Science and Exploration

CuriousMars: SAM and CheMin Ace Rock Analysis, Ready for More

By Craig Covault
February 28, 2013
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CuriousMars: SAM and CheMin Ace Rock Analysis, Ready for More
These Goddard technicians must have put all those wires in right, because just like at Rocknest last fall, SAM smoothly processed the drilled rock sample. Credit: GSFC
Credit: GSFC

The Mars rover Curiosity is this week in the midst of potentially historic discoveries as the full range of its capabilities are brought to bear for the first time on a gray powdered Martian subsurface rock sample.
The sample, drilled from within a mudstone type rock, was totally unexpected this early in the mission and could reveal whether this once water soaked region of Mars preserved organic carbon pertinent to past life on Mars.

The Mars rover Curiosity’s complex Sample Analysis at Mars (SAM) organic chemistry instrument and the CheMin X-ray system breezed through their first analysis of the sample this week and are already underway with, or preparing for additional processing runs on the same highly unusual material.

SAM is the most complex instrument ever sent on a planetary mission. Principal Investigator Paul Mahaffy leads the project from Goddard in collaboration with 51 co-investigators and 28 institutions including the University of Paris and The French Centre National de la Recherche Scientifique that provided the Gas Chromatograph. Credit GSFC.

The Ames Research Center Chemistry and Mineralogy CheMin instrument in fact is designed to fire X-rays through its samples multiple times to build up imagery revealing detailed mineralogy data.

“This stuff we drilled is new material to us,” said David Blake, CheMin Principal Investigator. “We are all sitting on pins and needles about what it is,” he told CuriousMars.

“SAM performed beautifully and we are busy analyzing data,” said Principal Investigator Paul Mahaffy of the NASA Goddard Space Flight Center, Greenbelt, Md.

Graphic shows how SAM silver pencil-like rods on rotating carousel hold top mounted quartz “sample cups” (out of view in drawing except right cutout). The large structure embracing the rods is a precision elevation and positioning device that pushes the rods with sample cup up into furnace out of view above . Credit: GSFC/Honeybee Robotics.

SAM works via heating the Martian rock powder to nearly 2,000 deg. F (1,093 deg. C) and analyzing the gas baked out of the sample.

“We are well on our way, our team is vigorously looking at the details of what we have gotten,” Mahaffy told Curious Mars.

“The experiment worked very well and we certainly want to very carefully look at the data and firm up any conclusions we have,” Mahaffy told CuriousMars. “As we get the best measurements there is certainly a desire to get them out and that is what the whole Curiosity Science Team wants to do,” Mahaffy said.

SAM carousel with sample carrier rods is shown upside down during test at Honeybee Robotics. Note helium transport gas tank. The “Bees” also developed Curiosity’s Dirt Removal Tool and the MER rovers’ RATs. Credit: Honeybee Robotics

If the rest of the Science Team agrees on the use of time and power resources required, the SAM team would like to run a second processing cycle with the same sample, plenty of which is still stored within the instrument turret on the rover’s arm. During this second run the temperature ramp up sequence would be varied allowing a different measure of the gases at different temperatures in Sam’s three instruments.

Gas Chromatograph

According to a Goddard SAM team review of its instrument suite, gas from the sample first travels to the Gas Chromatograph (GC) instrument. The purpose of this instrument is to sort out all the different molecules in the sample, and determine how much there is of each gas. It accomplishes this by using a stream of helium gas to push the sample down a long, narrow tube (which is wound into a coil to save space).

The inside of the tube is coated with a thin film. As molecules travel through the tube, they stick for a bit on the film–the heavier the molecule, the longer it sticks. Thus, the lighter molecules emerge from the tube first, followed by the middleweight molecules, with the heaviest molecules bringing up the rear.

The 88 lb. (40 kg.) SAM is a marvel of packaging and robotics. SAM’s sample gas processing system consists of the two turbo molecular pumps in a system of micro valves, manifolds, transfer tubes, heaters and multiple sensors. Of the tiny micro valves, 46 of the 52 were designed, fabricated, and tested in-house at NASA GSFC. The 6 remaining valves were manufactured by Aker Industries, Oakland, Calif. The micro-valves built in-house at GSFC were electron-beam welded into titanium manifolds. SAM utilizes 14 custom fabricated manifolds, tiny holding tanks for gas vented by rock samples. Credit GSFC

Quadrupole Mass Spectrometer

Since molecules of different weights emerge from the tube of the gas chromatograph at different times, the GC can send groups of different weights, one at a time, to SAM’s detection instrument, the Quadrupole Mass Spectrometer (QMS) instrument. It will determine exactly what kind of molecule makes up each of the groups.

The QMS fires high-speed electrons at the molecules breaking them up into fragments and giving the molecules and their fragments an electric charge. These electrically charged molecules and their fragments can be moved by electric fields.

The QMS uses both DC and AC fields to sort the electrically charged molecules and fragments based on their weight (mass). Molecules and fragments of different mass are counted by a detector at different times to generate a mass spectrum, which is a pattern that uniquely identifies molecules.

Tunable Laser Spectrometer

After the QMS identifies the molecules, the sample is directed into the Tunable Laser Spectrometer (TLS), which can identify and analyze certain volatile molecules, such as water and carbon dioxide.

Lower drilled hole produced the Gray powder from the mudstone type rock that SAM and CheMin are studying for the presence of organics or minerals that could help support life. Credit: NASA/JPL-Caltech/MSSS.

The sample enters a chamber with precisely positioned mirrors at both ends. A laser is fired through a tiny hole in one of the mirrors. As the laser light bounces between the mirrors, it illuminates the sample. Different molecules will absorb certain wavelengths (frequencies) of light, so the TLS identifies the molecules by which frequencies are blocked (since the laser is tunable, it can be adjusted to shine in a range of wavelengths). The TLS can also identify isotopes the same way.

Before SAM received its sample , “We cleaned our sample cup by heating it to a very high temperature (about 1,832 deg. F (1,000 deg. C) then left it in the hermetically sealed oven so it would not pick up any residual contamination, said Mahaffy.

The SAM instrument got its first 0.0026 oz (0.078 cc) of Martian powered subsurface rock on February 23 /Sol 196, a day after CheMin instrument received and processed a similar powdered rock sample. SAM then began a complex fully robotic processing run over 6 hours in length.

Pix 6 A generic turbo molecular pump design illustrates the extreme detail involved in the miniature pumps on SAM to move specific sample molecules. Credit: Liquidat

At the time of the delivery, the command software sequence enabled the funnel and transfer tube of the SAM inlet to vibrate utilizing a piezoelectric actuator. This motion facilitated the transfer of the sample into a quartz cup, which was then sealed in one of two ovens for the start of evolved gas analysis within minutes.

Although SAM had processed Martian sand dune material last fall, what Curiosity came to Mars to do was process samples from inside Martian rocks that could have preserved perhaps 3.5 billion year old organic carbon related to a period of possible Martian life only a billion years into the planet’s history – the same time that life arose on Earth.

The sample analysis sequence consisted of first thermally conditioning the SAM manifolds – small holding tanks – that were later exposed to the evolved gas. Background spectra was then obtained from both the Quadrupole Mass Spectrometer and the Tunable Laser Spectrometer to document their background conditions to subtract out of actual sample data.

Sample inlet for CheMin glistens during inspection by the rover’s Mars Hand Lens Imager. Credit: NASA/JPL-Caltech/MSSS

The SAM instrument’s Sample Manipulation System (SMS), a rotating carousel developed by Honeybee Robotics, transported the powder delivered from the Curiosity’s drill to the SAM inlet and into one of 74 sample ceramic cups mounted on stems. The stems are arranged in two circular rows around the carousel giving each side access to one of two ovens for redundancy.

The carousel, with an elevator to push the sample upward into the oven, then moved the powder in the quartz crucible to the SAM oven so the rock powder would release its constituent gases when heated to about 1,832 deg. F (1,000 deg. C). The oven is part of the Chemical Separation and Processing Lab (CSPL), which includes a lot of plumbing to transport, enrich and process gases so they can be measured and characterized.

A helium flow of approximately 0.03 atm-cc/sec was then initiated to sweep gases evolved from the sample past a capillary inlet of the mass spectrometer and into the Tunable Laser Spectrometer at selected sample temperature intervals.

The tiny sample delivery ports on the 60 lb. instrument turret dispensed a precise half-a-baby aspirin dose of 150 micron powdered rock to CheMin and SAM. Credit: NASA/JPL-Caltech/MSSS

“As the gas from the sample was transported to the TLS, we [deliberately] leaked just a little bit of it into the Mass Spectrometer on the way into the TLS, where we captured it,” Mahaffy said.

“We ran it over a trap that could [capture] the less volatile species–specifically the organic compounds we are looking for,” he told CuriousMars. He was unable to say, however, whether anything has been found at this early stage in the analysis.

“When all that was done, we measured the gas in the TLS by activating its lasers and doing scans of the gases from the sample.

“And when that finished we transferred the [possible] hydrocarbons or other simple inorganic gasses we trapped from the SAM trap over to a little equivalent trap on one of the six Gas Chromatograph ‘columns’,” Mahaffy said.

Graphic shows the relay satellite radio band capabilities from Curiosity up and down from the NASA Odyssey, MRO and European Mars Express orbiters. Credit: NASA/JPL-Caltech

SAM is part analytical chemistry instrument that can determine the compounds in the rock sample. To do that it uses gas chromatography where the helium carrying volatiles baked out of the rock powder, can be described as the mobile phase of the analysis.

But to work the mobile phase has to pass over a “stationary phase” of specialized material – a microscopic layer of polymer on an inert solid support, inside a piece of glass or metal tubing called a “column” in homage to the fractionating column used in distillation. On SAM there are six columns to provide enough differing stationary phase material for analytical chemistry tests throughout the minimum 2 year mission.

“The plan was to put a pulse of energy into what we call the hydrocarbon trap, right in front of the GC column,” said Mahaffy. “That started moving the gas through the column…and part of it separated out in time. But a portion of it went into the Mass Spectrometer where we were rapidly scanning the spectra with our ‘Smart Scan Algorithm’.”

As the rock drilling preparations were underway the rover’s mast mounted color Pancam imaged some interesting rocks, including this one that has apparently been scuffed and moved slightly by the 1 ton rover. Note the gray color on this rock also. Credit: NASA/JPL-Caltech

“That algorithm was looking for signals of intensity and zeroing in on those particular mass areas that really help carry out the search for organics and other simple organic compounds that are capable of making it through the GC column,” Mahaffy told CuriousMars.

“During this time we had lots of heaters on cause we wanted to make sure all the lines [were] hot enough to make sure some of the less volatile species [got] all the way from where they might have been hidden in the solid sample all the way through the GC column and into the Mass Spectrometer,” Mahaffy said.

The first instrument to process a portion of the sample was CheMin which will identify and measure the abundances of various minerals in the rock powder.

By determining the mineralogy of rocks and soils, CheMin will assess the involvement of water in their formation, deposition, or alteration. In addition the CheMin data will be important in the search for potential mineral biosignatures, energy sources for life or indicators of past habitable environments, Curiosity’s prime objective.

Dave Blake has been perfecting the CheMin instrument at Ames for use on Mars for 22 years. He built the first prototype in 1991 and the one on Curiosity is the “CheMin 4 design”. Geologists use ground based versions of his instrument for Earthly mineral identification in the field.

“Over that period and once we got the CheMin 4 instrument prototype we literally analyzed thousands of terrestrial materials all over the world to get their signatures,” Blake told CuriousMars.

Exploded view of CheMin instrument shows how a vertical mounted carousel rotates samples in front of X-ray beam. Credit: NASA/Ames

With that immense data base, Blake and his team should be able to accurately identify whatever mineral types exist on Mars using CheMin’s X-ray Diffraction and X-ray Fluorescence capabilities.

When the powder is poured by the arm into the instrument’s funnel atop the rover deck it collects in a reservoir above a wheel with 32 transparent cells, 27 of which can be used and reused for samples. Five of the cells hold calibration standards for repeated use throughout the mission.

Two different transparent materials make up the windows in the cells through which the X-rays are fired, because the different windows can bring out different X-ray Diffraction capabilities.

“Typically we will get 3 hr. of analysis per Martian night,” said Blake. “We operate at night because the instrument is cooler at night and the CCD that detects the X-ray protons is cooled with a cryocooler lowering the instrument’s temperature to -55 to -60 deg. F,” ( -48 to -51deg. C) he told CuriousMars.

Typically tests are run on back-to-back nights, although sometimes a longer test series is needed. The powder from this first subsurface drilling will likely be run several times as the team learns more about its composition, but significant results should be available by next week.

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