Mount Sharp – a mountain on Mars and the primary target of NASA’s Mars rover Curiosity – was built by sediments deposited in a large lake bed over tens of millions of years, says rover science team.
Gale crater is 155 km in diameter and now holds a layered mountain rising about 5 km above the crater floor. This illustration depicts a lake of water partially filling the crater. Image credit: NASA / JPL-Caltech / ASU / UA / Sci-News.com.
Mount Sharp, officially known as Aeolis Mons, forms the central peak within a very large crater named Gale that is 155 km in diameter.
The mountain stands about 5 km tall, its lower flanks exposing hundreds of rock layers. The rock layers – alternating between lake, river and wind deposits – bear witness to the repeated filling and evaporation of a lake much larger and longer-lasting than any previously examined close-up.
“If our hypothesis for Mount Sharp holds up, it challenges the notion that warm and wet conditions were transient, local, or only underground on Mars. A more radical explanation is that Mars’ ancient, thicker atmosphere raised temperatures above freezing globally, but so far we don’t know how the atmosphere did that,” said team member Dr Ashwin Vasavada of NASA’s Jet Propulsion Laboratory in Pasadena, California.
This view from the Mast Camera on board Curiosity shows an example of cross-bedding from water passing over a loose bed of sediment. The cross-bedding – evident as layers at angles to each other – reflects formation and passage of waves of sand, one on top of the other. These are known as ripples, or dunes. The direction of migration of these small ripples and dunes was toward the southeast. That direction is toward Mount Sharp and away from the area where Curiosity found evidence of delta deposits where a stream entered a lake. The directional flows recorded in the sediments are interpreted to have formed by currents moving down the deltas and into deeper lake water. The color has been approximately white-balanced to resemble how the scene would appear under daytime lighting conditions on Earth. Image credit: NASA / JPL-Caltech / MSSS.
“We are making headway in solving the mystery of Mount Sharp. Where there’s now a mountain, there may have once been a series of lakes,” added team member Dr John Grotzinger of the California Institute of Technology in Pasadena.
Curiosity currently is investigating the lowest sedimentary layers of Mount Sharp, a section of rock 150 m high, dubbed the Murray formation. Rivers carried sand and silt to the lake, depositing the sediments at the mouth of the river to form deltas similar to those found at river mouths on Earth. This cycle occurred over and over again.
This view from the Mastcam on Curiosity looks southward at the Kimberley waypoint. In the foreground, multiple sandstone beds show systematic inclination to the south suggesting progressive build-out of the sediments toward Mount Sharp. At this location, about 1.6 km north of the base of Mount Sharp, these inclined beds can be traced up to about 100 meters in the direction of build-out. These inclined beds are interpreted as small deltas building out into a shallow lake. As sediment-laden river water encountered a standing body of water, the river current was forced to abruptly decelerate, leading to rapid deposition of sediment at the river mouth. This deposition led to formation of a delta. Continued supply of sediment by rivers flowing from the crater rim led to deltas building out into the lake towards the south. The camera recorded the component frames of this mosaic on March 25, 2014, during the 580th Martian day, or sol, of Curiosity’s work on Mars. The color has been approximately white-balanced to resemble how the scene would appear under daytime lighting conditions on Earth. 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,” Dr Grotzinger said.
“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.”
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. This image was acquired by Curiosity at a target called Whale Rock in the basal geological unit of Mount Sharp. The camera took it during the 796th Martian day, or sol, of Curiosity’s work on Mars – November 2, 2014. The color has been approximately white-balanced to resemble how the scene would appear under daytime lighting conditions on Earth. This example of cross-bedding shows evidence of small climbing ripples that migrate on top of each other – just above 10 in scale bar. This suggests currents of water entered into a lake basin, possibly flowing down the front of a delta, and then spread out across the lake floor, slowing down, and depositing sediment. Image credit: NASA / JPL-Caltech / MSSS.
After the Gale crater filled to a height of at least a few hundred meters and the sediments hardened into rock, the accumulated 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 is now the edge of the mountain.
On the 8-km journey from the rover’s landing site to its current work site at the base of Mount Sharp, Curiosity uncovered clues about the changing shape of the crater floor during the era of lakes.
This image looks to the west of the Kimberley waypoint on the Curiosity’s route to the base of Mount Sharp. The mountain lies to the left of the scene. The image shows sets of sandstone beds all inclined to the south (left) indicating progressive build-out of sediment toward Mount Sharp. These inclined beds are overlain in the background by horizontally bedded fine-grained sandstones that likely represent river deposits. The camera recorded the component frames of this mosaic on April 4, 2014, during the 590th Martian day, or sol, of Curiosity’s work on Mars. The color has been approximately white-balanced to resemble how the scene would appear under daytime lighting conditions on Earth. Image credit: NASA / JPL-Caltech / MSSS.
“We found sedimentary rocks suggestive of small, ancient deltas stacked on top of one another. Curiosity crossed a boundary from an environment dominated by rivers to an environment dominated by lakes,” said team member Dr Sanjeev Gupta of Imperial College in London.
“Knowledge we’re gaining about Mars’ environmental evolution by deciphering how Mount Sharp formed will help guide plans for future missions to seek signs of Martian life,” said Dr Michael Meyer, lead scientist for the Mars Exploration Program at NASA headquarters in Washington, DC.
