The presence of water on Mars has become more evident, suggests a new NASA finding.

Scientists have revealed that the ground where NASA’s Mars Exploration Rover Spirit became stuck last year holds evidence that water, perhaps as snow melt, trickled into the subsurface fairly recently and on a continuing basis.

Stratified soil layers with different compositions close to the surface led the rover science team to propose that thin films of water may have entered the ground from frost or snow.

The seepage could have happened during cyclical climate changes in periods when Mars tilted farther on its axis. The water may have moved down into the sand, carrying soluble minerals deeper than less soluble ones. Spin-axis tilt varies over timescales of hundreds of thousands of years.

The relatively insoluble minerals near the surface include what is thought to be hematite, silica and gypsum. Ferric sulphates, which are more soluble, appear to have been dissolved and carried down by water.

“The lack of exposures at the surface indicates the preferential dissolution of ferric sulphates must be a relatively recent and ongoing process since wind has been systematically stripping soil and altering landscapes in the region Spirit has been examining,” said Ray Arvidson, investigator for the twin rovers Spirit and Opportunity.

The twin Mars rovers finished their three-month prime missions in April 2004, then kept exploring in bonus missions. One of Spirit’s six wheels quit working in 2006.

Researchers took advantage of Spirit’s months at Troy last year to examine in great detail soil layers the wheels had exposed, and also neighbouring surfaces.

“With insufficient solar energy during the winter, Spirit goes into a deep-sleep hibernation mode where all rover systems are turned off, including the radio and survival heaters,” said John Callas, project manager for Spirit and Opportunity at NASA.

“All available solar array energy goes into charging the batteries and keeping the mission clock running,” said Callas.

Spirit, Opportunity, and other NASA Mars missions have found evidence of wet Martian environments billions of years ago that were possibly favourable for life.

These newest Spirit findings contribute to an accumulating set of clues that Mars may still have small amounts of liquid water at some periods during ongoing climate cycles.

The findings were published in the Journal of Geophysical Research.