What Happened to Mars’ Lost Atmosphere? New Clues May Explain Why It Disappeared.

All evidence points to Mars having had a carbon dioxide-rich atmosphere billions of years ago, but insufficient carbonates in Martian soil challenge this theory.
Now, a new study using data from NASA’s Curiosity Rover claims that sulfate layers on the Martian surface may have been hiding siderite—iron carbonate—from orbital survey missions.
If similar levels of siderite are found in sulfate layers in other surveys, this could be a big missing piece to the puzzle of Mars’ prior habitability.

For all of human history, Mars has hung in the heavens as a tantalizing, red-hued mystery. Past civilizations associated the planet with the gods of war, and with the advent of telescopes in the 19^th century, some even came to believe that the Red Planet was criss-crossed with artificial canals. Today—even though we know more about our celestial neighbor than at any point in history—many, many mysteries remain.

One of the big remaining conundrums is what exactly happened in Mars’ ancient past that transformed the world from a wet and warm one to the cold, dry orb we see today. Plenty of evidence indicates that water once flowed on the surface of Mars and that the planet once had a carbon dioxide-rich atmosphere, and this set of conditions should’ve interacted with Martian rocks to create carbonate minerals. However, even with the 18 orbital spacecraft and six rovers we’ve sent to Mars—each laden with various spectroscopic and scientific tools—scientists haven’t found enough carbonate to support this theory.

Now, in a new study, a team of more than 30 scientists analyzing data from NASA’s Curiosity rover announce that they may have found the evidence they (and past teams just like them) have been looking for. The breakthrough comes from the discovery of siderite (an iron carbonate mineral) in the Martian topsoil that would’ve been “invisible in previous orbital observations,” the scientists wrote in a paper published in the journal Science.

“The discovery of large carbon deposits in Gale Crater represents both a surprising and important breakthrough in our understanding of the geologic and atmospheric evolution of Mars,” Ben Tutolo, the lead author of the study and geochemist from the University of Calgary, said in a press statement. “The abundance of highly soluble salts in these rocks and similar deposits mapped over much of Mars has been used as evidence of the ‘great drying’ of Mars during its dramatic shift from a warm and wet early Mars to its current, cold and dry state.”

While climbing up a mountain in the Gale Crater, NASA’s Curiosity rover—which originally arrived at the Red Planet back in 2011—analyzed three to four centimeters into the Martian soil by drilling into the subsurface and then dropping samples into its CheMin instrument. According to NASA, this instrument uses X-ray diffraction to analyze the chemical composition of the rocks and soil, and using this method, the rover discovered the iron carbonate siderate with sulfate-rich rocky layers. These highly water-soluble magnesium sulfate salts are what likely masked the siderite signature from orbital analysis.

This evidence points to an imbalanced carbon cycle on Mars—the rocks and soil pulled more carbon dioxide out of the atmosphere than they replenished, causing the planet to lose its ability to support an atmosphere. Result? A cold, dry, and dead Mars.

“The broader implications are that the planet was habitable up until this time, but then, as the CO₂ that had been warming the planet started to precipitate as siderite, it likely impacted Mars’ ability to stay warm,” Tutolo said in a press statement. “The question looking forward is how much of this CO₂ from the atmosphere was actually sequestered? Was that potentially a reason we began to lose habitability?”

It’s taken humanity millennia of star-gazing and scientific inquiry to understand its red-hued neighbor, and now its puzzling planetary history is quickly coming into focus.

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