In August of 2012, NASA's biggest, baddest, and most scientifically capable rover ever, Curiosity, will touch down on the surface of Mars.
But we wont be setting Curiosity down just anywhere; from a list of around 30 prospective locations, scientists have settled on Gale crater as the ideal landing site for the Mars Science Laboratory mission — a crater that Curiosity will subsequently explore for a minimum of one Martian year (or 697 Earth days) to assess whether Mars ever was, or is still today, an environment able to support microbial life. So what's so special about Gale crater? Let's find out.
Craters are commonly formed when a planet comes into violent contact with extra-planetary objects, and Gale crater is no exception; astronomers believe the crater was formed by an impact event somewhere in the neighborhood of 3.5—3.8 billion years ago.
Then again, so were plenty of the Red Planet's other craters. In fact, at first glance, there's not a whole lot about Gale that sets it apart from any other cavity in Mars' liberally pockmarked landscape. Sure, it's big — at 96 miles in diameter, the blemish occupies more of the planet's surface than Rhode Island and Connecticut would combined — but it's hardly the widest crater on Mars, and far from the deepest. But Gale's got some surprises up its sleeve that make it particularly attractive to scientists looking to address the question of whether Mars was ever habitable.
For one thing, it's got location. Situated just a few degrees south of the planet's equator, the ancient crater is found near the border of Mars' hemispheric dichotomy — a peculiar geological characteristic that divides the planet's relatively broad, flat, northern lowlands from its impact-riddled southern highlands. Shown here is a map of Mars' entire surface. Here the dichotomy is emphasized by the image's cooler and warmer colors, which depict the planet's lower and higher elevations, respectively.
Did you notice the map pin? That's where you'll find Gale. As we all know, water flows downhill. Scientists suspect that long ago, water may have flowed from highland to lowland, south to north across the hemispheric dichotomy. Gale, caught in the middle, may have provided a place for water to pool and further erode at the crater's bottom, in the process forming the lowest layers of rock in a basin than has long since gone dry.
But location isn't all this crater has going for it; it's also got a mountain. Check out this topographical image of Gale, courtesy of NASA (click for a hi-res view). Smack dab at its center is a summit that rises about three miles above the crater floor. Believe it or not, some of the mountain's highest points tower even higher in elevation than portions of the crater's rim.
Surrounding this central peak is a lopsided mound of material that slopes toward the crater's lower elevations. The ellipse in the image outlines a landing site that NASA once proposed for the Spirit rover.
How a crater comes to have a mountain in its middle — let alone one with peaks higher than the rim of the crater itself — has been the subject of a a good deal of speculation; the general consensus, however, is that the original, mountain-less crater was filled in with layer upon layer of sediments over vast geological time scales (a process known as deposition), but that the majority of these deposits have long since been carried away through the combined erosive forces of wind and water. What is left today is the mountain we find in Gale's center.
But to those scientists concerned with Mars' past, what remains is much more than a mountain; the erosive forces that sculpted the mound at Gale's center have created the perfect place for Curiosity to conduct its investigation into the Red Planet's ancient history.
Take a look at the images featured here. The left-most photograph is of a rock wall in Arizona's Grand Canyon; the middle picture is of rock exposed in a lower portion of Gale's mound; and on the far right is an image of formations near mound's higher elevations.
All three images reveal layers of sedimentary rock and soil that geologists refer to as strata. Here on Earth, we know that the strata in the Grand Canyon were laid down over the course of eons, only to later be exposed by the erosive power of what has come to be known as the Colorado River. Each layer of stratum in the Canyon provides a window into a specific point in its geological history, dating back incrementally through billions of years; and evidence suggests that the layers of stratum within Gale crater will provide us with a similar opportunity to understand Mars' past, and help us answer the question of whether the Red Planet was ever hospitable to life.
Images of the crater captured from orbit reveal that the exposed layers of Gale's central mound contain different minerals depending on their elevation. This image shows a lower portion of the mound, where clay and sulfate-bearing minerals, which are known to form in water, have been identified. Strata at higher elevations have been shown to contain little or no clay, suggesting these layers likely formed during a period when water on Mars was drying up.
In order to access these layers, Curiosity will first touch down in a relatively flat section of the northwestern crater floor. (Curiosity's landing zone, like Spirit's above, is marked with an ellipse. The size of Curiosity's landing ellipse relative to Spirit's is a testament to the precision necessary to land NASA's newest rover.)
After touching down, Curiosity will begin slowly working its way up Gale's mound, sampling layer upon layer of rock and soil along the way. In doing so, Curiosity will provide us with a much better understanding of the environments in which these layers formed, how they formed, and whether Mars is, or ever was, capable of supporting life.
To learn more about the impressive scientific payload that Curiosity will use to conduct its mission at Gale crater, check out our backgrounder on the rover: Meet Curiosity - NASA's Mars Science Laboratory