What happened to the water on Mars? How did the Red Planet's atmosphere become so thin over time? Nasa's MAVEN probe is scheduled to launch on Monday on a mission to find out.
The unmanned spacecraft aims to orbit Mars from a high altitude, studying its atmosphere for clues on how the Sun may have influenced gas to escape from the possibly life-bearing planet billions of years ago.
The probe is different from past NASA missions because it focuses not on the dry surface but on the mysteries of the never before studied upper atmosphere.
"There's a puzzle piece, I'll say, that's been missing with what's happening in that upper atmosphere," said David Mitchell, project manager of the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission.
"That is really what we are going after."
NASA's Goddard Space Flight Center in Maryland posted on its website an artist's video rendition of what Mars might have looked like in a long-gone era when its atmosphere was thick enough to support surface water.
The green lakes and white clouds it depicts offer a stark contrast to the barren desert planet of today, coated in reddish-pink grit and dry except for traces of briny subsurface streams.
Jim Green, director of the planetary science division at NASA headquarters said MAVEN, which cost $671 million, will reveal clues about what happened to make Mars' atmosphere too cold and thin to support water.
The planet that neighbors Earth "underwent a major climate change in its past," said Green.
"MAVEN will tell us why Mars went through the such dramatic atmospheric changes over the years."
Much of its year long mission will be spent circling the planet at a distance of 6,000 kilometers (3,800 miles) above the surface, but it will execute five deep dips to a height of just 125 kilometers (78 miles).
The square spacecraft weighs 5,410 pounds (2,453 kilograms) and will launch aboard an Atlas V 401 rocket from Cape Canaveral Air Force Station in Florida.
The launch could happen as early as 1:28 pm (1828 GMT) on Monday. The launch window stays open for two hours.
Rainy weather has a 40% chance of delaying the launch on Monday, and conditions deteriorate over the next two days, to 60% chance of delay Tuesday and 70% on Wednesday, NASA weather officials said.
If launch goes ahead as planned on Monday, the eight-foot (2.5-meter) cube with wing-like solar panels will arrive in Martian orbit September 22, 2014 after a journey of 10 months.
That would be an arrival two days earlier than India's Mars probe, launched November 5. MAVEN's science mission would begin in November 2014.
The science goals of the two do not overlap much, Mitchell said. The Indian probe will be searching for methane which could prove the existence of some ancient life form, while the US probe seeks answers about the planet's climate change.
"It is kind of a neat race and we wish them all the best," Mitchell told reporters.
"Down the road, the scientists will be collaborating on what they find there."
MAVEN is part of a series of rovers and probes that aim to return key data about Mars before a planned mission to send humans there as early as the 2030s, Nasa has said.