A long time ago, on a planet not too far away, there was a lake – and not just any lake, but one that could finally shed some light on that $64 million question: Was there ever life on Mars? New findings based on data from NASA's Curiosity rover mission reveal that the Gale Crater lake not only had properties similar to lakes on Earth, but that it also had all the trappings that would have made it a microbial playground.

Since landing inside the 96-mile-wide (155 km) Gale Crater in 2012, Curiosity has been tasked with finding out whether the planet has ever had the means to support a habitable environment. It found promising evidence of this early on in its mission, with the discovery at Yellowknife Bay of an ancient clay-rich lakebed where life might once have existed.

A study published earlier this week on "halos" found in the bedrock suggests how the area could have continued to support life even after the lake had disappeared and now, new data suggests that the lake might have had two habitable zones that would have made it an equal-opportunity hotbed for microbial life, regardless of whether they thrived in aerobic, anaerobic or anoxic conditions.

In this study, the researchers, led by geoscientist Joel Hurowitz at Stoney Brook University, analyzed the mudstones that Curiosity gathered during its first 1,300 Martian days (one Martian day is the equivalent of 24 hours and 37 minutes) to profile the ancient lake. One thing that caught their attention was the differences in the physical, chemical and mineral characteristics of the lakebed deposits at lower Mount Sharp, a mountain within the crater made of hardened mud and sediments that had piled up over time. Why did some rocks have thicker layering and a larger proportion of the iron mineral hematite while others had very fine layers and contained more magnetite?

This led the researchers to wonder whether fluctuating environmental conditions were at play since these differences indicate that the deposits were formed in very different environments. High concentrations of silica accompanied by magnetite, for example, indicate deposition in an anoxic environment, while the presence of hematite-phyllosilicate points to an oxidizing one.

"We could tell something was going on," says Hurowitz. "What was causing iron minerals to be one flavor in one part of the lake and another flavor in another part of the lake? We had an 'Aha!' moment when we realized that the mineral information and the bedding-thickness information mapped perfectly onto each other in a way you would expect from a stratified lake with a chemical boundary between shallow water and deeper water."

This diagram shows how the Gale Crater lake was divided into distinct segments with varying degrees of oxygen(Credit: NASA/JPL-Caltech/Stony Brook University)

In other words, just like the stratified lakes on Earth, the lake in the Gale Crater also had different layers with different amounts of oxygen, which was hinted at by the oxidation of the iron deposits found in the samples. Those taken from the edges of the crater tended to be rusty, indicating that there was more oxygen in these shallow depths, while those taken from the middle were not oxidized, meaning there was less of it.

If one considers how life on ancient Earth began in iron-rich and oxygen-free oceans, these findings help advance the case that ancient Mars also had equally habitable conditions for life.

"[We] have to remember that at the time of Gale Lake, life on our planet had not yet adapted to using oxygen – photosynthesis had not yet been invented," explains co-author Roger Wiens, a planetary scientist at Los Alamos National Laboratory. "Instead, the oxidation state of certain elements like manganese or iron may have been more important for life, if it ever existed on Mars. These oxidation states would be controlled by the dissolved oxygen content of the water."

Blowing hot and cold

The Mars of today resembles a cold desert wasteland. However, the presence of valley networks and basin lakes has prompted some scientists to argue that the planet was a much warmer and wetter place more than three billion years ago, leading to the rise of two competing theories regarding the climate of ancient Mars: "warm and wet" vs "cold and icy." This latest study suggests that far from an either-or scenario, the climate on early Mars was complex and prone to fluctuations, going through both cold and warm spells over the course of millions of years.

By comparing the differences in the chemical composition of the sedimentary rock layers, the researchers were able to deduce the climate conditions that led to their formation. The study suggests that when the Gale Crater lake first formed, the climate was very cold and dry, though younger rock layers point to warmer and wetter conditions that supported lakes. So what happened to all the water? That remains a mystery to be solved, but the presence of a salty overprint that flowed over the hardened sediments could hold clues to how it became the arid planet it is today, say the authors.

That said, given that this study was based on samples taken from just one section of the crater, more work will have to be done to prove that the findings of this study are applicable on a broader scale. In the meantime, Curiosity's mission continues and it is currently studying higher and younger layers of Mount Sharp to find out what drove the evolution of the ancient Martian lake environment.

The study was published in Science.

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