Scientists had long known that Mars' atmosphere was thin – about 1 percent as dense as Earth's atmosphere. But it took the Pioneer-Venus orbiter in 1978 to show that researchers could record the processes eroding another planet's atmosphere as they happened.
Researchers realized these were important measurements to make at Mars as well, says Dr. Jakosky, a researcher working at the interface of planetary geology and astrobiology at the University of Colorado at Boulder.
"Without going and measuring what's actually happening, you can't have any hope of understanding" the unique mix of factors that turned Mars from what many scientists hold as a wet, warm planet early in its history to the dry, radiation-bathed surface the planet presents today.
Eight sensors on the orbiter will allow scientists to tackle the atmosphere's history in three broad ways.
Orbiters currently at Mars have shown that atoms are being stripped from the atmosphere today – swept off as the solar wind, solar storms, and their magnetic fields flow past the planet. During its year-long primary mission, MAVEN will collect data on space weather's influence on Mars' upper atmosphere.
MAVEN also will measure extreme-ultraviolet radiation reaching Mars from the sun. This radiation breaks up molecules in the upper atmosphere, liberating lighter atoms, especially hydrogen, which then can easily be swept into interplanetary space.
Finally, MAVEN will measure the ratios of different forms, or isotopes, of five elements in the upper atmosphere – hydrogen, nitrogen, carbon, oxygen, and argon.
Typically planetary atmospheres are awash with normal hydrogen, the lightest atom, compared with its rarer, heavier sibling, deuterium. On Mars, however, the relative abundance of deuterium in the atmosphere is much higher than anywhere else in the solar system.
"That means that a lot of hydrogen has escaped," Jakosky says, since the heavier deuterium is less likely to leave the planet.