46th Lunar and Planetary Science Conference (2015)
M. Pitman1, C. S. Jamieson3, E. Z. Noe Dobrea1, J. B. Dalton III4, W. J. Abbey4. 1Planetary Science Institute, 1700
E. Fort Lowell Road, Suite 106, Tucson, AZ 85719 USA <[email protected]>, 2Space Science Institute, 4750 Walnut
Street, Suite 205, Boulder, CO 80301 USA <[email protected]>, 3SETI Institute, 189 Bernardo Ave., Suite
100, Mountain View, CA 94043 USA, 4Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak
Grove Drive, Pasadena, CA 91109 USA.
Introduction: Morphological evidence suggests
that several of the carbonates identified thus far on
Mars may have been exhumed from some depth (e.g.,
[1-2]), and others may have been exposed to hydrothermal conditions [3]. Constraining the mineralogy of
carbonates on Mars would prove important to our understanding of their genesis and the environmental
conditions to which they have been exposed. Increasing the number of optical constants datasets and available laboratory comparison spectra for a range of Ca-,
Na-, Mg-, and Fe-carbonates is necessary to better
identify the specific chemical compositions and quantify the abundances of the known carbonates on Mars.
What is needed are diffuse reflectance spectra of
(a) variations on carbonate polymorphs and hydration
states, (b) carbonates which have only been measured
in one particle size range or in poorly bounded grain
sizes, (c) other carbonates that would be stable on
Mars but have yet to be explored. 2014 marked a resurgence of interest in and production of carbonate
reflectance spectra for the Mars community (e.g., [46]). Most of these studies focused on aspect (a) but are
not suitable for deriving optical constants for various
reasons (e.g., the spectra were obtained for only one
grain size; diameter was not recorded or too roughly
estimated at the time of measurement). Continuing
from [7], our work addresses aspects (a-c) by characterizing Mars-relevant carbonate polymorphs and hydration states to fill in the gaps in chemistry and also to
measuring these in enough grain sizes to permit improved optical constants derivation.
Methodology: For this study, we focused on the
following: nahcolite [NaHCO3]), vaterite [hexagonal
CaCO3], nesquehonite [Mg(HCO3)(OH)•2(H2O)], and
brugnatellite [Mg6Fe3+(CO3)(OH)13·4H2O]. Transmission spectra of nesquehonite and nahcolite, and IR
modes of calcite, vaterite, and aragonite were considered by [8]. Nahcolite is also relevant to Type 1 brines
formed from basaltic weathering fluid evaporation [9].
Nesquehonite, a Mg-carbonate that forms on other
carbonates, serpentines, or volcanic breccias, was mentioned in [10], who noted that a comparison between
this compound and Mars data would potentially be of
interest but that sufficient laboratory spectral data do
not exist. Brugnatellite was cited in [11] as a Febearing carbonate worth studying for Mars.
We purchased a suite of samples from Minerals
Unlimited and Excalibur Mineral Corp., in sample
sizes of 10-100 grams per mineral. 10 grams is the
minimum for a sample that would perfectly separate
into our desired ASTM sieve fractions. Having up to
100 grams of material guards against losses of the
natural carbonates during sieving and allows us to better isolate the purest mineral grains. We synthesize
any carbonates that we cannot find in pure form from
vendors. For example, we synthesized the high purity
nahcolite for Fig. 1 from Sigma-Aldrich reagent.
Samples chemistries were confirmed via XRD analysis. Three to eight grain size fractions and three viewing geometries (incidence and emission angles i=0º,
e=10º; i=0º, e=15º; i=30º, e=0º) were measured for
each carbonate at the Planetary Ice Characterization
Laboratory at JPL (e.g., Fig. 2), with diameters confirmed via optical microscopy. The rationale for acquiring spectra on multiple grain size fractions is to
provide a robust estimate for the imaginary index of
refraction k [12]. Multiple viewing geometries permit
cross-checks on the optical constants values using different (Hapke vs. Shkuratov) derivation methods. We
then used the spectra to derive imaginary index of refraction k(λ) using the method of [13]. In Fig. 1-2, we
plot k and the real indices of refraction n(λ) derived via
subtractive Kramers-Kronig relations [14].
Significance: Optical constants are important for
generating synthetic spectra of intimately mixed minerals to compare to spacecraft data and quantify the
amount and distribution of carbonates on Mars. However, optical constants currently have not been published for the rarer forms of carbonates or are only
very roughly estimated. Because the hydration state
and mineralogy imply particular environmental and
chemical conditions as well as the duration of those
conditions, Mars abundances modeled using our carbonate analog data could be used to constrain these
conditions at different Mars epochs and used to infer
transport and alteration mechanisms at work on Mars
during their formation.
Acknowledgments: This work was supported by
NASA’s Mars Fundamental Research Program
(NNX13AG78G; PI Pitman) and partly performed at
the Jet Propulsion Laboratory, California Institute of
46th Lunar and Planetary Science Conference (2015)
Technology, under contract to the National Aeronautics and Space Administration.
References: [1] Michalski J. R. and Niles P. B.
(2010) Nature Geoscience, 3, 751-755. [2] Wray J. J.
et al. (2011) LPS XLII, 2635. [3] Ehlmann B. L. et al.
(2008) Science, 322, 1828-1832. [4] Applin D. M. et
al. (2014) LPS XLV, 1881. [5] Smyth S. et al. (2014)
LPS XLV, 2899. [6] Harner P. L. and Gilmore M. S.
(2015) Icarus, 250, 204–214. [7] Pitman K. M. et al.
(2014) LPS XLV, 1590. [8] Pollack J. B. et al. (1990) J.
Geophys. Res., 95, 14,595-14,627. [9] Tosca N. J. and
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(1994) J. Geophys. Res., 99, 14,659-14,675. [12] Lucey P. G. (1998) JGR, 103, 1703–1713. [13] Shkuratov
Y. et al. (1999) Icarus, 137, 235-246. [14] Warren S.
G. (1984) Appl. Opt., 23,1206–1225.
Fig. 2: Laboratory diffuse reflectance spectra at i=30º,
e=0º and optical constants for vaterite with calcite.
MRO CRISM wavelength range shown; full data range
extends to λ ~ 25 µm.
Fig. 1: Optical constants (real and imaginary indices of refraction k(λ) and n(λ)) for nahcolite. Solid
blue line indicates the average n, k based on several
sieve fractions.