At some point in its early history, Mars could have had a thin layer of icy, high-altitude clouds that caused a greenhouse effect, new research suggests. The theory helps explain one of the great puzzles of modern space science that the view from NASA’s Perseverance, which just landed on Mars, neatly sums up: Today Mars is a desert planet, and yet the rover is sitting right next to an ancient river delta. “Mars is important because it’s the only planet we know of that had the ability to support life—and then lost it.” The apparent contradiction has puzzled scientists for decades, especially because at the same time that Mars had flowing rivers, it was getting less than a third as much sunshine as we enjoy today on Earth.