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?
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.
“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.”
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.
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.
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.