Check out Ryan Anderson's description of Gale Crater at his blog, the Martian Chronicles. You can also download his scientific paper, Geologic mapping and characterization of Gale Crater and implications for its potential as a Mars Science Laboratory landing site
By now, probably most of you have read that NASA selected Gale Crater as the target for the Mars Science Laboratory that will land there next summer. Many news accounts repeated the story that all four landing site finalists would have made great sites, and that the science community wasn't able to come to a consensus about which one was a priority.
The journal Science has just published an article that gives the back story on the ultimate selection process. Unfortunately, a subscription is required, so anyone without a university library account is unlikely to be able to read the piece. So, I'll give a quick summary of the key points.
Gale Crater stands out for its geomorphic and compositional diversity. Unfortunately, no one knows how that big mountain in Gale Crater with all its layers formed. Wind blown dust? Volcanic ash? Impact debris? Science titled its story, "How an alluring geological enigma won the Mars rover sweepstakes." That term, enigma, Science reports, comes up a lot in discussions about Gale.
So, why did an enigma win? Here's what Science reports:
Mawrth has the most ancient materials (big plus), but as with Gale, no one knows how they formed. Also, the geology of Mawrth has been changed by a nearby impact, and the fear was that scientists wouldn't be able to tease out its history. As an additional problem, this site lacks geological diversity, and driving there might be like driving across the surface of Meridiani Planum, the Opportunity rover's site. (Fortunately, Opportunity has had craters to explore, but there's not a whole of diversity in between.)
Holden Crater lacks a delta to indicate that it ever had a lake, and the MSL mission is all about exploring sites modified by water.
Eberswalde Crater has a delta and eventually became the runner up site. While the layers in the delta would make an enticing target for exploration, this site lacked the diversity of Gale Crater.
In the end, the diversity of Gale Crater plus its abundance of water-altered strata won the day. As a bonus, the smooth plain where the rover will land may be a delta. For a mission of exploration, an enigma with diversity presents an opportunity.