Rounded Stones on Mars are Evidence of Flowing Water

NASA’s Curiosity rover has returned images of what may have been a riverbed on Mars. The observations were made with Curiosity’s stereo camera and reveal areas with densly packed gravel and pebbles. The pebbles are cemented together like concrete. After studying the size and shape of the pebbles, a team of scientists has reported that the features are clear evidence of flowing water on ancient Mars. According to the study, the depth of the liquid ranged between 0.03 meters to 0.9 meters.

“In order to have moved and formed these rounded pebbles, there must have been flowing water with a… flow rate of about 1 meter per second – or 3.6 km/h – slightly faster than a typical natural Danish stream.”

– Morten Bo Madsen, head of the Mars research group at the Niels Bohr Institute

The individual stones in the area are rounded like pebbles on Danish shores and were probably transported down through a ravine, Peace Vallis, which is carved in the southern slope of the northern crater wall. Credit: Malin Space Science Systems
The circle shows the proposed landing site for Curiosity inside Gale crater. Credit: NASA/JPL-Caltech/ASU/UA
The Mars rover Curiosity landed on 6th of August 2012 in the large crater, Gale, which lies right at the border between the northern and southern hemispheres. The crater is 154 kilometers in diameter and the walls of the crater are almost 5 kilometers high. In the middle of the crater lies Mount Sharp, which is approximately 5 km high. The area is particularly interesting, because here one finds ancient sediments more than 2-3 billion years old, when the environment was quite different from that found today. Credit: NASA/JPL-Caltech

Why is this research important to Astrobiology?

The potential for life on Mars (and other celestial bodies beyond Earth) is on of the major areas of study for astrobiologists. NASA’s Astrobiology Program lists the search for life in our solar system as one of the program’s major goals in the NASA Astrobiology Roadmap.

Because life as we know it requires liquid water to survive, the questions about life’s potential on Mars currently center around whether or not environments with liquid water once existed on the surface of the planet. This new research supports the idea that liquid was indeed present, indicating that habitats where life could have gained a foothold may have once been present on ancient Mars.


Where can you read more about this research?

The scientific paper, "Martian Fluvial Conglomerates at Gale Crater" was published in the journal Science under lead author R.M.E. Williams. The abstract is available at:

The study area, which has been named ‘Hottah’, is by all accounts the remains of sediments from the bottom of an ancient stream, which had a relatively strong current. Credit: Malin Space Science Systems
The study area, which has been named ‘Hottah’, is by all accounts the remains of sediments from the bottom of an ancient stream, which had a relatively strong current. Credit: Malin Space Science Systems



Soar over the crater on Mars where NASA’s Curiosity rover made its spectacular landing. Narrated by John Grotzinger, project scientist for the Mars Science Laboratory mission.

Astrobiology Magazine contacted Dr. Rebecca Williams, lead author of the study, with some additional questions about his research:

Astrobiology Magazine: Is there any other process that could have created these formations? Why is flowing water the most likely explanation?

Williams: The rounded shape of the pebbles is a testament to a transport process, either wind or water.  Rocks that exceed a few mm in size are too large to move by wind.  The typical pebble size is around 1 cm, and the largest rocks are 4 cm across.  Thus, at this size, fluvial transport is the most likely process.

AM: How widespread are these features in the Curiosity landing site? Are they commonly seen in the area that the rover has explored so far?

Williams: We’ve encountered a half-dozen outcrops of these water-transported conglomerated during our journey across Bradbury Rise. These are patchy occurrences and we are hoping to find more exposures as we travel back across this terrain en route to Mount Sharp. The MSL science team has a number of hypotheses for how these conglomerates relate to other rocks in this area, including the ones we have drilled near Glenelg, and seeing additional conglomerates will help refine our understanding of the overall ancient environment.

AM: Why is the study area named ‘Hottah’?

Williams: The MSL science team names rocks and other science targets for ease of reference.  Generally the names are for noteworthy geological sites on Earth.  At noteworthy sites, we also pay tribute to the memory of individuals who have contributed significantly to the mission.

AM: How can you tell the depth and flow rate of the water that created these features?

Williams: Using what we know about the physics of water flow and the motion of rocks in rivers, we can estimate the minimum water flow conditions required to move rocks of this size on Mars.

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