All signs pointed to an ancient lakebed in Gale Crater, where the Curiosity rover has been exploring Martian geology. But a new study suggests that wind, not water, created the tell-tale rock layers.

For almost two decades, NASA has been “following the water” on Mars. This is the reason Curiosity Rover was sent to Gale Crater in 2012. At the time of launch, scientists thought the site looked like a massive lakebed.

But now, an independent analysis of both Curiosity data and recent imagery taken from orbit suggests that the appearance of the rock layers previously thought to be deposits laid down by water actually come from wind and chemical weathering instead.

Gale Crater
Curiosity's image of Mount Sharp in Gale Crater shows layered sedimentary rocks. The rover has been driving from the floor of the crater up through these hills in order to understand how the rocks change from lower in the section (older) to higher in the section (younger).
NASA's Mars Curiosity Rover

The new study by PhD student Jiacheng Liu (The University of Hong Kong) and colleagues appears in the August 6th Science Advances. They begin by pointing out that despite early observations that indicated Gale Crater once held a large body of water, few of the rocks Curiosity has examined actually look like lake deposits.

The team thinks that this discrepancy is best explained if the crater only ever had a small smattering of little ponds. Then the majority of the ground explored to date would best be explained as basalt sand and silt. The layers of different kinds of rock, which were thought to have been formed by water activity, could instead have been created by wind in a caustic atmosphere.

Previously, global studies of Mars showed a mineralogical transition from iron and magnesium clays to aluminium clays to sulphate rocks. There are different theories about what this shifting ground composition might mean. Some think it might be evidence of a great environmental transition, perhaps the one that saw Mars change from a warmer and wetter world to the icy desert landscape it is today. Others think it might be more complicated than that.

“Sulphates are like a kind of salt,” says team member Joe Michalski. “So these younger minerals might represent a drying out of the planet. But this doesn't necessarily mean global climate change happened at that moment.”

At the very beginning of its journey, Curiosity identified some thin mudstone at the bottom of Gale Crater, which likely formed when mud settled out of calm water. But since then, as the rover has traveled up to the base of Mount Sharp, it has not seen any more lakeform rocks, but it has seen a slow change in the chemical content of the samples.

Data from Curiosity seem to indicate that elements, such as aluminum, that stay put under chemical weathering conditions become more prevalent at higher altitudes. Other elements that are more easily dispersed, such as iron, become scarcer. This push-and-pull relationship is similar to what we see on ancient, exposed rock formations on Earth, which used to have a very different atmosphere than it does today. The researchers argue that the rocks in Gale Crater were chemically weathered in a low-oxygen greenhouse atmosphere containing methane and hydrogen gas. 

Two scenarios for water in Gale Crater
These images of Gale Crater show elevation colorized in blue. The image on the left shows the standard model, in which Gale Crater once hosted a large lake. The image at the right shows what Liu and colleagues propose: Only small, shallow lakes ever existed on the floor of Gale Crater. Wind deposited most of the sediments, which weathered under precipitation or ice-melt. A star marks the rover’s landing site.

The Mars Science Laboratory (NASA’s Curiosity group) have attributed the compositional shift in the Gale crater rocks to the original “big lake hypothesis”. But this new analysis suggests that the data is more ambiguous.

“This new paper says you don't really need much water at all,” says Bradley Thomson (University of Tennessee-Knoxville), who was not involved in the study. “It points out a possible ambiguity in the data that only more data—a lot more samples or even sample return—could resolve.”

Perseverance Rover is attempting to collect samples for an eventual return to Earth, and it’s possible that these could provide more definitive clues to Mars’s water history. Until then, scientists must be content with watching Curiosity’s slow trek up Mount Sharp, data point by single data point, constructing a plausible theory of the crater’s past.




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