NASA is heading back to Mars: On Monday, the space agency launched the Mars Atmosphere and Volatile Evolution satellite, or MAVEN, from Cape Canaveral, Fla.
The spacecraft is expected to reach the Red Planet in September, where its goal is to help solve the mystery of the missing Martian atmospheric molecules.
About 4 billion years ago, scientists think Mars was covered with a thick atmosphere full of molecules such as water and carbon dioxide, elements that help sustain life here on Earth.
In fact, evidence suggests Mars was warm and wet, with lakes and oceans much like Earth. It may have even sustained life.
But over time that dense atmosphere leaked away, says Richard Zurek, co-investigator on the MAVEN mission and a scientist with NASA's Jet Propulsion Laboratory in Pasadena.
Zurek says solar wind may be partially to blame. Imagine a cloud of fast moving charged particles spit out by the sun.
"Those particles have energy and as they collide with the atmospheric molecules around Mars: They give some of that energy to the molecules," Zurek explained.
The theory is that once particles in the Martian atmosphere are charged up by this solar wind, they may have enough energy to escape the pull of Mars' gravity and float off into space.
Zurek says Earth is also regularly bathed in solar wind, but — unlike Mars — our planet has a strong magnetic field protecting the atmosphere. Mars had a similarly strong magnetic field in the past, Zurek, but somehow it diminished.
When the MAVEN satellite enters orbit around Mars next year, it will study the composition of the Martian atmosphere for clues about how and why these changes occurred.
Caltech planetary science professor Yuk Yung has studied Mars' atmosphere for nearly four decades. He says this mission is important, but it's unlikely it will close the case on Mars' mysterious atmospheric changes.
"Every great scientific mission always opens up new questions," Yung notes.
He adds that what we learn from the MAVEN mission may help us understand more about Earth's climate and atmosphere as well.
This story has been updated.