By Eric Hand of Nature magazine

Gale Crater, a 150-kilometer-wide depression named after an Australian banker-turned-amateur astronomer, has emerged as the preferred destination for the next spacecraft to set wheels on Mars.

The proposed landing site, which includes a tantalizing 5-kilometer-high mound of ancient sediments, may have once been flooded by water. Nature has learned that it rose to the top last month following a secret ranking of four candidate sites by co-investigators working with NASA's Mars Science Laboratory, a 900-kilogram rover dubbed "Curiosity" set to launch later this year.

The scientists' endorsement of Gale Crater does not ensure that it will be selected by NASA management. Another site, Eberswalde Crater, which contains a relic river delta and--perhaps--buried evidence of organics in the lakebed deposits into which the river flowed, was ranked a very close second. The other two sites under consideration include Mawrth Vallis, which records complex mineralogy in the oldest and longest sequence of rocks among the four sites, and Holden Crater, another site with an ancient lakebed.

NASA Associate Administrator Ed Weiler is expected to make the final decision on Friday with a formal announcement of the site to follow next week. His imprimatur will bring to a close the most elaborate selection process yet for a Mars lander, in which dozens of candidate sites were winnowed down to the four finalists. At a fifth and final site selection workshop last month, engineers told scientists that they were comfortable landing the rover safely in any of the four candidates sites. NASA engineers for the first time will use a "sky crane" technology, in which a floating, rocket-powered platform will gently lower the rover to a boulder-free spot somewhere within a landing ellipse 20 kilometers by 25 kilometers.

That vote of confidence from engineers means that the final site selection has come down to science. After the workshop, project scientist John Grotzinger of the California Institute of Technology in Pasadena gathered more than 50 principal and co-investigators from the science team for the closed-door ranking. "It was a very fair process," says one scientist involved.

On Friday, Grotzinger will present Weiler with the science team's preferences. In the past weeks, an external panel, headed by Gentry Lee, an engineer with the Jet Propulsion Laboratory in Pasadena, has conducted an independent review of the site selection process.

Lucky number 7?

If successful, Curiosity will be the seventh spacecraft to soft-land on Mars. However, this time the stakes for the $2.5 billion mission are especially high. The mission's key objective, to study the past habitability of Mars, is even more crucially tied to the selection of a worthy landing site than previous missions have been. Earlier landings, such as the Viking missions of the 1970s, were bound to reveal exciting details no matter what they found, simply by getting there first. More recently, during the Mars Exploration Rover mission, two camps of Mars scientists were placated by the selection of two very different landing sites for the mission's twin rovers. Spirit went to Gusev Crater, which appealed to geologists interested in its stratigraphy and sedimentology, while Opportunity touched down on Meridiani Planum, where remote sensing showed a concentration of the water-related mineral hematite.

This time, three of the four sites are craters and all three appeal more to the sedimentologists, whereas only the fourth site, Mawrth Vallis, holds much allure for the mineralogists. And there is only one rover. Some mineralogists are already frustrated that Mawrth Vallis has been ruled out, even as they acknowledge that its lack of scenic vistas--important in drawing the public into a mission--could be a major failing.

Ross Irwin, a geomorphologist who works for the Planetary Science Institute in Tucson, Arizona, says that the friction between sedimentologists and mineralogists doesn't just represent two different camps of Mars scientists, but two classes of geological activities on Mars: one with mechanisms that make and erode layers of rock and another with interesting geochemical mechanisms. "It's not just telling us something about ourselves," says Irwin, who has advocated for Holden Crater. "It's telling us something about Mars as well."

The selection also throws a posthumous spotlight on Walter Frederick Gale, an Australian banker who was an avid amateur astronomer and comet discoverer at the turn of the twentieth century. The crater that bears his name, with its finely layered mound of sediment, was also one of the sites considered by the Mars Exploration Rover science team.

This article is reproduced with permission from the magazine Nature. The article was first published on June 23, 2011.