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."
Considering Earth and Mars had a similar early past, scientists know what to expect from the many examples of lakes drying up on Earth " the dehydration leaves behind piles of pure salt crystals. This isn't what they've found in Sutton Island, where the salts were very different. They were mineral salts, not table salt; they were also mixed with sediment, suggesting that they likely crystallized in a wet environment " like the bed of a shallow pond with briny water.
NASA scientists have also drawn parallels between the salt-enriched rocks of Sutton Island and the saline lakes of Altiplano which exist in South America here on Earth.
Interestingly, this contrasting mineral composition in the Gale crater could also point to the reason for Mars' climate story. The planet, which we now know once had liquid water on it, is a rocky red desert today.
"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's likely that Mars' climate timeline over the past many million years wasn't straightforward " there were drier periods, like those at Sutton Island, followed by wetter periods like that observed in the 'clay-bearing unit' that Curiosity is currently exploring.