Spectral Identification of Hydrated Sulfates on Mars and Comparison with Acidic
Environments on Earth
Janice L. Bishop1,2, M. Darby Dyar3, Melissa D. Lane4, Jillian F. Banfield5
1
2
NASA-Ames Research Center, Exobiology Branch, Moffett Field, CA 94035
3
4
5
SETI Institute, 515 N. Whisman Rd, Mountain View, CA 94043
Mount Holyoke College, 50 College St., South Hadley, MA 01075
Planetary Science Institute, 1700 E. Fort Lowell Rd., Suite 106, Tucson, AZ 85719
Department of Earth and Planetary Science, University of California, Berkeley, CA 94720
*Corresponding Author: Janice L. Bishop, SETI Institute, 515 N. Whisman Rd, Mountain
View, CA 94043, Tel. (650) 810-0222, Fax (650) 961-7099, [email protected].
Running title: Hydrated Sulfates on Mars and Acidic Environments on Earth
Keywords: sulfates, Mars, reflectance spectroscopy, Mössbauer spectroscopy, emission
spectroscopy, water, life
Submitted Oct. 15, 2004; revised January 21, 2005
1
ABSTRACT
We interpret recent spectral data of Mars collected by the Mars Exploration Rovers to
contain substantial evidence of sulfate minerals and aqueous processes. We present
visible/near-infrared (VNIR), mid-IR and Mössbauer spectra of several iron sulfate
minerals and two acid mine drainage (AMD) samples collected from the Iron Mountain
site and compare these combined data to the recent spectra of Mars. We suggest that the
sulfates on Mars are produced via aqueous oxidation of sulfides known to be present on
Mars from the martian meteorites. The sulfate-rich rock outcrops observed in Meridiani
Planum may have formed in an acidic environment similar to AMD environments on
Earth. Because microorganisms typically are involved in the oxidation of sulfides to
sulfates in terrestrial AMD sites, sulfate-rich rock outcrops on Mars may be a good
location to search for evidence of life on that planet. Whether or not life evolved on
Mars, following the trail of sulfate minerals is likely to lead to aqueous processes and
chemical weathering. Our results imply that sulfate minerals formed in martian soils via
chemical weathering, perhaps over very long time periods, and that sulfate minerals
precipitated following aqueous oxidation of sulfides to form the outcrop rocks at
Meridiani Planum.
2
INTRODUCTION
Sulfates have been proposed to occur on Mars based on the presence of sulfur and the
oxidizing environmental conditions (e.g. Settle, 1979, Burns, 1987, Burns and Fisher,
1990, Clark and Baird, 1979, Clark and Van Hart, 1981). Estimates of the bulk chemistry
of Mars indicate the presence of 17.9% FeO (as Fe2+ and/or Fe3+) and 14.2% S (Dreibus
and Wänke, 1987), which is higher than the 8.9% FeO (as Fe2+ and/or Fe3+) and 9.0% S
estimated for Earth (Morgan and Anders, 1980). These elevated Fe and S levels on Mars
are consistent with the presence of even more Fe-and S-bearing minerals on Mars
compared to Earth. Sulfate contents of 6-8 wt.% have been observed in the fine-grained
surface material at the Viking and Pathfinder sites (Clark et al., 1982, Foley et al., 2003).
Sulfates have also been identified through direct observation of martian meteorites (e.g.
Treiman et al., 1993, Gooding et al., 1991). Early ground-based observations had
suggested the presence of sulfates in the spectra of Mars (Blaney and McCord, 1995,
Pollack et al., 1990) and more recent analyses suggest that sulfates are more globally
distributed in the dust (Bandfield, 2002) and cemented soil (Cooper and Mustard, 2001).
However, no strong signatures of either carbonate or sulfate bedrock were found in global
surveys using TES data (Bandfield, 2002).
Recent analyses by the Mars Exploration Rovers (MERs) at the Meridiani Planum
and Gusev crater landing sites and the Visible and Infrared Mineralogical Mapping
Spectrometer (OMEGA) on board Mars Express have provided much more detailed
information about the presence of sulfates on Mars. Measurements using the AlphaParticle X-ray Spectrometer (APXS) on outcrop rocks at Meridiani Planum suggest the
presence of greatly elevated S levels, even as much as 20-40% sulfate (Rieder et al.,
3
2004, Moore, 2004, Clark, 2004). The Mössbauer instrument uniquely identified jarosite
as one sulfate-bearing mineral present at Meridiani (Klingelhöfer et al., 2004a), however
geochemical modeling (Rieder et al., 2004, Clark, 2004) and spectral analyses (Lane et
al., 2004) suggest that other sulfate minerals are present as well. Recent analyses of the
Mini-Thermal Emission Spectrometer (Mini-TES) data include spectral models using a
mixture of hydrous and anhydrous sulfates to reproduce spectra of these outcrop rocks at
Meridiani (Christensen et al., 2004b). The presence of jarosite at Meridiani implies a
highly acidic (pH <3) formation environment (Bigham et al., 1992) and the sedimentary
character of the jarosite-bearing sulfur-rich outcrops there is consistent with a large
aqueous system. Diagnostic near-infrared absorptions of the sulfate minerals kieserite
and gypsum have been identified in data from the OMEGA instrument in several
locations on Mars (Bibring, 2004, Gendrin et al., 2004). The deposits range from small,
light-toned outcrops in Valles Marineris to regional layers rich in sulfate in the Meridiani
region. These recent discoveries highlight the importance of studying sulfate minerals in
particular as direct tracers of the alteration history of the red planet. A variety of sulfate
minerals are likely present on Mars and Fe2+ sulfates may be more likely than Fe3+
sulfates due to the oxidative pathways of these minerals, as described in a later section.
Because the upcoming Compact reconnaissance imaging spectrometer for Mars (CRISM)
will evaluate these sulfate-bearing sites at lower surface resolution (Murchie et al., 2003),
it is necessary to characterize potential sulfate minerals and naturally-occurring sulfate
deposits in order to identify them on Mars.
Burns (1987) suggested that the production of ferric (Fe3+) sulfate minerals on Mars
occurs through oxidative weathering of iron sulfides, and he also extended this alteration
4
model to describe the formation of gossans on Mars (Burns, 1988). These gossans
include poorly-crystalline or nanophase iron oxide and silicate phases as well as jarosite.
Morris et al. (1996, 2000) and Bishop et al. (1998a) have found jarosite, alunite, alunogen
and other sulfate species in a range of volcanic tephra and ash samples formed through
hydrothermal alteration. Such Fe- and Al- bearing sulfates also may have formed on
Mars through similar processes. The martian surface material could contain both
magnesium sulfate salts and Fe-, Al-, Ca- or Mg- bearing sulfate minerals. This would
explain why only a weak correlation exists between Mg and S (Clark, 1993). Because Fe
is assumed to be present as oxides and silicates, it may not correlate well with S
abundance even in the presence of a few wt.% SO3 in a jarosite-group mineral.
More recently, Burns (1994) suggested the possibility of formation of the ferric
oxyhydroxysulfate mineral schwertmannite in equatorial regions of Mars, where acidic
permafrost melts and is oxidized by the martian atmosphere. Streams flowing from acid
rock drainage sites or natural acidic environments often produce a variety of sulfate
minerals as the pH and water chemistry vary downstream (Schwertmann et al., 1995).
The formation of nanophase ferric oxides/oxyhydroxides and sulfates including
ferrihydrite, schwertmannite and jarosite occur under similar geochemical environments
and under specific, but partly overlapping, pH ranges, and some minerals can form
multiple morphologies and grain sizes (Bigham et al., 1994, 1996). Sulfides are oxidized
in these environments through either biomediated or inorganic pathways (e.g. Singer and
Stumm, 1970, Edwards et al., 2000, Baker and Banfield, 2003). Examples of terrestrial
acidic environments where iron oxide/oxyhydroxide and sulfate minerals form include
5
the Tinto River in Spain (e.g. Fernández-Remolar et al., 2004) and the Iron Mountain site
in California (e.g. Nordstrom and Alpers, 1999).
Because the sulfate outcrops in Meridiani are thought to have formed via wide-spread
aqueous activity (e.g. Squyres et al., 2004, Hynek, 2004) these are ideal sites to search for
evidence of life on Mars. Although precipitation of sulfates in acidic environments on
Earth does not require the presence of microorganisms, they are typically involved in the
process. This suggests that biomediation of the large sulfate outcrops might be considered
as a possibility on Mars as well.
METHODS
Samples
Sulfate precipitates were collected from acid mine drainage (AMD) outflow from the
Iron Mountain site in California. This is a superfund site located about 15 km north of
Redding off Interstate 5 in Shasta County (Figure 1) and has been the focus of previous
studies (e.g. Nordstrom and Alpers, 1999, Edwards et al., 2000, Nordstrom, 1985). One
sample collected for this study is from a sulfuric acid solution that was neutralized with
lime to form sediment and subsequently dried in air to form a dark orange-red crust
(JB577); the other is a bright green crystalline rock (JB626). The natural sample JB626
was identified as rozenite through X-ray diffraction (XRD) and the neutralized AMD
sample JB577 was identified as a mixture of gypsum and ferrihydrite through infrared
(IR) and Mössbauer analyses. Both samples were gently crushed and dry sieved in order
to characterize both the bulk and fine-grained fractions of these samples. Images of the
Iron Mountain site and samples collected for this study are shown in Figure 2. The
6
minerals ferricopiapite, coquimbite, and szomolnokite were obtained and characterized
for a related study (Lane et al., 2004) as examples of hydrated iron sulfates that could
form on Mars. The hand samples were walnut-sized and subsamples thereof were
powdered and dry-sieved to <125 µm. XRD patterns of the samples in this study were
measured using either a Rigaku Geigerflex powder diffractometer (at 40 kV and 35mA)
or a Rigaku Miniflex diffractometer at Mount Holyoke College. CuKα radiation over a
2θ of 0 to 70 and a step size of 0.02° 2Θ min-1 was used. The spectra of additional finegrained aliquots of minerals from previous studies are included here as well: ferrihydrite
(Bishop and Murad, 2002), jarosite (Bishop and Murad, 2004), schwertmannite (Bishop
and Murad, 1996), and gypsum (Bishop et al., 2004). Ideal mineral formulas for these
samples are given in Table 1.
Reflectance Spectra
Bidirectional visible/near-infrared (VNIR) reflectance spectra were measured relative
to Halon under ambient conditions at the Reflectance Experiment Laboratory (RELAB)
at Brown University. Biconical (off-axis) IR reflectance spectra were measured relative
to a rough gold surface using a Nicolet 740 FTIR spectrometer in a dry, controlled
humidity environment in order to remove adsorbed water from the samples. The samples
were placed in the sample chamber for ~12 hours under N2 in order to remove H2O and
CO2 adsorbed on the surface of the grains or in the air above the samples. This provides a
more Mars-like environment for the samples at the time of measurement than if the
samples were measured under ambient Earth conditions. Composite, absolute reflectance
7
spectra were prepared by scaling the FTIR data to the bidirectional data near 1.2 µm.
The spectral sampling is 5 nm for the bidirectional data and 2 cm-1 for the FTIR data.
Emission Spectra
Emission spectra were measured at the Mars Space Flight Facility at ASU using a
Nicolet Nexus 670 E.S.P. FTIR spectrometer. This spectrometer has been modified for
emission measurements and is equipped with a thermoelectrically stabilized DTGS
detector and a CsI beam splitter that enables measurement of emitted radiation over the
mid-infrared range of 2000 to 200 cm-1 (Christensen et al., 2000). Spectra of hand
samples were acquired at ~50 °C over the course of 160 scans at 2 cm-1 sampling.
Mössbauer Spectra
Mössbauer spectra were acquired at Mount Holyoke College using a WEB Research
constant acceleration Mössbauer spectrometer equipped with a Janis Research model 850
closed cycle He refrigerator. An 80-100 mCi 57Co in Pd source and 8-24-hour run times
were used; data were referenced to the midpoint of an α-Fe foil spectrum. Because
Mössbauer spectra are temperature-sensitive, some data were acquired at 260 K in order
to simulate temperatures on Mars during the 6-24-hour-long acquisition time of the MER
Mössbauer experiment. The samples were held in a He gas atmosphere, simulating the
dry conditions on Mars.
8
RESULTS
Iron Mountain site as example of precipitated hydrated sulfates
Within the Richmond Mine at Iron Mountain, AMD forms by dissolution of metal
sulfide minerals. Sulfides such as pyrite are oxidized quickly to aqueous Fe2+ and SO42when O2 and H2O are present (e.g. Singer and Stumm, 1970, Nordstrom, 1982, Holmes
and Crundwell, 2000). Resulting solutions contain near molar concentrations of Fe and
H2SO4 and millimolar levels of Zn, Cu, and As. Several hydrated sulfate minerals form
underground by evaporation of the AMD solution.
These include melanterite,
chalcanthite, coquimbite, rhomboclase, voltaite, copiapite and halotrichite (Nordstrom
and Alpers, 1999). Sulfides are also predicted to have oxidized on Mars given liquid
water and even low oxygen levels (Burns, 1987). Many sulfide minerals including
pyrrhotite, pyrite, chalcopyrite, troilite, marcasite and pentlandite have been identified in
martian meteorites (e.g. Bunch and Reid, 1975, Boctor et al., 1976, Floran et al., 1978,
Stolper and McSween Jr., 1979, Steele and Smith, 1982, Harvey et al., 1993, Greenwood
et al., 2000). Oxidation of sulfides is a complex process requiring multiple steps that
depend as much on the crystallinity, purity, and grain size of the sulfide as on the pH of
the aqueous environment (e.g. Nordstrom, 1982).
Abiotic reaction of sulfides to form ferrous (Fe2+) sulfates proceeds rapidly. However,
the formation of Fe3+ sulfates tends to be a slow reaction in abiotic sulfide environments
because the Fe3+ quickly reacts again with more sulfide, thus limiting the accumulation of
aqueous Fe3+ to react with the SO42- (Nordstrom, 1985). Microbes have long been known
to participate in the oxidation of sulfides at acid mine drainage sites (Colmer and Hinkle,
1947). At the Richmond Mine site, for example, organisms such as Leptospirillum
9
ferrooxidans catalyze the oxidation of ferrous to ferric iron, and this has been estimated
to increase rates of pyrite dissolution by a factor of ~ 2-4 (Edwards et al., 1998).
Aqueous oxidation of sulfides on Mars could be responsible for the presence of
copious amounts of sulfate observed in some regions of Meridiani (Gendrin et al., 2004,
Bibring, 2004, Klingelhöfer et al., 2004a). The light-toned crater rims associated with the
most degraded, oldest craters in Meridiani fall on an isochron dating the sulfate material
to approx. 4 Gya (Hartmann et al., 2001, Lane et al., 2003). This reaction could have
occurred on Mars over long time periods and biology need not be invoked in order to
explain the occurrence of Fe3+ sulfates.
However, given the association of
microorganisms such as Acidithiobacillus ferrooxidans with the oxidation of sulfides in
numerous localities on Earth (Nordstrom and Southam, 1997), deposits of sulfate
precipitates on Mars may be ideal sites to search for extinct life on that planet.
Spectra of AMD crust material
Reflectance spectra of the AMD crust sample (JB577-C) are compared with those of
the minerals gypsum and ferrihydrite in Figure 3. Part of the precipitated crust was
gently crushed and dry sieved to <125 µm particle size (JB577-B) and <45 µm particle
size (JB577-A). These are actually aggregate sizes composed of much smaller grains, at
least for the ferrihydrite particles, that are typically only a few nanometers in size.
Spectra of these three size separates exhibit different abundances of ferrihydrite and
gypsum as seen in the VNIR spectra in Figure 3a and the mid-IR spectra in Figure 3b.
The <45 µm AMD spectrum exhibits the strongest ferrihydrite features; these include the
Fe3+ transition near 0.93 µm, the broad water band near 3 µm, and the mid-IR features
10
near 1590 cm-1 (~6.3 µm), 1450 cm-1 (~6.9 µm) and 1280 cm-1 (7.8 µm). The <125 µm
AMD spectrum exhibits sharp VNIR gypsum bands near 1.44-1.54, 1.92-1.98, and 4.34.8 µm, plus additional characteristic bands near 1.77, 2.2 and 2.5 µm. A number of the
mid-IR spectral features of gypsum change as a result of particle size near 63 µm (Lane
and Christensen, 1998) and the <125 µm AMD sample has features between those of the
<63 µm and 63-90 µm gypsum near 1620 cm-1 (~6.2 µm), 1160-1200 cm-1 (~8.5 µm),
1010 cm-1 (~9.9 µm), and 580-680 cm-1 (~15-17 µm). The AMD crust spectrum contains
strong features due to both ferrihydrite and gypsum and the mid-IR gypsum features are
consistent with the coarser gypsum grains.
Room temperature Mössbauer spectra of the <45 µm AMD crust fraction and
ferrihydrite are shown in Figure 4. Mössbauer spectra of paramagnetic materials reveal
the electrostatic interactions between the nucleus of an Fe atom and the surrounding
electrons in the structure and are highly sensitive for discrimination of Fe coordination
polyhedra and Fe valence states in different mineral structures. The AMD sample has an
isomer shift of 0.28 mm/s and a quadrupole splitting of 0.70 mm/s, while ferrihydrite has
a very similar isomer shift of 0.31 mm/s and a quadrupole splitting of 0.72 mm/s. These
spectra, in combination with the reflectance spectra, show that the only iron-bearing
phase present in the AMD crust fraction is ferrihydrite.
Spectra of sulfate minerals including AMD rock
Shown in Figure 5 are VNIR spectra of the AMD rozenite sample and other acid
sulfate minerals including ferricopiapite, coquimbite, szomolnokite, schwertmannite and
jarosite. These spectra illustrate the variations in spectral features due to sulfate minerals
11
in this region, depending on the presence of Fe2+ and/or Fe3+ species, OH and H2O, in
addition to the SO4 groups. The iron bands occur from ~0.4-1.2 µm for these minerals
and are due to crystal field theory and charge transfer bands (Burns, 1993). Rozenite and
szomolnokite exhibit absorption bands near 1 µm due to Fe2+, while the others exhibit
absorptions closer to 0.9 µm due to Fe3+. The rozenite spectrum is very similar to that
observed for melanterite (not shown), which has a similar chemical formula except that it
bears less water. The AMD rozenite chip spectra include a band near 0.98 and a shoulder
at ~1.2 µm, while the <125 µm fraction has a broader feature in this region encompassing
both this band and the shoulder. Coquimbite has Fe3+ bands near 0.56 and 0.78 µm.
Many of these iron sulfate minerals also have a sharp absorption band near 0.43 µm due
to Fe3+. Broad water stretching bands are observed near 3 µm for all of these spectra and
broad water combination bands are observed near 1.45 and 1.95 µm. Sharper OH and
SO4 overtones and combinations are observed throughout this region and vary somewhat
depending on the bonding arrangement. These bands have been described in detail for
jarosite and alunite (Bishop and Murad, 2004). The AMD rozenite spectra include
combination bands near 2.42 and 2.54 µm, as well as a cluster of overlapping bands from
~4.2 to 4.8 µm.
Mid-IR reflectance spectra of the AMD rozenite, plus ferricopiapite, coquimbite,
szomolnokite, schwertmannite and jarosite are displayed in Figure 6. Both emittance and
reflectance spectra are shown for the AMD rozenite rock sample in order to illustrate the
similarity in these spectra. Reflectance spectra were measured for small rock chips (a
few mm across), while emittance spectra were measured for the whole rock sample
(~2 cm across).
12
Fundamental sulfate vibrations occur for the aqueous sulfate ion due to symmetric (ν1,
981 cm–1) and asymmetric (ν3, 1104 cm-1) stretching, and symmetric (ν2, 451 cm-1) and
asymmetric (ν4, 613 cm-1) bending motions (Ross, 1974). Pure tetrahedral (Td) sulfate
sites are rare in minerals and for these cases IR absorptions occur for ν3 and ν4, but not
for ν 1 and ν 2, which are IR inactive (Adler and Kerr, 1965). Transmittance spectra of
sulfate minerals are described by Ross (1974) and show a variety of IR absorptions due to
distortions in the Td structure. Emittance spectra of selected sulfates are described by
Lane and Christensen (1998) and Lane (2004). Mid-IR reflectance spectra of selected
sulfates have also been reported (e.g. Salisbury et al., 1991, Bishop and Murad, 2004,
Bishop and Murad, 1996). In the structures of sulfates such as jarosite, one O from each
SO42- Td is bound to an octahedral Al or Fe cation forming a unidentate ligand and thus
having C3v symmetry instead of Td symmetry. For jarosite and other C3v sulfates one
ν1, one ν2, two ν3, and two ν4 vibrations are expected (Adler and Kerr, 1965). The AMD
rozenite sample exhibits mid-IR spectral features as maxima near 1640 and 1500 cm-1,
and as minima near 1100, 808, 614, 510, and 290 cm-1. The features near 1640 and 1500
cm-1 give rise to emissivity minima near 1550 and 1435 cm–1. The band at 808 cm-1 is
broadened and shifted to ~900 cm-1 for the particulate AMD rozenite sample.
Figure 7 shows 260 K Mössbauer spectra of several sulfate minerals including the
AMD rozenite, olivine from the martian meteorite Chassigny and martian soils measured
at Gusev crater and Meridiani Planum (from Klingelhöfer et al., 2004a, Morris et al.,
2004). Mössbauer spectra of Mars are collected by miniature spectrometers on both MER
rovers (Klingelhöfer et al., 2003). Fe2+ sulfates such as szomolnokite and rozenite exhibit
similar Mössbauer doublets at 260 K to those observed for olivine at this temperature;
13
rozenite in fact has two closely-spaced, overlapping Fe2+ doublets (Lane et al., 2004).
Both of these mineral types are consistent with the Mössbauer spectra of martian soils
when appropriate error bars are considered.
An isomer shift of 1.32 mm/s and
quadrupole splitting of 3.02 mm/s were determined for the AMD rozenite sample.
Factors affecting the isomer shift and quadrupole splitting determined from Mössbauer
spectroscopy of these minerals include composition of the cations (Fe2+, Fe3+ and others)
bound to the sulfate or silica groups, the amount of disorder of the Td sites of the sulfate
or silica groups, particle size, and temperature. The Fe3+ in the jarosite identified in the
Mössbauer spectra of rocks at Meridiani Planum (Klingelhöfer et al., 2004a) produces a
much narrower doublet in Mössbauer spectra that can be readily distinguished from the
doublets of other sulfates. Mössbauer spectra at Meridiani Planum are interpreted to
contain iron silicates, iron oxides and jarosite (Klingelhöfer et al., 2004a).
We suggest that multiple iron sulfate minerals may be contributing to these spectra
(Lane et al., 2004) and that they are the components responsible for the broad Mössbauer
doublets in the Meridiani soil and outcrop spectra rather than the mineral olivine. We
also suggest that iron sulfate minerals rather than olivine are present in the Mössbauer
soil spectra observed at Gusev crater, although we accept the interpretation of olivine in
the spectra of rock interiors at Gusev. Our interpretations are based on observations of
olivine casts in the Gusev rocks suggesting that the olivine has weathered out of the rock
rinds (McSween et al., 2004) and the presence of several weight percent SO3 as sulfate in
the surface fines (Gellert et al., 2004). Olivine is one of the first minerals lost upon
alteration of basaltic rocks (e.g. Colman, 1982, Nesbitt and Young, 1984) and a recent
chemical weathering study of basalts indicates that sulfates form upon exposure of basalts
14
to acidic solutions (Tosca et al., 2004). Given evidence of alteration of large olivine
grains in rocks at Gusev crater, it is geologically unlikely that tiny olivine grains are
present in the sulfate-rich fine-grained surface material. Thus, identifying alternative
plausible minerals for the Mössbauer signature is important.
We support the
interpretation of Fe2+ sulfates as minerals contributing to the ferrous Mössbauer doublet
in the ubiquitous martian soil and in the sulfate-bearing outcrop rocks at Meridiani
Planum.
Analysis of Pamcam spectra of Mars
The Panoramic camera (Pancam) on each of the MER rovers measures spectral images
of Mars using up to eleven filters in the extended visible region (Bell et al., 2003). The
spectra collected by Pancam show somewhat more variability at Gusev crater (Figure 8a,
Bell et al., 2004a) than was observed by the Imager for Mars Pathfinder (IMP) that has
similar filters (e.g. Smith et al., 1997, Bell et al., 2000, Murchie et al., 2005). Pancam
spectra of Mars are compared with lab spectra convolved to the Pancam filters in Figure
8. These spectra are explained as a combination of basalt and altered phases including
poorly crystalline iron oxides and silicates (Bell et al., 2004a).
Sulfate minerals are also likely contributing to these spectra (Bishop et al., 1998b,
Murchie et al., 2005, Lane et al., 2004). Pancam-convolved spectra of several sulfate
minerals (Figure 8b), iron oxide/oxyhydroxide minerals (Figure 8c) and other samples
(Figure 8d) are shown for comparison. Some minerals, such as coquimbite, have
distinctive features in this spectral region and can be ruled out as dominant Fe-bearing
martian soil or rock components observed by Pancam. Other minerals such as jarosite,
15
schwertmannite and szomolnokite exhibit spectral trends that are consistent with the
spectral character of Mars and such sulfates could make up 10-20 percent of the soil.
Additional minerals such as olivine and pyroxene (present in the martian meteorites)
contain a strong absorption near 1 µm that is not observed in the spectra of martian rocks
and soils, thus limiting the presence of these minerals. For example, ratios of band 10
(935 nm) to band 6 (755 nm) give an average of 0.97 for the martian soil spectra shown
in Figure 8a. This band ratio for <125 µm coquimbite is 1.14, for <125 µm ferricopiapite
is 0.91, for <45 µm jarosite is ~0.81 (average of four jarosite spectra), for <45 µm
schwertmannite is 0.86, for <125 µm szomolnokite is 0.72 and for <125 µm AMD
rozenite is 0.56 (and 0.65 for coarser grains) shown in Figure 8b. Ca- or Mg-sulfates
would have a flat ratio here because the spectral feature near 900-1000 nm is due to iron.
For the <125 µm olivine spectrum shown in Figure 8d this band ratio is 0.70 (and 0.51
for coarser grains) and for the <125 µm pyroxene bearing martian meteorite it is 0.48
(and 0.40 for the coarser grains). In order to achieve a soil mixture that resembles the
spectral character of martian soils, minerals such as olivine and pyroxene (and some iron
sulfates such as rozenite and szomolnokite) would need to be mixed with spectrally flat
species such as altered volcanic material, Ca- or Mg-sulfates, or something like
coquimbite that has a ratio >1.
The spectra of the ferrihydrite- and gypsum-bearing AMD crust material presented in
this study and an altered volcanic soil from Haleakala, Maui, containing a number of
poorly crystalline silicate and iron oxide-bearing phases are consistent with the dominant
spectral character in this region. Normalized spectra of a Mars Pathfinder soil (from
Murchie et al., 2005) and a Meridiani soil (from Bell et al., 2004b) are shown in Figure
16
8e. This shows the similarity in general spectral shape of the martian soils, although
there are some differences in brightness and curvature near 0.6 and 1 µm for the soils at
the Pathfinder, Gusev crater and Meridiani Planum sites. A normalized soil mixture
spectrum derived from the spectra of the Haleakala volcanic soil, gray hematite,
szomolnokite and schwertmannite spectra (Lane et al., 2004) is also shown in Figure 8e
for comparison. Besides the weak band near 900-1000 nm, the gentle spectral slope
across bands 1 (434 nm) to 2 (482 nm) to 3 (535 nm) is difficult to model with many
mineral and martian meteorite spectra. This less pronounced spectral slope from 434 to
535 nm is observed for many iron oxides/oxyhydroxides and iron-bearing sulfates.
Adding more than ~10 wt.% hematite or goethite to a soil mixture produces other spectral
inconsistencies, so is not helpful in resolving this part of the spectrum. Minerals such as
ferrihydrite, jarosite, schwertmannite and szomolnokite model well the general spectral
character in this region, but are brighter than the martian soils. Thus, mixing some
sulfate minerals or the AMD crust sample studied here into an altered basaltic matrix
provides a good spectral match for the martian dust measured in several regions on the
planet. This is consistent with a combination of both alteration of basaltic rocks and
aqueous acidic alteration processes taking place on Mars.
Analysis of emission spectra of Mars
Global emission spectra have been collected on Mars from 200-1650 cm-1 by the
Thermal Emission Spectrometer (TES) on Mars Global Surveyor since 1999 (Christensen
et al., 2001). The Mini-TES is a related instrument on board each of the MER rovers
(Christensen et al., 2003). Shown in Figure 9 are TES and Mini-TES spectra of Mars
17
compared with emission spectra of sulfate minerals and reflectance spectra of AMD
materials. The dominant spectral character of the rocks observed by Mini-TES at Gusev
crater are basaltic rocks (Christensen et al., 2004a). This is consistent with the global
TES signatures observed for Mars that are interpreted as basalt and andesite (Bandfield et
al., 2000) or basalt and altered basalt (Wyatt and McSween, 2002).
A recent study of the global martian dust revealed the presence of two weak emissivity
minima near 1580 and 1390 cm-1. These were attributed by Bandfield et al. (2003) to the
presence of bound water and carbonate based on lab studies of mixtures of a small
amount of carbonate minerals in feldspar. A similar and somewhat stronger doublet was
observed by Christensen et al. (2004a) in Mini-TES soil spectra at Gusev crater. Another
explanation for this mid-IR martian soil doublet is hydrated iron sulfate minerals (Lane et
al., 2004). Doublets are observed in this spectral region for sulfate minerals such as
ferricopiapite, szomolnokite and rozenite. These minerals have been observed at AMD
sites and are typically associated with acidic aqueous environments. Mid-IR spectra of
natural ferrihydrites also exhibit a doublet in this region (Bishop and Murad, 2002) as
well as the AMD crust material collected at the Iron Mountain site (Figure 9). This
suggests that there are a few possible explanations for the mid-IR martian soil doublet
and that this spectral feature needs to be investigated further by analysis of more minerals
and analog materials.
Whether this weak emissivity doublet is interpreted as bound water plus carbonate or
hydrated iron sulfate minerals has important implications for Mars because these
minerals imply different environmental conditions. As discussed by Lane et al. (2004)
the presence of iron-bearing sulfates in the martian surface material is consistent with
18
chemical weathering of the surface via low-pH waters. This environment would preclude
the presence of carbonate and olivine in the soils that result from weathering, although
these minerals could be present and protected inside rocks. If this emissivity doublet is
due to sulfates, then these spectra are consistent with the presence of 10-20 wt.% sulfate
in the Pancam and Mössbauer spectra.
SUMMARY AND APPLICATIONS TO ASTROBIOLOGY ON MARS
This study includes VNIR, mid-IR and Mössbauer spectra of two AMD samples
collected from the Iron Mountain site in California and several iron sulfate minerals. We
compare these data to the new spectra of Mars collected by the MERs and interpret the
martian data to contain substantial evidence of sulfate minerals. Based upon comparison
of multiple types of spectral data and geologic reasoning, we assign the broad doublet
present in Mössbauer spectra of soils to be due to Fe2+ sulfates rather than olivine, and we
attribute the weak emissivity doublet near 1390 and 1580 cm-1 in martian soil spectra to
be due to hydrated iron sulfates (and/or oxyhydroxides) rather than carbonates. Our
spectral mixture analyses of Pancam and IMP data also indicate that 10-20 wt.% sulfate
is needed along with altered basaltic material in order to explain the martian soil spectra
observed. Identification of the minerals present in martian rocks and soils is important
for astrobiology because minerals are indicators of environmental factors such as aqueous
processes, pH, and temperature that are necessary to define on Mars in order to look for
potential habitable environments. Hydrated iron sulfate minerals are consistent with lowtemperature, acidic aqueous environments.
19
The dark red neutralized AMD crust material measured in this study is a mixture of
ferrihydrite and gypsum. The spectral properties of an air-dried aliquot of this sample
contain features of both minerals and favor those of ferrihydrite for the finer fraction.
The crusted material exhibits spectral properties of coarser particles suggesting that if
material like this were formed into cemented soil units on Mars it would appear to be a
coarser-grained material. The bright green AMD crystalline rock is composed of the iron
sulfate mineral rozenite and contains spectral features similar to the iron sulfate mineral
melanterite. In fact, it would be difficult to distinguish among rozenite and melanterite
on Mars with VNIR, mid-IR and Mössbauer spectra. The AMD rozenite spectra include
extended visible region bands near 0.43, 0.98 and ~1.2 µm, as well as isomer shift and
quadrupole splitting parameters of 1.32 mm/s and 3.02, respectively, due to the iron in
this sample. The AMD rozenite spectra include VNIR bands due to bound H2O near
1.45, 1.97, and 3 µm, plus OH and SO4 overtone and combination bands near 2.42 and
2.54 µm and from ~4.2 to 4.8 µm, as well as an additional feature near 2.1 µm for the
coarse-grained sample. Understanding the VNIR spectral features of natural sulfates and
sulfate-bearing materials is important for interpretation of the current OMEGA spectra
and upcoming CRISM spectra. The AMD rozenite sample exhibits emissivity minima
near 1550, 1435, 1100, 808, 614, 510, and 290 cm-1 for the rock chip and surface spectra;
the band at 808 cm-1 is broadened and shifted to ~900 cm-1 for the particulate sample.
Related features are observed for the other sulfate minerals shown in Figure 6, although
the band positions vary widely depending on the mineral structure. Continued study of
hydrated iron sulfates and oxyhydroxides is necessary in order to fully understand the
intriguing double observed near 1390 and 1580 cm-1 in martian emission spectra.
20
Aqueous oxidation of sulfides known to be present on Mars from martian meteorites
could be responsible for the presence of sulfates observed in the rock outcrops at
Meridiani and the global martian soils. This reaction could have occurred on Mars over
long time periods and biology need not be invoked in order to explain the occurrence of
Fe3+ sulfates.
However, given the association of microorganisms such as
Acidithiobacillus ferrooxidans with the aqueous oxidation of sulfides on Earth, deposits
of sulfate precipitates on Mars may be ideal sites to search for extinct life on that planet.
ACKNOWLEDGMENTS
Reflectance spectra were measured at the NASA-supported RELAB facility at Brown
University and emission spectra were measured at the NASA-supported Mars Space
Flight facility at Arizona State University. Thanks are due to G. J. Marchand for
assistance with the XRD, T. Hiroi for assistance with the reflectance spectra, and E.
Murad for helpful comments that improved the text. The authors are grateful for support
from NASA Astrobiology Institute (JLB, JFB), Mars Data Analysis program (JLB,
MDD), Cosmochemistry program (MDD), and Mars Odyssey Participating Scientist
program (MDL).
References
Baker, B. J. and Banfield, J. F. 2003, FEMS Microbiology Ecology, 44, pp. 139-152.
Bandfield, J. L. 2002, Journal of Geophysical Research, 107, pp. 10.1029/2001JE001510.
Bandfield, J. L., Glotch, T. D. and Christensen, P. R. 2003, Science, 301, pp. 1084-1087.
Bandfield, J. L., Hamilton, V. E. and Christensen, P. R. 2000, Science, 287, pp. 16261630.
Bell, J. F., III, McSween Jr., H. Y., Murchie, S. L., Johnson, J. R., Reid, R., Morris, R.
V., Anderson, R. C., Bishop, J. L., Bridges, N. T., Britt, D. T., Crisp, J. A.,
Economou, T., Ghosh, A., Greenwood, J. P., Gunnlaugsson, H. P., Hargraves, R.
21
M., Hviid, S., Knudsen, J. M., Madsen, M. B., Moore, H. J., Rieder, R. and
Soderblom, L. 2000, Journal of Geophysical Research, 105, pp. 1721-1755.
Bell, J. F., III, Squyres, S. W., Arvidson, R., Arneson, H. M., Bass, D., Blaney, D. L.,
Cabrol, N. A., Calvin, W. M., Farmer, J., Farrand, W. H., Goetz, W., Golombek,
M. P., Grant, J. A., Greeley, R., Guinness, E., Hayes, A. G., Hubbard, M. Y. H.,
Herkenhoff, K. E., Johnson, M. J., Johnson, J. R., Joseph, J., Kinch, K. M.,
Lemmon, M., Madsen, M. B., Maki, J. N., Malin, M., McCartney, E., McLennan,
S. M., McSween, H. Y., Jr., Ming, D. W., Moersch, J. E., Morris, R. V., Noe
Dobrea, E. Z., Parker, T. J., Proton, J., Rice, J. W., Jr., Seelos, F., Soderblom, J.,
Soderblom, L. A., Sohl-Dickstein, J. N., Sullivan, R. J., Wolff, M. J. and Wang,
A. 2004a, Science, 305, pp. 800-806.
Bell, J. F., III, Squyres, S. W., Arvidson, R. E., Arneson, H. M., Bass, D., Calvin, W. M.,
Farrand, W. H., Goetz, W., Golombek, M. P., Greeley, R., Grotzinger, J.,
Guinness, E., Hayes, A. G., Hubbard, M. Y. H., Herkenhoff, K. E., Johnson, M.
J., Johnson, J. R., Joseph, J., Kinch, K. M., Lemmon, M. T., Li, R., Madsen, M.
B., Maki, J. N., Malin, M., McCartney, E., McLennan, S. M., McSween, H. Y.,
Jr., Ming, D. W., Morris, R. V., Noe Dobrea, E. Z., Parker, T. J., Proton, J., Rice,
J. W., Jr., Seelos, F., Soderblom, J. M., Soderblom, L. A., Sohl-Dickstein, J. N.,
Sullivan, R., Weitz, C. M. and Wolff, M. J. 2004b, Science, 306, pp. 1703-1709.
Bell, J. F., III, Squyres, S. W., Herkenhoff, K., Maki, J., Arneson, H. M., Brown, D.,
Collins, S. A., Dingizian, A., Elliot, S. T., Hagerott, E. C., Hayes, A. G., Johnson,
M. J., Johnson, J. R., Joseph, J., Kinch, K., Lemmon, M. T., Morris, R. V., Scherr,
L., Schwochert, M., Shepard, M. K., Smith, G. H., Sohl-Dickstein, J. N., Sullivan,
R. J., Sullivan, W. T. and Wadsworth, M. 2003, Journal of Geophysical Research,
108, pp. 8063, doi:10.1029/2003JE002070.
Bibring, J.-P. 2004, In COSPARParis, pp. abstract#: 04-A-01888.
Bigham, J. M., Carlson, L. and Murad, E. 1994, Mineralogical Magazine, 58, pp. 641648.
Bigham, J. M., Schwertmann, U. and Carlson, L. (1992) In Biomineralization Processes
of Iron and Manganese - Modern and Ancient Environments(Eds, Skinner, H. C.
W. and Fitzpatrick, R. W.) Catena Verlag, pp. 219-232.
Bigham, J. M., Schwertmann, U. and Pfab, G. 1996, Applied Geochemistry, 11, pp. 845849.
Bishop, J. L., Fröschl, H. and Mancinelli, R. L. 1998a, Journal of Geophysical Research,
103, pp. 31,457-31,476.
Bishop, J. L. and Murad, E. (1996) In Mineral Spectroscopy: A tribute to Roger G.
Burns, Vol. Special Publication Number 5 (Eds, Dyar, M. D., McCammon, C. and
Schaefer, M. W.) The Geochemical Society, Houston, TX, pp. 337-358.
Bishop, J. L. and Murad, E. (2002) In Volcano-Ice Interactions on Earth and Mars(Eds,
Smellie, J. L. and Chapman, M. G.) Geological Society, Special Publication
No.202, London, pp. 357-370.
Bishop, J. L. and Murad, E. 2004, American Mineralogist, pp. submitted.
Bishop, J. L., Murad, E., Lane, M. D. and Mancinelli, R. L. 2004, Icarus, 169, pp. 331323.
22
Bishop, J. L., Scheinost, A., Bell, J., Britt, D., Johnson, J. and Murchie, S. 1998b, In
Lunar Planet. Sci. XXIX.Lunar Planet. Inst., Houston., pp. CD-ROM #1803
(abstr.).
Blaney, D. L. and McCord, T. B. 1995, Journal of Geophysical Research, 100, pp. 1443314441.
Boctor, N. Z., Meyer, H. O. A. and Kullerud, G. 1976, Earth Planet. Sci. Lett., 32, pp. 6976.
Bunch, T. E. and Reid, A. M. 1975, Meteoritics, 10, pp. 303-315.
Burns, R. G. 1987, Journal of Geophysical Research, 92, pp. E570-E574.
Burns, R. G. 1988, In Proceedings of the 18th LPSCLPI, Houston, TX, pp. 713-721.
Burns, R. G. 1993, Mineralogical Applications of Crystal Field Theory., Cambridge
University Press, Cambridge, UK.
Burns, R. G. 1994, In Lunar Planet. Sci. XXVLunar Planet. Inst., Houston., pp. 203-204.
Burns, R. G. and Fisher, D. S. 1990, Journal of Geophysical Research, 95, pp. 1416914173.
Christensen, P. R., Bandfield, J. L., Hamilton, V. E., Lane, M. D., Piatek, J. L., Ruff, S.
W. and Stefanov, W. L. 2000, Journal of Geophysical Research, 105, pp. 97359739.
Christensen, P. R., Bandfield, J. L., Hamilton, V. E., Ruff, S. W., Kieffer, H. H., Titus, T.
N., Malin, M. C., Morris, R. V., Lane, M. D., Clark, R. L., Jakosky, B. M.,
Mellon, M. T., Pearl, J. C., Conrath, B. J., Smith, M. D., Clancy, R. T., Kuzmin,
R. O., Roush, T., Mehall, G. L., Gorelick, N., Bender, K., Murray, K., Dason, S.,
Greene, E., Silverman, S. and Greenfield, M. 2001, Journal of Geophysical
Research, 106, pp. 23,823–23,871.
Christensen, P. R., Mehall, G. L., Silverman, S. H., Anwar, S., Cannon, G., Gorelick, N.,
Kheen, R., Jeffryes, J., O'Donnell, W., Peralta, R., Wolverton, T., Blaney, D. L.,
Denise, R., Rademacher, J., Morris, R. V. and Squyres, S. W. 2003, Journal of
Geophysical Research, 108, pp. 8064, 10.1029/2003JE002117.
Christensen, P. R., Ruff, S. W., Fergason, R. L., Knudson, A. T., Anwar, S., Arvidson, R.
E., Bandfield, J. L., Blaney, D. L., Budney, C., Calvin, W. M., Glotch, T. D.,
Golombek, M. P., Gorelick, N., Graff, T. G., Hamilton, V. E., Hayes, A., Johnson,
J. R., McSween, H. Y., Jr., Mehall, G. L., Mehall, L. K., Moersch, J. E., Morris,
R. V., Rogers, A. D., Smith, M. D., Squyres, S. W., Wolff, M. J. and Wyatt, M. B.
2004a, Science, 305, pp. 837-842.
Christensen, P. R., Wyatt, M. B., Glotch, T. D., Rogers, A. D., Anwar, S., Arvidson, R.
E., Bandfield, J. L., Blaney, D. L., Budney, C., Calvin, W. M., Fallacaro, A.,
Fergason, R. L., Gorelick, N., Graff, T. G., Hamilton, V. E., Hayes, A., Johnson,
J. R., Knudson, A. T., McSween, H. Y., Jr., Mehall, G. L., Mehall, L. K.,
Moersch, J. E., Morris, R. V., Smith, M. D., Squyres, S. W., Ruff, S. W. and
Wolff, M. J. 2004b, Science, 306, pp. 1733-1739.
Clark, B. C. 1993, Geochimica Cosmochimica Acta, 57, pp. 4575-4581.
Clark, B. C. 2004, In Second Conference on Early MarsJackson Hole, Wyoming, pp. CDROM (abstr.#8075).
Clark, B. C. and Baird, A. K. 1979, Journal of Geophysical Research, 84, pp. 8395-8403.
Clark, B. C., Baird, A. K., Weldon, R. J., Tsusaki, D. M., Schnabel, L. and Candelaria,
M. P. 1982, Journal of Geophysical Research, 87, pp. 10059-10067.
23
Clark, B. C. and Van Hart, D. C. 1981, Icarus, 45, pp. 370-378.
Colman, S. M. 1982, US Geolog. Survey #1246.
Colmer, A. R. and Hinkle, M. E. 1947, Science, 106, pp. 253-256.
Cooper, C. D. and Mustard, J. 2001, In Lunar Planet. Sci. XXXII.Lunar Planet. Inst.,
Houston., pp. CD-ROM #2048 (abstr.).
Cornell, R. M. and Schwertmann, U. 1996, The Iron Oxides, VCH, New York.
Dreibus, G. and Wänke, H. 1987, Icarus, 71, pp. 225-240.
Edwards, K. J., Bond, P. J., Druschel, G. K., McGuire, M. M., Hamers, R. J. and
Banfield, J. F. 2000, Chemical Geology, 169, pp. 383-397.
Edwards, K. J., Schrenk, M. O., Hamers, R. and Banfield, J. F. 1998, American
Mineralogist, 83, pp. 1444-1453.
Fernández-Remolar, D., Gómez-Elvira, J., Gómez, F., Sebastian, E., Martíin, J.,
Manfredi, J. A., Torres, J., González Kesler, C. and Amils, R. 2004, Planetary
Space Science, 52, pp. 239-248.
Floran, R. J., Prinz, M., Hlava, P. F., Keil, K., Nehru, C. and Hinthorne, J. R. 1978,
Geochimica Cosmochimica Acta, 42, pp. 1213-1229.
Foley, C. N., Economou, T. and Clayton, R. N. 2003, Journal of Geophysical Research,
108, pp. 8096, doi:10.1029/2002JE002019.
Gellert, R., Rieder, R., Anderson, R. C., Brückner, J., Clark, B. C., Dreibus, G.,
Economou, T., Klingelhöfer, G., Lugmair, G. W., Ming, D. W., Squyres, S. W.,
d'Uston, C., Wänke, H., Yen, A. S. and Zipfel, J. 2004, Science, 305, pp. 829-832.
Gendrin, A., Bibring, J.-P., Gondet, B., Langevin, Y., Mangold, N., Mustard, J. F. and
Poulet, F. 2004, In Second Conference on Early MarsJackson Hole, Wyoming,
pp. CD-ROM (abstr.#8030).
Gooding, J. L., Wentworth, S. J. and Zolensky, M. E. 1991, Meteoritics, 26, pp. 135-143.
Greenwood, J. P., Riciputi, L. R., McSween Jr., H. Y. and Taylor, L. A. 2000,
Geochimica Cosmochimica Acta, 64, pp. 1121-1131.
Hartmann, W. K., Anguita, J., de la Casa, M. A., Berman, D. C. and Ryan, E. V. 2001,
Icarus, 149, pp. 37-53.
Harvey, R. P., Wadhwa, M., McSween Jr., H. Y. and Crozaz, G. 1993, Geochimica
Cosmochimica Acta, 57, pp. 4769-4783.
Holmes, P. R. and Crundwell, F. K. 2000, Geochimica Cosmochimica Acta, 64, pp. 263274.
Hynek, B. M. 2004, Nature, 431, pp. 156-159.
Klingelhöfer, G., Morris, R. V., Bernhardt, B., Rodionov, D., de Souza, P. A. J., Squyres,
S. W., Foh, J., Kankeleit, E., Bonnes, U., Gellert, R., Schröder, C., Linkin, S.,
Evlanov, E., Zubkov, B. and Prilutski, O. 2003, Journal of Geophysical Research,
108, pp. 8067, doi:10.1029/2003JE002138.
Klingelhöfer, G., Morris, R. V., Bernhardt, B., Schröder, C., Rodionov, D., de Souza, P.
A. J., Yen, A. S., Gellert, R., Evlanov, E. N., Zubkov, B., Foh, J., Bonnes, U.,
Kankeleit, E., Gütlich, P., Ming, D. W., Renz, F., Wdowiak, T. J., Squyres, S. W.
and Arvidson, R. E. 2004a, Science, 306, pp. 1740-1745.
Klingelhöfer, G., Morris, R. V., Bernhardt, B., Schröder, C., Rodionov, D., de Souza, P.
A. J., Yen, A. S., Renz, F., Wdowiak, T. J. and Squyres, S. W. 2004b, In Lunar
Planet. Sci. XXXV.Lunar Planet. Inst., Houston., pp. CD-ROM #2184 (abstr.).
Lane, M. D. 2004, pp. in preparation.
24
Lane, M. D. and Christensen, P. R. 1998, Icarus, 135, pp. 528-536.
Lane, M. D., Christensen, P. R. and Hartmann, W. K. 2003, Geophysical Research
Letters, 30 (14), pp. 1770, doi: 10.1029/2003GL017183, 2003.
Lane, M. D., Dyar, M. D. and Bishop, J. L. 2004, Geophysical Research Letters, 31, pp.
L19702, doi:10.1029/2004GL021231.
McSween, H. Y., Jr., Arvidson, R. E., Bell, J. F., III, Blaney, D. L., Cabrol, N. A.,
Christensen, P. R., Clark, B. C., Crisp, J. A., Crumpler, L. S., Des Marais, D. J.,
Farmer, J. D., Gellert, R., Ghosh, A., Gorevan, S., Graff, T., Grant, J., Haskin, L.
A., Herkenhoff, K. E., Johnson, J. R., Jolliff, B. L., Klingelhöfer, G., Knudson, A.
T., McLennan, S. M., Milam, K. A., Moersch, J. E., Morris, R. V., Rieder, R.,
Ruff, S. W., de Souza, P. A. J., Squyres, S. W., Wänke, H., Wang, A., Wyatt, M.
B., Yen, A. S. and Zipfel, J. 2004, Science, 305, pp. 842-845.
Moore, J. M. 2004, Nature, 428, pp. 711-712.
Morgan, J. W. and Anders, E. 1980, Proc. Nat. Acad. Sci. USA, 77, pp. 6973-6977.
Morris, R. V., Golden, D. C., Bell III, J. F., Shelfer, T. D., Scheinost, A. C., Hinman, N.
W., Furniss, G., Mertzman, S. A., Bishop, J. L., Ming, D. W., Allen, C. C. and
Britt, D. T. 2000, Journal of Geophysical Research, 105, pp. 1757-1817.
Morris, R. V., Klingelhöfer, G., Bernhardt, B., Schröder, C., Rodionov, D. S., de Souza,
P. A., Jr., Yen, A. S., Gellert, R., Evlanov, E. N., Foh, J., Kankeleit, E., Güttlich,
P., Ming, D. W., Renz, F., Wdowiak, T. J., Squyres, S. W. and Arvidson, R. E.
2004, Science, 305, pp. 833-836.
Morris, R. V., Ming, D. W., Golden, D. C. and Bell III, J. F. (1996) In Mineral
Spectroscopy: A tribute to Roger G. Burns, Vol. Special Publication Number 5
(Eds, Dyar, M. D., McCammon, C. and Schaefer, M. W.) The Geochemical
Society, pp. 327-336.
Murchie, S., Arvidson, R., Beisser, K., Bibring, J.-P., Bishop, J., Boldt, J., Bussey, B.,
Choo, T., Clancy, R. T., Darlington, E. H., Des Marais, D., Fasold, M., Fort, D.,
Green, R., Guinness, E., Hayes, J., Heyler, G., Humm, D., Lee, R., Lees, J., Lohr,
D., Malaret, E., Morris, R., Mustard, J., Rhodes, E., Robinson, M., Roush, T.,
Schaefer, E., Seagrave, G., Silvergate, P., Smith, M., Strohbehn, K., Thompson,
P. and Tossman, B. 2003, In Sixth Int'l Conf. on MarsPasadena, CA, pp. CDROM #3062 (abstr.).
Murchie, S., Barnouin-Jha, O., Barnouin-Jha, K., Bishop, J. L., Morris, R. V., Johnson, J.
and McSween Jr., H. Y. 2005, Icarus, pp. in revision.
Nesbitt, H. W. and Young, G. M. 1984, Geochimica Cosmochimica Acta, 48, pp. 15231534.
Nordstrom, D. K. (1982) In Acid Sulfate WeatheringSoil Science Society of America,
Madison, WI, pp. 37-56.
Nordstrom, D. K. (1985) In Selected Papers in the Hydrological SciencesU.S. Geol.
Survey, pp. 113-119.
Nordstrom, D. K. and Alpers, C. N. 1999, Proc. Nat. Acad. Sci. USA, 96, pp. 3455-3462.
Nordstrom, D. K. and Southam, G. (1997) In Geomicrobiology: Interactions Between
Microbes and Minerals, Vol. Reviews in Mineralogy, Vol. 35 (Eds, Banfield, J. F.
and Nealson, K. H.) Mineralogical Society of America, Washington, D.C., pp.
361-390.
25
Pollack, J. B., Roush, T. L., Witteborn, F., Bregman, J., Wooden, D., Stoker, C., Toon, O.
B., Rank, D., Dalton, B. and Freedman, R. 1990, Journal of Geophysical
Research, 95, pp. 14595-14627.
Rieder, R., Gellert, R., Anderson, R. C., Brückner, J., Clark, B. C., Dreibus, G.,
Economou, T., Klingelhöfer, G., Lugmair, G. W., Ming, D. W., Squyres, S. W.,
d'Uston, C., Wänke, H., Yen, A. S. and Zipfel, J. 2004, Science, 306, pp. 17461749.
Ross, S. D. (1974) In The Infrared Spectra of Minerals(Ed, Farmer, V. C.) The
Mineralogical Society, London, pp. 423-444.
Salisbury, J. W., Walter, L. S., Vergo, N. and D'Aria, D. M. 1991, Infrared (2.1-25 µm)
Spectra of Minerals, Johns Hopkins University Press, Baltimore.
Schwertmann, U., Bigham, J. M. and Murad, E. 1995, Euro. J. Mineral., 7, pp. 547-552.
Settle, M. 1979, Journal of Geophysical Research, 84, pp. 8343-8354.
Singer, P. C. and Stumm, W. 1970, Science, 167, pp. 1121-1123.
Smith, P. H., Bell III, J. F., Bridges, N. T., Britt, D. T., Gaddis, L., Greeley, R., Keller, H.
U., Herkenhoff, K. E., Jaumann, R., Johnson, J. R., Kirk, R. L., Lemmon, M.,
Maki, J. N., Malin, M. C., Murchie, S. L., Oberst, J., Parker, T. J., Reid, R. J.,
Sablotny, R., Soderblom, L. A., Stoker, C., Sullivan, R., Thomas, N., Tomasko,
M. G., Ward, W. and Wegryn, E. 1997, Science, 278, pp. 1758-1765.
Squyres, S. W., Arvidson, R. E., Bell, J. F., III, Brückner, J., Cabrol, N. A., Calvin, W.
M., Carr, M. H., Christensen, P. R., Clark, B. C., Crumpler, L., Des Marais, D. J.,
d'Uston, C., Economou, T., Farmer, J., Farrand, W. H., Folkner, W., Golombek,
M. P., Gorevan, S., Grant, J. A., Greeley, R., Grotzinger, J., Haskin, L. A.,
Herkenhoff, K. E., Hviid, S., Johnson, J., Klingelhöfer, G., Knoll, A., Landis, G.,
Lemmon, M., Li, R., Madsen, M. B., Malin, M. C., McLennan, S. M., McSween,
H. Y., Jr., Ming, D. W., Moersch, J., Morris, R. V., Parker, T. J., Rice, J. W., Jr.,
Richter, L., Rieder, R., Sims, M., Smith, M., Smith, P., Soderblom, L. A.,
Sullivan, R., Wänke, H., Wdowiak, T. J., Wolff, M. J. and Yen, A. S. 2004,
Science, 306, pp. 1698-1703.
Steele, I. M. and Smith, J. V. 1982, Journal of Geophysical Research, 87, pp. A375A384.
Stolper, E. M. and McSween Jr., H. Y. 1979, Geochimica Cosmochimica Acta, 43, pp.
1475-1498.
Strunz, H. and Nickel, E. 2001, Strunz Mineralogical tables, 9th edition, E.
Schweizerbart'sche Verlagsbuchhandlung, Stuttgart.
Tosca, N. J., McLennan, S. M., Lindsley, D. H. and Schoonen, M. A. A. 2004, Journal of
Geophysical Research, 109, pp. E05003, doi:10.1029/2003JE002218.
Treiman, A. H., Barrett, R. A. and Gooding, J. L. 1993, Meteoritics, 28, pp. 86-97.
Wyatt, M. B. and McSween, H. Y., Jr. 2002, Nature, 417, pp. 263-266.
26
Figure Captions
Figure 1 Location of the Iron Mountain site near Lake Shasta. North is toward the top
of the figure. Rivers are shown in blue and roads in black. The I-5 freeway and S299
highway are labeled for orientation.
Figure 2 Image of the acid mine drainage (AMD) flowing out of the Iron Mountain site
(A), rozenite rock (B), and the AMD crust that precipitated along the creek bank (C).
Figure 3 Reflectance spectra of AMD crust sample, plus crushed and sieved versions of
this material and the minerals gypsum and ferrihydrite for comparison (a) VNIR region
and (b) mid-IR region.
Figure 4 Mössbauer spectra of the <45 µm AMD crust fraction and ferrihydrite
measured at 300 K.
Figure 5 VNIR spectra of the AMD rozenite and other acid sulfates:
ferricopiapite, coquimbite, szomolnokite, schwertmannite and jarosite.
Figure 6 Mid-IR reflectance and emittance spectra of the AMD rozenite and
other acid sulfates: ferricopiapite, coquimbite, szomolnokite, schwertmannite
and jarosite. Emittance spectra were measured of bulk rock samples and
reflectance spectra were measured of the fine-grained powders. The inverse
reflectance spectra can be equated with emittance spectra in most cases.
27
Figure 7 Mössbauer spectra of the AMD rozenite, plus ferricopiapite,
coquimbite, szomolnokite, schwertmannite, jarosite and an olivine separate
from the martian meteorite Chassigny at 260 K. For comparison spectra of
Gusev crater and Meridiani Planum on Mars from Klingelhöfer (2004b) are
shown.
Figure 8 Pancam spectra of Mars compared with convolved lab spectra. A)
Gusev spectra (from Bell et al., 2004a), B) sulfate minerals: coquimbite
(Coq), ferricopiapite (Fer), jarosite (Jar), schwertmannite (Sch), szomolnokite
(Szo) and the AMD rozenite (Roz), C) ferrihydrite (Fh), red hematite JB129
(Hm <5 µm), goethite Jb47 (Gt), coarse gray hematite JB41 (Hm gray), D)
olivine JB557 <125 µm, EETA79001 lithology B powder <125 µm, AMD
crust JB577 <45 µm, volcanic soil JB399 <45 µm, and E) normalized spectra
of a Mars Pathfinder soil (from Murchie et al., 2005), a Meridiani soil (from
Bell et al., 2004b) and a soil mixture spectrum derived from the spectra of the
volcanic soil, gray hematite, szomolnokite and schwertmannite spectra.
Figure 9 TES and Mini-TES spectra of Mars compared with emittance
spectra of sulfate minerals (offset for clarity) and reflectance spectra of AMD
materials. The TES global dust spectrum is from Bandfield et al. (2003), and
the Mini-TES Gusev crater dust spectrum is from Christensen et al. (2004a).
28
Table 1
Names and ideal chemical formulas for minerals in this
study
Sample ID
Mineral Name
General Formula
JB556,557
Gypsum
CaSO4 · 2 H2O
JB622
Szomolnokite
FeSO4 · H2O
JB626
Rozenite
FeSO4 · 4 H2O
JB621
Coquimbite
Fe2(SO4)3 · 9 H2O
JB440
Jarosite
K2Fe6(SO4)4(OH)12
JB620
Ferricopiapite
(Fe,Al,Mg)Fe5(SO4)6(OH)2 · 20 H2O
JB130,131
Schwertmannite
Fe16O16 (OH)12 (SO4)2 · n H2O
JB499
Ferrihydrite
Fe1.55O1.66(OH)1.33 · nH2O
Note: schwertmannite and ferrihydrite formulas from Cornell and
Schwertmann (1996), all other formulas from Strunz and Nickel (2001). Our
rozenite sample contains about Fe2+0.9 and Fe3+0.1 per sulfate.
29
Figure 1
Figure 2
30
Figure 3
Figure 4
31
Figure 5
32
Figure 6
33
Figure 7
34
Figure 8
35
Figure 9
36