Ancient Mars Crater Had Huge Lakes

They were good while they lasted

Curiosity Rover

Curiosity Rover

A self-portriat of the Curiosity rover from January 2015NASA/JPL-Caltech/MSSS

Last week, NASA announced that Mars has salty, ephemeral liquid water on its present-day surface. But Mars used to be a whole lot wetter. Today's martian tears are just drops compared to the lakes of Martian sols gone by.

In a paper published today in Science, researchers announced that the Curiosity rover found geological evidence of ancient lakes on Mars.

Scientists had long suspected that Mars had the possibility of lakes--there are plenty of basins and depressions on the Mars surface that would act like perfect bowls, able to hold water. Other observations from satellites has indicated that these depressions once held water.

Gale crater, where Curiosity landed, is one of those excellent basins, formed around 3.8 billion years ago when a meteorite slammed into the planet. Last year researchers announced preliminary evidence that there was likely a large lake in Gale crater that scientists thought might have lasted for millions of years, but a closer look reveals that instead of one monolithic lake, it was probably a shifting sequence of lakes that built up sediments over time.

The lakes in the crater were likely fed by a combination of groundwater and rivers. As the lakes filled, the rivers that brought water into the lake also brought sand, gravel and rocks into the region, building deltas out into the lake and leaving behind layers of sediment along the ancient shoreline. In the center of the lake, away from the turbulent river(s), fine sediments settled to the bottom of the lake, forming a thick layer of mud. Over time, the position of the rivers shifted, just like they do on Earth today. As the rivers shifted, they deposited sand and gravel in different places around the lake. These deposits of sandstone (from the river system) and mudstone (from a lake environment) are what the Curiosity Rover has found, giving scientists on Earth clues that there were once lakes on Mars.

The lake remains described in the new paper were temporary, only lasting on average between 100 and 10,000 years (a long time by human standards, but the blink of a geological eye). This happened over and over again, with lakes filling and drying up over the course of 10,000 to 10 million years within Gale crater.

Eventually, for still-unknown reasons, the lakes completely dried up. Slow and steady windstorms (remember, The Martian-style windstorms are purely science fiction) carried sand over the lake sediments, covering them, and probably built up the 3-mile tall Mount Sharp, the large mountain in the center of Gale crater. (Mount Sharp has been a mystery for a while, and other researchers have speculated that the mountain was built entirely by sediments deposited in water.) Then, over even more time, parts of the sediments that built Mount Sharp were eroded away, back down to the lake sediments (by more wind), leaving the current landscape for Curiosity to explore.

Clearly, in order for the lakes to form, Mars had to have a lot more liquid water than it does today, but most of the current models of the ancient Mars climate have painted the planet as extremely cold, with temperatures that should have turned liquid water to ice. But the geologists looking at the Curiosity data didn't find evidence of an icy, glaciated surface, they found evidence of rivers flowing into large lakes between 3 and 12 feet deep at the edges.

The news that Mars had long-lasting bodies of water on its surface is fascinating, and opens up a whole new field of questions, including new questions about the ancient Mars climate--could it have actually been as cold as we thought, if there was liquid water? But it also opens up a new area of inquiry looking for evidence of past life on the planet. As the authors write in the paper "One of the key criteria for planetary habitability is the duration for which water might have been accessible to enable microbial origination and evolution." With water present in pools for hundreds, if not thousands of years, there is a chance that microbial life could have developed.

Whether or not we'd be able to see any evidence of those microbes, even if they existed is still a matter of debate. Unlike large bones from animals like dinosaurs, tiny, single-celled organisms don't preserve well at all. But Mars having standing water, just like the early Earth, gives the idea of life on Mars new life. On Earth, large pools of water on the planet's surface are believed to be the setting for the origin of life, and seeing similar lakes and pools on Mars makes some scientists think that our planet might not be alone in giving rise to life after all.

As Marjorie Chan writes in an accompanying paper in Science, "The more the geology looks like Earth, the more likely it seems that some life-form(s) could have developed in the martian waters."