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Known as the “red planet”, Mars is a harsh desert world. Turbulent, continent-sized dust storms can last for weeks there, and the average temperature is minus 62 degrees Celsius (-80 degrees Fahrenheit). Liquid surface water is absent due to the cold temperature and thin atmosphere, but Mars is nonetheless covered with geologic clues to a warmer, wetter past: alluvial fans, canyons, and dry basins all point to a period in Martian history when surface water flowed across the planet. 

To piece together the sequence of events that led to modern-day Mars, researchers look for clues of past climatic conditions preserved in Martian sediments—just as how researchers study fossilized pollen grains or marine invertebrates to reconstruct paleoclimates on Earth.  

The biggest issue with this? Mars and Earth are separated by, on average, 140 million miles of space. Sediment samples are hard to come by. That’s where Mars rovers such as Curiosity come in, diligently taking measurements and beaming back high-resolution images from Gale Crater, where it has been operating since 2012. 

“Thanks to Curiosity, we’ve seen evidence of stream deposits and other fluvial landforms in and around Gale Crater,” said David Rubin, a USGS Emeritus and researcher in the Department of Earth and Planetary Sciences at the University of California, Santa Cruz. “From the photos, we can use standard field sedimentology procedures to infer many of the processes that shaped these landforms. One of the big questions we had was whether the lake that once filled Gale Crater was free of ice or covered in it, which has major climate implications.” 

By studying the Curiosity images and using a combination of sedimentary analysis and wave modeling, Rubin and collaborators, including the Pacific Coastal Marine Science Center (PCMSC), found evidence that ripples seen on lakebed deposits in Gale Crater were likely caused by waves at the lake surface. This implies that the lake surface was periodically ice-free. Lack of ice points to a warmer, denser atmosphere than exists on Mars today. Their findings are published in the Journal of Geophysical Research: Planets. (Read it: “Ancient winds, waves, and atmosphere in Gale Crater, Mars, inferred from sedimentary structures and wave modeling.”)

The study involved five different methods: modeling waves generated by predicted winds in a lake in Gale crater; predicting wind speeds and atmospheric pressures for use in the wave model; predicting what ripples the modeled waves could have produced; processing of images taken by Curiosity; and interpretation of candidate wave ripples observed in those images. 

A key question for researchers was determining how different environmental conditions on Mars affect waves on Martian lakes. “We had to tweak the numerical model we use on Earth to account for reduced gravity and atmospheric density to predict wave heights on Mars,” said Andrew Stevens, USGS Oceanographer at PCMSC and a co-author of the study. Due to its smaller size, the gravity of Mars is about a third of that on Earth, and the thinner atmosphere means that wind exerts less force as it blows over the water surface. “These two factors really change the way wind waves form on Mars compared to Earth,” said Andrew.   

The researchers also developed methods to differentiate between ripple deposits formed on land and those formed underwater. Wind-driven, or aeolian, ripples in sediment are formed under the dry, thin-atmosphere conditions present on Mars today. These are distinctly different in size and shape from wave-driven lakebed ripples, which form underwater on lake sediment.  

Water-carved geologic features on Mars are billions of years old. Today, liquid water on Mars only exists underground; surface water either froze at its polar regions or boiled away when Mars lost its magnetic field around 4 billion years ago, thinning the atmosphere and leaving the planet without thermal insulation. Atmospheric pressures on Mars are comparable to pressures found more than 32 kilometers (20 miles) above Earth’s surface. 

In Curiosity images taken in a region of Gale Crater known as the Kimberly formation, researchers found at least one deposit of thin-atmosphere, wind-driven ripples that are overlain by subaqueous ripples, suggesting that denser atmospheric conditions may have come and gone before disappearing entirely, taking the surface water with it.

“Much of what we know about past climatic conditions on Mars is based on modeling and interpretation of large morphological features on the surface,” Rubin said. “Large ice-free lakes such as the one that once filled Gale Crater have long been suspected, but this is the first time we’re seeing ripples in sediment that likely formed underwater, from waves on a lake on Mars.” 

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