LOS ANGELES - After years of poring through images from space and debating where on Mars the next NASA rover should land, it comes down to four choices.
Scientists in the close-knit Mars research community get one last chance to make their case this week when they gather before the "judges" - the team running the $2.5 billion mission that will soon suggest a landing site to NASA, the ultimate decider.
Location is everything when it comes to studying whether the red planet ever had conditions that could have been favorable for microbial life.
The upside is that all four candidates are relatively free of dangerous boulders and other hazards that would pose a threat to rover Curiosity upon landing. The size of a mini Cooper, Curiosity is scheduled to launch in late November after a two-year delay.
With no real engineering showstopper, scientists are haggling over the scientific merits of the locations and trying to convince the rest of the tribe why Curiosity should land at their preferred spot.
"All four of these places are compelling places on Mars to study. There's not a loser among them," said landing site scientist Matt Golombek of the NASA Jet Propulsion Laboratory, one of the meeting's leaders.
But like in any contest, there can be only one winner in the end. Here's a look at the final cut:
-Gale crater near the Martian equator possesses a 3-mile-high mound of layered mineral deposits.
-Mawrth Vallis is an ancient flood channel in the Martian northern highlands that is rich in clay minerals.
-Eberswalde crater in the southern hemisphere contains remnants of a river delta.
-Holden crater, close to Eberswalde, is the site of water-carved gullies and sediment deposits.
The short list was culled from nearly 60 hopefuls in a selection process that began in 2006. Some scientists broke up into teams to pore over close-up images snapped from the eagle-eyed Mars Reconnaissance Orbiter for the various locations and presented their findings at meetings. After technical woes pushed back Curiosity's launch, the community regrouped and considered more places.
Research scientist Steve Ruff of Arizona State University stayed on the sidelines for much of the debate. No longer.
He said he will make a pitch for Mawrth Vallis, the only spot where Curiosity can conduct science experiments as soon as it lands. For the other three sites, the rover would need to drive outside its landing zone to reach interesting targets.
Mawrth is the sole locale "that contains the scientific goodies," Ruff said.
After the community input, the team will meet privately to mull over the pros and cons of each site before recommending one to NASA. The space agency has the last say, but it usually follows the advice of its researchers. A final decision is not expected until late June or July.
"There's a big investment in this rover. We want to make sure that it goes to the best possible site," said Smithsonian geologist John Grant, co-chairman of the meeting.