Changing times in Syrtis Major

Early Mars appears to have been highly unlike today, being warmer and wetter. To find out why and how the environment changed, scientists look for inflection points in Martian history, where one geological regime gives way to another.

GEO-SLICE. A cross-section in northeast Syrtis tells a tale of change. Clay minerals (blue) lie on the bottom of the sediment stack, pointing to an early neutral or alkaline environment. On top them lies a veneer of carbonates (green), the product of different conditions. Sulfates (purple) topped with lava complete the picture of transition into a more acidic environment, thanks to abundant volcanic eruptions. (Image taken from Figure 2 in the paper.)

A report recently published in Geophysical Research Letters suggests that northeast Syrtis Major offers a sequence of deposits that straddles the boundary between the Noachian and Hesperian eras, the oldest and the next-youngest periods, from roughly 4.1 to 3.8 billion years ago. The paper is by Bethany Ehlmann (Caltech-JPL) and John Mustard (Brown University).

“The transition between the Noachian and Hesperian epochs on Mars is marked by evidence for a fundamental change in planetary-scale processes,” Ehlmann and Mustard write. Large volcanic provinces spread broadly across Mars, including the northern plains, Hesperia Planum, and Syrtis Major. Changes also included a shift from a chemically neutral, wetter environment with clay deposits to a drier and more acidic one in which sulfate minerals became common.

The researchers used data from the CRISM instrument on NASA’s Mars Reconnaissance Orbiter to explore the mineralogy of a tangled area where ancient Isidis basin deposits adjoin younger lavas from Syrtis Major.

“This geologic section captures three of the four major classes of water-related minerals discovered on Mars,” says Ehlmann. “Except for chloride minerals, which are missing here, they lie in a time-ordered stratigraphy.”

As the scientists reconstruct the history, igneous bedrock formed in the early Noachian, and water subsequently altered some of it to iron-magnesium bearing clays. This bedrock was disrupted by the Isidis basin impact, leaving deposits shattered in places. An olivine-rich unit was emplaced either from an impact melt sheet or as fluid lavas draped over the existing topography.

Then erosion carved channels, deposited clay- and carbonate-bearing sediments in craters and other topographic lows. Near-surface water activity created magnesium carbonates and aluminum-bearing clays. Sulfate-rich materials settled on top of the olivine-carbonate unit, filling in the low spots. Syrtis Major lavas then covered these.

“In contrast to the late Noachian to Amazonian stratigraphic section recorded in Gale Crater sediments,” they write, “the northeast Syrtis section captures an older period of impact bombardment, volcanism, and water alteration from the early Noachian through the Hesperian.”

The team notes this site was considered as a landing place for Curiosity, NASA’s Mars Science Laboratory rover, and it would make a good locale for a future rover mission looking at ancient climate change and habitability. Says the team, “This is one of the most well-preserved stratigraphic sections from early Mars and it provides a diverse set of units suitable for collecting samples to bring back to Earth.”

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