Mars was not always a frigid arid world. Billions of years ago, it had flowing water, lakes, and more. The planet may have been habitable, although it is uncertain if it ever hosted life or not. However, an important factor in the potential habitability of the Red Planet is the composition of its ancient waters.

New research published in Nature Communication suggests that the water of Mars had a salinity lower than Earth’s oceans but higher than freshwater on our planet. It also had a neutral pH level (the acidity of a substance) similar to seawater and contained an abundance of different minerals.

The team used sediment samples collected in the Yellowknife Bay by NASA’s Curiosity. The industrious rover has been studying samples of clays in Mars' Gale Crater, which is believed to have once been an ancient lake. Since Curiosity's arrival on Mars, researchers have been gaining insights into what the planet would have been like billions of years ago.

Many approaches have tried to reconstruct how Mars' water composition changed through the eons, but these require making assumptions about the aspects we don't know. The team from the Tokyo Institute of Technology decided to try a different way by looking at the properties of porewater, water trapped in pores within soils and rocks.

What can be gained by the Yellowknife Bay sediment is a representation of the last water in that area, what the researchers call the “last wetting event”. The porewater at that point was enriched with salt and minerals, and when it slowly disappeared it left those behind.

It's thought that Gale Crater was a lake with rivers flowing in but no rivers flowing out, so the water disappeared only through evaporation. Using this idea and the composition of rivers on Earth, the team estimated that the porewater composition was only possible if Gale Crater was a lake for up to 1 million years.

The presence of minerals and a long-lasting lake suggest that Mars could not only host life but it could also have been suitable for its birth. Future investigations by NASA's Mars 2020 Rover and the joint ESA-Roscosmos ExoMars rover, Rosalind Franklin, will provide new insights on clays from across the Red Planet, giving a more global idea of the composition of water across Mars.