NASA's Opportunity rover. Credit: NASA

The dry rock outcrops on the martian plains of Meridiani Planum are thought to have once been bathed in and shaped by acidic waters, based on evidence collected by NASA’s Opportunity rover in its seven years on the red planet.

How those ancient waters became so acidic, however, has been an open question. A new study suggests that these acid waters were created when iron-rich ground water rose to the surface of Mars and underwent chemical reactions changing the state of the iron and boosting the acidity of the waters.

As Opportunity has explored the Meridiani Planum area, it has analyzed sedimentary rocks thought to have been deposited about 3.7 billion years ago. The minerals present in the rocks suggest that they formed in a climate that was arid but that experienced periodic ground water upwelling. Certain telltale minerals, particularly one called jarosite, suggest that the waters that washed over the surface and formed the rocks were acidic.

Some scientists think that the water that shaped this area of the martian surface came from below the ground, periodically welling up. But the rock this ground water would have been surrounded by is basalt, and the chemistry of such a system would have likely made for neutral-pH water.

So just how did neutral water become acidic enough (with a pH estimated between 2 and 4, or around the acidity of lemon juice or orange juice) to leave that characteristic mineral signature?

Joel Hurowitz, an Opportunity team member at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., and his colleagues used chemical models and Opportunity’s observations to suggest a mechanism that could answer that question.

If the groundwater was rich in a particular form of iron, that iron might have undergone a chemical transformation when the water reached the surface and was exposed to the atmosphere, resulting in acidic waters. The iron in the water could have been oxidized where a molecule or atom loses an electron either by ultraviolet light or by reaction with dissolved oxygen. This process would have left the water with an acidic pH, and the iron would have come out of the water solution to eventually form the rocks that Opportunity observed.

Hurowitz and his colleagues crunched the numbers to see if this mechanism could produce enough acid to explain the conditions at Meridiani. The results seemed to match what Opportunity’s observations suggested very closely, and “we were sort of pleasantly surprised by that,” Hurowitz said.

Previous suggestions for how the waters became acidic include the idea that atmospheric sulfur compounds — possibly generated by volcanic eruptions — could have fallen as acid rain, much like can happen in polluted air on Earth. But Hurowitz thinks the iron oxidation explanation better fits the observed chemistry of Meridiani and does not require the onset of acid water flow to “perfectly coincide with volcanic exhalations and sulfur coming in from an external source for your acidity.”

One key question left, which Hurowitz and his colleagues are already investigating, is “whether or not you’re able to pick up enough iron along those groundwater flow paths and bring them to the surface and have all this acid chemistry take place,” Hurowitz said.