NASA's Curiosity rover has detected boron for the first time on the surface of Mars, which indicates that the groundwater may have possibly been suitable for hosting microbial life in the ancient past.
"If the boron that we found in calcium sulfate mineral veins on Mars is similar to what we see on Earth, it would indicate that the groundwater of ancient Mars that formed these veins would have been 0-60 degrees Celsius and neutral-to-alkaline pH," said Patrick Gasda, a postdoctoral researcher at Los Alamos National Laboratory in the US.
The temperature, pH, and dissolved mineral content of the groundwater could make it habitable.
The boron was identified by the rover's laser-shooting Chemistry and Camera (ChemCam) instrument
Boron is famously associated with arid sites where much water has evaporated away. However, environmental implications of the boron found by Curiosity are still open to debate.
Scientists are considering at least two possibilities for the source of boron that groundwater left in the veins: It could be that the drying out of part of Gale lake resulted in a boron-containing deposit in an overlying layer, not yet reached by Curiosity.
Some of the material from this layer could have later been carried by groundwater down into fractures in the rocks.
The changes in the chemistry of clay-bearing deposits and groundwater may have affected how boron was picked up and dropped off within the local sediments.
The discovery of boron is only one of several recent findings related to the composition of Martian rocks.
Curiosity is climbing a layered Martian mountain and finding rock-composition evidence of how ancient lakes and wet underground environments changed, billions of years ago, in ways that affected their favourability for microbial life.
As the rover has progressed uphill, compositions trend toward more clay and more boron.
These and other variations can tell us about conditions under which sediments were initially deposited and about how later groundwater moving through the accumulated layers altered and transported ingredients.
Groundwater and chemicals dissolved in it that appeared later on Mars left its effects most clearly in mineral veins that filled cracks in older layered rock.
However, it also affected the composition of that rock matrix surrounding the veins, and the fluid was in turn affected by the rock.
As the rover gets further uphill, researchers are impressed by the complexity of the lake environments when clay-bearing sediments were being deposited and also by the complexity of the groundwater interactions after the sediments were buried.
"We are seeing chemical complexity indicating a long, interactive history with the water. The more complicated the chemistry is, the better it is for habitability," said John Grotzinger, from California Institute of Technology in the US.
"The boron and clay underline the mobility of elements and electrons, and that is good for life," said Grotzinger.
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