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.

Gale crater on Mars

An oblique, southward-looking view of Gale crater shows the 20-by-25-km landing target for NASA's Mars Science Laboratory and the central mountain of layered rock that the rover will investigate. Gale is 96 miles (154 km) in diameter. Click here for a larger view.


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.

NASA managers announced the choice of Gale over three other final candidates during an hour-long press briefing on Friday.

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:

    Candidate landing sites on Mars

    Red circles on this topographic map of Mars show candidate landing sites for NASA's Curiosity rover; blue ones depict the final four considered by scientists. Among the mission's engineering constraints are requirements to land withing 30° of the equator and at an elevation below the planet's mean "sea level" (all higher elevations are shown in black; scale at lower right).

    John Grant / Smithsonian Inst.

  • 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.)

Topography of Gale crater

A topographic map of Gale crater, derived from Mars Express imagery, clearly shows the tall, broad central mound near the crater's center. The white ellipse denotes Curiosity's landing zone. Click here for a larger view.

Ryan Anderson & Jim Bell / HRSC

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."

Mars rovers compared

Weighing about a ton, the rover Curiosity (right)) is far larger than its predecessors Spirit and Opportunity (left)), which reached Mars in 2004; and tiny Sojourner (center)), which landed in 1997.


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.)


Image of Peter Wilson

Peter Wilson

July 25, 2011 at 2:07 pm

"Geologists like climbing on cliffs," comments Dawn Sumner, "and we get to go to those places with this rover for the first time on Mars." Will the rover repel down the cliffs from above, or ascend them from below?

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Image of James Grose

James Grose

July 25, 2011 at 8:02 pm

The MSL rover will actually ascend up into the canyon that is cut into the cliffs, from the landing zone that is just to the north of Gale crater's central peak.

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Image of Anthony Barreiro

Anthony Barreiro

July 27, 2011 at 4:29 pm

Curiosity is powered by plutonium? What are the chances of a failure during launch, and what safeguards would prevent plutonium from being released into Earth's atmosphere? Why plutonium rather than solar? And will future human explorers of Mars need geiger counters?

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August 5, 2011 at 4:31 pm

No offense, but stop annoying with these longtime overplayed woes. Such rover like Curiosity needs more power than solars can provide (size matters). On Mars there no power outlets nor generators, so stable and long living power source is required.

And putative nuclear radiation (especially from plutonium) is really not a danger for humans on Mars (just like background nature radioactivity on Earth). We have to protect ourselves there on Mars against cosmic rays i ultraviolet first. That's the challenge for Mars trip and visit.

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Image of Edward Schaefer

Edward Schaefer

August 17, 2011 at 10:05 pm

I would also add that plutonium has been regularly used on space missions ever since the Apollo missions. Examples include Apollo 12 (to power experiments left behind on the Moon, where the nights last for two weeks), Voyagers 1 and 2, Galileo, and Cassini-Huygens.

As for preventing contamination: These units are sealed in containers that cannot burn up in an atmospheric re-entry and can stand a high-speed impact. In fact, the unit intended to power experiments for Apollo 13 ended up being aimed towards to Tonga Trench after that mission's accident, and to this day there is no evidence that the container had (or has) been breached.

Beyond that Anthony did a nice job of explaining why plutonium is needed.

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