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Martian climate had ups and downs

While its sibling Perseverance has just landed on the Red Planet, the Curiosity rover – on Mars since 2012 – continues to make new discoveries. Some of these involve the beguiling Martian climate.

A French-US research team has used Curiosity’s ChemCam instrument to analyse layers of ancient rocks on Mount Sharp, finding that Mars underwent dramatic climate fluctuations early in its history.

“Compared to Earth, our understanding of the geological history of Mars is almost like a blank page,” says William Rapin, a French National Centre for Scientific Research (CNRS) researcher at the Université de Toulouse, who led the team. “Using rovers like Curiosity and Perseverance that have now access to large ancient stratigraphies for the first time […] we are about to unravel the story of a completely other world.”

Planetary scientists have known for a while that Mars wasn’t always such a cold, dry place – there is ample evidence that water flowed across its surface in the past, from minerals that could only have been formed by water to layered rocks made of dried mud and fluvial sands. By studying sulfates deposited on the surface, orbiting spacecraft have revealed that the last time water was active on Mars on a global scale was 3.5 billion years ago.

“Yet we didn’t know how the climate evolved during that crucial period,” Rapin says. “What was the nature of this changing global environment under which sulfates formed?”

View of hillocks on the slopes of Mount Sharp, showing the various types of terrain that will soon be explored by the Curiosity rover, and the ancient environments in which they formed, according to the sedimentary structures observed in ChemCam’s telescope images (mosaics A and B). Credit: NASA/JPL-Caltech/MSSS/CNES/CNRS/LANL/IRAP/IAS/LPGN.

The new study, published in the journal Geology, suggests that Mars didn’t simply change to a dry climate. Instead, the planet went through multiple transitions between sustained wetter and drier periods, before finally drying up completely.

“This is a very interesting result,” says Graziella Caprarelli, planetary scientist from the University of Southern Queensland who was not involved in the study. “It provides detailed analysis of the sedimentary sequence in Mount Sharp, which is almost as good as having human geologists on the ground to record observations of the cross-section of the outcrop.”

The study used observations made by ChemCam, an instrument designed to analyse the chemical composition of rocks and soil. From a distance, ChemChem focused on the steep slopes of Mount Sharp (also known as Aeolis Mons), composed of layers of sedimentary beds formed at different times and under different conditions.

The types of rocks changed radically along the several hundred metres studied, representing millions of years of deposition. The base of Mount Sharp, for example, is made up of clay laid down by ancient lakes. Above this are wide, tall, cross-bedded structures: signatures of wind-sculpted dunes from an arid period. Further up, thin strata alternate between brittle and resistant, which indicates deposits made by river floodplains – so wetter conditions prevailed again.

Rapin explains that it is difficult to date when these climate transitions occurred.

“Dating rocks is hard,” he notes. “It requires laboratory analyses that, for instance, will be done on the samples collected by Perseverance if the return to Earth is a success. It is not yet possible [to] date precisely the time of each sedimentary bed, but because they are forming a stratigraphy, laying on top of each other, we can know for sure the order of the climate evolution they trace.”

These results tie into previous research that has suggested Mars underwent environmental fluctuations – but this study observes changes on a smaller scale.

Curiosity will celebrate the 10th anniversary of its launch in November this year. Initially slated to last two years, the rover is now trucking on into its extended mission – part of which will involve climbing up Mount Sharp and drilling into the beds of sedimentary rocks that it spied from a distance.

The results of these more direct observations will test whether the climate fluctuation model holds water – and what might have driven these dramatic changes.

Caprarelli notes that while rovers are still limited in the types of analysis they perform, the data they gather are still essential to complement observations from orbiting satellites.

“Therefore, any piece of new ground evidence [or] observation adds to our knowledge and helps filling in the missing pieces of the puzzle,” she says.