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IU professor, Juergen Schieber, discovers evidence of sustained wet-dry cycling in Mars’ history

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IU earth and atmospheric sciences professor, Juergen Schieber, and his research team have discovered evidence of wet-dry cycling, a geological process advantageous to the formation of life, on the surface of Mars. In a paper released Aug. 9, 2023, in the scientific journal, “Nature,” Schieber and his team examined Martian rock dating to around 3.6 billion years old to better understand the red planet’s geological history.

IU chemistry department professor David Bish, who did not work on Schieber’s study, said understanding wet-dry cycling on Mars is just one piece of a larger puzzle in understanding the history of the planet. He said by comparing early Mars to early Earth, scientists can gain new insights into the processes of how the surfaces of each planet formed.

“There’s so many examples of things that we learned on Mars that clarified things on Earth,” Bish said.

In a previous paper co-authored by both Schieber and Bish, the scientists included a quote from science-fiction author Ray Bradbury’s notes on his popular “Martian Chronicles,” poetically explaining the significance of understanding Mars as an avenue to understanding our own planet, and where we come from.

“But there they go, off to Mars, just for the ride, thinking that they will find a planet like a seer's crystal, in which to read a magnificent future. What they will find, instead, is the somewhat shopworn image of themselves. Mars is a mirror, not a crystal,” Bradbury wrote.

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Roughly 3.6 billion years ago, the surfaces of Earth and Mars would have been completely unrecognizable to us today. A human transported back to this period on Earth would almost immediately die from a lack of oxygen and the presence of toxic gases such as methane and ammonia. But before dying, they may notice possible expansive oceans, and if looked upon closely enough, the beginnings of photosynthetic life on an otherwise barren surface.

Scientists believe that 3.6 billion years ago, Mars had liquid water covering much of its surface via lakes, rivers and possibly oceans. However, not much was known about the short-term climate of the planet until recently, when humans began to send rovers to study the Martian surface.

By studying climate in smaller scales, researchers hoped to understand more about what shorter-term weather cycles looked like, giving insight into possible attempts of life to form on the red planet.

On Aug. 6, 2012, NASA’s car-sized Curiosity Rover landed inside Gale Crater, the site of an ancient meteorite impact just south of the Martian equator. It soon began collecting data about the crater’s geological and climatic history, supplying researchers on Earth with data for analysis.

Using the rover’s data, Schieber and his team scoured through photo and chemical evidence to determine more about early Mars’ short-term climate. In this paper, the authors describe the planet at this time as having repeated wet-dry cycles, indicating a regular and possible seasonal climate in the observed area.

Bish said wet-dry cycles on Mars would have looked like non-permanent lakes in the arid southwestern United States, but colder. He said the process in Gale Crater would have been about the same, with water flooding in for a few months, then evaporating away. This process would repeat over and over, carrying sediments and minerals each time.

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Schieber said one piece of evidence for wet-dry cycling was the observation of polygon-shaped cracks in the mud rocks. He said as the mud dries after water exposure, it shrinks slightly, leaving T-shaped cracks. If this process happens repeatedly over long periods of time, they mature into Y-shaped cracks, generating hexagonal or polygonal patterns in the dried mud.

Schieber said the research team found a multitude of these patterns in the Gale Crater site, indicating repeated wet-dry cycling. Another piece of evidence Schieber’s team utilized was the accumulation of sulfates within the ancient lakebed.

Abhijit Basu, a retired IU sedimentary geology professor, said sulfates, a group of mineral salts, usually signify the presence of water. When water evaporates off rock, it can leave behind any minerals it carried with it, including magnesium or calcium sulfates, as seen at Gale Crater.

Basu said that not only do the sulfate measurements agree with the conclusion of wet-dry cycling, but they also show repetition via layering.

“The importance of the paper is that this is repeated through time,” Basu said. “Although we cannot measure the time, we can at least say that there are layers, one after the other.”

In the paper, Schieber’s team speculated that consistent wet-dry cycling could have been conducive to prebiotic evolution, the building of complex molecules that could eventually form life.

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Bish said wet-dry cycling means that simpler organic molecules, which can come from a myriad of sources, will end up dehydrating on mineral surfaces. When organic compounds lose their water, they can react with other elements, creating new, more complex compounds, leading further down the road to forming life.

However, as no evidence of life has been concretely observed thus far on Mars, Schieber referred to the differences in Earth and Mars’ early material conditions as a possible explanation of why this didn’t happen.

In the early years of Earth and Mars, water molecules could disassociate into hydrogen and hydroxide in the upper atmosphere, Schieber said. He said that if these molecules reached escape velocity, the speed necessary to escape a planet’s gravitational pull, they would leave the planets entirely.

Because Mars is only around a third the mass of Earth, it was much easier for these molecules to escape, causing more water and atmosphere to be lost. Schieber said the presence of photosynthesis on Earth prevents this from happening because the oxygen produced by plants and disassociated hydrogen can react to form more water.

“Absent life on Earth the bleed rate of hydrogen would be relatively small,” Schieber said. “Sooner or later, photosynthesis has time to kick in, and you’re saved. On Mars, this window was much shorter. If life didn’t evolve quickly enough, it could not have affected it’s atmosphere enough to sustain itself.”

CORRECTION: David Bish's title was updated to reflect his most recent position.

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