Mars: Once Upon A Day At The Beach

The Curiosity Rover discovered lots of pebbles next to broken slabs of conglomerate rock back in 2013. Here we see pebbles by the hundreds at the Hottah site, a sure sign of water that once flowed in rivers and accumulated in a large lake in Gale Crater. Credit: NASA
The Curiosity Rover discovered lots of pebbles next to broken slabs of conglomerate rock back in 2013. Here we see pebbles by the hundreds at the Hottah site, a sure sign of water that once flowed in rivers and accumulated in lakes in Gale Crater. Credit: NASA/JPL-Caltech/MSSS

I live in a tourist town. A favorite pastime of visitors is to head down to the pebble beach along Lake Superior and skip rocks on the water. The flatter they are, the better they skip. Each pebble’s rounded contours are visible evidence of the erosive power of the lake before them. Years and years of tumbling and slamming by wind-whipped waves have turned once gnarly rocks into sensual orange, black and pearly pebbles. Who can resist slipping one into a pocket?

LAKE GALE. High-resolution data and images reveal geological details that point to a former lake inside Gale Crater. According to the evidence, the lake's water level stood for a while at several elevations. The images show this lake at two levels: –2277 meters (where it joins a proposed ocean in the northern lowlands) and –3377 m. (Images from the abstract.)
Data and photos from the Curiosity Rover point to a former lake inside Gale Crater. According to the evidence, the lake’s water level stood for a while at several elevations as it shrunk and ultimately dried up. Click to read the paper on the topic. Credit: William Dietrich (University of California, Berkeley)

Now a new study from NASA’s Curiosity Rover team has confirmed that Mars, too once had lakes capable of storing water for a long time and rivers with currents powerful enough to wear down rough rock into pebbles. Using data from the Curiosity rover, the team has determined that long-lived streams and lakes that existed 3.3 to 3.8 billion years ago delivered and deposited sediment into Gale Crater, where the rover landed more than three years ago. Those layers built up over a long period of time to form the foundation for Mount Sharp, the wide mountain the dominates the center of the crater today.

Before Curiosity landed on Mars in 2012, scientists proposed that Gale Crater had filled with layers of sediments, but it was unclear whether they were left by water or accumulations of wind-blown dust and sand. The latest results from Curiosity indicate that at least for the lower levels of Mount Sharp, most of the material came by way of ancient rivers and lakes over a period of less than 500 million years.

A view from the "Kimberley" formation on Mars taken by NASA's Curiosity rover. The strata in the foreground dip towards the base of Mount Sharp, indicating flow of water toward a basin that existed before the larger bulk of the mountain formed. Credits: NASA/JPL-Caltech/MSSS
Did a lake once sparkle in the Martian sunlight here? A view from the “Kimberley” formation on Mars taken by NASA’s Curiosity rover. The strata in the foreground dip towards the base of Mount Sharp, indicating flow of water toward a basin that existed before the larger bulk of the mountain formed.
Credits: NASA/JPL-Caltech/MSSS

“During the traverse of Gale, we have noticed patterns in the geology where we saw evidence of ancient fast-moving streams with coarser gravel, as well as places where streams appear to have emptied out into bodies of standing water,” said Ashwin Vasavada, Mars Science Laboratory project scientist, in a recent press release. “The prediction was that we should start seeing water-deposited, fine-grained rocks closer to Mount Sharp. Now that we’ve arrived, we’re seeing finely laminated mudstones in abundance that look like lake deposits.”

This evenly layered rock photographed by the Mast Camera (Mastcam) on NASA's Curiosity Mars Rover on Aug. 7, 2014, shows a pattern typical of a lake-floor sedimentary deposit not far from where flowing water entered a lake. Aug. 7, 2014, during the 712th Martian day, or sol, of Curiosity's work on Mars. It shows an outcrop at the edge of "Hidden Valley," seen from the valley floor. This view spans about 5 feet (1.5 meters) across. Credit:
This evenly layered rock, photographed by the Curiosity Mars Rover on Aug. 7, 2014, shows a layered pattern typical of a lake-floor sedimentary deposit not far from where flowing water entered a lake. The outcrop lies at the edge of “Hidden Valley” as seen from the valley floor and spans about 5 feet (1.5 meters) across. Credit: NASA/JPL-Caltech/MSSS

Mudstone forms from sediments that accumulate over long periods of time in standing water in the form of lakes. How much mudstone and other sedimentary goodies has Curiosity found? Lots! So far about 250 feet (75 meters) of sedimentary fill and that may only be the tip of the mud-berg. Based on mapping data from NASA’s Mars Reconnaissance Orbiter and images from Curiosity’s camera, it appears that the water-transported sediments may reach at least 500 to 650 feet (150 to 200) meters above the crater floor.” Given Gale Crater’s 96-mile-wide (154 km) diameter, we’re talking a very large lake indeed. Add in those gusty Martian winds whipping up white-topped breakers and you’ve got a recipe for pebble-making.

An image taken at the "Hidden Valley" site, en-route to Mount Sharp, by NASA's Curiosity rover. A variety of mudstone strata in the area indicate a lakebed deposit, with river- and stream-related deposits nearby. Credits: NASA/JPL-Caltech/MSSS
An image taken at the “Hidden Valley” site, en-route to Mount Sharp, by NASA’s Curiosity rover. A variety of mudstone strata in the area indicate a lakebed deposit, with river- and stream-related deposits nearby.
Credit: NASA/JPL-Caltech/MSSS

But the big question remains. What was the source of the water? For flowing water to have existed on the surface, Mars must have had a thicker atmosphere and warmer climate several billion years ago. While evidence abounds for water flow across the Red Planet, no one is certain how Mars remained warm long enough for all those sediments to accumulate in lakes and rivers. The sun was 25-30% less luminous 3-4 billion year ago than it is today which would have made the planet decidedly chilly during the time we see the greatest evidence for standing water. Did Mars’ atmosphere contain more of the heat-trapping greenhouse gas CO2 back then? Maybe. If so, we’d see evidence for large exposures of carbonates or rock that form when CO2 interacts with water. We don’t.

At least some of the water may have been supplied to the lakes by snowfall and rain in the highlands of the Gale Crater rim, but it remains a mystery how water managed to exist as a liquid for such a long time in the distant past.

(a) Shape evolution of a single particle constantly colliding with a flat surface is described by Firey’s equation8 ν=cκ, where ν is the speed of abrasion in the inward normal direction, c is a constant and κ is the local curvature. This is illustrated on a quadrangle. b–d show example pebbles from each system studied, with comparable shape parameters as indicated beneath each image (IR: circularity, C: convexity, b/a: axis ratio, Fig. 3). (b) Sketch of the rotating drum experiment, limestone pebble samples (a≈15–35 mm) and mean shape parameter values after 0, 10.6 and 20.7% mass loss. (c) Aerial image of Dog Canyon fan, example limestone pebble contours (a≈20–40 mm) and mean shape parameter values at x=0, x=1.18 and x=2.10 km. Grains were collected from the active channel denoted by the blue line. (d) A few Martian grain contours (b≈2–32 mm; ref. 2) and mean shape parameter values at Sols 389, 27 and 356. Image credits: (c) Google Earth; (d) NASA/JPL-Caltech/MSSS.
Comparisons of how rocks are rounded by tumbling from the researchers’ experiment on how far Mars pebbles were transported. (a) How a single particle changes shape while constantly colliding with a flat surface; (b) Sketch of the rotating drum experiment with limestone pebble samples; (c) Aerial photo of Dog Canyon fan, material was collected from the active channel (blue line) and (d) A few small Martian rocks showing varying degrees of roughness. Credits: Nature Publishing Group / Tímea Szabó, Gábor Domokos,John P. Grotzinger and Douglas J. Jerolmack  (c) Google Earth; (d) NASA/JPL-Caltech/MSSS.

Let’s look at those pebbles again. A group of scientists recently published a study on how far Martian rocks would have to be transported by water to turn into the smooth stones first seen by the Curiosity Rover not far from its landing site in 2013. Co-author Douglas Jerolmack, a geophysicist at the University of Pennsylvania, and team examined pebbles at different points in a river in Puerto Rico and in the Dog Canyon alluvial fan in New Mexico to determine over what distance raw rocks evolve into smooth stones.

Taking into account Mars’ lesser gravity (only 38% that of Earth) and the hardness of its basaltic rock vs. the limestone of Dog Canyon, the researchers found that Curiosity’s pebbles must have traveled roughly 30 miles, a sure sign they were transported by a long-gone river. Will Mars climate change again in the distant future, offering the possibility for astronauts to stand on the shore of a new Martian lake and skip pebbles as lustily as we do on Lake Superior? We can only hope.