By studying the chemical elements on Mars today — including carbon and oxygen — scientists can work backwards to piece together the history of a planet that once had the conditions necessary to support life.

Weaving this story, element by element, from roughly 140 million miles (225 million kilometers) away is a painstaking process. But scientists aren’t the type to be easily deterred. Orbiters and rovers at Mars have confirmed that the planet once had liquid water, thanks to clues that include dry riverbeds, ancient shorelines, and salty surface chemistry. Using NASA’s Curiosity Rover, scientists have found evidence for long-lived lakes. They’ve also dug up organic compounds, or life’s chemical building blocks. The combination of liquid water and organic compounds compels scientists to keep searching Mars for signs of past — or present — life.

Despite the tantalizing evidence found so far, scientists’ understanding of Martian history is still unfolding, with several major questions open for debate. For one, was the ancient Martian atmosphere thick enough to keep the planet warm, and thus wet, for the amount of time necessary to sprout and nurture life? And the organic compounds: are they signs of life — or of chemistry that happens when Martian rocks interact with water and sunlight?

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