Data collected by the Curiosity Rover suggests Mars once featured a moderate climate capable of fostering lakes of liquid water and even a vast sea, and that this climate could have extended to many parts of the Red Planet.
NASA's Curiosity Rover is currently investigating the lowest sedimentary layers of Mount Sharp, a section of rock 500 feet (150 meters) high known as the Murray formation. Observations taken by the robotic probe suggests the mountain was produced by sediments deposited in a large lake bed over tens of millions of years. The observation strongly suggests that ancient Mars maintained a long-lasting water-friendly climate.
According to NASA scientists, it's an hypothesis that's challenging the notion that warm and wet conditions were transient, local, or only underground. It now appears that Mars' ancient, thicker atmosphere raised temperatures above freezing globally, but NASA scientists aren't entirely sure how the atmosphere produced the required effects.
Scientists have struggled to explain why the mountain sits inside a crater. Last year, a study suggested that the 3.5-mile-tall Mount Sharp formed as strong winds carried dust and sand into the crater in which it rests. It was actually bad news at the time because it suggested that the Gale Crater probably never contained a lake, which was one of the primary reasons why NASA sent Curiosity there in the first place.
But this new analysis has revived an older theory which suggests that Mount Sharp is the eroded remnant of sedimentary layers that once filled the crater — layers of silt that were originally deposited on a massive lakebed.
Unevenly layered rock showing a pattern typical of a lake-floor sedimentary deposit. Image credit: NASA/JPL-Caltech/MSSS.
Thanks to on-the-ground observations made by Curiosity, NASA scientists have now caught a glimpse of Mount Sharp's lower flanks, which feature hundreds of rock layers. These layers, which alternate between lake, river, and wind deposits, bear witness to the repeated filling and evaporation of a Martian lake. Rivers carried sand and silt to the lake, depositing the sediments at the mouth of the river to form deltas. It was a cycle that repeated over and over again.
Cross-bedding seen in the layers of this Martian rock is evidence of movement of water recorded by waves or ripples of loose sediment the water passed over. Image credit: NASA/JPL-Caltech/MSSS.
"The great thing about a lake that occurs repeatedly, over and over, is that each time it comes back it is another experiment to tell you how the environment works," noted Curiosity Project Scientist John Grotzinger in a NASA report. "As Curiosity climbs higher on Mount Sharp, we will have a series of experiments to show patterns in how the atmosphere and the water and the sediments interact. We may see how the chemistry changed in the lakes over time. This is a hypothesis supported by what we have observed so far, providing a framework for testing in the coming year."
After the sediments hardened to rock, the resulting layers of sediment were sculpted over time into a mountainous shape by wind erosion that carved away the material between the crater perimeter and what's now the edge of the mountain.
The new discovery has major implications for our understanding of the Red Planet. It suggests Mars was far warmer and wetter in its first two billion years than previous assumed. It also suggests that Mars experienced a vigorous and dynamic global hydrological cycle that involved rains or snows to maintain such moderate conditions.
A gif depicting a lake of water partially filling Mars' Gale Crater, receiving runoff from snow melting on the crater's northern rim. Image credit: NASA/JPL-Caltech.
It also means that Mars may have featured an ocean somewhere on its surface. And in fact, as noted by NASA scientist Ashwin Vasavada, "If we have a long-standing lake for millions of years, the atmospheric humidity practically requires a standing body of water like an ocean to keep Gale from evaporating."
Strangely, while evidence for liquid water on the surface continues to mount, simulations of the ancient Martian climate have yet to show the conditions that could have actually produced long periods warm enough for stable surface water.
According to Vasavada, even when accounting for a thicker atmosphere of carbon dioxide and other greenhouse gases, models can't seem to raise global temperatures high enough.
The researchers will continue to ponder these questions as Curiosity uncovers more invaluable data.