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Earth wasn鈥檛 always the only water-world in the solar system. Mars and Venus also appear to have started out wet but, as conditions changed, they lost their oceans. So how has Earth managed to avoid a similar fate?
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Our planet鈥檚 climate is remarkably stable, and has remained in a narrow, liveable, range for almost 4 billion years. The key appears to lie in the interplay between plate tectonics, carbon dioxide and the oceans (see 鈥淭he Earth鈥檚 thermostat鈥�).
The cycle begins with volcanoes spewing CO2 into the atmosphere, which helps keep the planet warm, thanks to the greenhouse effect. This warmth allows seawater to evaporate, forming clouds and rain. As the rain contains dissolved CO2 it is slightly acidic and so it reacts with surface rocks to dissolve carbon-containing minerals into the water.
This mixture is then washed out to sea, where the minerals build up and eventually precipitate out to form new carbon-containing rocks on the seabed. Sooner or later, plate tectonics carries these rocks into a subduction zone, where CO2 is baked out of them by heat of the Earth鈥檚 interior and later returns to the atmosphere via volcanoes.
This cycle turns out to be an extremely effective thermostat. When the planet is warm, rainfall increases, speeding the rate of atmospheric CO2 removal and cooling the planet. When it is cold, rainfall decreases, allowing volcanic gases to build up in the atmosphere, warming the planet.
Venus and Mars probably had similar thermostats early on. Venus, though, was too close to the sun and the extreme heat overloaded its thermostat. A warmer atmosphere can hold more water than a cooler one before it must rain, and since water vapour acts like a greenhouse gas, it contributes to further warming. Eventually these factors stacked up until the planet warmed enough for its oceans to evaporate. At the same time, solar radiation high in the Venusian atmosphere split water into hydrogen and oxygen, allowing the lightweight hydrogen atoms to escape into space. So Venus lost its water for good, and with it any control over its thermostat.
Mars, on the other hand, was too small to maintain its thermostat. Its relatively weak gravity made holding on to heat-retaining gases in its atmosphere difficult. Meanwhile, with a higher surface-to-volume ratio than Earth, the core cooled quickly, shutting down plate tectonics and eliminating the source of planet-warming CO2.
The cooling of the core also turned off the Red Planet鈥檚 magnetic field 鈥� a by-product of an active core. Without a magnetic field, Mars is exposed to the full force of solar radiation. This breaks down water molecules into hydrogen and oxygen, leading to the loss of water from Mars鈥檚 atmosphere in a similar process to that which occurred on Venus.
On Earth, the moon has played an additional role in keeping the climate habitable. It damps wobbles that would otherwise cause Earth鈥檚 axis to tilt wildly. Even small wobbles are enough to launch ice ages, but the ones we have experienced are nothing compared to those on Mars, which flops over on its side under the influence of Jupiter鈥檚 gravitational pull.
Now, of course, humans are playing their part. The changes we make to the climate by burning fossil fuels could last millions of years but, after we鈥檝e gone, Earth鈥檚 underlying thermostat should be able to regain control. That is not guaranteed, however. Both Venus and Mars were habitable once. Perhaps we should heed their warning and take better care of the thermostat our planet has so generously provided.
