From high above the planet, two orbiters – Mars Global Surveyor and Mars Odyssey (the first mission in a revamped NASA Mars exploration program following the 1999 debacles) – returned images of fans and flows of sediment from the earliest period of Martian history. The teams interpreting the images, published in 2003, concluded that the features represented evidence of long-lived flows of water moving across the surface.
At the same time, images from Mars Odyssey showed channels cut into the sides of craters and along other slopes, suggesting brief catastrophic releases of water in the geologically recent past.
A year later, after Opportunity landed, it found tiny spheres of minerals sprinkled across the surface and embedded in a layer of sediment that was part of a rocky outcropping the rover was examining. Researchers dubbed the spheres "blueberries," later identified as made of the mineral hematite, which form in water-saturated soil deposits. Last September, the team said it had discovered similar spheres in a formation at Endeavour Crater, but with different compositions. In addition, the rover has detected veins of gypsum and clays in the rocks, further suggesting water in the area early on.
Curiosity has added to the growing evidence. In its brief sojourn, the rover has already helped scientists identify rounded rocks, as well as rocks bound up in cemented clumps of soil, that point to the presence of an ancient riverbed etched on the floor of Gale Crater billions of years ago.
Comparing timelines for the geological evolutions of Earth and Mars, as well as for the emergence of life on Earth, "it becomes abundantly clear that, yes, there probably was persistent standing water on Mars long enough" for life to have evolved in the same fashion it did on Earth, Dyar says.