Scientists have no doubts that oceans and rivers once pooled on the surface of Mars, but what happened to all that water is a long-standing mystery.
The prime suspect is the sun, which has been peeling away the planet's atmosphere, molecule by molecule, for billions of years.
Exactly how that happens is the goal of NASA's new Mars Atmosphere and Volatile Evolution Mission, or MAVEN, which is scheduled for launch at 1:28 p.m. EST on Monday from Cape Canaveral Air Force Station in Florida.
Upon arrival in September 2014, MAVEN will put itself into orbit around Mars and begin scrutinizing the thin layer of gases that remains in its skies.
"MAVEN is going to focus on trying to understand what the history of the atmosphere has been, how the climate has changed through time and how that has influenced the evolution of the surface and the potential habitability - at least by microbes - of Mars,' said lead scientist Bruce Jakosky, with the University of Colorado at Boulder.
Specifically, MAVEN will look at how much and what type of radiation is coming from the sun and other cosmic sources and how that impacts gases in Mars' upper atmosphere.
Scientists have glimpsed the process from data collected by Europe's Mars Express orbiter and NASA's Curiosity rover, but never had the opportunity to profile the atmosphere and space environment around Mars simultaneously.
"We'll get a window on what is happening now so we can try and look backward at the evidence locked in the rocks and put the whole story together about Martian history and how it came to be such a challenging environment," said Mars scientist Pan Conrad, with NASA's Goddard Space Flight Center in Greenbelt, Maryland.
EARTH'S LOST TWIN?
The evidence for a warmer, wetter, more Earth-like Mars has been building for decades. Ancient rocks bear telltale chemical fingerprints of past interactions with water. The planet's surface is riddled with geologic features carved by water, such as channels, dried up riverbeds, lake deltas and other sedimentary deposits.
"The atmosphere must have been thicker for the planet to be warmer and wetter. The question is where did all that carbon dioxide and the water go?" Jakosky said.
There are two places the atmosphere could go: down into the ground or up into space.
Scientists know some of the planet's carbon dioxide ended up on the surface and joined with minerals in the crust. But so far, the ground inventory is not large enough to account for the early, thick atmosphere Mars would have needed to support water on its surface.
Instead, scientists suspect that most of the atmosphere was lost into space, a process that began about 4 billion years ago when the planet's protective magnetic field mysteriously turned off.
"If you have a global magnetic field, it causes the solar wind to stand off. It pushes it away so it isn't able to strip away atmosphere," Jakosky said.
Without a magnetic field, Mars became ripe pickings for solar and cosmic radiation, a process that continues today.
MAVEN's prime mission is expected to last one year, enough time for scientists to collect data during a variety of solar storms and other space weather events.
Afterward, MAVEN will remain in orbit for up to 10 years serving as a communications relay for Curiosity, a follow-on rover slated to launch in 2020 and a lander that is being designed to study the planet's deep interior.
If MAVEN is launched as planned on Monday, it is due to reach Mars on Sept. 22 - two days before India's Mars Orbiter Mission, which launched on November 5. India's probe has been raising its orbit around Earth and should be in position on December 1 to begin the journey to Mars.
If weather or technical problems prevent Monday's launch, NASA has 20 days to get MAVEN off the ground while Earth and Mars are favorably aligned for the probe to reach Mars.