Mud on Mars shows evidence of wet-dry cycles that support life

New research into mud in the Gale Crater, a 95-mile-wade basin on Mars, may be evidence that the red planet was once able to sustain life, according to a study published earlier this month. 

It was not the composition of the mud that intrigued scientists, but the orderly hexagonal patterns that formed when the mud dried and cracked.

Usually, when mud dries it forms a square pattern, however, when mud goes through a repeated cycle of drying, being exposed to moisture, and drying again, it forms a hexagonal pattern.

Scientists have long held the theory that the Gale Crater was once a large lake but have not been able to find definitive proof. The paper, which was published in the peer-reviewed Nature journal explains how the distinctive pattern of the mud cracks offers the first real evidence of wet-dry cycles occurring on Mars.

“These particular mud cracks form when wet-dry conditions occur repeatedly – perhaps seasonally,” said the paper’s lead author, William Rapin of France’s Institut de Recherche en Astrophysique et Planétologie.

How wet-dry cycles support life

“This is the first tangible evidence we’ve seen that the ancient climate of Mars had such regular, Earth-like wet-dry cycles,” Rapin said. “But even more important is that wet-dry cycles are helpful – maybe even required – for the molecular evolution that could lead to life.”

Since 2012, a Mars rover called Curiosity has been tasked with exploring the Gale Crater. NASA scientists chose the Gale Crater as the landing site for Curiosity because of the many signs that water was once present there. 

In 2016, Curiosity took clay samples from the crater that turned out to be glauconitic clay, which was significant as this type of clay can only form under low-acidity conditions that are hospitable to life. 

And in 2021, Curiosity made another amazing discovery during its ascent of the three-mile-high Mount Sharp in the center of the Gale Crater when it found the hexagonal mud cracks. 

The cracks were found in a transitional area positioned between two different types of sediment. Underneath was a clay-rich layer of mud that forms through contact with water, and below was a salted layer of mud that forms in dry conditions. The hexagonal mud had alternated between wet and dry, which caused its hexagonal pattern to form. 

The hexagons were preserved within 3.6 billion-year-old mudstone, cemented in place by sulfate salts, and extended 4 inches down into the stone for at least 4 inches, indicating that the sediments had been layered onto the surface in repeated wet and dry cycles.

“As these elements and organic molecules are forced closer and closer together with increasing salinity, they may start polymerizing and make longer chains, creating the conditions for spontaneous chemistry that may start the complex chemical evolution that could lead to living organisms," said Juergen Schieber, professor of geology at Indiana University Bloomington, in a statement. 

“Wet-dry cycles are helpful,” Rapin said, and “maybe even required for the molecular evolution that could lead to life.”