Sen—Nearly 1000 sols (Martian days) into her mission, Curiosity may be poised to take a major step forward through Martian time. The mission began in a very flat region of Gale crater that was safe for the rover's radar-assisted landing mechanism. For about two years, the robot has been progressing steadily southward toward the crater's gigantic central mountain. She has left behind the flat plains and is now roving through a landscape of subdued buttes and canyons. As the topography steepens, more geologic time is represented in each vertical stack of rock layers. And as the rover climbs in elevation, she rolls forward through Martian history. Soon, Curiosity will reach canyon walls that expose a major discontinuity in that history. A view from orbit provides context.
An image from the Mars Reconnaissance Orbiter covers the area in which Curiosity is now driving. The color swath is about 1.4 kilometers wide. HiRISE image ESP_021610_1755. Image credit: NASA/JPL/UA/Emily Lakdawalla
Curiosity descended from the crater-scarred rocks of the Zabriskie Plateau into a landscape of sand-filled valleys on sol 740. Soon after, the rover approached the light-toned rocks of Pahrump Hills. The Curiosity science team believes these rocks represent the base of Mount Sharp, and therefore the earliest phase of Martian history accessible to the mission. They spent nearly six months at Pahrump Hills, drilling twice into the soft, finely layered rocks whose sediments must have formed in relatively slow-moving or deep water.
After Pahrump Hills, Curiosity drove into a valley called Artists' Drive. The walls of that valley represented some of the same materials that Curiosity had passed through before—the rocks that formed Zabriskie Plateau, the scarred highland to the north. But as Curiosity progressed to the west, the rover began to get glimpses a different type of rock sitting atop the Pahrump materials. This upper unit has been named the "Washboard Unit" because it has weathered into sets of strikingly parallel ridges. It occurs all over the interior of Gale crater. Here is a large patch of the stuff located south of Gale's central mound:
Washboard materials south of the Gale central mound. Cropped from HiRISE image PSP_005998_1745. Image credit: NASA/JPL/UA.
Its 'Washboard' appearance suggests that it could be sand dunes, but if so, they are sand dunes that have been turned into solid rock; the Washboard unit clearly preserves impact craters. Such large dunes must have formed by wind, not water action, but to turn them into rock seems to require water. What caused the ancient environment within Gale crater to change from a wet, lake-forming one to one that supported enormous fields of sand dunes? What process cemented the dunes into rock? Did moisture percolate down from above, dissolving and reprecipitating salts and oxides from Martian atmospheric dust? Or did groundwater rise into the crater from below, bringing minerals that glued the sand dunes together?
The Curiosity mission planned to encounter the contact between Pahrump materials and the Washboard unit with a drive up from the valley into a place called Logan Pass. But on sol 958 the geologists spotted something that brought them up short. In a news update, project scientist Ashwin Vasavada described the outcrops at Mount Shields as looking like "what's called an 'incised valley fill,' which is where a valley has been cut into bedrock and then filled in with other sediment." If so, then the boundary between the lower Pahrump materials and the higher Washboard unit could be an erosional one, where water carved into the Pahrump materials after they had been turned to rock. Here is a distant view of Mount Shields, in context with Logan Pass:
Curiosity took in the view toward Logan Pass and Mount Shields with her telephoto camera on sol 957. Image credit: NASA/JPL/MSSS
Curiosity drove over to those rocks and found them to be exquisitely finely layered. It's not clear yet what they mean for the story of Gale crater's history. The mission has not yet informed us whether they believe the sediments formed in a river environment or if they were blown in by wind.
Mount Shields outcrop, sol 976. Image credit: NASA/JPL/MSSS/Kevin Gill
Curiosity has now returned to her originally planned path, uphill through Logan Pass toward a particularly clear view of the contact between the Pahrump unit and the overlying Washboard unit. Here is a view of a hill named Mount Stimson, where the Washboard unit clearly lies directly on top of Pahrump materials.
Mount Stimson in the middle ground, with Pahrump materials on its lower flanks and Washboard materials forming its top. In the background are the wind-carved slopes of Mount Sharp. Image credit: NASA/JPL/MSSS/Emily Lakdawalla.
When Curiosity drives to and over that contact, the rover will be rolling forward through Martian history—but by how many years, we don't know. Hopefully, there will be clues in the Washboard rocks, which Curiosity can read by drilling into them. Washboard rocks could contain a record of a different environment, with different chemistry than we've studied so far in Gale crater. On the other hand, we've seen in the Pahrump rocks a record of several distinct episodes of groundwater altering the rock, leaving behind thick veins of precipitated minerals. Maybe these wetting episodes happened long after both the Pahrump and Washboard units formed, and the chemistry of that later alteration will obscure whatever distinct character the rocks originally had.
The contact between the Pahrump unit and the Washboard unit is the last major geological boundary Curiosity will pass before crossing the black sand dunes that ring the base of Mount Sharp. From orbit, it looks like Curiosity will cross back into Pahrump materials as she roves across a landscape that's been molded by the action of scouring wind. But we could find the rocks to be quite different—maybe their proximity to the mountain affected the fluids that flowed through them. Even if they're the same materials, it's clear that the views are going to be incredible as Curiosity rolls through the valleys among picturesque buttes and brooding black sand dunes. The adventure continues!