Curious Mars


CuriousMars: Curiosity Begins Rock Triage to Avoid Dangers of "Martian Honey"

The NASA Mars rover Curiosity used its Mastcam on Dec. 7, sol 120, to image a spectacular outcrop dubbed "Shaler”. The outcrop's striking layers, some at angles to each indicate cross bedding. NASA/JPL-Caltech/Marco Di Lorenzo/Ken Kremer

The first use the rover Curiosity's drill to obtain subsurface samples from inside a rock on Mars will be delayed until mid to late January to reduce risk to the rover during its first drilling operations.

The delay is needed to complete extensive target rock "triage" to ensure that the heat from drilling friction will not cause the pounded rock sample to turn into a kind of gooey "Martian honey" that would foul rover components, perhaps fatally.

"We have to make sure anything we put into our system does not destroy our system," said Robert C. Anderson, of the Curiosity Surface Sampling System (SSS) Team at the Jet Propulsion Laboratory, Pasadena, Calif.

"We probably would have already drilled [by now] but we are being very cautious," Anderson told CuriousMars.

Once drilling occurs, the initial subsurface sample analyzed will be a milestone in planetary exploration history.

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Internal workings of a Curiosity drill bit (top) are annotated in the graphic at bottom. Credit: NASA/JPL

It will mark the first in situ subsurface examination of any material beyond the Earth-Moon system and the first mineralogy assessment of any Martian rock on the planet's surface.

JPL geologists and engineers are taking extreme care to avoid rocks that could change phase into a honey-like material that would seriously gum up rover mechanisms costing time and threatening science operations. This triage will take extra time that, coupled with a Christmas break and final drill checkout, will push the first drilling well into January.

"There are numerous papers in the literature now about weird minerals on Earth where they look like a solid rock, but when you put some heat into them they turn into something like honey," Anderson told CuriousMars. "Deliquescence is what it's called, and similar rock chemistry and mineralogy could be on Mars," he said.

Materials like iron sulfates and perchlorates could have these characteristics, according to recent scientific literature. Chemistry similar to both could occur on Mars and perchlorate salts have been found at both the Curiosity equatorial and 2008 Phoenix north polar lander sites.

Curiosity this week arrived in Yellowknife Bay following a 26 meter (86 ft.) drive from a spectacular layered rock outcrop known as "Shaler". The rover science team is first looking for potential rocks to drill among the flat light colored rocks at Yellowknife Bay. But if nothing suitable is found at Yellowknife, they will U-turn Curiosity and continue looking back up an incline toward the spectacular Shaler layered outcrop.

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Rover view of Yellowknife Bay taken Dec. 17, sol 130, shows flat light colored rock at right swept clean by wind or water and bordered on the left by much darker material. Credit: NASA/JPL-Caltech/Marco Di Lorenzo/www.KenKremer.com

The site is known within the team as "The Triple Point". According to Joel Hurowitz, another member of the SSS team, the three different terrains can be summarized as:

- Light toned fractured unit: The material stands out in Mars Reconnaissance Orbiter images. "The theories on what it could be have been are all over the map from mudstone formed in a lake to lava flows," Hurowitz said. The rover is now sitting on this light toned fractured unit that looks like a wide avenue swept clean by water or wind with a left bank of distinctly darker slightly higher elevation material.
- Dark Cratered unit. "It is not clear that we have actually driven yet on this unit that lies just above the light toned fractured rocks. " Hurowitz told CuriousMars. "In general it appears to be made of a volcanic basaltic rock that appears fairly coherent and hard. It could be an interbedded lava flow," he said. The dark material makes what looks like a river bank to the light toned rock at Yellowknife.
- Alluvial fan material. "We landed on it Aug. 5 and it seems to overtop the two underlying units. It is material composed of sedimentary conglomerate rocks," said Hurowitz.

"We have got 400 scientists on this mission and I guarantee you we have got 400 hypothesis of what those rocks are," said Anderson.

While descending an incline 26 meters (85 ft. ) from Shuler to Yellowknife, the rover began the triage process.

Rover camera images and data from Curiosity's arm mounted Alpha Particle X-ray Spectrometer (APXS) and the powerful MAHLI "hand lens" imager are being coupled with data from the mast mounted ChemCam Laser-Spectrometer to determine the composition of potential targets.

The composition information from the contact science instruments will help keep the rover from drilling rocks that threaten deliquescence. This will be instrumental for triage on various rocks for us to determine "if a sample is good enough," Anderson said.

But it will be much less informative than the detailed mineralogy data that the CheMin spectrometer and the SAM instrument will provide once a safe sample is provided to those internal Curiosity laboratories.

"We are trying to get an understanding of what the elemental composition of the target rocks are and their textural composition for comparison with our engineering constraints," Anderson said.

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Mars Reconnaissance Orbiter image inset map at bottom right shows rover positions on different sols including final points where Curiosity is at the demarcation point between the dark material and flat light colored rocks.
Credit: NASA/JPL

The term "drill" is a bit of a misnomer for Curiosity's tool which is much more like a rotating a chisel with a percussive action. The hammering blows break the subsurface material into powder that is augured upward within a hollow sleeve surrounding the drill bit that leads to circular storage area atop the bit.

In addition to finding a large, safe rock to drill, a relatively fine grained texture is desired along with a flat surface for rover stability. Both the arm and rover must be kept within limits for tilt and elevation.

The fine grain size is important to cleaning the drill system of potential contaminates acquired during final assembly, launch and the 8 month cruise to Mars.

The first 3-5 drilling operations will be used to clean the drill. The selected rock will be drilled and dumped at least three times including transport of the sample into the circular can atop the bit which holds the powdered sample.

Each time the drill system will be vibrated to ensure the wall of the system are coated with Martian dust. Once that has been completed, rover arm motions will use gravity to dump a fourth or fifth sample into the adjoining CHIMRA collection and handling system that will filter the sample to the 150 micron size particles acceptable to CheMin and SAM. It will then be distributed to those two laboratories.

NASA and JPL Management must approve use of the drill and that will not happen until the multi-layered rover team goes through its own assessments.

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Curiosity Mastcam image of drill bit shows percussion bit at center flanked by treaded prongs to hold the bit steady on rock target being drilled for subsurface sample. Credit: NASA/JPL

The triage results will be used for comparison with dozens of terrestrial rocks tested with the same instruments on Earth to give an idea of how hard the Martian rocks are.

Anderson put together and entire catalogue of rocks from very soft rocks to hard abrasive basalts. "For the last five years we have been running drill samples on hard basalts and other rocks and also on clays to see how they could clog the system or move within the auger. All of this has given us an idea about how the drill will behave on certain rock types," he said.

(The next edition of CuriousMars will appear Thursday January 3 and focus in detail on Curiosity's complex drilling hardware.)

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