Available online at www.sciencedirect.com
ScienceDirect
Geochimica et Cosmochimica Acta 127 (2014) 25–38
www.elsevier.com/locate/gca
Contrasting styles of water–rock interaction at the
Mars Exploration Rover landing sites
Joel A. Hurowitz a,⇑, Woodward W. Fischer b
b
a
Department of Geosciences, Stony Brook University, 100 Nicolls Road, Stony Brook, NY 11794-2100, United States
Division of Geological and Planetary Sciences, California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125,
United States
Received 21 September 2012; accepted in revised form 17 November 2013; Available online 24 November 2013
Abstract
The nature of ancient hydrological systems on Mars has been the subject of ongoing controversy, driven largely by a disconnect between observational evidence for flowing water on the Martian surface at multiple scales and the incompatibility of such
observations with theoretical models that predict a cold early Martian environment in which liquid water is unstable. Here we
present geochemical data from the Mars Exploration Rovers to evaluate the hydrological conditions under which weathering
rinds, soils, and sedimentary rocks were formed. Our analysis indicates that the chemistry of rinds and soils document a
water-limited hydrologic environment where small quantities of S-bearing fluids enter the system, interact with and chemically
alter rock and soil, and precipitate secondary mineral phases at the site of alteration with little to no physical separation of primary and secondary mineral phases. In contrast, results show that the sedimentary rocks of the Burns Formation at Meridiani
Planum have a chemical composition well-described as a mixture between siliciclastic sediment and sulfate-bearing salts derived
from the evaporation of groundwater. We hypothesize that the former may be derived from the recently investigated Shoemaker
Formation, a sequence of impact breccias that underlie the Burns Formation. This result has important implications for the style
of chemical weathering and hydrology recorded by these sedimentary materials, revealing long-range transport of ions in solution in an open hydrological system that is consistent only with subsurface or overland flow of liquid water.
Ó 2013 Elsevier Ltd. All rights reserved.
1. INTRODUCTION
The Martian geologic record contains a wide variety of
geomorphological features consistent with flowing and
standing water on the surface (e.g., outflow channels, valley
networks, alluvial fans and deltas, sediment-filled craters
and basins), with the generation of water-related landforms
generally accepted to have been more prevalent in early
Martian history and waning towards the modern day (for
recent reviews, see Carr, 2012; Grotzinger and Milliken,
2012). The discovery of a broad suite of aqueous secondary
minerals produced from alteration of the Martian crust by
fluids has added further constraints on the nature of
⇑ Corresponding author. Tel.: +1 631 632 6801.
E-mail address: [email protected] (J.A. Hurowitz).
0016-7037/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.gca.2013.11.021
water–rock interactions. Syntheses of these mineralogic
observations from orbital data have driven a hypothesis for
fundamental secular changes in secondary mineral forming
processes over time, from early clay-mineral forming environments to intermediate sulfate-mineral forming environments to more modern environments in which anhydrous
ferric iron oxides are produced (Bibring et al., 2005; Mustard
et al., 2008; Murchie et al., 2009; Ehlmann et al., 2011). These
geomorphic and mineralogic observations are broadly
thought to reflect a change in environmental conditions at
the Martian surface from an early wet (and possibly warm)
state to the pervasive dry, cold conditions observed today.
Despite geomorphologic, sedimentological, and mineralogical evidence for the presence of liquid water at the
Martian surface, at least in its early geologic history,
significant questions remain regarding the state and evolution of the Martian atmosphere (e.g., Greenwood et al.,
26
J.A. Hurowitz, W.W. Fischer / Geochimica et Cosmochimica Acta 127 (2014) 25–38
2008; Cassata et al., 2012), specifically whether it was sufficiently thick and had an adequate greenhouse budget for liquid water to have been stable at the surface of Mars. A
number of models have been proposed that can provide
the warm, clement conditions required for liquid water stability (e.g., Forget and Pierrehumbert, 1997; Sagan and Chyba,
1997; Halevy et al., 2007), though none of these models have
garnered universal acceptance (for a recent review see Niles
et al., 2013). The apparent inconsistency between observations and theory has led to suggestions that Mars may only
have experienced transient warm climate episodes driven
by volcanism (e.g., Johnson et al., 2008, 2009) or large impact events (Segura et al., 2002). Others have questioned
the sedimentary interpretations for liquid water (e.g., Gaidos
and Marion, 2003; Niles and Michalski, 2009; Kite et al.,
2013), hypothesizing instead that widespread sedimentary
deposits were produced in a cold, polar-type climate for
much of Martian geologic history. In these conceptual models, the previously described geomorphic and mineralogic
features are largely generated by processes such as groundwater escape from beneath a global cryosphere, melting of
snowpacks, or glacial melting, as examples.
The ancient sedimentary rocks of the Burns Formation
at Meridiani Planum, Mars have recently become a touchstone in the debate over whether climate conditions were
ever appropriate to allow for stable liquid water in surface
environments at the time these deposits were formed (ca.
3.5–3.7 Ga). Based on extensive in situ observations by
the Mars Exploration Rover (MER) Opportunity, the sedimentary rocks of the Burns Formation were interpreted to
have formed under dry, desert-like conditions in which
chemically weathered basaltic sediment was transported in
an aeolian environment and cemented by evaporating, sulfate-rich groundwater (Grotzinger et al., 2005; Squyres and
Knoll, 2005). This interpretation is consistent with models
of global groundwater transport that indicate that Meridiani Planum was a locus for groundwater upwelling and
evaporation, requiring a climate system that would have
supported meteoric recharge to drive groundwater transport (Andrews-Hanna et al., 2007, 2010 although see
Michalski et al., 2013). If this interpretation is correct, then
the Burns Formation places an important constraint on
Martian climate conditions, and indicates that during late
Noachian to early Hesperian time, the climate was sufficiently warm and wet to allow for the existence of at least
a limited hydrologic cycle in which aquifers could be recharged by rainfall and/or snowmelt.
The Burns Formation is characterized by enigmatic
chemistry and mineralogy: its bulk composition has been
broadly described as a mixture of “basalt plus sulfur”, and
it contains as a ubiquitous mineral component the mineral
jarosite ðK; Na;H3 Oþ ÞFe3þ
3 ðSO4 Þ2 ðOHÞ6 , which requires
acidic (pH = ca. 2–4) conditions to precipitate. This chemistry is somewhat surprising given the process hydrology interpretation of the deposit that implies aquifer recharge and
subsequent groundwater upwelling and evaporation. In such
a setting, elemental fractionation along the flow path due to
differential mineral solubility and the generation of alkalinity
from silicate weathering might reasonably be expected.
Highlighting the apparent inconsistency between the charac-
teristics of the Burns Formation and an open hydrologic system, a number of alternative hypotheses have been advanced
that interpret the Burns Formation as the product of sulfuric
acid-driven weathering in volcaniclastic (McCollom and
Hynek, 2005), impact (Knauth et al., 2005), or glacial ice
(Niles and Michalski, 2009) environments.
These alternative hypotheses, driven by the recognition
of a “basalt plus sulfur” bulk chemical composition; invoke
chemical weathering processes described as “closed system”, “isochemical”, or “cation-conservative”. What is
meant by these terms from a process perspective is: sulfuric
acid-charged water is added to basalt, a suite of secondary
minerals is generated, the water leaves the system in the gas
phase by vaporization or sublimation (and/or is partially
retained in hydrated secondary minerals), and left behind
is a mixture of newly formed secondary minerals and residue that have not been physically separated from each
other. The term “cation-conservative” (Niles and Michalski,
2009) is the most accurate for the hydrochemical conditions
described above, which are neither truly “closed” nor truly
“isochemical”; accordingly, we employ the term “cationconservative” here.
“Acid fog” hypotheses also describe a form of cation-conservative chemical weathering that yield basalt plus sulfur
chemistries, and are often invoked to explain the chemical
composition of soils on Mars. In this model, acidic volcanic
aerosols settle out of the atmosphere onto rock, soil, and
dust, altering the surfaces they come into contact with
through low water-to-rock ratio, low pH weathering (e.g.,
Settle, 1979; Clark and Van Hart, 1981; Banin et al., 1997;
King and McSween, 2005; Hurowitz et al., 2006b; Ming
et al., 2008; King and McLennan, 2010). Similar mechanics
could arise from impact processes in which the decomposition of sulfate minerals generates “recycled” sulfuric acid
to drive silicate weathering, similar to what is thought to
have occurred in the terrestrial Chixulub impact event
(Kring, 2007; Zolotov and Mironenko, 2007; King and
McLennan, 2010). The weathering process associated with
the acid fog model is similar to the hypothesis proposed by
McCollom and Hynek (2005), and more recently by Michalski
and Bleacher (2013), who suggested that the chemistry of the
Burns Formation was generated by the co-deposition of acid
volatiles and basalt in volcaniclastic base surge deposits.
Building on analogues in terrestrial polar environments,
Niles and Michalski (2009) proposed that Burns Formation
sediment was generated during alteration in small pockets of
meltwater hosted in massive ice sheets in the equatorial latitudes of Mars (including over Meridiani Planum). In these
melt pockets, low temperature and low water-to-rock ratio
chemical weathering occurred between co-deposited basaltic
dust and volcanic acid aerosols trapped in the ice as it accumulated. Later sublimation of the ice left behind altered sediments with a basalt plus sulfur bulk composition.
In this report we present evidence from rover geochemical data that the sedimentary record of Mars preserves evidence for both cation-conservative weathering processes
and open system weathering conditions in which there is
sufficient liquid water present to mobilize and transport cations in solution over more significant length scales. The
geochemistry of modern soils and weathered rock surfaces
J.A. Hurowitz, W.W. Fischer / Geochimica et Cosmochimica Acta 127 (2014) 25–38
is well explained by cation-conservative weathering processes. In stark contrast, the ancient sedimentary rocks of
the Burns Formation at Meridiani Planum capture opensystem hydrochemical processes that require long range solute transport and associated fractionation of elements. The
examples we describe from the MER landing sites lay out a
useful framework to discover and understand the chemical
fractionations induced by weathering and alteration under
a broad spectrum of hydrologic conditions, and will provide examples against which analyses by the Mars Science
Laboratory Curiosity can be compared as it begins exploration of the 5 km-thick sedimentary section at Gale Crater.
2. METHODS
The MERs Spirit and Opportunity collected a wealth of
data on the composition, mineralogy, and textural properties
of soils and rocks on Mars. Spirit operated in Gusev Crater
from January 2004 through March 2010, when contact with
the rover was lost. Opportunity continues its science mission
at Meridiani Planum, 9+ years since landing in January
2004. Over a combined traverse distance of >40 km, these
twin rovers have collected well in excess of 300 rock and soil
chemical analyses using their Alpha-Particle X-ray Spectrometers (APXS). During analysis, the APXS instruments
determine concentrations of the elements Si, Ti, Al, Fe,
Mn, Mg, Ca, Na, K, P, S, Cl, Zn, Ni, and Br. The APXS
instruments are deployed on the end of each rover’s arm
and provide contact analyses of rocks and soils with a field
of view of 3.8 cm and a penetration depth of 1–20 lm
depending on the material and atomic weight of the element
being interrogated (Rieder et al., 2004; Gellert et al., 2006).
Increased sampling depth for rocks is achieved through the
use of the Rock Abrasion Tool (RAT), also mounted on
the end of each the rover’s arm, which can be employed to
brush rock surfaces clean and abrade rock surfaces to a maximum depth of 15 mm (Gorevan et al., 2003). APXS analyses are performed on natural soil and rock surfaces, brushed
rock surfaces, and abraded rock surfaces, and are typically
referred to as “undisturbed” or “as is”, “brushed”, and
“RAT-ed”, respectively.
For our analysis of the geochemical data, we employ
diagrams that plot molar Al2O3/(FeOT + MgO + CaO +
Na2O + K2O) versus SiO2/SO3. This parameter set is
particularly valuable because it captures much of the compositional variability inherent in unaltered igneous rocks
and also enables an assessment of the degree to which
basaltic rocks and soils have been affected by chemical
(e.g., mineral dissolution during chemical weathering) and
physical (e.g., mineral fractionation during physical transport) alteration processes subsequent to emplacement.
The ordinate axis was primarily chosen to capture the relationship between an oxide that is often described as relatively immobile during chemical weathering processes
(Al2O3) and oxides that tend to be substantially more mobile (FeO, MgO, CaO, Na2O, K2O) during those same
weathering processes. Oxidation will significantly decrease
Fe-solubility and under conditions that promote oxidation,
Fe and Al mobility can be considered to be quite similar.
Ternary diagrams, which plot (FeO + MgO) – Al2O3 –
27
(CaO + Na2O + K2O) at the apices, also offer a juxtaposition of immobile and mobile element oxides and have been
particularly useful for understanding chemical weathering
processes on Earth (Nesbitt and Wilson, 1992) and Mars
(Hurowitz et al., 2006b). However, because the Martian
soils, igneous rocks, weathering rinds, and sedimentary
rocks under consideration here do not separate from each
other on such ternary diagrams, we have chosen to collapse
these oxide interrelationships onto a single axis and plot
them against the ratio SiO2/SO3. The inability of these ternary diagrams to segregate the Burns Formation from
other soils and rocks has led to an interpretation that the
sediments that comprise the Burns Formation formed from
low water-to-rock ratio chemical alteration processes (e.g.,
Hurowitz and McLennan, 2007); a hypothesis that can be
tested by casting the data into a different chemical space.
The abscissa (SiO2/SO3) enables an evaluation of the
mobility of SiO2 and the degree to which sulfur has influenced the geochemical characteristics of a given soil or rock
sample. This ratio was employed by McLennan (2003) to
evaluate soil and rock geochemical variations at the Pathfinder landing site at Ares Valles, Mars, and Hurowitz
et al. (2006a) to evaluate rock geochemistry at Husband Hill,
in Gusev Crater, Mars. In fact, SO3 could be exchanged for
chemical species such as Cl or CO2 in order to evaluate the
influence of hydrochloric acid or carbonic acid on rock and
soil geochemistry. We have deliberately chosen to use SO3
as the denominator since S is the most abundant anionic constituent of the rocks and soils under consideration. In general, less abundant Cl mimics the behavior of S, shown by
the consistent SO3:Cl ratio of almost all samples in our dataset (7.5 ± 1.4, n = 50), excluding the Burns Formation and
subsurface soils in trenches, which are discussed more below.
APXS does not analyze C (or any other elements with masses
below that of Na), and so we cannot evaluate the impact of
carbonate-forming processes.
3. RESULTS
The APXS data used in our study can be found in the
Electronic Annex as a tabulated *.csv file. The full APXS
dataset for both MERs can be downloaded from the
MER Analyst’s Notebook on the Planetary Data System
Geosciences node with the exception of data for the Shoemaker Formation at Meridiani Planum, which was reported in Squyres et al. (2012).
3.1. Adirondack Class basalts
Adirondack Class basalts are olivine-rich (picritic) basalts emplaced as ejecta from impacts into the Hesperianaged basalt flows that blanket the floor of Gusev Crater
at the Spirit landing site. No in-place examples of this rock
class were ever encountered by the Spirit rover, thus they
are termed “float” rocks. These basalt lithologies were
encountered during Spirit’s traverse across the floor of
Gusev Crater between sol 0 and 150 of the MER Spirit mission; their characteristics and petrogenesis are fully
described in McSween et al. (2006). The RAT-ed surfaces
of Adirondack Class basalts are characterized by a uniform
28
J.A. Hurowitz, W.W. Fischer / Geochimica et Cosmochimica Acta 127 (2014) 25–38
Al 2O 3/(FeO T + MgO + CaO + Na 2O + K 2O) = 0.151 ±
0.002 (1r, n = 4), and more variable SiO2/SO3 = 49 ± 6.
Brushed surfaces of Adirondack Class basalts show slightly
elevated Al2O3/(FeOT + MgO + CaO + Na2O + K2O) =
0.165 ± 0.002 (n = 3) and significantly lower SiO2/
SO3 = 22 ± 7 relative to their RAT-ed counterparts. In
one location, at the rock named “Mazatzal”, brushing
revealed the presence of an apparent salt coating (Haskin
et al., 2005), and the Al2O3/(FeOT + MgO + CaO + Na2O + K2O) and SiO2/SO3 ratios of this surface are lower
still, at 0.138 and 7.8, respectively.
3.2. Surface soils and subsurface soils in trenches
We examined surface soil analyses from Gusev Crater
and Meridiani Planum that were collected between sols 1–
2071 and 1–2670, respectively. We subsampled the full soil
dataset to avoid samples that show clear geochemical evidence for contributions from local lithologic components.
For example, some soils at Gusev Crater have high P2O5
contents as a result of incorporation of local P2O5-enriched
“Wishstone-class” bedrock (Squyres et al., 2006a), and
some soils at Meridiani Planum are enriched in FeOT as a
result of the incorporation of hematite concretions derived
from erosion and physical concentration of concretions
from the underlying Burns Formation. The resulting dataset (n = 32) captures the bulk geochemical characteristics
of “average” Martian soil, and exhibits Al2O3/(FeOT +
MgO + CaO + Na2O + K2O) = 0.163 ± 0.007, similar to
that of brushed Adirondack class basalts, with still lower
SiO2/SO3 = 10 ± 3. For comparison, we also include APXS
data from the target “Portage” in the “Rocknest” eolian
sand shadow examined by the Mars Science Laboratory rover Curiosity (Blake et al., 2013). Although this analysis was
collected on a disturbed soil target (a rover wheel “scuff”
into the Rocknest deposit), the Al2O3/(FeOT + MgO + CaO + Na2O + K2O) and SiO2/SO3 ratios are
remarkably similar to that of undisturbed soils from Gusev
and Meridiani (0.14 and 10.5, respectively).
Finally, we also examined the six APXS analyses that
were collected at various depths beneath the surface of
three trenches that were excavated using the wheels on
the MER Spirit at Gusev Crater. These three trenches are
called “Road Cut”, “Big Hole”, and “The Boroughs”,
and they were excavated to depths of 6–7 cm, 9 cm, and
11 cm, respectively (see Wang et al., 2006). In contrast to
Adirondack Class basalts and surface soils, the trench soil
analyses exhibit a range in Al2O3/(FeOT + MgO + CaO +
Na2O + K2O) = 0.12–0.16 and SiO2/SO3 = 3.7–10.8, with
lower values for both of these ratios corresponding to
increasing trench depth (i.e., on average, both ratios are
highest for Road Cut, lower for Big Hole, and lowest for
The Boroughs).
3.3. The Burns and Shoemaker Formations
The Burns Formation is composed of sulfate-rich sandstones and siltstones; their characteristics and origin are
described in detail below (Section 5.2); we examined the
36 APXS analyses collected by the MER Opportunity from
RAT-ed surfaces between sols 1 and 2670. Burns Formation sandstones exhibit a range in Al2O3/(FeOT +
MgO + CaO + Na2O + K2O) = 0.09–0.15 and low SiO2/
SO3 ranging from 1.5 to 3.4.
We also analyzed the limited (n = 8) APXS dataset recently published by Squyres et al. (2012) for the Shoemaker
Formation at Meridiani Planum. The Shoemaker Formation was encountered in impact debris deposits that surround the 22 km diameter Endeavour Crater, located
30 km south of Opportunity’s landing site. It lies stratigraphically beneath the Burns Formation and consists of
lithic and suevite impact breccias with Al2O3/(FeOT +
MgO + CaO + Na2O + K2O) = 0.15–0.17 and SiO2/SO3
ranging from 6.6 to 12.7 for “as is” rock surfaces. One
RAT-ed Shoemaker Formation analysis was also reported
in Squyres et al. (2012), it has Al2O3/(FeOT +
MgO + CaO + Na2O + K2O) = 0.13 and SiO2/SO3 = 19.6.
4. DISCUSSION
4.1. Soils, weathered basalts and the importance of cationconservative weathering on mars
Soils provide an integrated record of geological processes at the surface of planetary bodies and the origin
and evolution of Martian soils has been the focus of a large
body of research over many decades (e.g., Clark et al., 1976;
Morris et al., 2000; McSween et al., 2010). A general consensus has emerged that the soils of Mars represent a broad
compositional average of the exposed crust of Mars, generated through a combination of impact brecciation and comminution, eolian erosion, transport, and mixing, and to
varying degrees, interaction with water and other volatiles.
The conditions under which water-driven processes have taken place remain somewhat less clear, particularly where
interpretations of the amount and duration of liquid water
interaction (e.g., Amundson et al., 2008; McGlynn et al.,
2012) and the pH of those interactions (e.g., Hurowitz
and McLennan, 2007; Kounaves et al., 2009), are
concerned.
Adirondack Class basalts have a broadly similar chemical composition to the soils in our dataset; accordingly,
they represent a reasonable starting point from which to
consider the chemical effects of weathering processes that
generated soils on Mars. Fig. 1A illustrates data for
Adirondack Class basalts from Gusev Crater and soils from
Gusev Crater and Meridiani Planum. The rock and soil
data are characterized by a relatively narrow range of
Al2O3/(FeOT + MgO + CaO + Na2O + K2O) = 0.14–0.18
and soils are not significantly fractionated from the basaltic
rocks on the ordinate axis. To aid comparison, this ratio is
0.51 for the Earth’s granodioritic upper continental crust
(Taylor and McLennan, 1985). The low ratio in Martian
rocks and soils is consistent with a tholeiitic composition
for which ferromagnesian minerals are a major constituent
of the modal mineralogy (McSween et al., 2009). In contrast, the ratio SiO2/SO3 shows significant variation, ranging from 8 to 60. The SiO2 concentrations of the data
values shown in Fig. 1A only vary by 4%, relative
(46.0 ± 1.8 wt%, 2r, n = 41); therefore, the large horizontal
J.A. Hurowitz, W.W. Fischer / Geochimica et Cosmochimica Acta 127 (2014) 25–38
29
Fig. 1. Plots of Al2O3/(FeOT + MgO + CaO + Na2O + K2O) versus SiO2/SO3 (A-C, E) and (FeOT + MgO + CaO + Na2O + K2O)/SO3
versus SiO2/SO3 diagrams (D, F), all data plotted as mole percentages of the oxides. Figures A and B demonstrate the effects of cationconservative weathering (CCW) on relatively unweathered basalt and regolith, respectively. On Figure B, El_D = El Dorado, Fo50 = olivine
with the composition (Fe0.5,Mg0.5)2SiO4. Figures C and E show mixing models between a groundwater-derived component (rich in Fe, Mg,
Ca, and S) and a variety of siliciclastic protoliths. Figures D and F are used to validate the mixing relationships in C and E (see text), and
highlight the potential diagenetic disturbance of some samples of the Burns Formation (samples denoted “Group 3”). The arrow on Fig. 1E
indicates the vector from RAT-ed to undisturbed surfaces of Shoemaker suevite breccias.
30
J.A. Hurowitz, W.W. Fischer / Geochimica et Cosmochimica Acta 127 (2014) 25–38
deviation observed in the soil data from Meridiani Planum
and Gusev crater must have been caused by a process that
added differing amounts of sulfur without significant mobilization and fractionation of cations.
On Fig. 1A we show the results of modeling a simplified
chemical weathering process in which olivine, in this case
Fo50Fa50, is mathematically subtracted from the analysis
Humprey_RAT1 (the rock analysis with the highest SiO2/
SO3). This intermediate olivine composition is consistent
with measured and modeled olivine compositions for
Adirondack Class basalts from Gusev Crater (McSween
et al., 2008), and we emphasize olivine dissolution as it is
the most kinetically labile primary igneous phase present
in the Adirondack class basalts (Stopar et al., 2006; Zolotov
and Mironenko, 2007; Hausrath et al., 2008). In this calculation, for every mole of MgO, FeO, and SiO2 subtracted, a
mole of MgSO4, FeSO4, and SiO2 was added back into the
residual rock chemical composition. Effectively, this algorithm captures the dissolution of olivine by a S-bearing
fluid (e.g., sulfuric acid), followed by the precipitation of
divalent metal-sulfates and silica, and produces a horizontal
trend away from the RAT-ed interior surfaces of Adirondack class basalts that explains the remaining data well.
Olivine removal in the absence of secondary mineral precipitation has also been modeled on Fig. 1A and is, in part, required to explain the observation that the brushed rock
surfaces are offset vertically on Fig. 1A relative to the
RAT-ed rock surfaces. This analysis reveals that the differences in chemical composition between the interior and
exterior of rock surfaces is best explained by the formation
of thin (mm-scale) weathering rinds on rock surfaces under
conditions in which olivine is removed and sulfur is added
without significant cation fractionation relative to the protolith. Altogether, these results support chemical weathering processes hypothesized in earlier works to explain the
alteration of rock surfaces at Gusev crater (Haskin et al.,
2005; Ming et al., 2006; Hurowitz et al., 2006b; Hausrath
et al., 2008; King and McLennan, 2010). In our updated
analysis, however, the importance of the precipitation of
secondary minerals at the site of primary mineral dissolution is highlighted.
We note that the actual choice of secondary product
formed is immaterial to the alteration trajectory realized
under these conditions. For instance, a scenario in which
Fe3+-oxide is substituted for Fe2+-sulfate again results in
a horizontal trajectory, but in this case, half the SO3 is
added for every mole of olivine dissolved. In effect, this
doubles the amount of cation-conservative alteration
needed to cause a unit change in SiO2/SO3 relative to the
model depicted on Fig. 1A. We can also envision a scenario
in which S-bearing acid volatiles are added to the system
forming a suite of amorphous secondary mineraloids from
the dissolution of any one of the primary igneous mineral
phases present in the host rock (e.g., plagioclase, pyroxene,
olivine). Indeed, the horizontal trend in the data could even
be explained simply as sulfur addition, with no chemical
interactions between sulfur and the basaltic substrate to
which it is added, though such a scenario seems geologically
implausible. What can be stated with confidence is that a
horizontal array is produced when sulfur is added to the
system and primary and secondary phases are not physically separated from each other following interaction
between sulfur and basalt. In this way, the Al2O3/(FeOT +
MgO + CaO + Na2O + K2O) ratio remains relatively
unfractionated from that of the protolith, while SiO2/SO3 decreases. Changing the primary and secondary minerals involved simply changes the degree of cation-conservative
weathering needed to move a given distance along the horizontal trajectory. Interestingly, undisturbed soil analyses
from Gusev Crater and Meridiani Planum fall further along
the continuum of cation-conservative weathering, indicating
a commonality of weathering processes in the generation of
alteration rinds on rocks and the regolith on Mars.
Additional processes can be recognized from the soil
chemical data, which are plotted alone with a linear abscissa in Fig. 1B. Here, we re-calculate the cation-conservative
weathering trajectory starting from an analysis of a soil target named “El Dorado”. This target was part of a field of
soil “ripples” encountered on the south-facing slope of Husband Hill in Gusev Crater. The nature of the ripple field
was fully described in Sullivan et al. (2008); the salient point
for our analysis here is that this soil represents an endmember composition in SiO2/SO3 space, and given what is
known about its mineralogical and geochemical properties
(Ming et al., 2008; Morris et al., 2008), appears to be one
of the least chemically altered soils encountered at either
MER landing site. Accordingly, it presents an appropriate
endmember from which to examine the effects of weathering processes recorded by soils. The cation-conservative
weathering trajectory from El Dorado also matches the
remaining soil data well, with some vertical scatter about
this trend. It has been proposed that hydrodynamic sorting,
particularly of olivine, plays an important role in determining the chemical and mineralogical properties of soils at the
MER landing sites (McGlynn et al., 2012). The olivine loss
trajectory on Fig. 1B can effectively be cast as either a physical (hydrodynamic) removal mechanism or a chemical (dissolution) removal mechanism. We also model the effect of
olivine addition on Fig. 1B, which might occur as a result
of physical concentration during transport. While olivine
loss/gain can certainly account for the variation from a horizontal cation-conservative weathering trajectory, it cannot
account for the widespread observation of sulfur addition
to the system. This suggests that hydrodynamic loss and
gain processes played a subordinate role to cation-conservative weathering in determining the chemical composition
of these soils.
Soils exposed in the three wheel trenches that were dug
by the Spirit rover on the traverse across the basaltic plains
of Gusev Crater reveal an interesting but different trend,
specifically due to the accumulation of excess secondary
mineral components. In broad agreement with the conclusions of Wang et al. (2006), the chemical composition of
subsurface soils at these trench investigation sites implies
the addition of a mixture of sulfate salts and silica, here calculated as 1/3 SiO2 and 2/3 (Mg, Fe, Ca)SO4 to an analysis
of soil in a trench wall, which has a composition similar to
that of undisturbed surface soils (target name: RoadCut_Wall). Such salt accumulations provide an important
example of open system hydrological processes on a local
J.A. Hurowitz, W.W. Fischer / Geochimica et Cosmochimica Acta 127 (2014) 25–38
scale. Here these might reflect groundwater infiltration of
the soils and precipitation of secondary mineral phases
(Wang et al., 2006). The surface of the brushed Adirondack
Class basalt “Mazatzal” provides another example of salt
accumulation on rock surfaces, perhaps occurring at a time
when this particular rock was buried beneath a mobile sand
deposit, as suggested by Haskin et al. (2005).
In summary, weathered basalt surfaces from Gusev Crater and undisturbed soils from both MER landing sites bear
the imprint of cation-conservative chemical weathering,
with sulfur (most likely volcanically-sourced SO2 and its
oxidative derivatives) as the primary acid volatile driving
the alteration. While minor changes in primary mineral
abundance (in this case, modeled as olivine fractionation)
can be recognized in concert with this weathering, the geochemistry of these materials is strongly influenced by Saddition and the generation of secondary phases that are
not physically separated from their substrate. This geochemistry is broadly consistent with acid fog hypotheses
but does not distinguish these from what might be predicted
for other cation conservative weathering models, such as
those proposed by McCollom and Hynek (2005), Niles
and Michalski (2009), or Kite et al. (2013). Soils and weathered rock surfaces therefore provide a useful measure of
environments in which cation-conservative weathering
was an important process.
4.2. Open system hydrology and the sedimentary rocks of the
Burns formation
“The Burns Formation is a sequence of sedimentary
rocks” that the Opportunity rover studied extensively between sols 0 and 2500 of its science mission to Meridiani
Planum, encompasses an area of 2 105 km2 and up to
800 m thickness (Hynek and Phillips, 2008), of which
Opportunity has explored 6100 m thickness along a 35 km
traverse. Details of the sedimentology, provenance, diagenesis, geochemistry, and mineralogy of the Burns Formation
were described in a series of publications (Clark et al., 2005;
Grotzinger et al., 2005, 2006; McLennan et al., 2005; Squyres and Knoll, 2005; Tosca et al., 2005; Morris et al., 2006;
Squyres et al., 2006b, 2009; Metz et al., 2009; Hayes et al.,
2011); we provide a brief overview of the salient details required for our analysis here.
The sedimentary rocks of the Burns Formation comprise medium-grained (0.3–0.8 mm) sandstones and diagenetically re-crystallized mudstones (<100 lm) (Edgar
et al., submitted for publication). These sedimentary rocks
are composed of a mixture of approximately equal proportions by mass of chemical (esp., sulfates, Fe-oxides, chlorides) and siliciclastic material (Clark et al., 2005;
McLennan et al., 2005; Glotch et al., 2006; Squyres et al.,
2006b). The siliciclastic material is fine-grained, as no discrete siliciclastic grains have been observed at the resolution
limit (100 lm) of the Microscopic Imager camera system
onboard Opportunity. The facies preserved within the Burns
Formation record aeolian dune, aeolian sand sheet, wet
interdune, and more recently investigated playa (Edgar
et al., submitted for publication) paleoenvironments. In
these environments, variations in water table level and/or
31
sand supply mediated the development of different depositional modes, with dry, sediment-rich conditions resulting
in the formation of aeolian dune bedforms; damp-to-wet,
sediment-poor conditions resulting in sand sheet formation;
and groundwater breaching of the topographic surface
resulting in wet interdune or playa development (Grotzinger et al., 2005). The sedimentary fabrics also record multiple episodes of syn-to-post depositional groundwatermediated diagenesis (McLennan et al., 2005).
Illustrated by the data array in Fig. 1C, the RAT-ed
sandstones of the Burns Formation display a trend that is
nearly orthogonal to the cation-conservative weathering
trajectory displayed by the soil data, and appear more infamily with those subsurface soil analyses characterized
by secondary mineral accumulation (Fig. 1B). In order to
explain the chemical composition of the Burns Formation
as “basalt plus sulfur” from cation-conservative weathering
of a basaltic protolith (McCollom and Hynek, 2005; Niles
and Michalski, 2009), a horizontal trajectory is required.
Instead, the data display a trend that records the addition
of soluble elements (Mg, Fe, Ca, Na, K, and S) to a lithology with an Al2O3/(FeOT + MgO + CaO + Na2O + K2O) P 0.15. It is critical to note that the sedimentary
geochemistry of Burns Formation cannot be explained by
the addition of S alone, but requires the simultaneous addition of cations.
We can further constrain the compositions of the salt
and siliciclastic components by employing a mathematical
approach that enables: (1) an assessment of whether or
not 2-component mixing is indicated by the dataset, and
(2) extrapolation outside the bounds of the data themselves
to identify the nature of the endmembers that control the
observed variation by differential mixing. The approach
and the equations it employs are described in great detail
in Langmuir et al. (1978) for the case of 2-component mixing in igneous rocks, and in Hurowitz et al. (2006a) for the
case of 2-component mixing in sedimentary systems and
environments in which secondary weathering and/or diagenetic processes have played a significant role. The results of
this mixing model are shown on Fig. 1C for two cases. In
the first case the most S-rich analysis (Gagarin, 28.6 wt%
SO3) and S-poor analysis (MacKenzie, 17.0 wt% SO3) were
used to define a hyperbolic mixing array, displayed with a
dashed line. The data are consistent with a 2-component
mixture between an endmember with low Al2O3/(FeOT + MgO + CaO + Na2O + K2O) and low, non-zero SiO2/SO3
ratio, and another endmember with higher Al2O3/
(FeOT + MgO + CaO + Na2O + K2O) and SiO2/SO3. We
suggest, in accord with previous conclusions regarding the
origin of the Burns Formation (Grotzinger et al., 2005;
McLennan et al., 2005; Tosca et al., 2005), that the low
SiO2/SO3 endmember is an aqueous component derived
from groundwater that infiltrated surface materials and
evaporated to form an assemblage of Al-poor sulfate salt
minerals, silica, and other precipitates such as chlorides
and secondary ferric iron minerals. As illustrated by the
mixing array, the second (high SiO2/SO3) endmember could
have a range of compositions; an intriguing possibility is
that this component lies on the cation-conservative weathering trend, and might represent a somewhat more chemi-
32
J.A. Hurowitz, W.W. Fischer / Geochimica et Cosmochimica Acta 127 (2014) 25–38
cally-weathered version of the modern basaltic regolith that
is omnipresent at the MER landing sites. Previous work has
suggested that the Burns Formation is composed of distinct
chemical and siliciclastic endmembers, with the siliciclastic
component represented by basalt that had 50–60% of its
soluble Ca, Mg, and Fe removed by leaching (Squyres
et al., 2006b). Interestingly, these results suggest that far
less leaching is required to explain the data, as the mixing
array crosses the weathering array at 15% cation-conservative weathering. We note again, however, that estimates
of the absolute degree of cation-conservative weathering
implied by our modeling are dependent on both the choice
of starting composition and secondary minerals formed.
This finding of a lower degree of alteration is therefore
non-unique, and should not be taken as necessarily inconsistent with the conclusions reached by Squyres et al.
(2006b) regarding the degree to which the siliciclastic component of the Burns Formation has been altered. These differences highlight the difficult problem of assessing the
precise chemical characteristics of the various components
in a fine-grained sedimentary rock using bulk chemical data
alone.
In order to assess the quality of the results from our mixing analysis, we recast the Burns Formation data on
Fig. 1D, which plots (FeOT + MgO + CaO + Na2O + K2O)/SO3 against SiO2/SO3. In this space, data exhibiting a
true 2-component mixing relationship should display linear
behavior, and each data point should plot on the mixing array (dashed line) in the same relative order as they do on
Fig. 1C (Langmuir et al., 1978). We find that the mixing
relationship between Gagarin and MacKenzie fails the latter test for 2-component mixing, and that MacKenzie (the
low SO3 endmember in the mixing relationship) is displaced
from the remaining samples of the Burns Formation. In total, there are six such samples that display anomalous geochemical behavior similar to MacKenzie. Four of them, the
aforementioned “MacKenzie”, “Inuvik”, and “Diamond
Jenness” (which was analyzed after a first RAT abrasion
and again after a second, deeper abrasion) are analyses
from RAT-ed rocks that lie beneath a recognized stratigraphic boundary in Endurance Crater (informally named
the “Wellington contact”). This contact separates the overlying sand sheet and interdune facies from the dune facies at
the bottom of the stratigraphic section. Two additional
samples, named “Cha” and “Gilbert”, also cluster with
these four samples; notably, Gilbert was the deepest sample
analyzed in the stratigraphic section analyzed at Victoria
Crater. These six samples are all relatively depleted in
SO3 and enriched in SiO2, Al2O3 and K2O. These traits
are not unexpected given that these samples lie closer to
the siliciclastic end of the mixing array on Fig. 1C, but these
samples are also anomalously low in MgO (ranging from
5.1 to 6.5 wt%), which displaces them from the remaining
Burns Formation samples (MgO = 6.8–9.2 wt%) on
Fig. 1D, a trait noted previously by Clark et al. (2005).
The CaO, Na2O, and FeOT concentrations of these six samples are indistinguishable from the rest of the Burns Formation. These six samples all share a unique and
distinguishing textural trait: all appear to have undergone
a recrystallization process that gave rise to the presence of
blocky isopachous cements, overgrowth precipitates on
hematitic concretions, and nodular textures (Clark et al.,
2005; McLennan et al., 2005; Herkenhoff et al., 2008). This
suggests that the loss of MgO in these samples may have
been the result of diagenetic recrystallization processes.
Another attribute that these six samples have in common is elevated Cl concentrations, ranging from 0.90 to
1.90 wt%. Notably, however, there is another group of 9
samples in the dataset that also have similar high Cl concentrations, ranging from 1.45 to 1.70 wt%, but that otherwise exhibit no particularly unusual characteristics relative
to the remaining 21 samples of the Burns Formation, which
have Cl concentrations between 0.46 and 0.91 wt%.
On Fig. 2, we show Microscopic Imager mosaics of the
RAT-ed samples Gagarin (Fig. 2A), Dramensfjord
(Fig. 2B), and MacKenzie (Fig. 2C) in order to illustrate
the texture exhibited by the most SO3-rich endmember
(Gagarin), the texture associated with a high chlorine sample (Dramensfjord), and the previously described blocky
recrystallization texture (MacKenzie). Here it can be seen
that MacKenzie is dramatically different in appearance.
On Fig. 2D, we show that in (FeOT + MgO + CaO + Na2O + K2O)/SO3 versus SiO2/SO3 space, the anomalous geochemical behavior of those samples that exhibit nodular
recrystallization textures is consistent with the loss of
MgSO4.
Based on these geochemical and textural attributes, we
have separated the Burns Formation into “Group 1”,
“Group 2”, and “Group 3” on Fig. 1C and D. Group 1
are “normal” Burns Formation samples, Group 2 are the
9 samples with high chlorine abundance, but which do
not exhibit “disturbed” geochemical characteristics or
blocky, nodular textures, and Group 3 are the remaining
six samples that do exhibit such textural and geochemical
disturbance. For the remainder of the discussion we will
ignore the six Group 3 samples, as they appear not to faithfully record the chemistry of the endmembers in the 2-component mixing relationship that defines the geochemistry of
the rest of the Burns Formation (Groups 1 and 2). Finally,
one other sample that is anomalously high in FeOT, named
Penrhyn, will also be ignored in our analysis.
We can now recalculate the mixing array using as an
alternate endmember the most SO3-poor analysis that is
not offset from the linear array displayed on Fig. 1D
(Cercedilla, 19.1 wt% SO3, Group 1). An MI mosaic from
this sample is also shown on Fig. 2 which shows no evidence of blocky, nodular texture. Returning to Fig. 1C,
the updated mixing array (shown by a solid line) points
to the same “groundwater-derived” endmember, but the
siliciclastic endmember could now plausibly be a material
having a chemical composition similar to that of modern
basaltic regolith. From a parsimony perspective, this is an
enticing possibility: that the Burns Formation simply represents a mixture between lightly weathered basaltic regolith,
similar to that observed in abundance at the modern-day
surface of Mars, and a groundwater-derived evaporative
salt component that acted as a cement to bind the basaltic
grains together. However, in light of the fact that modern
basaltic regolith at the MER sites contains abundant olivine (modeled at 10–15% of the modal mineralogy,
J.A. Hurowitz, W.W. Fischer / Geochimica et Cosmochimica Acta 127 (2014) 25–38
33
(FeOT + MgO + CaO + Na2O + K2O) / SO3
5
4
3
2
val
emo
MgSO4
R
gSO 4
M
1
0
0
1
2
3
4
5
SiO2 / SO3
Fig. 2. Microscopic Imager mosaics of the RAT-ed Burns Formation targets Gagarin (A), Dramensfjord (B) and MacKenzie (C), taken on
sols 403, 162, and 177, respectively. These samples are members of geochemical Group 1, Group 2, Group 3, respectively (see text for details).
MacKenzie exhibits blocky, nodular textures, potentially associated with a diagenetic recrystallization event(s) that the other type examples
did not experience. This recrystallization process also resulted in a change in the geochemical properties of Group 3 samples, most notably a
loss of MgO, consistent with the removal of MgSO4, as shown on Fig. 2D, which plots (FeOT + MgO + CaO + Na2O + K2O)/SO3 versus
SiO2/SO3. Fig. 2E is a microscopic imager mosaic, taken in partial shadow, of the Group 1 sample “Cercedilla” on sol 1182. Each MI mosaic
contains 4 images, resulting in mosaics that are 5 cm 5 cm in size. Image credit: NASA/JPL/Cornell/USGS.
McSween et al., 2010), yet Burns Formation sandstones
contain very little detectable olivine (Morris et al., 2006),
this simple relationship seems somewhat untenable. A calculation based on the average FeOT concentration of
RAT-ed Burns Formation outcrop (15.9 wt% FeOT, Electronic Annex) and the average abundance of olivine in
RAT-ed Burns Formation (2% of all Fe is present as fayalite, Morris et al., 2006), indicates a maximum of 0.32 wt%
of FeOT in the Burns Formation could come from Fe in
olivine. This is not consistent with a mixture in which modern basaltic soil is a 50% component, because the expected
contribution from Fe in olivine would amount to 2.1–
34
J.A. Hurowitz, W.W. Fischer / Geochimica et Cosmochimica Acta 127 (2014) 25–38
3.1 wt% of FeOT (assuming 10–15% olivine having the
composition Fo50Fa50). Therefore, we conclude that a siliciclastic component having the composition of modern
basaltic regolith is an unlikely participant in the mixing
relationships observed.
More recently, Opportunity has begun exploration of
Noachian bedrock of the Shoemaker Formation, which
underlies the Burns Formation and is exposed in outcrops
rimming the 22 km diameter Endeavour Crater. Thus far,
the rocks encountered are reported to have textures consistent with impact-derived lithologies, including shallow suevite breccias and deeper lithic breccias, both of which may
have been affected by hydrothermal alteration processes
(Squyres et al., 2012). Analyses from these two lithologies
are plotted on Fig. 1E and F and reveal that these breccias,
particularly the lithic breccias, are a plausible siliciclastic
endmember for the Burns Formation. If correct, a genetic
detrital link between the Burns Formation and the underlying Shoemaker Formation can be drawn, indicating that
sediment derived from the Shoemaker Formation was cemented and lithified by upwelling evaporating groundwater
to form the siliciclastic component of the Burns Formation.
It should be noted that, to date, only one analysis of a
RAT-ed Shoemaker Formation rock has been published;
the remaining analyses are of undisturbed surfaces. There
is a significant offset between RAT-ed and undisturbed suevite breccia analyses (Fig. 1E), indicating that the undisturbed surface analyses may not properly record the bulk
compositions of the rock interiors. Accordingly, further
exploration of the possible linkages between the siliciclastic
component of the Burns Formation and the underlying
Shoemaker Formation will await the results of Opportunity’s future exploration of this new geologic terrane.
5. CONCLUSIONS
Geochemical data from weathered basaltic rock surfaces
and soils provide important examples of a style of cationconservative chemical alteration that occurs when acidified
waters come into contact with basaltic substrates under
what are very likely water-limited environmental conditions
(e.g., Banin et al., 1997; Tosca et al., 2004; King and
McSween, 2005; Hurowitz and McLennan, 2007; King
and McLennan, 2010). Under these conditions, altering
fluid enters the system and interacts with rock and soil,
but does not migrate away to a significant extent, allowing
for the precipitation of secondary phases at or very nearby
the site of chemical alteration, and preserving the clear geochemical fingerprint of cation-conservative weathering processes. It seems sensible that for those interactions that
result in S-addition, the pH of the fluid was lower than is
typical for terrestrial water–rock interactions, given that
sulfuric and sulfurous acids are stronger acids than carbonic acid. It is highly likely, however, that on a planet with
a CO2 atmosphere and intermittent volcanic activity, water
that was not acidified by S-volatiles has interacted with
rock and soil. In such cases, it is reasonable to suspect that
other secondary mineral phases with compositions that the
APXS is not particularly sensitive to (e.g., carbonates) are
formed. Under these conditions, sulfate minerals may be
recycled through the weathering process, and the long-term
stability of a given secondary mineral assemblage will be
dictated by factors such as: relative mineral solubilities,
the integrated quantity of water and “fresh” acid to that
of soil or rock, and the degree to which redox-based acid
chemistries have been titrated.
In marked contrast, the geochemical data collected from
the ancient sedimentary rocks of the Burns Formation preserve evidence for a hydrological environment that was
capable of fractionating soluble cations from basalt, transporting them in a sulfur-rich solution, and precipitating sulfate salts and silica that acted as a cementing agent for a
still somewhat enigmatic, siliceous sediment that was
undergoing physical transport in a predominantly dry aeolian surface environment. An intriguing possibility raised
by our analysis is that the source of this sediment was recently discovered by the Opportunity rover in the impact
breccias of the Shoemaker Formation that underlies the
Burns. It is important to note that study of terrestrial analogues, experiments, and models of Burns Formation chemistry and mineralogy have already demonstrated that under
certain conditions, acidic groundwaters are an expected
product of basalt–groundwater interaction on Mars (Tosca
et al., 2005, 2008; Baldridge et al., 2009; Marion et al., 2009;
Hurowitz et al., 2010). Accordingly, our results support
hypotheses that the groundwater-derived component in
the Burns Formation obtained its chemical composition
as a result of interactions between S-charged groundwater
and the basaltic crust of Mars under open system hydrologic conditions, fractionating elements along a path length
from the catchment to the basin. These results do not support those models in which the sedimentary rocks were
formed under water-limited, cation-conservative conditions. We conclude, therefore, that the Burn Formation records evidence of a Martian climate that was capable of
supporting a hydrologic system characterized by ground
(or surface) water recharge and transport, placing a valuable temporal constraint on the activity and availability
of water on the late Noachian to early Hesperian surface
of Mars.
ACKNOWLEDGEMENTS
This research was carried out in part at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with
the National Aeronautics and Space Agency (J.A.H.). This work
was supported by NASA award NNX10AM84G to J.A.H. The
authors thank associate editor Penny King, Mikhail Zolotov, and
two anonymous reviewers for their helpful comments, which substantially improved this manuscript. The authors thank Ken Herkenhoff (USGS) for producing the Microscopic Imager mosaics
on Fig. 2, and Edwin Kite for feedback on an early draft of the
manuscript.
APPENDIX A. SUPPLEMENTARY DATA
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.gca.2013.11.021.
J.A. Hurowitz, W.W. Fischer / Geochimica et Cosmochimica Acta 127 (2014) 25–38
REFERENCES
Amundson R., Ewing S., Dietrich W., Sutter B., Owen J.,
Chadwick O., Nishiizumi K., Walvoord M. and McKay C.
(2008) On the in situ aqueous alteration of soils on Mars.
Geochim. Cosmochim. Acta 72, 3845–3864.
Andrews-Hanna J., Zuber M., Arvidson R. and Wiseman S. (2010)
Early Mars hydrology: Meridiani playa deposits and the
sedimentary record of Arabia Terra. J. Geophys. Res. 115,
E06002, doi:06010.01029/02009JE003485.
Andrews-Hanna J. C., Phillips R. J. and Zuber M. T. (2007)
Meridiani Planum and the global hydrology of Mars. Nature
446, 163–166.
Baldridge A. M., Hook S. J., Crowley J. K., Marion G. M., Kargel
J. S., Michalski J. L., Thomson B. J., de Souza C. R., Bridges
N. T. and Brown A. J. (2009) Contemporaneous deposition of
phyllosilicates and sulfates: using Australian acidic saline lake
deposits to describe geochemical variability on Mars. Geophys.
Res. Lett. 36, 6.
Banin A., Kan H. I. and Cicelsky A. (1997) Acidic volatiles and the
Mars soil. J. Geophys. Res. 102, 13341–13356.
Bibring J.-P., Langevin Y., Gendrin A., Gondet B., Poulet F.,
Berthe M., Soufflot A., Arvidson R. E., Mangold N., Mustard
J. F. and Drossart P.OMEGA-Team (2005) Mars surface
diversity as revealed by the OMEGA/Mars express observations. Science 307, 1576–1581.
Blake D. F., Morris R. V., Kocurek G., Morrison S. M., Downs R.
T., Bish D., Ming D. W., Edgett K. S., Rubin D., Goetz W.,
Madsen M. B., Sullivan R., Gellert R., Campbell I., Treiman A.
H., McLennan S. M., Yen A. S., Grotzinger J., Vaniman D. T.,
Chipera S. J., Achilles C. N., Rampe E. B., Sumner D., Meslin
P.-Y., Maurice S., Forni O., Gasnault O., Fisk M., Schmidt M.,
Mahaffy P., Leshin L. A., Glavin D., Steele A., Freissinet C.,
Navarro-González R., Yingst R. A., Kah L. C., Bridges N.,
Lewis K. W., Bristow T. F., Farmer J. D., Crisp J. A., Stolper
E. M., Des Marais D. J., Sarrazin P. and Team M. S. (2013)
Curiosity at Gale Crater, Mars: characterization and Analysis
of the Rocknest Sand Shadow. Science 341.
Carr M. H. (2012) The fluvial history of Mars. Philos. Trans. R.
Soc. A: Math. Phys. Eng. Sci. 370, 2193–2215.
Cassata W. S., Shuster D. L., Renne P. R. and Weiss B. P. (2012)
Trapped Ar isotopes in meteorite ALH 84001 indicate Mars did
not have a thick ancient atmosphere. Icarus 221, 461–465.
Clark B. C., Baird A. K., Rose H. J., Toulmin P., Keil K., Castro
A. J., Kelliher W. C., Rowe C. D. and Evans P. H. (1976)
Inorganic analyses of Martian surface samples at the Viking
landing sites. Science 194, 1283–1288.
Clark B. C., Morris R. V., McLennan S. M., Gellert R., Joliff B. L.,
Knoll A. H., Squyres S. W., Lowenstein T. K., Ming D. W.,
Tosca N. J., Yen A., Christensen P. R., Gorevan S., Bruckner
J., Calvin W., Dreibus G., Farrand W. H., Klingelhoefer G.,
Wanke H., Zipfel J., Bell, III, J. F., Grotzinger J., McSween,
, H. Y. and Rieder R. (2005) Chemistry and mineralogy of
outcrops at Meridiani Planum. Earth Planet. Sci. Lett. 240, 73–
94.
Clark B. C. and Van Hart D. C. (1981) The salts of Mars. Icarus
45, 370–378.
Edgar L., Grotzinger J., Bell III J. and Hurowitz J. (submitted for
publication) Hypotheses for the origin of fine-grained sedimentary rocks at Santa Maria Crater, Meridiani Planum. Icarus.
Ehlmann B. L., Mustard J. F., Murchie S. L., Bibring J. P.,
Meunier A., Fraeman A. A. and Langevin Y. (2011) Subsurface
water and clay mineral formation during the early history of
Mars. Nature 479, 53–60.
35
Forget F. and Pierrehumbert R. T. (1997) Warming early Mars
with carbon dioxide clouds that scatter infrared radiation.
Science 278, 1273–1276.
Gaidos E. and Marion G. (2003) Geological and geochemical
legacy of a cold early Mars. J. Geophys. Res. 108, 19.
Gellert R., Rieder R., Brückner J., Clark B. C., Dreibus G.,
Klingelhoefer G., Lugmair G., Ming D. W., Wanke H., Yen A.,
Zipfel J. and Squyres S. W. (2006) The Alpha Particle X-ray
Spectrometer (APXS): results from Gusev Crater and calibration report. J. Geophys. Res. 111. http://dx.doi.org/10.1029/
2005JE002583.
Glotch T. D., Bandfield J. L., Christensen P. R., Calvin W. M.,
McLennan S. M., Clark B. C., Rogers A. D. and Squyres S. W.
(2006) Mineralogy of the light-toned outcrop at Meridiani
Planum as seen by the Miniature Thermal Emission Spectrometer and implications for its formation. J. Geophys. Res. 111,
E12S03, doi:10.1029/2005JE002672.
Gorevan S. P., Myrick T., Davis K., Chau J. J., Bartlett P.,
Mukherjee S., Anderson R., Squyres S. W., Arvidson R. E.,
Madsen M. B., Bertelsen P., Goetz W., Binau C. S. and Richter
L. (2003) Rock Abrasion Tool: Mars Exploration Rover
mission. J. Geophys. Res. 108, 8.
Greenwood J. P., Itoh S., Sakamoto N., Vicenzi E. P. and
Yurimoto H. (2008) Hydrogen isotope evidence for loss of
water from Mars through time. Geophys. Res. Lett. 35, 5.
Grotzinger J., Bell J., Herkenhoff K., Johnson J., Knoll A.,
McCartney E., McLennan S., Metz J., Moore J., Squyres S.,
Sullivan R., Ahronson O., ArvIdson R., Joliff B., Golombek
M., Lewis K., Parker T. and Soclerblom J. (2006) Sedimentary
textures formed by aqueous processes, Erebus crater, Meridiani
Planum, Mars. Geology 34, 1085–1088.
Grotzinger J. and Milliken R. (2012) In Sedimentary Geology of
Mars (series eds. G.J. Nichols and B. Ricketts). SEPM Special
Publication # 102. SEPM Society for Sedimentary Geology,
Tulsa. p. 270.
Grotzinger J. P., Arvidson R. E., Bell, III, J. F., Calvin W., Clark
B. C., Fike D. A., Golombek M., Greeley R., Haldemann A. F.
C., Herkenhoff K. E., Joliff B. L., Knoll A. H., Malin M. C.,
McLennan S. M., Parker T., Soderblom L. A., Sohl-Dickstein
J. N., Squyres S. W., Tosca N. J. and Watters W. A. (2005)
Stratigraphy and sedimentology of a dry to wet eolian
depositional system, Burns formation, Meridiani Planum,
Mars. Earth Planet. Sci. Lett. 240, 11–72.
Halevy I., Zuber M. T. and Schrag D. P. (2007) A sulfur dioxide
climate feedback on early Mars. Science 318, 1903–1907.
Haskin L. A., Wang A., McSween H. Y., McLennan S. M., Cabrol
N. A., Crumpler L. S., Jolliff B. L., Yen A. S., Clark B. C.,
DesMarais D. J., Squyres S. W., Arvidson R. E., Ming D. W.,
Morris R. V., Tosca N. J., Hurowitz J. A., Gellert R.,
Klingelhöfer G., Bell, III, J. F., Herkenhoff K. E., Christensen
P. R., Ruff S. W., Blaney D. L., Farmer J. D., Grant J. A.,
Greeley R., Grin E. A., Landis G. A., Rice J. W., Richter L.,
Schröder C., Soderblom L. A. and de Souza, Jr., P. A. (2005)
Water alteration of rocks and soils from the Spirit rover site,
Gusev Crater, Mars. Nature 436. http://dx.doi.org/10.1038/
nature03640.
Hausrath E. M., Navarre-Sitchler A. K., Sak P. B., Steefel C. I. and
Brantley S. L. (2008) Basalt weathering rates on Earth and the
duration of liquid water on the plains of Gusev Crater, Mars.
Geology 36, 67–70.
Hayes A. G., Grotzinger J. P., Edgar L. A., Squyres S. W., Watters
W. A. and Sohl-Dickstein J. (2011) Reconstruction of eolian bed
forms and paleocurrents from cross-bedded strata at Victoria
Crater, Meridiani Planum, Mars. J. Geophys. Res. 116, 17.
36
J.A. Hurowitz, W.W. Fischer / Geochimica et Cosmochimica Acta 127 (2014) 25–38
Herkenhoff K. E., Grotzinger J., Knoll A. H., McLennan S. M.,
Weitz C., Yingst A., Anderson R., Archinal B. A., Arvidson R.
E., Barrett J. M., Becker K. J., Bell J. F., Budney C., Chapman
M. G., Cook D., Ehlmann B., Franklin B., Gaddis L. R.,
Galuszka D. M., Garcia P. A., Geissler P., Hare T. M.,
Howington-Kraus E., Johnson J. R., Keszthelyi L., Kirk R. L.,
Lanagan P., Lee E. M., Leff C., Maki J. N., Mullins K. F.,
Parker T. J., Redding B. L., Rosiek M. R., Sims M. H.,
Soderblom L. A., Spanovich N., Springer R., Squyres S. W.,
Stolper D., Sucharski R. M., Sucharski T., Sullivan R. and
Torson J. M. (2008) Surface processes recorded by rocks and
soils on Meridiani Planum, Mars: Microscopic Imager observations during Opportunity’s first three extended missions. J.
Geophys. Res. 113, 39.
Hurowitz J. A., Fischer W., Tosca N. J. and Milliken R. E. (2010)
Origin of acidic surface waters and the evolution of atmospheric
chemistry on early Mars. Nat. Geosci. 3, 323–326.
Hurowitz J. A. and McLennan S. M. (2007) A 3.5 Ga record of
water-limited acidic weathering conditions on Mars. Earth
Planet. Sci. Lett. 260, 432–443.
Hurowitz J. A., McLennan S. M., McSween H. Y., DeSouza P. A.
and Klingelhofer G. (2006a) Mixing relationships and the
effects of secondary alteration in the Wishstone and Watchtower Classes of Husband Hill, Gusev Crater, Mars. J.
Geophys. Res. 111, 18.
Hurowitz J. A., McLennan S. M., Tosca N. J., Arvidson R. E.,
Michalski J. R., Ming D. W., Schröder C. and Squyres S. W.
(2006b) In-situ and experimental evidence for acidic weathering
of rocks and soils on Mars. J. Geophys. Res. 111. http://
dx.doi.org/10.1029/2005JE002515.
Hynek B. M. and Phillips R. J. (2008) The stratigraphy of
Meridiani Planum, Mars, and implications for the layered
deposits’ origin. Earth Planet. Sci. Lett. 274, 214–220.
Johnson S. S., Mischna M. A., Grove T. L. and Zuber M. T. (2008)
Sulfur-induced greenhouse warming on early Mars. J. Geophys.
Res. 113, 15.
Johnson S. S., Pavlov A. A. and Mischna M. A. (2009) Fate of SO2
in the ancient Martian atmosphere: implications for transient
greenhouse warming. J. Geophys. Res.: Planets 114, E11011.
King P. L. and McLennan S. M. (2010) Sulfur on Mars. Elements
6, 107–112.
King P. L. and McSween, Jr., H. Y. (2005) Effects of H2O, pH, and
oxidation state on the stability of Fe-minerals on Mars. J.
Geophys. Res. 110, E12S10, doi:10.1029/2005JE002485, 002005.
Kite E., Halevy I., Kahre M., Wolff M. and Manga M. (2013)
Seasonal melting and the formation of sedimentary rocks on
Mars, with predictions for the Gale Crater mound. Icarus 223,
181–210.
Knauth L. P., Burt D. M. and Wohletz K. H. (2005) Impact origin
of sediments at the opportunity landing site on Mars. Nature
438, 1123–1128.
Kounaves S., Catling D., Clark B., DeFlores L., Gospodinova K.,
Hecht M., Kapit J., Ming D. and Quinn R. (2009) Aqueous
chemistry of the Martian soil at the Phoenix landing site. 40th
Lunar and Planetary Science Conference. Houston, TX.
Kring D. A. (2007) The Chicxulub impact event and its environmental consequences at the Cretaceous–Tertiary boundary.
Paleogeogr. Paleoclimatol. Paleoecol. 255, 4–21.
Langmuir C. H., Vocke R. D. J., Hanson G. N. and Hart S. R.
(1978) A general mixing equation with applications to Icelandic
basalts. Earth Planet. Sci. Lett. 37, 380–392.
Marion G. M., Crowley J. K., Thomson B. J., Kargel J. S., Bridges
N. T., Hook S. J., Baldridge A., Brown A. J., da Luz B. R. and
de Souza C. R. (2009) Modeling aluminum–silicon chemistries
and application to Australian acidic playa lakes as analogues
for Mars. Geochim. Cosmochim. Acta 73, 3493–3511.
McCollom T. M. and Hynek B. M. (2005) A volcanic environment
for bedrock diagenesis at Meridiani Planum on Mars. Nature
438, 1129–1131.
McGlynn I. O., Fedo C. M. and McSween H. Y. (2012) Soil
mineralogy at the Mars Exploration Rover landing sites: an
assessment of the competing roles of physical sorting and
chemical weathering. J. Geophys. Res. 117, 16.
McLennan S. M. (2003) Sedimentary silica on Mars. Geology 31,
315–318.
McLennan S. M., Bell, III, J. F., Calvin W. M., Christensen P. R.,
Clark B. C., De Souza, Jr., P. A., Farmer J., Farrand W. H.,
Fike D. A., Gellert R., Ghosh A., Glotch T. D., Grotzinger J.
P., Hahn B. C., Herkenhoff K. E., Hurowitz J. A., Johnson J.
R., Johnson S. S., Joliff B. L., Klingelhoefer G., Knoll A. H.,
Learner Z. A., Malin M. C., McSween, Jr., H. Y., Pocock J.,
Ruff S. W., Soderblom L. A., Squyres S. W., Tosca N. J.,
Watters W. A., Wyatt M. B. and Yen A. (2005) Provenance and
diagenesis of the evaporite-bearing Burns formation, Meridiani
Planum, Mars. Earth Planet. Sci. Lett. 240, 95–121.
McSween H. Y., McGlynn I. O. and Rogers A. D. (2010)
Determining the modal mineralogy of Martian soils. J.
Geophys. Res. 115, 10.
McSween H. Y., Ruff S. W., Morris R. V., Gellert R., Klingelhofer
G., Christensen P. R., McCoy T. J., Ghosh A., Moersch J. M.,
Cohen B. A., Rogers A. D., Schroder C., Squyres S. W., Crisp
J. and Yen A. (2008) Mineralogy of volcanic rocks in Gusev
Crater, Mars: Reconciling Mossbauer, Alpha Particle X-ray
Spectrometer, and Miniature Thermal Emission Spectrometer
spectra. J. Geophys. Res. 113, 14.
McSween H. Y., Taylor G. J. and Wyatt M. B. (2009) Elemental
Composition of the Martian Crust. Science 324, 736–739.
McSween H. Y., Wyatt M. B., Gellert R., Bell J. F., Morris R. V.,
Herkenhoff K. E., Crumpler L. S., Milam K. A., Stockstill K.
R., Tornabene L. L., Arvidson R. E., Bartlett P., Blaney D.,
Cabrol N. A., Christensen P. R., Clark B. C., Crisp J. A., Des
Marais D. J., Economou T., Farmer J. D., Farrand W., Ghosh
A., Golombek M., Gorevan S., Greeley R., Hamilton V. E.,
Johnson J. R., Joliff B. L., Klingelhofer G., Knudson A. T.,
McLennan S., Ming D., Moersch J. E., Rieder R., Ruff S. W.,
Schroder C., de Souza P. A., Squyres S. W., Wanke H., Wang
A., Yen A. and Zipfel J. (2006) Characterization and petrologic
interpretation of olivine-rich basalts at Gusev Crater, Mars. J.
Geophys. Res. 111, 17.
Metz J. M., Grotzinger J. P., Rubin D. M., Lewis K. W., Squyres
S. W. and Bell J. F. (2009) Sulfate-rich eolian and wet interdune
deposits, Erebus Crater, Meridiani Planum, Mars. J. Sediment.
Res. 79, 247–264.
Michalski J. R. and Bleacher J. E. (2013) Supervolcanoes within an
ancient volcanic province in Arabia Terra, Mars. Nature 502,
47–52.
Michalski J. R., Cuadros J., Niles P. B., Parnell J., Rogers A.
D. and Wright S. P. (2013) Groundwater activity on Mars
and implications for a deep biosphere. Nat. Geosci. 6, 133–
138.
Ming D. W., Gellert R., Morris R. V., Arvidson R. E., Brukner J.,
Clark B. C., Cohen B. A., d’Uston C., Economou T., Fleischer
I., Klingelhofer G., McCoy T. J., Mittlefehldt D. W., Schmidt
M. E., Schroder C., Squyres S. W., Treguier E., Yen A. S. and
Zipfel J. (2008) Geochemical properties of rocks and soils in
Gusev Crater, Mars: Results of the Alpha Particle X-Ray
Spectrometer from Cumberland Ridge to Home Plate. J.
Geophys. Res. 113, 28.
J.A. Hurowitz, W.W. Fischer / Geochimica et Cosmochimica Acta 127 (2014) 25–38
Ming D. W., Mittlefehldt D. W., Morris R. V., Golden D. C.,
Gellert R., Yen A., Clark B. C., Squyres S. W., Farrand W.
H., Ruff S. W., Arvidson R. E., Klingelhoefer G., McSween
H. Y., Rodionov D. S., Schroder C., de Souza, Jr., P. A. and
Wang A. (2006) Geochemical and mineralogical indicators for
aqueous processes in the Columbia Hills of Gusev Crater,
Mars. J. Geophys. Res. 111. http://dx.doi.org/10.1029/
2005JE002560.
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) Mineralogy,
composition and alteration of Mars Pathfinder rocks and soils:
evidence from multispectral, elemental and magnetic data on
terrestrial analogue, SNC meteorite, and Pathfinder samples. J.
Geophys. Res. 105, 1757–1817.
Morris R. V., Klingelhofer G., Schroder C., Fleischer I., Ming D.
W., Yen A. S., Gellert R., Arvidson R. E., Rodionov D. S.,
Crumpler L. S., Clark B. C., Cohen B. A., McCoy T. J.,
Mittlefehldt D. W., Schmidt M. E., de Souza P. A. and Squyres
S. W. (2008) Iron mineralogy and aqueous alteration from
Husband Hill through Home Plate at Gusev Crater, Mars:
Results from the Mossbauer instrument on the Spirit Mars
Exploration Rover. J. Geophys. Res. 113, 43.
Morris R. V., Klingelhofer G., Schroder C., Rodionov D. S., Yen
A., Ming D. W., de Souza P. A., Wdowiak T., Fleischer I.,
Gellert R., Bernhardt B., Bonnes U., Cohen B. A., Evlanov E.
N., Foh J., Gutlich P., Kankeleit E., McCoy T., Mittlefehldt D.
W., Renz F., Schmidt M. E., Zubkov B., Squyres S. W. and
Arvidson R. E. (2006) Mossbauer mineralogy of rock, soil, and
dust at Meridiani Planum, Mars: opportunity’s journey across
sulfate-rich outcrop, basaltic sand and dust, and hematite lag
deposits. J. Geophys. Res. 111, E12S15, doi: 10.1029/
2006JE002791.
Murchie S. L., Mustard J. F., Ehlmann B. L., Milliken R. E.,
Bishop J. L., McKeown N. K., Dobrea E. Z. N., Seelos F. P.,
Buczkowski D. L., Wiseman S. M., Arvidson R. E., Wray J. J.,
Swayze G., Clark R. N., Marais D. J. D., McEwen A. S. and
Bibring J. P. (2009) A synthesis of Martian aqueous mineralogy
after 1 Mars year of observations from the Mars Reconnaissance Orbiter. J. Geophys. Res. 114, 30.
Mustard J. F., Murchie S. L., Pelkey S. M., Ehlmann B. L.,
Milliken R. E., Grant J. A., Bibring J. P., Poulet F., Bishop J.,
Dobrea E. N., Roach L., Seelos F., Arvidson R. E., Wiseman
S., Green R., Hash C., Humm D., Malaret E., McGovern J. A.,
Seelos K., Clancy T., Clark R., Des Marais D., Izenberg N.,
Knudson A., Langevin Y., Martin T., McGuire P., Morris R.,
Robinson M., Roush T., Smith M., Swayze G., Taylor H.,
Titus T. and Wolff M. (2008) Hydrated silicate minerals on
Mars observed by the Mars reconnaissance orbiter CRISM
instrument. Nature 454, 305–309.
Nesbitt H. W. and Wilson R. E. (1992) Recent chemical weathering
of basalts. Am. J. Sci. 292, 740–777.
Niles P. B., Catling D. C., Berger G., Chassefiere E., Ehlmann B.
L., Michalski J. R., Morris R., Ruff S. W. and Sutter B. (2013)
Geochemistry of Carbonates on Mars: implications for Climate
History and Nature of Aqueous Environments. Space Sci. Rev.
174, 301–328.
Niles P. B. and Michalski J. (2009) Meridiani Planum sediments on
Mars formed through weathering in massive ice deposits. Nat.
Geosci. 2, 215–220.
Rieder R., Gellert R., Anderson R. C., Bruckner J., Clark B. C.,
Dreibus G., Economou T., Klingelhofer G., Lugmair G. W.,
Ming D. W., Squyres S. W., d’Uston C., Wanke H., Yen A. and
Zipfel J. (2004) Chemistry of Rocks and Soils at Meridiani
Planum from the Alpha Particle X-ray Spectrometer. Science
306, 1746–1749.
37
Sagan C. and Chyba C. (1997) The early faint sun paradox: organic
shielding of ultraviolet-labile greenhouse gases. Science 276,
1217–1221.
Segura T. L., Toon O. B., Colaprete A. and Zahnle K. (2002)
Environmental effects of large impacts on Mars. Science 298,
1977–1980.
Settle M. (1979) Formation and deposition of volcanic sulfate
aerosols on Mars. J. Geophys. Res. 84, 8343–8354.
Squyres S., Arvidson R., Bell J., Calef, III, F., Clark B., Cohen B.,
Crumpler L., deSouza, Jr., P., Farrand W., Gellert R., Grant J.,
Herkenhoff K., Hurowitz J., Johnson J., Joliff B., Knoll A., Li
R., McLennan S., Ming D., Mittlefehldt D., Parker T., Paulsen
G., Rice M., Ruff S., Schroder C., Yen A. and Zacny K. (2012)
Ancient impact and aqueous processes at Endeavour crater,
Mars. Science 336. http://dx.doi.org/10.1126/science.1220476.
Squyres S. W., Arvidson R. E., Blaney D. L., Clark B. C.,
Crumpler L., Farrand W. H., Gorevan S., Herkenhoff K. E.,
Hurowitz J. A., McSween, Jr., H. Y., Ming D. W., Morris R.
V., Ruff S. W., Wang A. and Yen A. (2006a) Rocks of the
Columbia Hills. J. Geophys. Res. 111. http://dx.doi.org/
10.1029/2005JE002562.
Squyres S. W. and Knoll A. H. (2005) Sedimentary rocks at
Meridiani Planum: Origin, diagenisis, and implications for life
on Mars. Earth Planet. Sci. Lett. 240, 1–10.
Squyres S. W., Knoll A. H., Arvidson R. E., Ashley J. W., Bell, III,
J. F., Calvin W. M., Christensen P. R., Clark B. C., Cohen B.
A., de Souza, Jr., P. A., Edgar L., Farrand W. H., Fleischer I.,
Gellert R., Golombek M. P., Grant J., Grotzinger J., Hayes A.,
Herkenhoff K. E., Johnson J. R., Jolliff B., Klingelhofer G.,
Knudson A., Li R., McCoy T. J., McLennan S. M., Ming D.
W., Mittlefehldt D. W., Morris R. V., Rice, Jr., J. W., Schroder
C., Sullivan R. J., Yen A. and Yingst R. A. (2009) Exploration
of Victoria Crater by the Mars Rover Opportunity. Science 324,
1058–1061.
Squyres S. W., Knoll A. H., Arvidson R. E., Clark B. C.,
Grotzinger J. P., Joliff B. L., McLennan S. M., Tosca N. J.,
Bell, III, J. F., Calvin W. M., Farrand W. H., Glotch T. D.,
Golombek M. P., Herkenhoff K. E., Johnson J. R., Klingelhoefer G., McSween, Jr., H. Y. and Yen A. S. (2006b) Two
Years at Meridiani Planum: Results from the Opportunity
Rover. Science 313, 1403–1407.
Stopar J. D., Taylor G. J., Hamilton V. E. and Browning L. (2006)
Kinetic model of olivine dissolution and extent of aqueous
alteration on Mars. Geochim. Cosmochim. Acta 70, 6136–6152.
Sullivan R., Arvidson R., Bell J. F., Gellert R., Golombek M.,
Greeley R., Herkenhoff K., Johnson J., Thompson S., Whelley
P. and Wray J. (2008) Wind-driven particle mobility on mars:
insights from Mars Exploration Rover observations at “El
Dorado” and surroundings at Gusev Crater. J. Geophys. Res.
113, 70.
Taylor S. R. and McLennan S. M. (1985) The Continental Crust: its
Composition and Evolution. Blackwell Scientific Publications,
Oxford.
Tosca N., McLennan S., Dyar M., Sklute E. and Michel F. (2008)
Fe oxidation processes at Meridiani Planum and implications
for secondary Fe mineralogy on Mars. J. Geophys. Res. 113.
http://dx.doi.org/10.1029/2007JE003019, 002008.
Tosca N. J., McLennan S. M., Clark B. C., Grotzinger J. P.,
Hurowitz J. A., Knoll A. H., Schroder C. and Squyres S. W.
(2005) Geochemical Modeling of Evaporation Processes on
Mars: insight from the Sedimentary Record at Meridiani
Planum. Earth Planet. Sci. Lett. 240, 122–148.
Tosca N. J., McLennan S. M., Lindsley D. H. and Schoonen M. A.
A. (2004) Acid-sulfate weathering of synthetic Martian basalt:
the acid-fog model revisited. J. Geophys. Res. 109, E05003, doi:
05010.01029/02003JE002218.
38
J.A. Hurowitz, W.W. Fischer / Geochimica et Cosmochimica Acta 127 (2014) 25–38
Wang A., Haskin L. A., Squyres S. W., Joliff B. L., Crumpler L.,
Gellert R., Schroder C., Herkenhoff K. E., Hurowitz J. A.,
Tosca N. J., Farrand W. H., Anderson R. C. and Knudson A.
T. (2006) Sulfate deposition in subsurface regolith in Gusev
crater, Mars. J. Geophys. Res. 111. http://dx.doi.org/10.1029/
2005JE002513.
Zolotov M. Y. and Mironenko M. V. (2007) Timing of acid
weathering on Mars: a kinetic–thermodynamic assessment. J.
Geophys. Res. 112, 20.
Associate editor: Penelope L. King