New measurements of Mars' thin atmosphere show that most of it has been lost to space due to bombardment from solar wind; this was the likely driver of the transition in Martian climate from an early, warm, wet environment to today's cold, dry, thin atmosphere. The results, derived from the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission, provide a better understanding of the history of the red planet, shedding light on its geological evolution and potential habitability. The composition of a planet's atmosphere contributes greatly to its climate. To better understand how Mars' atmosphere has changed over time, Bruce Jakosky and colleagues measured the abundance of two argon isotopes at different heights in the atmosphere. The lighter argon isotope (36Ar) is more abundant than its heavier counterpart (38Ar) at higher altitudes, and so is more susceptible to being ejected from the atmosphere by incoming solar wind. Based on measurements from the MAVEN spacecraft, the authors say that 66% of Mars' argon has disappeared from the atmosphere since the planet's formation. The researchers used these measurements of argon loss to determine the amount of other gases that would have been lost by the same mechanism. They suggest that Mars' atmosphere was probably once as thick as Earth's, but made primarily of carbon dioxide. Most of it has since been lost, leading to huge changes to the red planet's climate since its formation.