Disturbed soil surrounding the Mars rover Spirit…….Credit: NASA/JPL-Caltech/Cornell
A heading for this story could be: Dead Spirit reports from a moist grave. Almost technically correct but too much wordplay. The reality is more prosaic and potentially more important. The issue is water on Mars. Evidence is overwhelming that Mars had, as in two billion years ago, a large amount of water on or near the surface. What’s left of it is frozen, some of it subsurface at middle latitudes and most definitely at the poles. The question, still in active exploration and debate, is there any contemporary water activity on or near the surface, especially at non-polar locations?
New evidence on this question is even more intriguing because of the story behind its acquisition. A weakening Mars rover, Spirit, its batteries running down and struggling to free itself from the sand but essentially digging its own grave, broke through the surface to reveal something unexpected: Stratified layers of materials such as ferric sulfate, which are known to be deposited by water.
The story of the Mars Exploration Rover (MER) Spirit is like the Lazarus of robotic devices. Surviving a broken wheel-track and resurrecting itself from deep-shutdown during three winters on Mars, Spirit finally may have ended its useful operation stuck in a small crater (8 meters/ 25 feet wide) in what is called the Troy site in March, 2010. However, during the efforts to move out of the crater, Spirit dug itself a small pit. Analysis of the images taken of the pit area revealed newly exposed layers of subsoil (see image above).
Reporting in the Journal of Geophysical Research [ Spirit Mars Rover Mission: Overview and selected results from the northern Home Plate Winter Haven to the side of Scamander crater] a team of 36 scientists decided that the geological features – a variety of depositions mostly of ferric sulfates (reddish iron-bearing material) – indicate that water moved downward through the soil, dissolving and carrying with it the ferric sulfate. The pattern of layering suggest that the movement of water was cyclic – seasonal – and could have occurred recently. ‘Recently’ in Mars geological parlance might mean anything from a recent millennium to a few million years ago. Nevertheless, seasonal climatic activity still takes place on Mars, and it is not inconceivable that water (or water vapor) may be involved.
So far there is no incontrovertible evidence of life on Mars, now or ever. It seems likely, given the cold, arid conditions at the Martian surface that if there is life, it will be subsurface. This makes patterns of water settling through the Martian soils potentially significant.
“The lack of exposures at the surface indicates the preferential dissolution of ferric sulfates 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 of Washington University in St. Louis, deputy principal investigator for the twin rovers Spirit and Opportunity.