The Mars Atmosphere and Volatile EvolutioN (MAVEN), a NASA spacecraft to be launched to Mars later this month, will try to decipher billions of years of planetary history from careful study of the martian atmosphere. Eons ago, planetary scientists believe, Mars had a thick atmosphere that sheltered a surface awash with water—conditions in which life could have emerged and thrived. Today, that atmosphere is thin and depleted, and Mars is a cold, barren desert. What caused this remarkable transformation? Until now, planetary scientists have attempted to answer such questions mainly from the planet's surface. MAVEN will take a new course: flying through the outer fringes of Mars's atmosphere, measuring gases and monitoring conditions with eight instruments. The measurements should help researchers figure out how the solar wind, asteroid impacts, and chemical reactions gradually depleted the Red Planet's atmosphere.