The Curiosity rover on Mars has sampled ancient lake deposits from Gale Crater on the Red Planet, showing water that once existed there was dense with salt and minerals. By studying mineral deposits on Mars, researchers hope to better understand the composition of water on the surface of that world in the ancient past. Water, on Earth or on Mars, leaves distinctive chemical traces, and examination of mineral deposits can reveal significant information about long-lost seas.

Researchers study the salinity of deposits — the concentrations of salt in formations that were once exposed to water. They also look at the redox state of minerals — essentially, the balance between hydrogen and oxygen—in samples to better understand the sample material. Here on Earth, material that is exposed to the atmosphere becomes oxygen-rich, while samples from just a few inches underground show higher concentrations of reducers like hydrogen.

An artist’s concept of the Curiosity rover on Mars, drilling into the crust, attempting to understand the material which sits just below the surface. Image credit: NASA

“Salinity, pH, and redox states are fundamental properties that characterize natural waters. These properties of surface waters on early Mars reflect palaeoenvironments and thus provide clues on the palaeoclimate and habitability,” researchers from the Tokyo Institute of Technology report in the journal Nature.

[Read: Scientists explain what on earth could live on a salt water lake on Mars]

Examination of material from Gale Crater shows the water which once occupied the paleolake had a similar chemical composition to the oceans of Earth. This suggests that modern life on Earth may have once been able to thrive in the ancient seas of Mars.

However, this finding brings about another puzzle — if Martian water was so amicable to life, why have we not yet found conclusive evidence of life on Mars?