Even after Mars' surface water was long gone, it survived underground. The legacy has been detected by the Curiosity rover, and suggests the period during which life could have flourished on the Red Planet was substantially longer than we assumed.
Fractures in rocks in Gale Crater are surrounded by lighter colored “halos”. The coloring has been attributed in Geophysical Research Letters to high concentrations of silica, which were almost certainly deposited by flowing groundwater.
"What we're seeing is that silica appears to have migrated between very old sedimentary bedrock and into younger overlying rocks,” said Dr Jens Frydenvang of the Los Alamos National Laboratory in a statement. Curiosity previously demonstrated that Gale Crater once held a great lake, and Frydenvang added that “even when the lake eventually evaporated, substantial amounts of groundwater were present for much longer than we previously thought – thus further expanding the window for when life might have existed on Mars.”
Curiosity used the Chemistry and Camera laser to determine the composition of the halos and their difference from rocks both above and below them. Silica content was between 60 and 80 percent, by weight, compared to 45 percent elsewhere. They were located near older high-silica sediments. The most common analogous silica deposits on Earth are quartz, but Mars' conditions apparently didn't allow this.
The authors don't know what caused the water to flow through rock fractures to the surface, but they are not surprised it brought silica with it. Silica's solubility in water is heavily dependent on acidity and temperature, so a change in either of these would lead to sudden deposition. The fact that this occurred after the lake had evaporated can be established from the fact that some of the halos are seen in dunes that were shaped by wind after the crater dried out.
The silica discovery confirms suspicions of long-lasting groundwater raised by a recent analysis of boron distribution within Gale.
We don't yet know when the halos were formed, and therefore how long after the lake evaporated that such outbursts continued, but the paper concludes: “The timescale for potential habitability, at least in the subsurface of Gale, must be substantially extended.”
Water does not necessarily mean life, of course. However, without it life seems unlikely, if not impossible. Moreover, if we learn that such a long conjunction of water and many of the other conditions we think are essential for life were not sufficient to see it evolve, it will tell us that Earth is more rare and precious than we thought.
