NASA's Curiosity rover has been exploring Mars ever since it arrived on our Red neighbour in 2012. In the seven years since, Curiosity has sent constant updates on its findings. It has thrown light on radiation levels on Mars, which will be helpful in deciding how safe astronauts and human colonies will be living there. The rover also discovered beds of sediment that suggest liquid water once flowed on Mars and could support life.

Now, the rover has found evidence that there were several ponds dotting the surface of its current subject — the Gale crater on Mars' Mount Sharp.

The network of cracks in this Martian rock slab called Old Soaker may have formed from the drying of a mud layer more than 3 billion years ago. Image: NASA

Scientists suspect the Gale crater was once a lake with brine water, has rocks rich in mineral salts. Around 150 kms wide, this ancient basin was formed when a meteor hit Mars in its early days. NASA has already established that the Gale crater has many layers of sediments, carried by water and wind that covered the bedrock for a period of time. Adding some clarity to this early image of Mars, Curiosity has found that these ponds were both shallow and briny (super salty).

Here on Earth, these salts are found in nature and formed in layers by the erosion of rocks, flowing water and deposition in river beds. On Mars, these mineral salt deposits are evidence of fluctuating periods of overflowing water and what we refer to as droughts.

While it may be hard to imagine today, Mars was once a 'wet' planet — far from the freezing, desolate 'desert' planet it is today. Scientists at NASA are trying to understand how this transition took place and at what rate it all unfolded. By looking at multiple different regions on Mars with different rovers, NASA is also trying to piece together another overarching question: why and when did the dramatic change in climate over the planet's evolution?

"Imagine ponds dotting the floor of Gale Crater, the 150-km-wide ancient basin that Curiosity is exploring. Streams might have laced the crater's walls, running toward its base," the statement reads. "Watch history in fast forward, and you'd see these waterways overflow then dry up, a cycle that probably repeated itself numerous times over millions of years."

Researchers have found a curious mix of minerals and sediments elsewhere on Mars — the salt-enriched rocks of Sutton Island. These, according to the study's scientists are similar to the saline lakes of Altiplano which exists in South America, here on Earth.

Filled with briny lakes, the Quisquiro salt flat in South America's Altiplano represents the kind of landscape that scientists think may have existed in the Gale Crater, which NASA's Curiosity rover is exploring. Image credit: Maksym Bocharov/NASA

"As we climb Mount Sharp, we see an overall trend from a wet landscape to a drier one," says Ashwin Vasavada, Curiosity Project Scientist at NASA's JPL, which leads the Mars Science Laboratory mission of which the Curiosity rover is a part. "But that trend didn't necessarily occur in a linear fashion."

The researchers think it is more likely that the timeline was a lot messier — with drier periods, like those at Sutton Island, followed by wetter periods like that observed in the 'clay-bearing unit' that Curiosity is currently exploring.