This week, NASA’s Curiosity rover stumbled across the best evidence yet that liquid water once covered much of Mars in the planet’s distant past: undulating rippled rock formations – now frozen in time – that were sculpted by the waves of an ancient shallow lake. But perhaps the biggest surprise is that they were discovered in an area that researchers expected to be dry.
NASA’s Curiosity rover has been exploring Gale crater on Mars since 2014, studying the geologic history of the region and assessing whether the planet could ever have supported an environment suitable for organic life. Starting from the crater floor, the rover has been slowly climbing the foothills of Mount Sharp, the five-kilometer-tall central peak at the heart of Gale crater. As it climbs, it traverses Mars’ geologic history. The lower rock layers at the bottom of the crater represent the oldest periods in Mars’ history, and as it climbs higher it is able to study more and more recent deposits.
The ripples discovered this week were found in a region nicknamed the “Marker Band”, which sits about 800 meters above the crater floor. From orbit, the Marker Band looks dark in color, standing out from the slopes of Mount Sharp. It’s well above the region where scientists expected to find evidence of water.
“We climbed through thousands of feet of lake deposits and never saw evidence like this – and now we found it in a place we expected to be dry,” said Ashwin Vasavada, Curiosity’s project scientist at the Jet Propulsion Laboratory.
The rocks of the Marker Band contain salty minerals called sulfates, which are usually found in regions where previously wet regions are drying up. But the ripples are clear evidence of sediment stirred up by waves, just like the ripples found in the sand of shallow water beaches here on Earth.
Curiosity is decked out with a suit of geology tools, but two attempts to drill into the Marker Band have been unsuccessful – the rock is too hard to penetrate. Future attempts will be made in the weeks ahead. In the meantime, there are other intriguing formations nearby that tell more of the story.
Nearby rocks, positioned just above the ripples, feature overlapping layers that are unusually regular in their spacing and thickness. Such regularity suggests a cyclical cause – perhaps repeated weather events or atmospheric phenomena like dust storms. It’s an indication of a period of variable climate conditions at some point in Mars’ history.
Later this year, Curiosity is expected to have a chance to examine debris from a wet landslide in the nearby Gedis Valley, a depression carved partially by wind and partially by a long-vanished river.
This landslide debris is likely to be the most recent water-driven geology in Gale crater, deposited in a period long after the era of liquid lakes.
Taken together, these varied rock formations are helping build a climate timeline for Gale crater. “The wave ripples, debris flows, and rhythmic layers all tell us that the story of wet-to-dry on Mars wasn’t simple. Mars’ ancient climate had a wonderful complexity to it, much like Earth’s,” said Vasavada.
Featured Image: Ripples formed billions of years ago by waves on a shallow lake, imaged by NASA’s Curiosity rover. Credit: NASA/JPL-Caltech/MSSS