NASA’s current rovers, Curiosity and Opportunity, are continuing to explore their respective regions of Mars, with new findings that are providing yet more clues as to the geological history and potential past habitability of this fascinating world. They have also both just completed significant steps in their journeys and are now entering new and exciting phases of their missions. Both missions have found yet more evidence that the Mars we see today—cold and dry—was once much wetter and potentially habitable, at least for microorganisms.
Curiosity has been trundling across Naukluft Plateau for several weeks now, since passing by the Bagnold Dunes earlier. Naukluft Plateau is a remnant plateau of fractured sandstone which used to be more extensive in this area. While on the plateau, the rover completed its 12th drilling campaign, in mudstone bedrock, as well as re-examining its 1oth and 11th drill holes, to repeat an experiment which compared material within and away from pale zones around fractures. Curiosity also took additional self-portraits while in this region.
But now, the rover has finished crossing the plateau and has descended back down to lower terrain, as well as turning south onto a path which will take it directly to Mount Sharp.
“Now that we’ve skirted our way around the dunes and crossed the plateau, we’ve turned south to climb the mountain head-on,” said Curiosity Project Scientist Ashwin Vasavada, of NASA’s Jet Propulsion Laboratory, Pasadena, Calif. “Since landing, we’ve been aiming for this gap in the terrain and this left turn. It’s a great moment for the mission.”
Curiosity has been examining the Murray formation near the base of Mount Sharp which is about one-eighth of a mile (200 meters) thick, and to date has studied about one-fifth of its vertical extent. Naukluft Plateau covers a portion of the Murray formation; previous drillings and other experiments by the rover in this region have revealed that this area inside Gale crater used to be a lot more habitable than it is now, with both lakes and rivers.
“The story that the Murray formation is revealing about the habitability of ancient Mars is one of the mission’s surprises,” Vasavada said. “It wasn’t obvious from pre-mission data that it formed in long-lived lakes and that its diverse composition would tell us about the chemistry of those lakes and later groundwater.”
Previous drilling by the rover were in a sandstone bedrock called the Stimson formation. Stimson is thought to have been created by wind which left a portion of lower Mount Sharp covered in sand dunes. Later, water moving through cracks in the sandstone created “haloes” of brighter-colored sandstone near the fractures.
“We were about to drive off the Naukluft Plateau and leave the Stimson formation forever as we go up Mount Sharp,” said Curiosity science-team member Albert Yen of JPL. “A few of us were concerned. The fracture-associated haloes were becoming more prevalent, and we had only one data point. With just one data point, you never know whether it is representative.”
Curiosity has also found evidence for multiple episodes of groundwater in this region billions of years ago, by comparing the samples taken from the drill sites at Lubango and Okorusu. Higher levels of both silica and sulfate were found in samples closest to the fractures.
“The big-picture story is that this may be one of the youngest fluid events we’re likely to study with Curiosity,” Yen said. “You had to lay down the Murray, then cement it, then lay down the Stimson and cement that, then fracture the Stimson, then have fluids moving through the fractures.”
Combined with previous evidence from earlier in the mission, it has become clear to scientists that this region in Gale crater was once once much more habitable than it is now, at least for microbes, with groundwater, lakes, and rivers. Whether it ever actually was inhabited, however, is still unknown.
Another more recent finding by the rover is the discovery of the mineral tridymite, which was unexpected and surprising to mission scientists. On Earth, it is produced by silicic volcanoes, such as Mount St. Helens in Washington. Volcanoes on Mars, however, tend to be of the basaltic type, like the ones in Hawaii, so it’s not clear yet just how the mineral formed in Gale crater.
“It’s really nifty, but we were shocked,” said Richard Morris, a scientist with NASA who led the study. “If it stands up, then something is going on on Mars that we don’t fully appreciate,” he added.
Curiosity also recently found evidence that the Martian atmosphere used to be much richer in oxygen than it is now. On Earth, most of the oxygen is produced by living organisms. Whether that was true on Mars also isn’t yet known, as it could also have been produced without life, but it’s possible. As interestingly noted by Damien Loizeau of the University of Lyon in France, “O2 is bad for life as we know it, but we only know life to be able to create large amounts of O2.”
The change in direction is exciting since Mount Sharp itself has been the long-term goal of the mission. So far, Curiosity has been busy traveling on the outskirts of the mountain, since landing in 2012. The rover technically reached the outer base of the mountain in 2014, but now will be actually ascending the mountain itself, once it passes through the gap in Murray Buttes. The foothills of Mount Sharp are a very scenic landscape of mesas, buttes, and canyons, similar to what is seen in the American Southwest.
Meanwhile, elsewhere on Mars, the Opportunity rover is now finishing up its study of Marathon Valley on the edge of Endeavour crater. The shallow valley, which cuts through the western edge of the crater, was a prime target for the rover, since studies from orbit showed that it contained an abundance of clay minerals, which would be further evidence for past liquid water in this area. That water would have been less acidic and salty then the ancient water elsewhere in Meridiani Planum, which the rover had previously found evidence for. Upon closer examination, the clays were found to be present in reddish streaks inside the valley, and the material was very weathered and crumbly. While Curiosity has a drill to penetrate several centimeters into rocks, Opportunity has the RAT, which can grind into just the surface layers of rocks.
“What we usually do to investigate material that’s captured our interest is find a bedrock exposure of it and use the RAT,” Squyres said. “What we didn’t realize until we took a close-enough look is that this stuff has been so pervasively altered, it’s not bedrock. There’s no solid bedrock you could grind with the RAT.”
“Fractures running through the bedrock, forming conduits through which water could flow and transport soluble materials, could alter the rock and create the pattern of red zones that we see,” Squyres also noted.
The rover can also scuff surface soil material with its wheels, to expose what lies just underneath.
“In the scuff, we found one of the highest sulfur contents that’s been seen anywhere on Mars. There’s strong evidence that, among other things, these altered zones have a lot of magnesium sulfate. We don’t think these altered zones are where the clay is, but magnesium sulfate is something you would expect to find precipitating from water,” Squyres added. “Fractures running through the bedrock, forming conduits through which water could flow and transport soluble materials, could alter the rock and create the pattern of red zones that we see.”
Opportunity has been exploring the Meridiani Planum region since landing there in 2004. The amazing longevity of the mission has been a welcome surprise—the nominal mission was expected to last for hopefully three months! As of June 14 this year, Opportunity has driven 26.59 miles (42.79 kilometers).
Last spring, Opportunity also saw a large dust devil, a rare occurrence for the rover, although its former twin rover Spirit saw many of them in Gusev crater.
So what’s next for Opportunity? It seems the plan is to continue traveling along the southeast edge of the crater, for the time being at least.
“We are wrapping up our last few activities in Marathon Valley and before long we’ll drive away, exiting along the southern wall of the valley and heading southeast,” said Opportunity Principal Investigator Steve Squyres, of Cornell University in Ithaca, N.Y.