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. 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. 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. “The question looking forward is how much of this CO 2 from the atmosphere was actually sequestered?