The Curiosity Mars rover has found intriguing veined rocks just below tilted cross-bedded layers indicating water once flowed and "percolated" through fractured terrain near the landing site in Gale Crater, scientists said Tuesday, additional evidence of a watery past on the red planet,
Taking their time evaluating a surprising variety of scientific targets, mission scientists and engineers now are gearing up for the first tests of a powerful impact drill that will be used to collect samples from inside targeted rocks.
The drill tests are a final major milestone before the rover begins creeping toward Mount Sharp, a towering 3.4-mile-high mound of layered rocks in the center of the broad crater that represents the mission's primary objective.
The base of Mount Sharp is just six miles away as a martian crow would fly. But given the wealth of scientific targets expected along the way, the complexity of the rover and an understandably conservative operational philosophy, it's going to take a while to get there.
"I would guess it would be a good goal for us to try to get there by the end of the calendar year," Project Scientist John Grotzinger told reporters Tuesday. "But this mission is 100 percent discovery driven. If we find some really good stuff, we're going to take the time it requires to do it right."
Curiosity currently is working in an area known as Yellowknife Bay about a quarter of a mile from where the rover landed Aug. 6. Its cameras have beamed back razor-sharp images of fine-grained sedimentary rocks with intriguing veins, nodules and cross bedding that indicate the effects of flowing water.
"This lowest unit that we are at in Yellowknife Bay, the very farthest thing we drove to, turns out to be kind of the jackpot unit here," said Grotzinger. "It is literally shot through with these fractures."
The fractures eventually were filled in by a white material that precipitated out at some later time.
"What these vein fills tell us is water percolated through these rocks, through these fracture networks and then minerals precipitated to form the white material that ChemCam (a rover instrument) has concluded is very likely a calcium sulfate, probably hydrated in origin," Grotzinger said.
"So this is the first time in this mission that we have seen something that is not just an aqueous environment, but one that also results in precipitation of minerals, which is very attractive to us."
Adding to the emerging picture of a once wet environment in Gale Crater, a layer of rocks just above the veined material shows cross bedding, in which multiple thin layers are oriented in different directions.
"What this basically records is the passage of sediment in a moving current that created a small dune and as the dunes migrate they preserve their avalanche faces and those get preserved in the rock record as this cross bedding," Grotzinger said.
The grains that make up the cross-bedded rocks "are too course for the wind to be the current that pushed the grains along," he said. "So we think this formed in water."
Based on orbital photography, scientists already knew Curiosity landed on an alluvial fan where water probably flowed in the distant past. Moving downstream, or east-southeast from the landing site, Curiosity made its way to an area dubbed Glenelg where three different types of rock come together.
Along the way, scientists noted outcrops of conglomerate rock made up of large fragments that once were transported by water. Moving into Glenelg and then into Yellowknife Bay, the rover descended slightly, moving into older terrain, dominated by much finer-grained material, possibly indicative of a less forceful, more tranquil flow.
The cross bedding implies a stream flow "probably a few tens of centimeters per second, maybe a meter per second (2.2 mph) flow, something like that," Grotzinger said. "As far as the depth, it's difficult to know at this point.
"It would certainly be similar to what we were talking about with the conglomerates (closer to the landing site). Maybe just a little farther away, farther down current, or at a time when the current was slowing so it could no longer transport the gravel but it was able to still transport the sand and the finer bits of gravel."
Richard Cook, the project manager of the Mars Science Laboratory mission, said the rover is healthy and that drill tests using the veined rock in Yellowknife Bay should begin soon.
The drill is capable of penetrating rocks to a depth of about two inches. As it grinds into a target, pulverized samples from the interior will be fed into Curiosity's two major laboratory instruments.
The Sample Analysis at Mars instrument, known as SAM, uses a gas chromatograph and two spectrometers to look for signs of organic compounds. The Chemistry and Mineralogy experiment, or CheMin, uses X-ray diffraction to identify minerals in soil and rock samples.
The drill is the last of Curiosity's sampling systems to be put to the test.
"Because of all the interesting things the scientists have been finding, the start of the drilling campaign has been delayed by a few days," Cook said. "But at this point, we are all ready to go to do that.
"We're undoubtedly going to learn a lot about how to drill things on Mars as it's the first time we've ever done that and it'll probably go slowly. But I think by the time we get through this campaign and deliver some sample to the CheMin and SAM, I'm sure scientifically it'll be a great set of measurements."
The primary scientific goals of Curiosity's mission are to look for signs of past or present habitability and to search for the organic compounds that are essential to life as it is known on Earth. Water is a key factor in habitability and there now is little doubt it once flowed in Gale Crater.
But how much water might have been present, and for how long, is not yet known.