Oh, how scientific times have changed!
When NASA's Vikings reached Mars 35 years ago, scientists and engineers had only vague ideas about where the mission's twin life-seeking landers should set down on the surface. Remarkable in hindsight, members of the site-selection team gave themselves just two weeks to find the best landing spot for Viking 1 — and ultimately had to scrap their provisional Plans A and B (too many big rocks!) and delay the landing by two weeks as they scrambled to find a suitable Plan C.
There'll be no such hurry-up offense for the space agency's next Red Planet adventure. When the Mars Science Laboratory (a.k.a. "Curiosity") departs Earth on or about November 25th, the mission's 263 scientists will know with certainty that it's headed for 4.4868ºS and 137.4239ºE — a target on the broad floor of Gale crater.
Whereas the Viking team had only relatively crude orbital imagery, some water-vapor measurements, and a few ground-based radar scans to work with, MSL's site selection involved deliberations over five years by 150 scientists, who hashed over 60 possible sites using 16 sets of detailed measurements and met in five dedicated workshops. The final four candidates, which underwent intense scrutiny after being picked in 2008, were:
- Eberswalde, a crater 39 miles (62 km) across, once served as a holding pond for water carried in by ancient rivers. Today its floor is covered with arguably the planet's richest delta of sediments, chock full of clay minerals and considered an especially good environment for preserving organic materials.
- Holden, an even larger crater (diameter: 95 miles or 153 km) that likely once brimmed with water. Its interior contains stacks of finely layered clay sediments that appear to have formed in a relatively placid setting.
- Mawrth Vallis, the most ancient setting, is a winding canyon some 400 miles (650 km) long. Its walls expose clay-rich layers of rock that might reveal details about the period in early Martian history when wetter conditions prevailed.
- Gale, named for Australian banker-turned-astronomer Walter Frederick Gale (1865–1945), is 96 miles (154 km) across. Likely at least 3½ billion years old, it's distinguished by a massive layered mound at its center that rises 3 miles (5 km) above the crater floor. To astrobiologist Nathalie Cabrol (SETI Institute), the geology inside Gale suggests a water-rich environment that changed "from warm and wet to cold and ice-covered water that could have provided suitable oases for various communities of microorganisms."
Interestingly, Gale crater was not an early contender in the MLS sweepstakes, nor did it rate as highly as a couple other final candidates did on an 11-point checklist of desirable geologic attributes. All four proved acceptable to mission engineers, and all would be worthy scientifically. "We'd be happy to go to any one of them," says John Grant (Smithsonian Institution), who led the site-selection effort.
Ultimately "there was no hard yes-or-no answer," admits John Grotzinger, a Caltech geologist and MSL's project scientist. "In the end we picked the one that feels best." (And even after all that, NASA brass needed to give the winner a two-thumb's-up endorsement.)
The scientific team "feels best" about Gale largely because of that massive mound in its center. Orbital scrutiny shows that the towering stack has layers of clay minerals near its base (just 1,000 feet above the crater floor), sulfates above those, and an enigmatic cap of still-younger material that seems to be a sediment-filled system of fractures. The Gale site offers a chance to understand water's role in a sequence of ancient environments that the MSL team just couldn't pass up — "an opportunity," observes Grant, "to read chapters in a book of the history of past deposition on Mars."
Curiosity should have direct access to this layer-cake topography thanks to a canyon incised into the mountain's northern flank. "Geologists like climbing on cliffs," comments Dawn Sumner (University of California, Davis), "and we get to go to those places with this rover for the first time on Mars."
After its planned arrival in August 2012, the plutonium-powered rover — roughly twice as long and five times as heavy as Spirit and Opportunity — is expected to travel at least 12 miles (20 km) during its two-year basic mission. Its complement of instruments (which includes 17 cameras!) is heavily biased toward analyzing rock and soil chemistry in what was once a water-rich environment — Curiosity isn't equipped to test for life, though it can reveal the presence of organic carbon through isotopic analysis. "The primary goal," Grotzinger emphasizes, "is to explore a habitable environment."
There's no shortage of online resources about the MSL mission. You can peruse NASA's MSL website or zoom in on the landing zone in a 2½-minute animation narrated by Grotzinger. An overview of what makes Gale so special is here, and you can delve into the nitty-gritty of the selection process here and here.
(I'll save discussion about why the launch was delayed two years and why the mission's total cost ballooned by more than 50% to $2½ billion for another time.)