The hunt for traces of ancient life on Mars has yielded some promising results. Observations from instruments aboard NASA’s Curiosity rover suggest that 3.5 billion years ago, Gale crater held a low-acidity, low-salinity lake. Meanwhile, the Mars Reconnaissance Orbiter has found evidence of an ancient lagoonlike landscape with mild acidity and elevated salinity in the Robert Sharp crater. Both settings may have been hospitable to life.
These instruments also have revealed the presence of akaganeite in the craters, an oxide mineral found in various locations on Earth. Terrestrial akaganeite usually forms in acidic, saline wet environments and likely also formed in wet conditions in the Martian craters. Exactly how it could have formed in two very different environments on Mars is still unclear.
To tackle this question, Peretyazhko et al. studied the combined effects of acidity and salinity on akaganeite formation in a laboratory setting. They grew the mineral in a solution containing iron(III) perchlorate and sodium chloride because tentative evidence suggests that these two substances exist on the Red Planet’s surface.
Running this experiment under different chloride ion (salinity) and pH (acidity) levels, the scientists used X-ray diffraction and other detection techniques to determine whether and how much akaganeite formed and whether it formed alone or in mixtures with other substances. They then analyzed these results in light of what they already knew about the ancient Martian environment.
Their analysis suggests that the akaganeite detected in a part of Gale crater known as Yellowknife Bay may have formed from minerals present in volcanic basalts in solutions with a chloride ion molarity higher than 0.05 and a pH between 1.6 and 8.
In Robert Sharp crater, akaganeite could have arisen from basaltic minerals or from a process involving the exposure of iron sulfides to air in a partially dried lagoon, similar to a process known to produce akaganeite on Earth. This formation on Mars would have occurred in acidic solutions with a pH below 4 and a chloride ion molarity of about 0.1.
These findings provide new insights into wet settings on ancient Mars while supporting the possibility that the Red Planet once held life. (Journal of Geophysical Research: Planets, https://doi.org/10.1029/2018JE005630, 2018)
—Sarah Stanley, Freelance Writer