"There was a river, and there was a lake. That's pretty awesome," Vasavada said.
It was firm proof, scientists believed, that water once flowed on Mars — a critical notion, because every environment that contains water, at least on Earth, sustains microbial life.
But scientists already knew that water once existed on the surface of Mars. Older missions had offered plenty of suggestions, and the twin rovers NASA sent to Mars in 2003, Spirit and Opportunity, had proved it in spectacular and definitive fashion.
The new rover's mission was more complex: a search for all of the basic ingredients of life, particularly carbon-carrying organic molecules.
What's more, there were percolating questions about what the delta might prove. There was a risk that the delta was not a record of Mars' history, but a "one-trick pony," Vasavada said, an aberration forged in a relatively short period of time, at least by geologic standards.
As discussions continued, NASA's Mars Reconnaissance Orbiter, which launched in 2005, was quietly circling the planet. The MRO, as it is known in space science parlance, was designed to pave the way for landed missions on Mars by mapping the planet's surface and conducting a higher-fidelity investigation of Mars' weather and environment than had ever been accomplished.
Starting a couple of years ago, the MRO began sending home a data dump about the landing site finalists. The data made it clear that all of the finalists contained vast scientific possibilities, but none more than Gale Crater and its mountain.
The MRO was equipped with lasers that could peer into the planet's surface and detect the presence of minerals.
The mountain, it turned out, was once not a mountain at all. It was part of a giant, layered plateau, which was then eroded — perhaps by wind, perhaps by water — into a cone.
On its slopes, the MRO found distinct layers of rock, like those John Wesley Powell found in the Grand Canyon.
The lowest tier of the mountain was built of clay, a remnant of what appear to have been Mars' water years, topped with a layer of mineral-rich sulfates created when the water disappeared, topped with layers of sand and dust.
Here could be a record of Mars' environmental history — of two planetary-scale geologic phenomena, the first when water appears to have covered Mars with sediment, the second when Mars dried up and the sediment was stripped away. Last summer, NASA made it official: Curiosity would land in Gale Crater and, from there, would head for the mountain.
"Gale was a racehorse that was in the back of the pack and then jumped ahead," Vasavada said. "You want to look at piles of stuff — that's where history is recorded. The rocks at the bottom are old. The rocks above are younger. And some amount of time is recorded for us in between. We think Gale has many stories to tell. We're going to ask it what happened."