NASA’s troubled Mars Exploration Rover Spirit has found evidence that liquid water once seeped underground from the surface, despite the rover still being stuck in a spot of bother.

The mystery of where Mars’ water went has been one of the most enduring mysteries of the red planet. Evidence abounds that billions of years ago Mars may have had oceans, and during its great variations in tilt ice may have extended down to the tropics. Evidence has been found, first by the European Space Agency’s Mars Express probe, and then by NASA’s Phoenix lander and the Mars Reconnaissance Orbiter, of the existence of water-ice below the surface. Now Spirit has seen evidence that the water moved underground by flowing through the Martian dirt, probably from the melting of frost or snow.

Spirit is currently caught in a sand-trap, desperately trying to outlast the Martian winter. As the rover’s wheels churned in the sand, they dug trenches that exposed different layers in the dirt. Upon inspection, these layers corresponded to different minerals. The minerals least likely to dissolve in water, such as silica, hematite and gypsum, are found closer to the surface, whilst minerals of increasing solubility, such as ferric sulphates, are found deeper and deeper.

“The preferential dissolution of ferric sulphates must be a relatively recent and ongoing process since wind has been systematically stripping oil and altering landscapes in the region Spirit has been examining,” says the rover’s Deputy Principal Investigator Ray Arvidson, of Washington University in St Louis, USA.

‘Relatively recently’ means one of the episodes of climate change over the last few hundred thousand years caused by the change in the tilt of Mars’ spin-axis. Without a large moon to stabilise it, the red planet undergoes large wobbles in tilt. Mars’ current tilt is 25 degrees, but could potentially tip over as much as 82 degrees (see the November 2010 issue of Astronomy Now for more on planets with high obliquity). During one of Mars’ warmer phases, ice, snow or frost may have gradually melted, with the resulting runoff soaking the dirt.

Spirit is currently hibernating to conserve precious power as its dust encrusted solar panels, tilted away from the low winter sun, struggle to gather enough solar energy. “All rover systems are turned off, including the radio and survival heaters,” says the rover’s Project Manager John Callas of NASA’s Jet Propulsion Laboratory. “All available solar array energy goes into charging the batteries and keeping the mission clock running.” Without its heaters Spirit is currently experiencing temperatures far colder than it has ever done before, and it remains to be seen if the little rover will ever wake up again. If it does, it will find itself a stationary science platform, permanently stuck in the sand with two broken wheels. However, prior to hibernation but its spinning wheels also dug trenches behind it that are currently unexamined. If Spirit rises from its slumber, it will be presented with these new trenches for new investigation.

To follow the exploits of the Mars Exploration Rovers, visit their website here, and read our previous story relating to Spirit’s plight here.