Potassium-Argon Laser Experiment (KArLE)

Absolute dating of planetary samples is an essential tool to establish the chronology of geological events, including crystallization history, magmatic evolution, and alteration. Traditionally, geochronology has only been accomplishable on samples from dedicated sample return missions or meteorites. The capability for in situ geochronology is highly desired, because it will allow one-way planetary missions to perform dating of large numbers of samples. The success of an in situ geochronology package will not only yield data on absolute ages, but can also complement sample return missions by identifying the most interesting rocks to cache and/or return to Earth. In situ dating instruments have been proposed, but none have yet reached TRL 6 because the required high-resolution isotopic measurements are very challenging.


Our team is now addressing this challenge by developing the Potassium (K) – Argon Laser Experiment. KArLE uses a combination of several flight-proven components that enable accurate K-Ar isochron dating of planetary rocks. The K-Ar isochron approach is a relatively simple, and therefore more feasible approach for in situ geochronology. K is far more abundant in typical rocks than U, Rb or Sm, so it is readily detectable. The daughter Ar has a different physical state than the parent K, so the mass spectrometer component does not need to measure isobaric species. Finally, Ar diffuses less readily than He, so is more likely to be retained for a long time within a planetary surface rock. All of these attributes enable the K-Ar system to achieve desirable measurement accuracy using currently-available flight components and techniques.


KArLE will ablate a rock sample, determine the K in the plasma state using laser-induced breakdown spectroscopy (LIBS), measure the liberated Ar using quadrupole mass spectrometry (QMS), and relate the two by the volume of the ablated pit using an optical method. For this breadboard, we are using commercial off-the-shelf parts with performance similar to flight parts. These COTS parts are currently used in our collaborators’ laboratories for low-cost testing for their flight instruments (Ocean Optics LIBS in the LANL LIBS laboratory; Hiden RGA in the GSFC mass spectrometer laboratory).

(Above) KArLE operational schematic and laboratory setup. LIBS = Laser-Induced Breakdown Spectroscopy; OM = Optical Metrology; QMS = Quadrupole Mass spectrometer.

It is necessary to relate the absolute QMS and relative LIBS measurements to each other. Laser U-He measurements accomplish this by measuring the volume of the ablated material and converting it to mass via an assumed density, which for the majority of planetary samples is acceptable without introducing significant uncertainty. There are many possible methods to measure the pit volume pit without the physical contact of a probe, including scanning electron microscopy, phase shifting interferometry, and vertical scanning interferometry. We are evaluating the applicability of these methods to the LIBS and QMS setup to achieve the desired measurements.

KArLE Applications

The geochronology instrument must be integrated into a suite of other instruments and measurements to give the rock context. Appropriate measurements include remote sensing for geologic setting, imaging and microscopic imaging for petrology, and microanalytical techniques for chemical and mineralogic composition and variation. The instruments for making these measurements are expected to be present on any lander or rover as part of a standard measurement suite. However, the KArLE components themselves achieve many of these common analyses as well (e.g. full elemental characterization via LIBS, microscopic imaging via the optical component). Secondly, measurements must be made with as much contextual information about the sample’s location, composition, and properties as possible to ensure that the fundamental dating assumptions are valid, namely that the samples forming the isochron are cogenetic and that the system is closed. Finally, the measurement must generate an age that enables a geologic interpretation that clearly improves upon current knowledge. Many problems in geochronology require the resolution and sensitivity of a terrestrial laboratory and therefore cannot be solved by in situ instrumentation. Our preliminary work indicates that the KArLE instrument will be capable of determining the age of several kinds of planetary samples to ±100 Myr, sufficient to address a wide range of geochronology problems in planetary science, including dating the beginning and end of lunar volcanism, investigating the lunar impact history, and bounding Mars’ climate history.


Further Reading


Barbara A. Cohen (2015) Stories in Stone: Reading a planet’s history in its rocks. The Planetary Report,volume 35, number 1. (PDF)


Barbara A. Cohen, J. Scott Miller, Zheng-Hua Li, Timothy D. Swindle, and Renee A. French (2014) The Potassium-Argon Laser Experiment (KArLE): In Situ Geochronology for Planetary Robotic Missions. Geostandards and Geoanalytical Research. DOI: 10.1111/j.1751-908X.2014.00319.x. Open access article: http://onlinelibrary.wiley.com/doi/10.1111/j.1751-908X.2014.00319.x/abstract


B. A. Cohen, et al. (2014) The Potassium-Argon Laser Experiment (KArLE): In situ geochronology for Mars and beyond. Eighth International Conference on Mars, abstract #1482. (Abstract) (e-Poster)


B. A. Cohen, et al. (2014) The Potassium-Argon Laser Experiment (KArLE): In situ geochronology for Mars and beyond. International Workshop on Instrumentation for Planetary Missions, paper #1040. (PDF)


B. A. Cohen, D. Devismes, J. S. Miller, T. D. Swindle (2014) The Potassium-Argon Laser Experiment (KArLE): In situ geochronology for planetary robotic missions. 44th Lunar and Planetary Science Conference, abstract #2363. (PDF) (e-Poster)


E. Hand (2012) Planetary Science Time Machine. Nature 487, 422–425 (26 July 2012) doi:10.1038/487422a. Nature article describing in situ dating efforts, including KArLE.


B. A. Cohen (2012) Development of the Potassium-Argon Laser Experiment (KArLE) instrument for in situ geochronology. 43rd Lunar and Planetary Science Conference, abstract #1267. (PDF)


B. A. Cohen, Z.-H. Li, J. S. Miller, W. B. Brinckerhoff, S. M. Clegg, P. R. Mahaffy, T. D. Swindle, and R. C. Wiens (2012) Development Of The Potassium-Argon Laser Experiment (KArLE) Instrument For In Situ Geochronology. International Workshop on Instrumentation for Planetary Missions, paper #1018. (PDF)


F. S. Anderson, J.H. Waite, J. Pierce, K. Zacny, B. Cohen, G. Miller, T. Whitaker, K. Nowicki, P. Wilson, and H. Y. McSween (2012) In Situ Life Detection and Dating: A MSR Precursor Mission Concept. Concepts and Approaches for Mars Exploration, Abstract #4324. (PDF)



KArLE Acknowledgements

The KArLE project has been funded under the NASA Planetary Instrument Definition and Development Program (PIDDP).

In the LIBS method, a pulsed laser beam is focused on a target to ablate a small mass of material, forming a plasma. In the high temperature of the plasma, the atoms are electronically excited to emit light. Elements in the target sample are identified by collecting, spectrally resolving, and analyzing the plasma emission, and their abundance is related to the peak height. The advantages of using LIBS for KArLE are the absence of sample preparation and the liberation of daughter Ar in the plasma. The KArLE LIBS follows advances in development of LIBS for a variety of in situ planetary applications, including the ChemCam instrument on the Mars Science Laboratory.


(Right) LIBS spectra of microcline and rhyolite test samples showing K and O peaks (the oxygen peak is a triplet with a maximum intensity at 777.3 nm). Sample spectra in air were acquired with 100 shots each. The feldspar spectrum in vacuum (1.7E-07 torr) was acquired with 200 shots, and the rhyolilte spectra in vacuum with 370 shots. Each spectrum was acquired individually and averaged. Backgrounds were taken each day consisting of at least 100 blank runs and averaged, then subtracted from the averaged sample spectra. LIBS signal intensity varies with confining pressure; at Mars pressure, the intensity is greater than in terrestrial atmosphere, but as pressure decreases further the signal intensity diminishes. However, even under the high vacuum conditions of the Moon, the signal is clearly differentiated from background and measurements can be made with confidence.

Laboratory noble-gas measurements are traditionally accomplished by magnetic sector mass spectrometers. It is infeasible for flight instruments to use this magnetic sector instruments because of the large mass and power required by the high voltage and strong magnetic fields; instead, missions typically use quadrupole mass spectrometry. QMS techniques typically have lower sensitivity but their mass and power attributes are well-suited to in situ applications. Recent advances, especially improvements in resolution, have increased the performance of these instruments to the extent that they have demonstrated utility in some geochronologic applications. The KArLE mass spectrometer draws on neutral mass-spectrometer instruments developed for several planetary missions (CONTOUR, MSL, LADEE) at Goddard Space Flight Center.


(Left) 40Ar abundance in microcline and rhyolite test samples. The QMS was set in multiple ion detection mode using the secondary electron multiplier and continuous measurements were collected during the LIBS measurements. 40Ar buildup from background is small compared to the amount released from the sample. The microcline measurement is the total release from 200 laser shots and the rhyolite from 370 laser shots.

Volume of ablation pits using vertical scanning interferometry (left) and laser confocal microscopy (right). These measurements were acquired using two different optical methods on samples different from the LIBS/MS samples so are not directly comparable. However, they show the suitability of both methods for measuring pit volume.