Last updated January 11, 2018 at 10:32 am
The reason Earth has water and Mars is a desert may be underfoot.
Tens of kilometres under the barren, dried out surface of Mars may exist the water that astronomers have long suspected it should contain but somehow lost, turning it into a lifeless desert while Earth retained its oceans and allowed life to evolve. 
Recent discoveries by robotic explorers and orbiters of Mars have revealed that the red planet was once blue like Earth, containing oceans and atmosphere.
However, over the course of millions of years the atmosphere was eroded away by the actions of the Sun. With no atmosphere the water that once freely flowed over its surface was lost, it was believed, to space.
One hope had been that the water may have found its way into vast undiscovered underground ice deposits, ready and waiting for humans to exploit for our own survival and exploration on the planet.
New results published in Nature reveal, however, that not all the water may have been lost, and in fact may still be on Mars after all. Or rather, in it. But unlike the subterranean ice, this water reservoir is locked away from humanity forever.
And the results also show how Earth and Mars, once so similar, diverged.
Geological models suggest that Martian basalt, a form of volcanic rock, can hold more water than basalt on Earth. If the latest results are right, then it suggests that surface water once reacted with lava from volcanic eruptions to form a mafic (or hydrated) basaltic crust.
The iron-rich basalt of Mars was able to hold 25 per cent more water than the equivalent rocks of Earth, essentially locking the water inside the rock.
Over time the basalt was covered by the surface rocks and sank into the mantle. Because of its higher iron content, under the temperature and pressure of the upper crust it sank much deeper into the planet than basalt on Earth, trapping the locked-away water at vast depths inside the planet.
The same process on Earth leads to a different result, with the hydrated rocks remaining at a shallower depth. Early Earth had a much steeper geothermal gradient (i.e. a more dramatic temperature change with depth) than Mars, meaning that the upper crust was more buoyant, and allowed the water-rich rocks to remain closer to the surface and be recycled.
The study found that overall the difference in relative volume of water is enormous. The researchers find that the “Martian mantle can be in excess of 9 per cent by volume … compared to about 4 per cent by volume of Earth’s mantle.”, all locked away in hydrous minerals.
Being locked in rock and far below the surface, this water will remain unreachable by human explorers on Mars.
Such an enormous difference between the planets is astounding, as current theories of planet formation would lead to an expectation that they should be the same or closely similar.
The basic building blocks of the rocky inner worlds are similar in the accretion picture of an early solar system, leading to the expectation that the internal rocks of Mars, Earth, Venus and Mercury should be similar as well. Which they are, except for the all important level of iron.
The amount of iron in rocks like basalt is influenced by the oxygen present in the mixture, also known as fugacity. This small difference in fugacity in the early history of Mars and Earth had an enormous knock on effect, with Mars basalts containing 17 per cent iron (II) oxide, whereas equivalent rock on Earth is half that at 7 – 10 per cent by weight.
The difference in amount of iron leads to the difference in water content, and led to the divergence of the two worlds. Both began as watery worlds, and end up as a Red Planet and a Blue Marble. All due to little more than rust.
The research has been published in Nature
Featured image courtesy of Jon Wade and James Moore
Graph courtesy of Wade et al, Nature 2017
Mars image courtesy of NASA/JPL-Caltech/MSSS
