MAVEN

When Mars was young, the planet was probably pretty cozy. It had a warm, thick atmosphere, and liquid water flowed across its surface. Yet the air quickly disappeared. And without a thick atmosphere, water couldn’t survive on the surface — it vaporized.

So far, though, scientists aren’t sure just how and why the atmosphere disappeared. But a new Mars mission should provide some answers.

The launch window opens tomorrow for MAVEN — Mars Atmosphere and Volatile Evolution. The craft will enter orbit around the Red Planet next September, then begin a one-year primary mission.

MAVEN’s altitude will vary from several thousand miles to less than a hundred. When it’s closest to the surface, its instruments will directly measure the composition of the upper atmosphere. And when it’s farthest, an ultraviolet camera will see the entire hemisphere, providing a global view of the atmosphere. The combination will help scientists understand how the atmosphere escapes into space, and the role the S