Determining Key Isotopic Ratios In The Field

On August 6th, 2012 the automatic Mars Science Laboratory rover named Curiosity landed on Mars. One of the scientific instruments on board is ChemCam, which has a pulsed laser capable of ablating a focused spot on a remote sample to create a glowing plasma plume of target material. Light from plasma is collected by rover’s telescope on a mast, and the optical spectra are then analyzed by an internal spectrometer. ChemCam can take thousands of spectra per day from a distance of about 7 meters, thus making chemical analyses on the surface of Mars with unprecedented speed. Operationally, the ChemCam data facilitates decisions of where the rover should be driven.

In 2009, Applied Spectra was awarded a NASA SBIR grant to develop an innovative technology that enables not only chemical elemental but also isotopic analysis using ChemCam or a similar instrument. Applied Spectra collaborated in this work with the Lawrence Berkeley National Laboratory. As a result of the joint research efforts, a new elegant technology was born and branded “Laser Ablation Molecular Isotopic Spectrometry” or LAMIS.^[1-4] LAMIS shares all the same technical benefits of its predecessor, Laser Induced Breakdown Spectroscopy (LIBS), including rapid analysis and the elimination of sample preparation. LIBS measures atomic emission spectra during the first microsecond after an ablation pulse. LAMIS measurement follows later when the plasma cools down. Then molecules form in the plasma and the intensity of molecular spectra increases and persists for some time (Figure 1). Mo l e cul a r spectra are useful for isotopic analysis because the isotopic shifts in molecular emission are significantly larger than in atomic spectra (Figure 2). The difference in isotopic masses has only a small effect on electronic transitions (as in atoms) but appreciably affects the vibrational and rotational energy levels in molecules.^[4] There is no need for a large, high-resolution spectrometer; a compact spectrometer can resolve isotopic spectra. LIBS and LAMIS techniques can be accomplished on the same instrument, thus extending ChemCam with a new dimension of isotopic analysis.

The merits of LAMIS were quickly recognized within the scientific community. LAMIS achieves rapid and direct chemical and isotopic characterization without any acidic dissolution or deep evacuation of samples as required in typical mass spectrometry. Furthermore, rasterizing surface scans and depth profiling are easily realized with high spatial definition (~10nm in depth and ~10μm lateral).

Isotopic analyses have proven to be a powerful scientific tool. Isotopes formed at the origin of the Universe and are produced in stars including catastrophic events of supernovae. Studies of the sources and driving mechanisms of isotopic variations can provide answers to fundamental questions on the development and evolution of stars and planets, as well as our solar system. Large fractionation in stable isotopes of H, C, N, O can be particularly indicative of a range of diverse processes in the biosphere, hydrosphere and lithosphere. Life processes lead to distinctive isotope patterns thereby providing clues to the origin of life and evolution in a galactic context. Isotopic information in paleoclimatology plays a critical role for the reconstruction of variations in past climate conditions. Consequently, isotopic records hold keys to the prediction of future climate changes that may influence global temperature, energy needs, availability of drinking water, and food supplies.

“Overarching issues that could have a significant impact on... strategies for Mars include the absolute chronology of the planet.” This assertion was strongly emphasized in the National Research Council review Assessment of NASA’s Mars Architecture 2007—2016. Radioisotopic age dating is the primary method in which accurate geochronological ages can be established. Measurements of the isotopic ratios of ⁸⁷Rb/⁸⁶Sr and ⁸⁷Sr/⁸⁶Sr isotopes using LAMIS can provide the age at which rocks solidified by applying the well-known radiometric isochron dating method. Presently, the dissolution and chromatographic separation of strontium from rubidium is necessary prior to conventional mass spectrometric analysis because of nonresolvable isobaric interference between the ⁸⁷Sr and ⁸⁷Rb. The results of LAMIS measurements of ⁸⁶Sr, ⁸⁷Sr and ⁸⁸Sr isotopes were recently published,^[1,2] while optical spectra of ⁸⁵Rb and ⁸⁷Rb in laser ablation plasmas were obtained earlier.^[5] Accordingly, a ChemCam-like de vice can potentially be used for age determination. Until now, there was no means by which to make direct age dating measurements on other planets (indirect age estimates for Mars have uncertainties in billions of years, the validity of which is unknown).