The Sun continuously sends a fast stream of charged particles through space, a phenomenon known as the solar wind. This leaves the planet's upper atmosphere exposed, allowing the solar wind to knock atmospheric particles away and carry them into space. At Mars, the solar wind appears to produce a similar effect by "stirring" the outer boundary of the upper atmosphere. This interaction generates enormous boundary waves called Kelvin-Helmholtz waves. A Clue to Mars' Lost HabitabilityUnderstanding atmospheric escape could help explain the dramatic transformation Mars experienced over its history.