The atmosphere of Mars was destroyed four billion years ago by a mysterious "catastrophic" event, scientists say.
Data returned by the Curiosity rover suggests there was a major upheaval, caused by volcanic eruptions or a massive collision.
The rover, which landed a year ago, has returned its first measurements of the makeup of gases in the martian atmosphere. The results are published as part of two papers in the journal Science.
Dr Chris Webster, from Nasa's Jet Propulsion Laboratory in Pasadena, said the data allows for direct comparisons with the Earth's climate.
He said: "As Mars became a planet and its magma solidified, catastrophic outgassing occurred while volatiles were delivered by impact of comets and other small bodies.