Laser-Induced Breakdown Spectroscopy Promises a Quicker, More Thorough Analysis of Martian Rock

Barry E. DiGregorio, Cardiff Centre for Astrobiology

One thing was clear from the surface images provided by the Mars Pathfinder and Mars Exploration rover missions: Mars is a dusty planet. Boulders, rocks, pebbles and soils have coatings of dust that are millimeters to centimeters thick.

Steps in the interaction between the ChemCam laser pulse and the target show the removal of surface dust coating.

Unfortunately for planetary scientists, the small Sojourner rover on the Mars Pathfinder mission did not have a tool to clean off the dust, so all that could be measured with any certainty were the coatings themselves. To correct this, the Mars Exploration rovers Spirit and Opportunity were equipped with spinning abrasion tools, which cleaned rocks right down to their matrices. However, this method requires that the rovers drive to an individual rock for cleaning and sampling. This means that the rover engineers must spend a lot of time for just one alpha-particle x-ray or Mössbauer spectrometer analysis of a single rock: For each measurement, it must calculate the distance to the rock and plot a course that avoids obstacles such as crevices and boulders.

ChemCam fires a sequence of laser pulses at an otherwise unreachable rock outcrop. The ability of the ChemCam laser-induced breakdown spectroscope to analyze rocks and soils at a distance will make it an indispensable tool in the exploration of Mars. Copyright ©1998-2003 The Regents of the University of California. Images courtesy of Roger C. Wiens.

Now, an instrument package known as ChemCam has been approved to become part of the next landing mission to Mars. And rather than having to drive up to a rock and brush off its dust mechanically, ChemCam will ablate the dust at a distance with a laser.

Since its inception 35 years ago, laser-induced breakdown spectroscopy (LIBS) has evolved into a sophisticated analytical tool with applications in such fields as medical diagnostics, environmental monitoring, and meteorological, archaeological and geological sciences. Since the mid-1990s, the technology has improved to the point where no sample preparation is required and where the results are obtained in real time. Besides being easy to use, LIBS can reveal information on virtually any element in any form.

The LIBS instrument that will be part of ChemCam, which is suitable for spectroanalysis at low atmospheric pressures, is being provided as a joint effort of the Los Alamos National Laboratory in Albuquerque, N.M., and the Centre d’Etude Spatiale des Rayonnements (CESR) in Toulouse, France — supported, respectively, by NASA and the French space agency, CNES. ChemCam will fly on NASA’s Mars Science Laboratory, a long-range, long-duration rover due to launch in 2009 and to arrive on Mars in 2010. The rover will explore the planet for at least two years and will use a nuclear power source to generate the electricity to run its complement of scientific instruments and cameras.

How ChemCam works

ChemCam is a suite of two instruments mounted to the rover’s mast, with electronics in the rover’s body. One instrument is the LIBS component, the other a remote microimager 2 m above ground level. The LIBS laser unit is based on the Diva Nd:YAG made by Thales Laser Diodes SA of Orsay, France. It uses a pulsed laser providing 30 mJ to a 1-mm spot, with a pulse duration of <8 ns. It will generate a series of variable-frequency pulsed bursts to produce a glowing plasma (typically around 10,000 K) from any targeted rock, pebble or soil sample at distances of 2 to 13 m from the rover. The vaporized plasma will then be analyzed by the spectrometer and compared with a spectral library of Earth samples.

Roger C. Wiens of the Los Alamos lab, the ChemCam team leader, described a test conducted on a dusty rock: “The rock was shot from a distance of 5.3 m with an ~30-mJ laser, similar to ChemCam. The dust was removed over an area about 1.5 cm in diameter with 50 laser shots. The dust was removed by the expanding shock wave from the LIBS plasma. The LIBS on ChemCam will be able to penetrate through several millimeters of dust coatings on Mars, and in rocks it can excavate at least 1 mm.”

Greater reach

The remote microscopic imager, which was built by CNES, is a telescope with a 100-mm mirror combined with a 1024 × 1024-pixel CCD camera that can focus between 1 m and infinity. The ability to focus to 1 m will allow the imager to examine samples on the rover deck that subsequently will be fed into the in situ analytical instruments.

Its resolution is expected to surpass that of the Mars Exploration rover’s panoramic cameras by fourfold or better. At 10 m, the imager will have a pixel resolution of 300 μm, or 3 cm at 1 km. With the closest samples at ground level (~2 m from the instrument), it will have a pixel resolution of around 60 μm. The imager’s exposure times range between 2 ms and 8 s, and it also can take black-and-white context images of the areas on rocks that have been cleaned by the laser and analyzed by LIBS.

According to Wiens, although alpha-particle x-ray and Mössbauer spectroscopy can’t zero in on fine detail as well as LIBS can, they have an easier time accurately determining whole-rock composition because they cover more area at once and are significantly easier to calibrate. But alpha-particle x-ray spectroscopy is expected to achieve more accurate results because it is a contact measurement. LIBS is a remote sensing technique and, therefore, a complementary one.

Once the rover’s camera has captured images of the landing site area and ChemCam’s instruments have analyzed rocks within their ranges, the rover will be sent to the most interesting rocks. When it arrives at a rock to be sampled, the full suite of scientific instruments onboard, including the alpha-particle x-ray spectrograph, mass spectrometer, x-ray diffractometer and microscopic imager, will be employed.

In the martian atmosphere, the size and brightness of the visible plasma (inset) will be greater than in Earth’s atmosphere because of the lower average pressure of 6 to 10 mbar. The low pressure will allow greater expansion of the emitting plasma and will result in much greater ablation rates. Copyright ©1998-2003 The Regents of the University of California.

Another advantage of stand-off remote sensing with ChemCam will be the ability to analyze martian rocks that cannot be reached by the rover at all — such as outcrops high up and out of reach on a crater wall. Rocks in crevices also will be easily examined.

Meet the author

Barry E. DiGregorio is a research associate at the Cardiff Centre for Astrobiology in the UK.