The rock-coring drill fixed to the end of the Curiosity rover’s robot arm has suspended operations to allow engineers on the ground to diagnose, and officials hope correct, a problem traced to the mechanism that pushes the drill bit onto rocks to collect powder samples.
Rover controllers based at NASA’s Jet Propulsion Laboratory in Pasadena, California, first encountered the issue Dec. 1 when Curiosity was unable to complete a planned drilling on the lower flank of Mount Sharp, a three-mile-high (5-kilometre) mountain the robot is climbing to study how the environment evolved on ancient Mars.
Managers decided to keep the six-wheeled rover in its current position — prohibiting driving and use of its robotic arm — until experts can determine the cause of the problem.
Ashwin Vasavada, Curiosity’s project scientist at JPL, said the ground team believes the issue has been traced to a brake on the drill feed mechanism, which is supposed to extend and place the drill bit on the surface of the rock targeted for drilling.
“You press against the surface to keep the drill in place, and then a mechanism moves the actual drill up and down to do the drilling,” Vasavada said in a press conference Dec. 13 at the fall meeting of the American Geophysical Union. “That’s called the drill feed. That mechanism exhibited a stall … and since then we’ve been running activities on the rover to diagnose that issue.”
Engineers originally thought the problem might be rooted in an encoder associated with electrical sensors that tell the rover’s computer how the drill is functioning. Vasavada said the problem apparently is with the brake, which is “very much internal to the motor itself.”
Vasavada said Curiosity’s team had “partial success” in unstalling the drill feed, but now the problem is recurring.
“It went away and we were very excited, but then it unfortunately has returned again in just the last day or so,” Vasavada told reporters last week. “We’re in the process of still figuring out how to go recover the operation of that drill feed.”
Curiosity’s drill works by boring into rock targets with a combination of a percussive, hammering motion and the rotation of the drill bit. Rock powder excavated by the drill goes into a collection chamber, where the material is sifted and sieved for delivery to miniature laboratory instruments on the rover’s science deck.
The target selected for drilling more than two weeks ago was to be the 16th rock drilled by the rover since it landed on Mars in August 2012. It would have been the seventh drilling operation of 2016, according to NASA.
Ground controllers programmed the drill to only use its rotating mechanism on the latest sampling attempt. The percussive mechanism that chisels into rock has had an intermittent electrical short since early 2015, and while that function is still available, officials prefer to avoid using it unless necessary.
“We still have percussion available, but we would like to be cautious and use it for targets where we really need it, and otherwise use rotary-only where that can give us a sample,” Vasavada said in a press release.
Rock samples collected by the drill feed two of Curiosity’s main science instruments — the Sample Analysis at Mars payload and the Chemistry and Mineralogy package — to look for organic materials and measure mineral content.
While Curiosity employed its drill more sparingly in the first three years of its mission, the device has had more use in recent months. Early in the mission, scientists turned to the drill to probe into specific rocks that captured their interest.
The last four drillings have been spaced around 80 feet (25 metres) apart in vertical elevation, allowing for a more systematic geologic investigation, according to Joy Crisp, Curiosity’s deputy project scientist at JPL.
“This landing site has definitely exceeded our expectations,” Crisp said Dec. 13.
Curiosity is in its fifth year on Mars, well past its two-year prime mission in Gale Crater, an impact basin stretching 96 miles (154 kilometres) across that harbored a salt water lake around 3.5 billion years ago.
After driving nearly 10 miles (15 kilometres) across the crater floor, bypassing a dune field and blazing a path through rocky outcrops and buttes, Curiosity is now climbing up Mount Sharp, where it will explore progressively younger layers of sedimentary rock.
“Now in our second extended mission, we’re well into the layers that were the prime reason for selecting Gale Crater as our landing site,” Crisp said.
Curiosity is currently located in the Murray formation, a band that consists primarily of mudstone, a type of rock that hardened from mud at the bottom of ancient lakes.
“You might think mudstones would be boring, but they’re definitely not,” Crisp said.
The exploration of younger layers of Mount Sharp, each deposited at a different time on ancient Mars, gives scientists a window into how the climate of the planet changed over hundreds of millions of years.
“It gives us a sense for the chemical reactions that could have been utilised for any life that was around to make a living,” said Thomas Bristow, a scientist based at NASA’s Ames Research Center in California who works on the rover’s CheMin instrument. “By surveying the variety of minerals as we climb up Mount Sharp, we’re essentially surveying the possibilities for making a living within the lakes and rivers (that were in) Gale Crater.”
As Curiosity ascends through Murray formation, the rover’s instruments have detected changing levels of minerals that tell scientists about the conditions of the water that once stood inside the crater.
“It turns out this Murray formation is really sort of a bananza of all the things we intended to study when we picked the landing site,” said John Grotzinger, a member of Curiosity’s science team from Caltech and the mission’s former project scientist. “We just got it a lot earlier than we thought.”
Grotzinger said the Murray formation more than 600 feet (about 200 metres) higher in elevation than the Yellowknife Bay region Curiosity visited in the first year of the mission. On Earth, such vertical relief represents tens to hundreds of millions of years of time, and that “conservative estimate” holds true on Mars, he said.
“A sedimentary basin such as this is a chemical reactor,” Grotzinger said. “Elements get rearranged. New minerals form and old ones dissolve. Electrons get redistributed. On Earth, these reactions support life.”
Bristow said CheMin measurements over time have shown evidence of increased “aqueous alteration” as Curiosity climbs higher through the Murray formation. The mineral magnetite, a type of iron oxide, was more prevalent lower on the mountain, but hematite is the prominent iron oxide found at Curiosity’s most recent drill sites.
“Both samples are mudstone deposited at the bottom of a lake, but the hematite may suggest warmer conditions, or more interaction between the atmosphere and the sediments,” Bristow said in a NASA press release.
Grotzinger said the presence of magnetite is an indicator of an environment that could have fed nutrients to micro-organisms, building on Curiosity’s discovery that Mars was habitable to microbial over long periods of time.
“These are micro-organisms that don’t need sunlight,” Grotzinger said at the Dec. 13 press conference. “They just feed off the materials that are in the rocks.”
According to Bristow, the results from the latest drilled samples show a return of clay minerals, which were prevalent early in the mission and disappeared as Curiosity started its climb up Mount Sharp. But the clay minerals Curiosity is currently studying have a different crystalline structure, he said, with a shift from magnesium and reduced iron to aluminum and oxidized iron.
The CheMin payload on the Curiosity rover fires a beam of X-rays as fine as a human hair into rock powder dumped inside the instrument. The atoms in the sample re-emit the X-rays at particular energies depending on their type of element, and the X-ray signal will bounce off each mineral’s structure in a certain way.
Bristow said the “X-ray diffraction” technique allows scientists to study the structure of minerals at nano-meter scales.
Curiosity is also finding more calcium sulfate and silica at higher elevations in the Murray formation, which tells scientists that the water content present when those minerals were deposited was different than lake environment that existed earlier in Mars’ history.
“The waters are getting saltier but they’re nowhere near the salt levels that we had at Meridiani Planum for the Opportunity rover,” Grotzinger said, referring to the terrain explored elsewhere on Mars by NASA’s Opportunity mission. “So it’s not so salty that it can’t be habitable. It might be sea water salt.”
The question of habitability was the key objective of Curiosity mission scientists before the rover landed, and Grotzinger said the evidence continues to indicate that ancient Mars could have sustained life forms.
“We see all of the properties in place that we really like to associate with habitability,” he said. “There’s actually nothing really extreme here, for the most part, so this is all very good for habitability over very long periods of time.”
In fact, Grotzinger said said the silica-rich rock where Curiosity is now located might be a good destination to snatch up samples for return to Earth on a future mission. NASA’s Mars 2020 rover is designed to collect and cache specimens in pencil-shaped tubes for recovery and return by a future spacecraft.
“We’ve got loads of silica in a finely laminated kind of rock that would actually be the kind of rock you’d want to get a sample of and bring back to Earth to look for fossil microbes actually preserved in there, because on Earth laminated silica is the best type of material we have for recording evidence of life in ancient rocks,” Grotzinger said.
Curiosity, like previous Mars missions, has turned up no sign of ancient Martian life, but such a goal is beyond the scope of its instrumentation.
Gale Crater is not currently one of the candidate landing sites for consideration on the Mars 2020 mission.
“This type of silica on Earth will form in environments where it will precipitate very quickly, and if there is a micro-organism there it will entomb them before water, which is the essential substance of life, interacts enough that the organic matter breaks down,” Grotzinger said.
Scientists also announced last week the detection of boron in veins crisscrossing through rock on the slopes of Mount Sharp. The veins, mainly made of calcium sulfate, formed as ground water flowed through bedrock after the lakes in Gale Crater dried up.
“The presence of boron can tell us about the chemistry of the lake water, or it can tell us about the later ground water that produced these calcium sulfate veins,” said Patrick Gasda from the Los Alamos National Laboratory in New Mexico.
Curiosity’s rock-zapping laser tool, ChemCam, detected trace levels of boron inside the veins. It is the first time boron has been found on Mars, Gasda said.
On Earth, boron is found in arid sites like Death Valley, California.
Scientists have also discovered boron in a meteorite that traveled from Mars to Earth, but Gasda said researchers could only learn so much from that sample.
“The only problem from the meteorite is you don’t have all the context of Gale Crater,” Gasda said. “That’s that benefit of having the measurement in Gale Crater, is that we know a lot about the crater, and there was a lake, so we have this context that we can bring to the table and leverage that to understand what the boron is doing in the ground water, for example.”
Boron may also play a part in the genesis of RNA, a building block of life.
“It is thought by some people … that you would need boron in a certain form called a borate, and these borates will react with chemicals, and these chemicals can eventually become RNA,” Gasda said.
But the laser-shooting ChemCam instrument is not capable to determining the type of boron in the mineral veins. The CheMin instrument could make that measurement if Curiosity is able to drill out a sample containing boron on the path higher up Mount Sharp.
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