The astrobiological hunt begins once the $2.5 billion Mars Science Lab Rover Curiosity lands itself beside a towering mountain that rises from the floor of a vast, ancient impact basin called Gale Crater.
Touchdown, monitored from mission control at Nasa's Jet Propulsion Laboratory in California, is scheduled for 10.31pm on Sunday Pacific time (1.31am EDT on Monday/0531 GMT on Monday).
"It's a big science goal. We're not just looking for water anymore," said California Institute of Technology geologist John Grotzinger, the lead mission scientist.
"The expectations go up. The scientific challenge is much greater. It's just going to be harder to address this question of habitability," he told Reuters.
In this 2011 file artist's rendering, a "sky crane" lowers the Mars Science Laboratory Curiosity rover onto the surface of Mars. (AP/Nasa/JPL-Caltech)
Scientists considered hundreds of landing sites before choosing Gale Crater, which probably formed when an asteroid or comet crashed into the planet some 3.5 billion to 4 billion years ago.
From high-resolution images taken by orbiting satellites, Gale Crater's central mound, known as Mount Sharp, appears to consist of layers of sediment rising like a stack of cards 3 miles (5 km) into the sky, taller than the crater's rim.
The most likely origin of the mountain is that it formed from the remains of whatever material filled up the basin long ago. How it was left standing in the middle of Gale Crater, a 96-mile-(154-km)wide bowl located near the planet's equator, is a mystery, one of many scientists hope to answer during Curiosity's two-year science mission.
Regardless of how it formed, scientists consider Mount Sharp a gift of time.
Nothing like it exists on Earth, where plate tectonics, erosion and other natural phenomena constantly reshape the planet's surface.
"We have the opportunity to start in the past, rove up the surface of Mount Sharp and come through time to see how the environments have changed," said Michael Meyer, Nasa's Mars exploration program scientist.
Warmer, wetter Martian past
A succession of previous rovers, landers and orbiting spacecraft have gathered compelling evidence that Mars, which is about half the size of Earth and 50% farther away from the sun, was not always the dry, acidic, cold desert that appears today.
Nasa's strategy since resuming Mars exploration following the 1970s-era Viking missions there has been to look for the chemical and physical fingerprints of water, which is necessary for life - at least as it has evolved on Earth.
The second ingredient in the recipe for life is carbon, which provides organic structure. Carbon will be far more difficult to detect on Mars, if it exists, because the same processes that produce rock tend to destroy organics.
The planet's harsh radiation environment doesn't help either.
"We have a radiation-rich environment on Mars that can destroy organics, so even if it was there, it may be hard to find a place where it's been preserved," Meyer said.
This Aug 26, 2003 image shows Mars photographed by the Hubble Space Telescope on the planet's closest approach to Earth in 60,000 years. (AP Photo/Nasa)
On Earth, the oldest evidence for life dates back about 3.5 billion years. Fossilized remains of single-celled microorganisms were found in 1958 inside a type of rock known as chert. This glass-like rock may exist on Mars as well, and it is not the only material that can preserve organics like a time capsule.
"The challenge for Mars exploration is to first try to identify environments that might have been habitable and then ask, 'Is this the kind of place where organic carbon could have been preserved?'" Grotzinger said.
"With Curiosity, we don't have the ability to look for life, or even fossil life, but we do have the ability to look for organic carbon, so we try to find those environments conducive for preservation. That's the hard part," he said.
The oldest sections of Mount Sharp may overlap the window when life emerged on Earth, a time when Mars is believed to have been warm and wet.
Curiosity's landing site inside Gale Crater is one of the lowest regions on Mars, stacking the odds that water, if it existed there, flowed down to the basin's floor. Mount Sharp
may be the remains of this ancient lake bed and perhaps a place that life once called home.
For details on Curiosity rover mission, click here.
Speeding toward a landing on Mars
After an 8 1/2-month voyage through space, Nasa's souped-up Mars spacecraft zoomed toward the red planet for what the agency hopes will be an epic touchdown.
The fiery punch through the tenuous Martian atmosphere at 13,000 mph on Sunday night marks the beginning of "seven minutes of terror" as the Curiosity rover aims for a bull's-eye landing inside a massive crater near the equator.
The latest landing attempt is more nerve-racking than in the past because Nasa is testing out a new routine. Curiosity will steer itself part of the way and end on a dramatic note: Dangling by cables until its six wheels touch the ground.
That's the plan at least.
"Can we do this? Yeah, I think we can do this. I'm confident," Doug McCuistion, head of the Mars exploration program at Nasa headquarters, said Saturday. "We have the A-plus team on this. They've done everything possible to ensure success, but that risk still exists."
Despite humanity's fascination with Mars, the track record for landing on it is less than stellar. Out of the 14 attempts by space agencies around the world to touch down on Earth's neighbor, only six have succeeded. Nasa has fared better — with only one failure out of seven tries.
In keeping with a decades-old tradition, peanuts will be passed around the mission control room at the Nasa Jet Propulsion Laboratory for good luck.
Nasa will need it. The $2.5 billion mission comes as the space agency faces a financial crunch. It abandoned a partnership with the European Space Agency to send missions in 2016 and 2018 and, instead, is charting a new future for Mars exploration.
For now, Nasa is counting on Curiosity to nail the landing.
"We're now right on target to fly through the eye of the needle" at the top of the Martian atmosphere, said mission manager Arthur Amador.
Earlier in the week, a dust storm swirling to the south of the landing site gave the team some pause. Ashwin Vasavada, the mission's deputy project scientist and Mars weather forecaster, said the storm basically went "poof" and posed no threat.
"Mars appears to be cooperating very nicely with us. We expect good weather for landing Sunday night," he said.
As Curiosity plummets to the surface, it will rely on the precisely choreographed use of a heat shield and supersonic parachute to slow its descent. Less than a mile from the ground, the hovering spacecraft will unspool cables to lower the rover.
Due to the signal time lag between Mars and Earth (it takes about 14 minutes for a signal on Mars to zip to Earth), Curiosity will execute the landing autonomously, following the half a million lines of computer code designed by Earthlings.
Touchdown was set for 10.31pm. PDT. Nasa warned that spotty communication during landing could delay confirmation for several hours or even days.
On the eve of landing day, mission control was quiet with only a handful of flight controllers on duty. Two jars of peanuts were on display on the front console. As the countdown to landing nears, the place will be humming.
"I get butterflies every now and then," said flight director Keith Comeaux.
If successful, Curiosity will join another roving spacecraft, Opportunity, which has been exploring Mars since 2004.
The most high-tech Mars spacecraft ever built, the nuclear-powered Curiosity is equipped with more than a dozen cameras, a weather station and tools to drill, taste and sniff the environment in search of the chemical building blocks of life.
Its target is Gale Crater near the equator, which scientists think is a place where water once flowed — a good starting point to learn whether microbes could exist there. Rising from the floor of Gale is an impressive mountain where mineral signatures of water have been spied at the base.
Life as we know it requires three ingredients: Water, energy and carbon. The missing piece so far is finding carbon. One of Curiosity's main tasks is to drive to the mountain, chisel rocks and dig into soil in search of the elusive element.
During its cruise to Mars, Curiosity turned on its radiation sensor and sent back data, which should help scientists better understand the risks that astronauts would face on a manned mission.
Before Curiosity can further explore, it must first stick the landing.
Weighing nearly 2,000 pounds, it is much heavier than Opportunity and can't bounce to a stop swaddled in air bags; it would break apart if it did. So engineers devised a new trick. Sunday will be the first time that the novel landing routine will make its debut.
Engineer Steve Sell said his eyes will be glued to his computer screen on landing day.
"I just have to keep reminding myself to keep breathing," Sell said.
For details on Mars missions, click here.
(With inputs from Reuters, AP)