, very low:

“At that pressure, any liquid water would just vaporize, and water ice does not melt when warmed: it hops straight to vapour without passing through the liquid state.”

Worse, most of that water vapour then escaped the planet’s gravity altogether.

 

Now and then, Mars has enjoyed “transient warm spells”. This would cause the ice caps to melt, in turn releasing CO2, and the resulting greenhouse effect would melt more ice. Unfortunately, the problem is that the liquid water would then erode rocks, which takes us back to the first reason (erosion led to carbonate formation, which sucked CO2 out of the air, thus reducing the greenhouse effect, which meant the temperature would fall, and the water would freeze again).

 

And so Mars is doomed to never be able to hold onto any water.

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