In 2004, NASA's rover Opportunity discovered a peculiar mineral on Mars: jarosite. This is a sulfate of iron and potassium and it needs water and acidic conditions to form. There was so much jarosite that it baffled planetary scientists. How could it have formed on Mars, a planet with little water and alkaline soil? The explanation was apparently buried in Antarctica.
As reported in Nature Communication, researchers have discovered jarosite deep beneath the ice of the southernmost continent. The presence of this mineral deep beneath Antarctica's glaciers suggests that one of the hypotheses for its formation on Mars is probably correct.
Scientists think it's possible the jarosite deposits formed billions of years ago when the Red Planet wasn’t really red but white – covered in thick glaciers. These glaciers would have been contaminated by rocky dust from the planet, and the ice could have slowly turned the dust into jarosite.
The jarosite crystals in Antarctica were found inside an ice core from a depth of about 1,620 meters (5,315 feet), when the ice was deposited about 300,000 years ago. The ice core was drilled in Talos Dome, an ice dome on the edge of the East Antarctic plateau, and given its depth, is considered a good analog for Mars. It is isolated from Earth’s atmosphere so it is independent of its pressure, temperature, chemistry, and pH.
According to the study, there is a mechanism that explains the transformation of mineral powder trapped in deep ice into jarosite. The mineral needs to be trapped in the ice at least 1,000 meters (3,281 feet) deep, where the temperature is around -10°C (14°F). Under these conditions, the liquid water needed to transform jarosite is present in the form of concentrated acid solutions whose freezing point is much lower than water's.
The team thinks that the presence of basaltic dust and aerosols rich in sulfurs from volcanos would be a common trait on both Earth and Mars, so the mechanism would work. However, there were just a few grains of jarosite in the Earthly ice core. On Mars, on the other hand, the mineral is abundant and present almost everywhere. The team believes that the dustier nature of the Red Planet could explain such a difference.
"The discovery is destined to revolutionize the current understanding of the origin of the widespread deposits of jarosite on Mars,” lead author Giovanni Baccolo, from the Università di Studi di Milano-Bicocca, said in a statement. "Although they are now gone, the ancient glaciers and the dust trapped in them appear to have left a clear geological trace on the Red Planet, evidence of climate events from a bygone past.”
The team would now like to investigate ice cores for other minerals that might crop up on Mars. With a new wave of Martian missions set to arrive there in February 2021, it is certainly a great time to investigate how some of Mars's other minerals came to be.