Set for a 1:28 p.m. liftoff Monday, Maven will study the Red Planet's upper atmosphere and history of climate change.
Water is believed to have flowed through rivers and lakes on ancient Mars, supported by a thicker, warmer atmosphere.
Microbial life might have flourished in that climate more than 3.5 billion years ago.
But today the Red Planet is a cold, dry desert.
"Why do we appear to have had a climate on Mars that could have supported life at the surface early on, and we don't today?" asked Bruce Jakosky of the University of Colorado's Laboratory for Atmospheric and Space Physics, the lead scientist for NASA's Maven mission. "Something fundamentally changed, and we want to understand what those changes were."
The $671 million Maven mission hopes to begin solving that puzzle with a 1:28 p.m. EST blastoff Monday atop an Atlas V rocket from Launch Complex 41 at Cape Canaveral Air Force Station, Fla.
There's a 60% chance of favorable weather during the two-hour launch window.
After a successful launch, the Maven spacecraft will take 10 months to reach orbit around Mars, where it will perform science observations for at least one Earth year.
Scientists say the water and greenhouse gasses once abundant in Mars' thicker atmosphere could have gone in two directions: down, to be absorbed by the planet's crust, or up, escaping to space through the top of the atmosphere.
Maven — short for "Mars Atmosphere and Volatile EvolutioN" — will focus on the latter as the most likely driver behind Mars' loss of atmosphere and resulting climate change.
Eight science instruments carrying nine sensors will study the makeup of the upper atmosphere, and how it reacts to solar wind and storms thought responsible for stripping molecules away over time.
"The sun is a major player in this mission," said Janet Luhmann, Maven deputy principal investigator from the University of California at Berkeley. "The atmosphere is more or less exposed most everywhere to the direct onslaught of what's coming from the sun."
That's because Mars lacks a protective magnetic field like Earth's. It disappeared when Mars was about a billion years old, and the loss of atmosphere began around the same time, when solar activity was more intense.
The length of a school bus with its solar arrays deployed, the Lockheed Martin-built Maven spacecraft will soar as high as 3,900 miles and as low as 93 miles above the surface.
Five times it will perform "deep dip" campaigns, dropping to an altitude of 78 miles to sample denser regions.
By understanding what's eating away at the atmosphere at slow rates today, and combining that with historical models of solar activity, mission scientists will be able to extrapolate how Mars' atmosphere and climate changed over billions of years.
They say that knowledge has broader implications for understanding whether life could have been possible on Mars or other planets, particularly as more and more planets are being discovered, including some potentially like Earth.
"We're understanding processes by which a planetary environment can change through time," Jakosky said. "We want to understand what makes a planet habitable, and what makes a planet go from being habitable to not being habitable?"
In addition to its primary science mission, Maven carries an antenna that will serve as a backup communications link between Earth and the rovers and landers operating on Mars.
The science mission could be extended after the initial year is up.
"It's going to answer a key question about Mars' evolution," said Michael Meyer, lead Mars scientist at NASA headquarters.