Geologic evidence suggests that the surface of Mars has been dominated by cold, dry, and relatively stable environmental conditions over the past ~3.5 Ga. These conditions differ from those pre-sumed to be present prior to ~3.5 Ga, when observa-tions indicate that the martian surface was at least in-termittently able to support the prolonged flow of liq-uid water. Despite the more than 75% of martian his-tory dominated by cold, dry, and stable conditions, few investigations have studied weathering and alteration processes that may influence the martian surface dur-ing this time. Please see attachment

Clegg, S.

Dehouck, E.

Lanza, N.

Mangold, N.

Rampe, E.

Roszell, K.

Salvatore, M.

Truitt, K.

Wiens, R.

NASA Technical Reports Server

CONSTRAINTS ON THE MODE AND EXTENT OF SEDIMENTARY ROCK ALTERATION IN HYPER-
ARID AND HYPO-THERMAL ENVIRONMENTS.  M. Salvatore
1
, K. Truitt
2
, K. Roszell
2
, N. Lanza
3
, E. 
Rampe
4
, N. Mangold
5
, E. Dehouck
6
, R. Wiens
3
, and S. Clegg
3
. 
1
Northern Arizona University, 
mark.salvatore@nau.edu, 
2
University of Michigan-Dearborn, 
3
Los Alamos National Laboratory, 
4
NASA Johnson 
Space Center, 
5
LPG-Nantes, Université de Nantes, France, 
6
LGL-TPE, Université de Lyon, France. 
 
Introduction:  Geologic evidence suggests that the 
surface of Mars has been dominated by cold, dry, and 
relatively stable environmental conditions over the past 
~3.5 Ga [1].  These conditions differ from those pre-
sumed to be present prior to ~3.5 Ga, when observa-
tions indicate that the martian surface was at least in-
termittently able to support the prolonged flow of liq-
uid water.  Despite the more than 75% of martian his-
tory dominated by cold, dry, and stable conditions, few 
investigations have studied weathering and alteration 
processes that may influence the martian surface dur-
ing this time. 
[2] investigated the role of surface alteration on 
dolerites in the McMurdo Dry Valleys (MDV) of Ant-
arctica (the most Mars-relevant environment on Earth), 
demonstrating that immature oxidative weathering 
products can be uniquely preserved on erosion-
resistant lithologies under these cold, dry, and stable 
environmental conditions.  However, this study did not 
consider the role of similar weathering processes on 
sedimentary lithologies, which vary significantly in 
their parental composition and physical properties 
(e.g., porosity, induration).  While [3] analyzed poten-
tial oxidative weathering trends in a single sandstone 
from the Transantarctic Mountains, variations in altera-
tion products or processes in different sedimentary 
lithologies were not considered in detail. 
Might oxidative weathering be hindering our abil-
ity to determine the true chemistry and mineralogy of 
martian sedimentary surfaces?  Is it possible that 
measurements of the geochemistry of brushed rock 
surfaces by the Mars Science Laboratory (MSL) Curi-
osity rover in Gale crater do not represent the true 
composition of the inherent rocks, but instead the 
products of hypo-thermal and hyper-arid oxidative 
weathering?  To test whether such processes might be 
influencing sedimentary rock surfaces, we conducted a 
suite of chemical and mineralogical analyses on sedi-
mentary rocks from throughout the Transantarctic 
Mountains [4].  Should oxidative weathering signa-
tures significantly mask the underlying composition of 
these sedimentary rocks, these processes must be taken 
into consideration when deciphering the formation 
environments of sedimentary rocks on Mars.  If these 
processes do not dominate sedimentary rock surfaces 
in Antarctica, however, then it is unlikely that our in-
terpretation of martian sedimentary surface composi-
tions are to be significantly influenced by such pro-
cesses potentially occurring on Mars. 
Methods:  We collected a suite of Antarctic sedi-
mentary rock samples from the Polar Rock Repository 
at the Ohio State University.  In total, 178 samples 
were collected, which include 32 mudstones, 30 sand-
stones, 33 tuffs, and 26 conglomerates. 
Visible/near-infrared (VNIR, 0.35 - 2.50 μm) and 
thermal infrared (TIR, ~8 - 50 μm) spectra were ac-
quired from surface and interior rock fragments.  
VNIR data are relevant to both martian orbital (e.g., 
OMEGA, CRISM) and surface (e.g., Mastcam) obser-
vations, as are the acquired TIR data (e.g., THEMIS, 
TES, Mini-TES).  Rock fragments were also analyzed 
using laser-induced breakdown spectroscopy (LIBS) at 
Los Alamos National Laboratory on the ChemCam 
engineering model to derive sample chemistry.  Major 
element chemistry was also derived from powdered 
samples using inductively coupled plasma-optical 
emission spectroscopy (ICP-OES) on samples prepared 
using the flux fusion methodology.  X-ray diffraction 
(XRD) was also used on both surface and interior 
powders to determine bulk mineralogy. 
All performed analyses are directly relevant to mar-
tian orbital and surface measurements, as the purpose 
of this investigation is to identify significant alteration 
processes and products associated with cold, dry, and 
stable environmental conditions. 
Results:  We assess the chemical alteration of rock 
surfaces relative to their interiors using the Weathering 
Intensity Scale (WIS), which compares the behavior of 
monovalent cations, divalent cations, and silica [5].  
This method includes the ability to identify trends in 
Si, which is absent from most other weathering indices 
(e.g., Chemical Index of Alteration, CIA).  The WIS 
compares M
+
 (Na
+
 + K
+
 + 2Ca
2+
), R
2+
 (Fe
2+
 + Mg
2+
), 
and 4Si (Si/4) in a ternary diagram, where trends 
away/towards different apices indicate different modes 
of alteration.  
The WIS requires knowledge of the abundance of 
Fe
2+
/FeTotal, which has not yet been derived for the sed-
imentary samples measured in this study.  As a result, 
we refer to the work of [2], who found that igneous 
rocks that underwent oxidative weathering processes in 
the MDV have 14% more Fe
3+
 at their surfaces relative 
to their unaltered interiors.  We assumed the same rela-
tive enrichment in Fe
3+
 at the surfaces of the sedimen-
tary samples and assumed that, on average, the sedi-
mentary rocks contain ~75% Fe
3+
.  These assumptions 
allow us to plot the WIS on a ternary diagram, where 
the sedimentary rocks in this study are plotted against 
the igneous rocks from [2] (Fig. 1).  Mössbauer spec-
1265.pdf49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083)
https://ntrs.nasa.gov/search.jsp?R=20180006501 2019-08-31T18:48:14+00:00Z
troscopy is underway to better quantify the Fe
2+
/FeTotal 
in these sedimentary samples. 
Building upon our earlier study [4], we now recog-
nize at least three different weathering processes oc-
curring at the surfaces of the samples analyzed to test 
the hypothesis that oxidative weathering dominates the 
surfaces of sedimentary rocks throughout the 
Transantarctic Mountains (Fig. 1). 
Aqueous Alteration.  Traditional aqueous weather-
ing appears to influence several of the samples ana-
lyzed in this study.  These samples exhibit a trend to-
wards the 4Si apex from their original starting compo-
sitions, indicating a preferential leaching of mobile 
cations relative to less mobile silica. 
Oxidative Weathering.  Oxidative weathering, as 
identified in Antarctic basalts, is indicated by a minor 
shift away from the R
2+
 apex as the abundance of 
monovalent cations preferentially increases relative to 
divalent cations, and as Fe
2+
 transitions to Fe
3+
 [2].  
While some of the sedimentary samples analyzed in 
this study show comparable chemical trends, these 
processes do not dominate our suite of samples. 
Coatings/Patinas.  Unlike the Antarctic basalts, 
which show no evidence for coatings preserved on 
their surfaces, several samples analyzed in this investi-
gation exhibit coatings or patinas that are dominated 
by widely different compositions than their interiors. 
For example, sample PRR-25633 exhibits a signifi-
cant increase (~15 wt.%) in FeOT and an associated 
decrease (~13 wt.%) in SiO2 at its surface relative to its 
interior.  LIBS measurements also show that the geo-
chemistry measured at the rock’s surface rapidly de-
creases with increasing depth into the sample, indicat-
ing the presence of a relatively thin surface coating.  
Lastly, XRD and VNIR analyses confirm the presence 
of goethite on the surface of this sample, which is ab-
sent from the sample interior.  In Figure 1, this sample 
appears to trend significantly towards the R
2+
 apex, 
although this trend is almost certainly the result of the 
assumed Fe
3+
/FeTotal and not a representation of the 
actual geochemical behavior. 
Similarly, four tuff samples show evidence for Ca-
sulfate at their surfaces, as indicated by a ~4-10 wt.% 
increase in CaO.  Although sulfur was not directly 
measured in our chemical analyses, the corresponding 
decrease in silica, the observed trend towards the M
+
 
apex in Figure 1, and VNIR signatures consistent with 
gypsum on the surfaces of these samples support the 
presence of a Ca-sulfate phase. 
Discussion:  Our hypothesis that sedimentary rocks 
throughout the Transantarctic Mountains should be 
dominated by oxidative weathering processes can be 
confidently rejected based on the analyses performed 
in this investigation.  It is apparent that the wide range 
of starting compositions, the diversity of weathering 
environments, and the range of rock porosity and indu-
ration all contribute to the type and extent of alteration 
at work on these samples, as is highlighted in Figure 
1.  Despite these different types of chemical alteration, 
nearly all of the measured samples (with the exception 
of those with coatings or patinas) do not show signifi-
cantly different compositional signatures between rock 
surfaces and interiors, indicating that chemical weath-
ering in cold, dry, and stable environments is limited. 
These results also suggest that oxidative weather-
ing processes are unlikely to be significant contributors 
to the observed spectral and compositional signatures 
of regions of Mars dominated by sedimentary litholo-
gies.  While surface alteration under hyper-arid and 
hypo-thermal conditions may be possible, it is unlikely 
that such alteration would complicate our interpreta-
tion of the bulk composition, or significantly alter our 
understanding of martian surface processes.  A notable 
exception would be the presence of coatings or patinas, 
as observed in some of the Antarctic samples.  While 
evidence for these features have been noted on the 
martian surface [6,7], they appear to be the exception 
rather than the rule, and can be successfully identified 
using equipment common to most Mars rovers. 
References: [1] Bibring, J.-P., et al. (2006) Science 
312, 400. [2] Salvatore, M.R., et al. (2013) GCA 115, 
137. [3] Cannon, K.M., et al. (2015) EPSL 417, 78. [4] 
Truitt, K., et al. (2017) LPSC 48, 1243. [5] Meunier, 
A., et al. (2013) AmJSci 313, 113. [6] Haskin, L.A., et 
al. (2005), Nature 436, 66. [7] Lanza, N.L., et al. 
(2016), GRL 43, 7398. 
 
Figure 1.  A WIS (M
+
-R
2+
-4Si) ternary diagram 
highlighting different weathering trends in Antarc-
tic sedimentary rock surfaces (red circles) relative 
to their interiors (grey circles).  Interiors and sur-
faces of the same rock are linked, and the colors 
of these links represent different dominant altera-
tion processes (see legend).  Igneous dolerites 
from [2] are also plotted on this diagram as a ref-
erence for oxidative weathering trends.  See [5] 
for more detailed discussions on M
+
-R
2+
-4Si ter-
nary diagrams and the WIS. 
1265.pdf49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083)


Constraints on the Mode and Extent of Sedimentary Rock Alteration in Hyper-Arid and Hypo-Thermal Environments

Constraints on the Mode and Extent of Sedimentary Rock Alteration in Hyper-Arid and Hypo-Thermal Environments

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