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Curiosity tasked with hunting for elusive Mars organics

Posted: 22 May 2013

Back in action after a month out of contact with Earth, NASA's Curiosity rover is renewing its quest to excavate a definitive signal of organic molecules - the building blocks of life - from the red planet's regolith and bedrock after a first taste of Martian soil turned up inconclusive results.

Artist's concept of the Curiosity rover. Credit: NASA/JPL-Caltech
Scientists say there should be plenty of organic molecules on Mars. The red planet has been pummeled by asteroids and comets since it formed, delivering organics to the surface over billions of years.

So far, Curiosity has not encountered the levels of organic material scientists expected to find, but the mission has a long way to go before reaching a verdict on the presence of organics at the rover's Gale Crater exploration site.

The six-wheeled robot's drill bored into a slab of Martian mudstone Sunday to collect the mission's second sample of rock powder. The rover's drill and scoop, mounted on the craft's robotic arm, will deliver the sample to Curiosity's analytical instruments to scrutinize the material for the signature of life-forming organic molecules.

The drill site is at a rock named "Cumberland" about 9 feet away from the location where Curiosity extracted the mission's first powder rock sample from a target called "John Klein."

The drill campaign at Cumberland is the rover's first major scientific research activity since it renewed full communications with Earth in early May. Mars was on the opposite side of the sun from Earth for most of April, and managers instituted a moratorium on commands to the rover in case the sun disrupted or corrupted signals passing between Curiosity and ground antennas.

Scientists will use the Cumberland sample to confirm results from an analysis of Curiosity's John Klein sample collected in February, which showed the rover is in a region that was once habitable for life on ancient Mars.

The second drill sample will help ensure the findings from the John Klein analysis were not affected by contamination from other types of Martian soil.

"The idea is to follow up on all these discoveries we've been making at this John Klein drill site," said Ashwin Vasavada, the rover's deputy project scientist, in an interview before the May 19 drill at Cumberland. "The team has done a really extensive analysis of the first powder that was acquired during the drilling, but to confirm some of those results and maybe slightly tweak some of experiments for even better analysis, we'd like to do a second drill hole somewhere around the current drill site."

Both drill locations are in a shallow depression named Yellowknife Bay, where Curiosity has explored rocks and soil since autumn. Scientists believe the Yellowknife Bay site is at the end of an ancient river system.

Curiosity's robot arm camera took a series of images to compose this up-close view of the rover's second drill site May 19. Credit: NASA/JPL-Caltech/MSSS
Despite the promising discovery that Yellowknife Bay was once habitable for microbial life, studies of drill and scoop samples gathered by Curiosity have been inconclusive on the hunt for organic compounds - the chemical building blocks necessary for the formation of life.

Vasavada said he expects Curiosity will spend a few more weeks at Yellowknife Bay, then drive toward Mount Sharp, a three-mile-high peak nearby that might offer a more definitive measurement for Curiosity's instruments sensitive to organic molecules.

Since landing on Mars in August, Curiosity has driven about a half-mile across rocks and dusty soil to the Yellowknife Bay region. The rover's on-board instruments tasted powder from mudstone and found an array of elements essential for life.

Carbon? Check. Hydrogen? Absolutely. Oxygen? Yep. Nitrogen? Sure. Phosphorus? You bet. Sulfur? Plenty.

"You add it all up, and the presence of minerals in various states of oxidation would provide a source of energy for primitive biology," said John Grotzinger, Curiosity's lead scientist from NASA's Jet Propulsion Laboratory.

That finding is enough for Curiosity's mission to be considered a success, Grotzinger told a NASA advisory panel in April.

"This meets our mission success criteria, and we're having a good time," he said.

But in its first thorough check of Martian soil, Curiosity's SAM sample-analyzing instrument, designed to sniff out the building blocks of life, did not detect the kind of complex organic molecules many scientists hoped to find.

"So far, we detected some carbon in the rocks and some very simple carbon-containing molecules, but nothing you would call the organics that people were excited to find," Vasavada said. "That's kind of a mystery because we expect there to be carbon or even organics on Mars delivered naturally. We know that, for example, asteroids and comets have organic molecules in them. They form naturally in space and would be delivered to Mars."

Grotzinger and Vasavada have directed their science team to develop a systematic search for organic molecules, driving the rover to locations thought to best sustain carbon. Curiosity is not equipped to find extant life, but there is much to learn about the red planet's ability to preserve organics and how to uncover them.

It won't be easy, Grotzinger said.

"I just think we'd be nuts to go around promising people that we have even a good chance of finding organics," Grotzinger said in April. "On the other hand, I think, as a mission, we have to undertake this search systematically, so that if we don't find anything, we can do a proper post-mortem and say here's what we tried, here's what we discovered, and here's our best attempt to explain why we might have failed."

And if scientists are lucky, the rover could make a discovery, he said.

Curiosity returned this view of layered ground on Mount Sharp in August. This is near the rover's next destination. Credit: NASA/JPL-Caltech/MSSS
Curiosity is first continuing the work at Yellowknife Bay.

Managers selected the Cumberland drill site because it is near the location Curiosity retrieved the mission's first powder sample.

But one difference is the Cumberland rock is covered with tiny spherical concretions, which look similar to 'blueberries' discovered by NASA's Opportunity rover on the other side of Mars.

Opportunity found the blueberry concretions were made of hematite - a strongly oxidized iron-bearing mineral - left behind as water saturated Martian bedrock in an earlier wetter period in the red planet's history.

While Curiosity's finding looks similar to Opportunity's blueberries, scientists say the granules are made of different material. One possibility is the concretions are made of magnetite - an iron-based mineral with less oxidation than hematite - and formed in an aquatic environment in a similar way to the hematite blueberries.

Curiosity's chemical and mineralogical instrument indicated the rocks near the rover are rich in magnetite, and trailings from both of the rover's drill holes are gray, suggesting the material is less oxidized than the reddish rock present at Opportunity's landing site.

Scientists will have to wait several weeks to learn whether Curiosity's instruments find organic molecules in the Cumberland concretions. Their wait is even longer before Curiosity searches for organics amid the layers of clay on Mount Sharp, which researchers suspect holds the mission's best shot for finding more complex organic molecules than found so far.

Curiosity will start driving toward Mount Sharp as soon as June, making brief stops to investigate rocks and other interesting research targets before beginning the climb up the flank of the central peak of Gale Crater, the rover's landing site.

When NASA selected the Curiosity landing site, the prevailing theory was Mount Sharp was formed in a long-gone lake. But new research from scientists at Princeton University and the California Institute of Technology suggests Mount Sharp was assembled over eons by silt lifted into the Martian sky by winds.

The theory of wind formation for Mount Sharp would throw into doubt whether Mount Sharp is the best spot on Mars for a rover to seek evidence for past life, according to Kevin Lewis, a Princeton associate research scholar in geosciences and a participating scientist on the Curiosity rover mission.

This image from Curiosity's navigation camera captured a view of the rover's robot arm and turret, which holds the drill and other instrumentation. Credit: NASA/JPL-Caltech
Based on researchers' understanding of how organics are transported through the solar system, Vasavada said there should be many more carbon compounds than what Curiosity has discovered to date.

Paul Mahaffy, principal investigator for Curiosity's Sample Analysis at Mars instrument package, said ancient stream systems and layered clays like Yellowknife Bay and Mount Sharp are typically good preservers of organics on Earth.

"We do believe that there should be a background rain of abiotic carbon that comes in from the cosmos," Grotzinger said. "We would expect, at some point, to actually find, and be able to measure, something more complicated than a single chlorine chlorohydrocarbon - just a one-carbon atom structure."

Although Curiosity's first tastes of soil produced no conclusive results for organics, officials are not close to giving up. But scientists are questioning why the rover did not detect carbon where it was expected.

"Are we even able to detect that background amount we expect to be there, apart from life or anything else? If not, why not? The team has been discussing various ways that organics are destroyed on Mars," Vasavada said. "One possibility is that this particular site we're at wasn't conducive to preserving organics over time. There could be things like UV light, natural high-energy radiation, or different chemical oxidants, all of which could destroy the evidence before we get a chance to detect it with our instruments.

"That's where we're at with this particular site, not having seen much of a signal," Vasavada said. "Now the job is to take out the pencil and paper and figure out what we would have expected to see even from natural sources. Is it telling us this is not a place that would preserve that kind of evidence for us? That would point us to look elsewhere."

Michael Meyer, chief scientist for NASA's Mars program, said the rover's results will not be the final ruling on whether life or organic material existed in the red planet's distant past.

"Even if you understand everything and you don't find any evidence, that doesn't mean there wasn't something going on when those rocks were laid down," Meyer said May 6 in a public discussion at the Humans 2 Mars Summit in Washington.

According to Grotzinger, the rover's science team is thinking of ways "up the bar" and explore for carbon with an eye toward informing upcoming missions of where organics tend to reside on Mars.

"We're learning about how to find areas and samples that would preserve that kind of evidence," Vasavada said. "That helps not only us, but a sample return mission."

NASA's next Mars rover, due for launch in 2020, will likely carry equipment to collect and store soil samples for retrieval by a future spacecraft for return to Earth.

"I see this as something important for us to do because this isn't going to be easy," Grotzinger said. "Somewhere there should be something preserved. I think as we go to Mount Sharp, we'll be able to push more buttons there, and not only explore different habitability scenarios but also explore options that may have preserved organic carbon differently."