[meteorite-list] Focusing a Laser on Martian Rocks and Soils (LIBS on MSL)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri Oct 27 16:07:57 2006
Message-ID: <200610272007.NAA28577_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/Oct06/libs.html

Instrumets of Cosmochemistry
LIBS: Remote Chemical Analysis
Planetary Science Research Discoveries

Written by Linda M. V. Martel
Hawai'i Institute of Geophysics and Planetology

In this series of articles, "Instruments of Cosmochemistry," PSRD
highlights the essential tools and amazing technology used by talented
scientists seeking to unravel how the solar system formed. You will find
information on how the instruments work as well as how they are helping
new discoveries come to light.lightbulb

------------------------------------------------------------------------
Laboratory Caliber Instrument on a Rover

Laser-induced breakdown spectroscopy (LIBS) is an active remote sensing
technique used for the rapid characterization of elemental compositions
of materials. Used for years in laboratory and industry applications, it
will make its debut performance on rocks and soils on another planetary
surface in 2010 as part of the ChemCam instrument package onboard NASA's
Mars Science Laboratory (MSL) rover scheduled for a 2009 launch to Mars.
A combined Raman-LIBS is also planned to be part of the Pasteur
instrument payload on the ExoMars rover mission planned by the European
Space Agency for a 2011 launch.

In preparation for use on Mars, a team of scientists at Los Alamos
National Laboratory, Roger Wiens, Justin Thompson, James Barefield,
David Vaniman, Sam Clegg, and colleague Horton Newsom (Institute of
Meteoritics at the University of New Mexico) have tested the LIBS
technique on two Martian meteorites and a terrestrial analog rock. Their
work confirms that LIBS is capable of determining even subtle
differences in rock types from a stand-off distance of 5.4 meters. This
high-quality remote sensing on the surface of Mars is exactly what's
needed to push the state-of-the-art of cosmochemical investigations as
we prepare for follow-up Mars sample return missions.

Reference:

    * Thompson, J. R., R. C. Wiens, J. E. Barefield, D. T. Vaniman, H.
      E. Newsom, and S. M. Clegg (2006) Remote Laser-Induced Breakdown
      Spectroscopy Analyses of Dar al Gani 476 and Zagami Martian
      Meteorites. Journal of Geophysical Research, v. 111, doi:
      1029/2005JE002578,2006.

------------------------------------------------------------------------
How LIBS Works

Laser-induced breakdown spectroscopy (LIBS) uses a high power pulsed
laser, focused on the target, to provide more than a megawatt of power
on a small spot less than a millimeter diameter for a few billionths of
a second. The target rock can be up to 13 meters away from the
instrument (otherwise known as the stand-off distance). Each laser pulse
vaporizes thin layers of the target rock--a process known as laser
ablation--producing a hot spark or plasma. This supersonically expanding
plasma glows with electronically excited ions, atoms, and small
molecules from the target rock (see image below.)

[frame of LIBS plasma movie]
<http://www.psrd.hawaii.edu/Oct06/LIBS_LosAlamos_plasma.mov> Click on
the movie frame to view a QuickTime movie in a new window.

This picture shows the glowing LIBS plasma produced in air during a
laboratory test where the laser was five meters away from the rock. The
high-temperature ablated material breaks down into electronically
excited atoms and ions, giving off light when they decay back to lower
energy levels. The light emitted by the plasma can be collected and
analyzed through spectrometers to resolve the characteristic emission
lines of the elements that are present in the target rock.

Original source: http://libs.lanl.gov/LIBS_movies.html. Note that the
apparent wandering of the plasma position on the rock is due to motion
of the rock during the test. There is no positional instability of the
laser relative to the spark size.

The plasma light is collected by a reflecting telescope and directed
through a fiber-optic cable to spectrometers, which resolve and measure
the elemental emission lines in the plasma spectrum. In a typical
analysis, the spectra from multiple pulses (for example 75 to 100
pulses) are averaged for greater statistical accuracy into one final
spectrum for the analysis spot.

The LIBS technique yields detailed, quantitative information on
compositions of the elements (high and low atomic numbers), including
some minor and trace elements, that are present in the target rock. This
information is obtained very quickly, within minutes, and will allow
scientists to identify rocks on the surface of Mars that are of greatest
interest and may be chosen for further investigation by instruments that
require physical contact or for collection.

Why LIBS is an outstanding tool for planetary surface analyses:

    * no sample preparation is required
    * operates at a stand-off distance (typically 2-13 meters), which
      permits remote analysis of inaccessible rocks (perhaps up on cliff)
    * the laser removes dust from target surfaces, again without the
      need to drive to and touch the surface
    * repetitive laser pulses on the same analysis spot permits ablation
      down through weathering rinds to measure composition through depth
      profiling and examine the pristine rock chemistry
    * simultaneous multi-element detection (major, minor, trace elements)
    * rapid analysis (a typical analysis sequence is six minutes)
    * good detection sensitivity; 10 ppm detection limits for some elements
    * laser requires only an average of 3 Watts of power during the
      several minutes of instrument operation time

    (For more details see the ChemCam Fact Sheet
    <http://www.psrd.hawaii.edu/Oct06/ChemCam_Fact_Sheet.pdf> produced
    by Los Alamos National Laboratory. Link opens in a new window.)

[cliff at crater edge]
<http://photojournal.jpl.nasa.gov/catalog/PIA08810>

This view of a cliff of layered rocks at Victoria crater was made by the
panoramic camera on NASA's Mars Exploration Rover Opportunity in
September, 2006. The cliff is about six meters (20 feet) tall. In the
future, scientists will be able to use LIBS to analyze the rocks up on
the cliff that are out of reach of the rover. Theoretically, LIBS could
produce elemental profiles of the entire wall of the outcrop. [Click the
image for higher resolution versions.]

------------------------------------------------------------------------

LIBS Cosmochemical Applications to Mars

The ability to identify and quantify the elemental compositions of rocks
and soils on Mars is of paramount importance to understanding important
issues of the planet's formation and alteration. LIBS spectra will play
a vital role in allowing scientists to address these key issues:

    * igneous processes and what they tell us about planetary
      differentiation and evolution of magma compositions through time
    * sedimentary processes and what they tell us about the interactions
      between rock and water or atmosphere
    * hydrothermal and weathering processes that have modified (or are
      currently altering) the Martian crust and what they tell us about
      the history of water on Mars
    * movement and deposition of materials
    * climate and habitability of Mars

In preparation for use on the Martian surface, the research team at Los
Alamos and University of New Mexico tested the LIBS technique in their
laboratory on natural rock samples under simulated Martian conditions.
The ability of LIBS to distinguish between rocks of widely differing
compositions is well known. But what Thompson, Wiens, and colleagues
wanted to test specifically was the ability to remotely distinguish a
range of igneous compositions on Mars. So, they chose to study two
basaltic Martian meteorites with slightly different compositions and
textures and an andesite rock powder standard. The team analyzed
Dar al Gani 476 and Zagami, two basaltic shergottite meteorites. Both
samples were in the form of sawn slabs, shown below.

[slab of DaG476 ]
[slab of Zagami]

Dar al Gani 476 is a basaltic shergottite with olivine and pyroxene
phenocrysts up to 5 millimeters in size set in a fine-grained groundmass
of average grain size of 0.13 millimeters. This sample is roughly 1
centimeter across.

Zagami is a basaltic shergottite described as a composite of up to three
related lithologies and minor
shock-melted glass. Overall this rock is finer grained than DaG 476.
This sample is roughly 1.5 centimeters across.

The LIBS analyses were conducted under simulated Martian conditions,
with the Martian meteorites and andesite sample each placed in a vacuum
chamber maintained at a static pressure of ~7 Torr CO2. Each analysis
spot was shot 100 times with a pulsed Nd:YAG laser resulting in ablation
pits that ranged in diameter from 0.4 to 0.5 millimeter.

Fourteen analysis spots were recorded on DaG 476 (five are shown in the
figure below) and nine spots were used on Zagami (also shown below).
Thompson and his coauthors chose a larger number of analyses on DaG 476
to try to compensate for the rock's larger grain sizes.

pictures of rock slabs with analysis spots
These pictures show some of the LIBS analysis spots or pits (~400
micrometers diameter) on slabs of DaG 476 (top left) and Zagami (top
right). Each pit was produced by 100 pulses of the laser beam. The red
arrow from spot 2 on DaG 476 points to a magnified back-scattered
electron image of the pit. Each LIBS spectrum used by the research team
was produced by averaging 100 laser pulses.
 

A typical LIBS spectrum for an analysis spot on DaG 476 collected by the
research team is shown below.

[LIBS spectrum of DaG476]
Major elemental emission lines in the LIBS spectrum for DaG 476 are
labeled in this plot. The Roman numeral in parentheses refers to the
ionization state of the atom. (I) is an excited neutral atom, (II) is a
singly-ionized atom. LIBS spectra obtained for Martian meteorite Zagami
and the andesite rock powder standard (JA-2) are similar at the level of
detail shown. The spectrum was collected from a stand-off distance of
5.4 meters.

The diagrams below show how well the LIBS results compare with the
average of published chemical analyses (averages of several analyses) of
the two Martian meteorites and the andesite. In general, the LIBS
results are within 10% of the oxide compositions reported in the
literature.

[LIBS data vs. published lab analyses]

Elemental compositions obtained by the research team using LIBS (shown
as weight percent on the x-axis) are compared to literature whole-rock
compositions (y-axis) of DaG 476, Zagami, and andesite JA-2. An exact
match in oxide composition between the LIBS tests and published
laboratory results would plot on the straight line. The oxides plotted
here are SiO2, FeO, CaO, MgO, TiO2, Al2O3, and Na2O. The lower plot is
an enlargement of the left half of the top plot. For clarity, we have
labeled the CaO and MgO data points, which are discussed below. Click
here <LIBS_tables1-3.html> to see a table of values that were used to
create these plots (table will open in a new window.)

In their study, Wiens, Thompson, and colleagues demonstrated that the
LIBS technique is capable of determining subtle differences in rock
types from a remote distance of 5.4 meters. The differences between LIBS
and values determined by traditional laboratory techniques differed by
less than 12% (relative) for most of the major elements (most were
better than 6%). The agreement for titanium and sodium were not as good,
but their concentrations are low (less than 1 wt%), hence the emission
signals are low. The comparison with the andesite was particularly good
with all elements agreeing to within 5% (relative), except for titanium.
Even for titanium the difference between the LIBS and accepted
concentration for the andesite was only about 8%. The andesite was a
powdered sample, hence homogeneous on the scale of the LIBS laser pit.
In contrast, the Martian meteorites were slabs of rock. Getting a good
average analysis in this case requires numerous analytical points
because sometimes the laser zaps only one mineral, other times a
mixture, and so on.

The most important result of this test of LIBS analytical capability is
that it is possible to distinguish between the two Martian meteorites.
This is particularly noteworthy for MgO and CaO, as shown in the diagram
above. DaG 476 and Zagami are clearly different in composition. This
shows that LIBS will be capable of distinguishing rock types as the
rover journeys across the Martian surface.

------------------------------------------------------------------------

What's Planned for LIBS on Mars

LIBS instruments are planned for NASA's Mars Science Laboratory rover
(scheduled to launch in the fall of 2009) and ESA's ExoMars rover
(slated for launch in mid-2011).

Mars Science Laboratory rover is designed to operate for a full Martian
year, which is almost two Earth years. LIBS, as part of the ChemCam
instrument package, is expected to make thousands of measurements to
help scientists characterize the geology of the landing region, help
analyze surface ices or salts or evaporite minerals or rocks and soils
that have been altered by water, help identify possible organic
materials if they exist or ever existed, and help check for toxic
materials. A typical analysis sequence for LIBS will begin when the
science team identifies a target rock and commands ChemCam to fire a
burst of up to 75 laser pulses at a ??? 1 millimeter spot on the target.
The rover's onboard spectrometers will determine the elemental
compositions of the ablated plasma. Acquiring a LIBS spectrum for an
analysis spot is expected to take six minutes. This is very rapid
compared with previous techniques that have taken up to three Martian
sols for analogous dust-free analyses that
required contact with the target rock.

Like all scientific instruments, LIBS has to be calibrated using
standards of known composition. A challenge on Mars faced by the ChemCam
science team is that they will not have a suite of calibration standards
that they can expose at the same conditions (distance, etc.) Roger Wiens
and team are currently working on determining how the calibration curves
vary with instrument-to-sample distance. They will have some calibration
standards on the rover, which will be at a close range of 1.4 meters.
Over time, the team will also be able to cross-calibrate between LIBS
and the other ChemCam instruments APXS and CheMin. [See this ChemCam web
page <http://marsprogram.jpl.nasa.gov/msl/mission/sc_instruments.html>
for additional information on the science instruments.] These aspects
should allow LIBS to be an extremely useful quantitative geochemical and
geological mapping tool.

The research team at Los Alamos and University of New Mexico who tested
LIBS on the Martian meteorites and terrestrial analog rock, as well as
other teams of cosmochemists who are working to better calibrate LIBS
for the entire variety of rocks it will encounter on Mars, are eagerly
anticipating what they'll see in the glowing LIBS light.

ADDITIONAL RESOURCES LINKS OPEN IN A NEW WINDOW.

    * ChemCam LIBS instrument description
      <http://marsprogram.jpl.nasa.gov/msl/mission/sc_instru_chemcam.html>
      from Jet Propulsion Laboratory.
    * ExoMars <http://www.esa.int/esaMI/Aurora/SEM1NVZKQAD_0.html>
      mission homepage from the European Space Agency.
    * LIBS planetary science applications website
      <http://libs.lanl.gov/> from Los Alamos National Laboratory.
    * Mars Science Laboratory <http://mars.jpl.nasa.gov/msl/> rover
      homepage from Jet Propulsion Laboratory.
    * Thompson, J. R., R. C. Wiens, J. E. Barefield, D. T. Vaniman, H.
      E. Newsom, and S. M. Clegg (2006) Remote Laser-Induced Breakdown
      Spectroscopy Analyses of Dar al Gani 476 and Zagami Martian
      Meteorites. Journal of Geophysical Research, v. 111, doi:
      1029/2005JE002578,2006.
Received on Fri 27 Oct 2006 04:07:54 PM PDT