An image taken at the 'Hidden Valley' site, en-route to Mount Sharp, by Curiosity. Image credit: NASA/JPL-Caltech/MSSS
A series of lakes and streams existed on Mars at some point over three billion years ago, according to observations made by NASA’s Curiosity rover.
The team behind NASA’s Mars Science Laboratory have concluded that water on the Red Planet helped deposit sediment into Gale Crater: the site where the rover landed over three years ago.
This sediment then formed layers that became the foundation for Mount Sharp, which is in the middle of the crater today.
The study builds on recent news of liquid water currently flowing under the Martian surface.
“What we thought we knew about water on Mars is constantly being put to the test,” says Michael Meyer, lead scientist for NASA’s Mars Exploration Program.
“It’s clear that the Mars of billions of years ago more closely resembled Earth than it does today.
Our challenge is to figure out how this more clement Mars was even possible, and what happened to that wetter Mars.”
“Observations from the rover suggest that a series of long-lived streams and lakes existed at some point between about 3.8 to 3.3 billion years ago, delivering sediment that slowly built up the lower layers of Mount Sharp,” adds Ashwin Vasavada, Mars Science Laboratory project scientist at NASA’s Jet Propulsion Laboratory.
Before Curiosity began its survey of Mars, it was proposed that Gale Crater was filled with layers of sediments.
Some suggested the sediment had been built from dust and sand blown by wind, while others explored the idea that the layers had been deposited in ancient lakes.
Results from Curiosity show the latter to be the case for the lower sections of Mount Sharp.
The recent study has determined that the filling of the bottom layers of the mountain was a result mostly of ancient rivers and lakes, and that it took place over a period of less than 500 million years.
“During the traverse of Gale, we have noticed patterns in the geology where we saw evidence of ancient fast-moving streams with coarser gravel, as well as places where streams appear to have emptied out into bodies of standing water,” Vasavada says.
“The prediction was that we should start seeing water-deposited, fine-grained rocks closer to Mount Sharp.
Now that we’ve arrived, we’re seeing finely laminated mudstones in abundance that look like lake deposits.”
NASA says the mudstone shows that bodies of standing water in the form of lakes remained on the Red Planet for long periods of time, and that they may have expanded and contracted repeatedly over hundreds to millions of years.
These lakes deposited the sediment that eventually formed the lower portion of the mountain.
“Paradoxically, where there is a mountain today there was once a basin, and it was sometimes filled with water,” says John Grotzinger, the former project scientist for Mars Science Laboratory at the California Institute of Technology.
“We see evidence of about 75 metres of sedimentary fill, and based on mapping data from NASA’s Mars Reconnaissance Orbiter and images from Curiosity’s camera, it appears that the water-transported sedimentary deposition could have extended at least 150 to 200 meters above the crater floor.”
It is thought that the total thickness of sedimentary deposits that indicate interaction with water could even extend up to 800 metres above the crater floor.
Past this point, the mountain shows no sign of the presence of water, and this portion is the bulk of what forms Mount Sharp.
NASA says this higher segment of the crater’s history could once have been dominated by dry, wind-driven deposits; the same as was previously posited for the lower section.
But it is still unclear where the sediment-carrying water came from.
For flowing water to have existed on the surface, Mars must have had a thicker atmosphere and warmer climate than has previously been theorised for the time period in question.
Some of the water may have run into the lakes from snowfall and rain in the highlands of the Gale Crater rim.
There could also have been an ocean in the plains north of the crater, but this theory would not account for how the water existed as a liquid for extended periods of time on the surface.
“We have tended to think of Mars as being simple,” Grotzinger says.
“We once thought of the Earth as being simple too.
But the more you look into it, questions come up because you’re beginning to fathom the real complexity of what we see on Mars.
This is a good time to go back to reevaluate all our assumptions.
Something is missing somewhere.”