Geological features indicative of processes related to the hematite

Icarus 171 (2004) 295–316
www.elsevier.com/locate/icarus
Geological features indicative of processes related to the hematite
formation in Meridiani Planum and Aram Chaos, Mars:
a comparison with diagenetic hematite deposits in southern Utah, USA
Jens Ormö a,∗ , Goro Komatsu b , Marjorie A. Chan c , Brenda Beitler c , William T. Parry c
a Centro de Astrobiología, Instituto Nacional de Técnica Aeroespacial, Ctra de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain
b International Research School of Planetary Sciences, Universita d’Annunzio, Viale Pindaro 42, 65127 Pescara, Italy
c Department of Geology & Geophysics, University of Utah, 719 WBB, 135 S. 1460 E. Salt Lake City, UT 84112-0111, USA
Received 4 December 2003; revised 9 June 2004
Available online 28 July 2004
Abstract
In order to understand the formation of the few but large, hematite deposits on Mars, comparisons are often made with terrestrial hematite
occurrences. In southern Utah, hematite concretions have formed within continental sandstones and are exposed as extensive weatheredout beds. The hematite deposits are linked to geological and geomorphological features such as knobs, buttes, bleached beds, fractures
and rings. These terrestrial features are visible in aerial and satellite images, which enables a comparison with similar features occurring
extensively in the martian hematite-rich areas. The combination of processes involved in the movement and precipitation of iron in southern
Utah can provide new insights in the context of the hematite formation on Mars. Here we present a mapping of the analogue geological and
geomorphological features in parts of Meridiani Planum and Aram Chaos. Based on mapping comparisons with the Utah occurrences, we
present models for the formation of the martian analogues, as well as a model for iron transport and precipitation on Mars. Following the
Utah model, high albedo layers and rings in the mapped area on Mars are due to removal or lack of iron, and precipitation of secondary
diagenetic minerals as fluids moved up along fractures and permeable materials. Hematite was precipitated intraformationally where the fluid
transporting the reduced iron met oxidizing conditions. Our study shows that certain geological/geomorphological features can be linked
to the hematite formation on Mars and that pH differences could suffice for the transport of the iron from an orthopyroxene volcanoclastic
source rock. The presence of organic compounds can enhance the iron mobilization and precipitation processes. Continued studies will focus
on possible influence of biological activity and/or methane in the formation of the hematite concretions in Utah and on Mars.
 2004 Elsevier Inc. All rights reserved.
Keywords: Mars; Mineralogy; Geochemistry; Geological processes; Mars surface
1. Introduction
1.1. Aim of study
The existence of large hematite deposits on Mars is now
established, yet enigmatic (e.g., Christensen et al., 2001;
Hynek et al., 2002). In order to understand the processes
behind their formation, comparisons are often made with
* Corresponding author. Fax: +34-91-5201621.
E-mail address: [email protected] (J. Ormö).
0019-1035/$ – see front matter  2004 Elsevier Inc. All rights reserved.
doi:10.1016/j.icarus.2004.06.001
terrestrial hematite occurrences (e.g., Banded Iron Formations; Christensen et al., 2001). In this study we compare
surface features of an area with extensive hematite deposits
in southern Utah, USA (Fig. 1), with conspicuous features
visible in high-resolution Mars Orbiter Camera (MOC) images from the Meridiani Planum and Aram Chaos hematite
regions on Mars (Fig. 2). Here the focus is on regional mapping distributions and similarities (versus detailed hematite
mineralogies that have yet to be more thoroughly examined
in both the terrestrial and Mars examples).
In Utah, a large variety of diagenetic iron oxide (e.g.,
hematite and goethite) concretions occur within Jurassic
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Fig. 1. Utah map overlain with multispectral data from five overlapping Landsat 7 ETM + scenes acquired between 8 September 1999 and 3 October 3 1999.
Bands 7 (mid infrared, reflected), 4 (near infrared, reflected), and 2 (visible green, reflected) displayed in grayscale. Original spatial resolution 30 m. Outline
of Glen Canyon Group outcrop locations are modified from Hintze et al. (2002). Photo localities are indicated by numbers 1 through 8. 1. Snow Canyon
(Figs. 3B, 5B); 2. Kaibab Uplift (Fig. 11A); 3. Mollies Nipple (Fig. 12B); 4. Paria River (Figs. 3A, 7B, 6A, 6B); 5. Lake Powell (Figs. 7A, 6C, 6D, 8A);
6. Escalante (Fig. 4E, 5D); 7. Dubinky Well and Moab area (Figs. 3C, 11B, 11C, 12A, 8B, 4A–4D, 5A); 8. San Rafael Swell.
Diagenetic hematite deposits in Utah and on Mars
297
Fig. 2. The location of geological and geomorphological features in the mapped sectors of Meridiani Planum and Aram Chaos analogous to features in southern
Utah hematite-rich regions. Images include all released images from the mission phases dating from August 2001 to January 2002 and February 2002 to July
2002 and image 0301633 from an earlier phase. Each symbol represents the most dominant feature in each MOC image, and is located at the geographic
center of the image. Simplified outlines of the hematite-rich plains unit (P2) and the etched unit (E) mapped by Hynek et al. (2002) are indicated (P2 and
hematite-rich area in Aram Chaos = dark gray, E = light gray). The outline of the hematite-rich area in Aram Chaos is adopted from Christensen et al. (2001)
and Catling and Moore (2003). Image numbers for images discussed in the text are indicated.
continental sandstones and are exposed on the surface as
resistant, weathered-out beds or accumulations. These iron
oxide-cemented zones in the sandstone commonly generate spheres, pods, beds, and cylindrical pipes and columns
that cut across the surrounding, primary bedding. Some deposits are associated with large pipe-like bodies of fluidized
sand that intruded through the sedimentary bedding. The
pipes have acted as aquifer conduits for the fluids that transported the iron. These bodies are in general cylindrical and
can be several tens of meters in both width and vertical extent. In certain areas, weathered-out pipes generate clusters
of towers a few meters high, and some even cap and protect
hundred meter high butte-like mountains. The iron from the
sediment may be carried by brines which can cause extensive alteration of strata along permeable layers, pipes, and
fractures that are visible as albedo differences between the
host and altered rock. Landforms and other geological features strongly resembling those of southern Utah, such as
high-albedo rings and layers, knobs, domes, and cemented
ridges along vertical joints and faults, also dominate the
landscape features in the hematite-rich regions on Mars.
In this study we seek an explanation for these geological
features in Meridiani Planum and Aram Chaos by comparing with the mode of formation of the Utah examples. The
hematite occurrences in southern Utah are not nearly as extensive as the hematite deposits on Mars, and may be smaller
by a magnitude or more in size. However, the process behind
their formation is of interest as it provides a model for iron
transportation and precipitation in a low-temperature hydrologic groundwater system (Ormö and Komatsu, 2003). The
strength of the model is that it is supported by the large
variety of geological and geomorphological features. The
model does not require a geologically and paleoclimatologically complicated environment (e.g., hydrothermal systems,
long-lived large bodies of standing water etc.), however the
presence of such an environment does not exclude the plausibility of our model.
1.2. Martian hematite occurrences
Hematite is one of few minerals found on Mars, which
can be linked directly to processes involving water. The
first concentration of crystalline hematite mineral was identified in West Candor Chasma of Valles Marineris using
Viking color and Phobos II data (Komatsu et al., 1993;
Geissler et al., 1993). Examination of this site by the Mars
Global Surveyor Thermal Emission Spectrometer (TES)
has not yet confirmed this finding, and an alternative explanation is tentatively given as poorly crystalline FeOOH
polymorphs including goethite (Christensen et al., 2001).
However, the TES instrument has identified relatively high
discrete concentrations of martian hematite deposits at
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Meridiani Planum, Aram Chaos, and small areas within
Valles Marineris (Christensen et al., 2001). Christensen et al.
(2001) identified the mineral as crystalline, coarse-grained
(> 5–10 µm), gray hematite, which may have axis-oriented
crystals (Lane et al., 2002). Hematite coatings and finegrained consolidated hematite have also been suggested
based on spectral studies matching the TES signatures
(Kirkland et al., 2003, 2004).
The Mars Exploration Rover (MER), Opportunity, is
currently investigating an area in the western part of the
hematite-rich unit in Meridiani Planum (Golombek et al.,
2002; Squyres and the Athena Science Team, 2004a, 2004b).
These studies have confirmed the existence of hematite
concretions in a sedimentary host rock and apparent high
concentrations of sulfur-rich minerals (i.e., sulfates) at the
Opportunity landing site crater (“Eagle Crater”) although
calibrated results have not yet been published. The friable
layered unit in Meridiani Planum where hematite is concentrated (∼ 10–15%) was interpreted to be sedimentary
(Christensen et al., 2001) or pyroclastics (Chapman and
Tanaka, 2002; Hynek et al., 2002). The hematite-rich unit
appears to be confined within a single stratigraphic horizon concentrated as a thin surface layer (Christensen et al.,
2001), which now appears to be confirmed by the observations by Opportunity (Christensen et al., 2004). Thermal
inertia data indicate that the unit is made of, or covered
by, sand-sized or larger materials (e.g., Ramsey and Christensen, 1998). The hematite-rich unit is superimposed on
Middle to Late Noachian cratered terrain (Hynek et al.,
2002, and this study). In Meridiani Planum, a Geographic
Information System (GIS) study of Mars Orbiter Laser Altimeter (MOLA) topography and hematite concentrations indicate that the highest abundance levels of hematite are concentrated along the southern half of a 200 km long, 70 km
wide, NE-trending, ancient ridge and a small area just to the
east of the ridge (Chapman and Hare, 2002).
Christensen et al. (2001) assessed five different mechanisms of hematite deposit formation as (1) chemical precipitation (a)low-temperature precipitation in Fe-rich water,
(b) laterite style weathering, (c) direct precipitation from circulating hydrothermal fluid, (d) coating formation by weathering and (2) thermal oxidation of magnetite-rich lavas.
Mechanisms (1)(a) and (1)(b) require diagenetic processes to
convert Fe-oxide/oxide assemblages to coarse-grained, gray
hematite. They preferred chemical precipitation [(1)(a) or
(1)(c)] based on the geological settings assuming that the
hematite-bearing unit is sedimentary. Hynek et al. (2002),
on the contrary, considered that the hematite-bearing unit is
pyroclastic in origin and advocated thermal oxidation of volcanic ash or precipitation from circulating fluids. In the case
of the latter hypothesis, the flow may have been under hydrothermal or ambient conditions (Christensen et al., 2001).
Catling and Moore (2003), argue for a hydrothermal origin of the hematite-rich unit in Aram Chaos. The Aram
Chaos hematite deposits are concentrated inside a heavily
degraded crater that shows a complex geological history and
the deposits are observed within plateaus dotted with mesas
(Glotch and Christensen, 2003). From the abundance map by
Christensen et al. (2001) it is clear that hematite is confined
in the northeastern part of the crater. Although this part of the
crater basin has a widespread hematite distribution, there is a
wide variety of geological units within the hematite concentrations. Glotch and Christensen (2003) suggest the hematite
to have formed when iron in water ponding in the crater became saturated.
1.3. The southern Utah hematite deposits: geological
setting, distribution, and related landforms
The most prominent large-scale (km-scale visible in
satellite images) features in southern Utah related to the
hematite formation are the strong contrasts between red
strata and white (bleached) strata. It is the iron oxides (e.g.,
hematite, goethite) that account for the vivid coloration in
the permeable Jurassic Navajo, Page, and Entrada sandstones. Sandstone coloration is an index to both the character of the subsurface fluids which caused precipitation
(oxidation) and bleaching (reduction), fluid flow pathways,
and the overall paleohydrologic setting (Chan et al., 2000;
Beitler et al., 2003).
Most of the Utah sandstones were likely originally red
from small amounts of early, disseminated iron oxides and
clays that create thin grain coatings during deposition or
early burial (e.g., Walker, 1975). The original iron source
for the red coloration is the breakdown of ferromagnesium
silicates with iron in the ferrous state. This iron is released
from the minerals and oxidized to form the red coloration
of the “redbeds.” This oxidized ferric iron is highly immobile. The iron needs to be reduced to Fe2+ (by donations
of electrons) in order to be put into solution and transported. Bleaching by reducing solutions later removes iron
on nearly microscopic scales such as those along individual eolian laminae, deformation bands, and lithologic contacts, to regional scales extending through a formation over
tens of kilometers. Bleaching is dependent on the mobility
of iron, fluid composition and flow paths. Flow paths are a
function of permeability, lithology, sedimentology, stratigraphy, and structure. Field, laboratory, and numerical modeling studies on iron mineralization in southern Utah suggest
that iron is mobilized and removed by reducing water that
moved along conduits (e.g., faults or fractures) and then
outward into adjacent permeable rocks (Chan et al., 2000;
Beitler et al., 2003; Parry et al., 2004). The fluids in the
Utah example were reducing possibly due to the presence
of hydrocarbons, methane, organic acids, or hydrogen sulfide. Iron mobilizing fluids (sometimes brines) need only to
contain small quantities of the reductant.
When reduced waters carrying the iron meet and mix with
shallow, oxygenated ground water, iron oxides are precipitated to form a variety of iron-oxide concretionary cements
in the porous sandstones. Multiple iron-oxide mineralization
events and concretionary geometries are evident and can be
Diagenetic hematite deposits in Utah and on Mars
299
Fig. 3. Examples of large-scale bleaching in southern Utah. (A) Color aerial photo of Paria River area (supplied by the U.S. Bureau of Land Management).
Right enlargement shows the colorful redox reaction front in the Jurassic Navajo Sandstone, with bleached sandstone at right. (B) Bleached white (distant)
and red-colored (foreground) Navajo Sandstone in Snow Canyon State Park, Utah (GPS coordinates 37◦ 13 N, 113◦ 39 W). (C) Dark gray tar sand (center)
with bleached (yellow) selvages on top and bottom, within red-colored Jurassic Entrada Sandstone, Dubinky Well area, Utah (GPS coordinates 38◦ 41 N,
109◦ 55 W).
explained as the result of permeability heterogeneities in the
host rock, a self-organization process, and/or the influence
of microbes (Chan et al., 2000; Chan and Parry, 2002). In
many Utah concretions, hematite is the primary iron oxide identified by XRD (Chan et al., 2000) although other
iron oxides (e.g., goethite) are also important components in
the broad, regional spectrum of Navajo concretions. Quartz
grains are typically coated with hematite, precipitated into
pore space or voids, with radial crystal growth perpendicular to the grain boundaries (Fig. 5D). Some hematite is very
pervasive and may form as euhedral, hexagonal plates (Chan
et al., 2000).
The arid climate and lack of vegetation and differential
ground cover in southern Utah creates unique conditions
where lithology can often be distinguished with multispectral data (Beitler et al., 2003). Paleo-reservoir characteristics
and diagenetic bleaching in the Jurassic Navajo Sandstone
of southern Utah can be evaluated using Landsat Thematic
Mapper (TM) satellite imagery, aerial photos (Fig. 3) and
detailed field mapping (Beitler et al., 2003). Analyses indicate that diagenetic removal of iron oxides in the Jurassic Navajo Sandstone occurs on regional formational scales
to localized laminae scales. The most extensive bleaching
is commonly associated with zones of higher permeabil-
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ity associated with large-scale structures and anticlinal uplifts such as the Navajo Sandstone around the San Rafael
Swell, the East Kaibab Uplift, and the Escalante Monocline
(Fig. 1). Regional bleaching patterns indicate that Laramide
structures likely served as conduits for hydrocarbons (Beitler
et al., 2003). Field relationships also indicate small-scale
bleaching that is controlled by primary sedimentary structures. Fluid migration patterns are complex and can be both
constrained and/or independent of internal stratification.
Chan et al. (2000) suggest that in the Moab area, the
fluid responsible for bleaching of the redbeds was a reduced,
low pH, high salinity brine from the Pennsylvanian Paradox Formation that moved up along faults and percolated
the permeable sandstones where it met oxidizing meteoric
water with higher pH. The iron was transported in a reduced
state until it precipitated as hematite when encountering the
oxidizing water. This has caused concentric structures, and
in some cases the flow direction of the redox fluid front is
visible (Fig. 4B). Manganese concretions occur in less extensive deposits in the area. 40 Ar/39 Ar dating indicates that
the manganese mineralization (probably coincident with the
hematite mineralization) occurred about 25 Ma, possibly in
connection to a period of regional uplift and increased volcanism in 30 km distant volcanoes (La Sal Mountains) (Chan
et al., 2001a).
The Utah hematite deposits occur at a variety of scales
(mostly cm to m-scale), sometimes as individually small,
but highly concentrated (up to ∼ 30 wt% Fe2 O3 ), concretions that form spheres, pipes and columns, to sheets
and joint/fracture fills (Fig. 4). Many of the iron oxide occurrences are also affected by stratigraphy, sedimentology,
and structure, for instance within the toes of some of the
fossil dune sets. The concretions weather out and concentrate, although as thin deposits, in large sheets along flat
surfaces. Accumulations (Chan et al., 2004) of weatheredout concretions can cover larger areas on the scale to tens
of meters to hundreds of meters (Figs. 5A–5C). Another
example (Fig. 4E) is from Grand Staircase Escalante National Monument and shows iron oxide cementation in
bleached sandstone beds. The iron cementation follows laterally along stratigraphically controlled permeable surfaces.
Both Figs. 4D and 4E demonstrate that hematite-cemented
material can reach considerable thickness.
There is a remarkable display of intraformational, syndepositional deformation structures, with a large concentration of sandstone pipes in the Jurassic Carmel and Entrada
Sandstone intervals around Lake Powell, Utah (Netoff and
Shroba, 2001; Netoff, 2002). Some pipes are clustered, and
weather out of a finer-grained or less-cemented host rock
creating a knob-like morphology (Figs. 6A and 6B), but
pipes can also take the shape of sand-volcanoes after the host
sediments have been eroded (Figs. 6C and 6D). These pipes
are different from the hematite cemented columns shown in
Figs. 4A–4C. It is important to note that the features seen
in Figs. 6A and 6B are sand intrusions, and not petrified
tree trunks, despite some initial similarities in appearance.
There is no organic material associated with these features,
and they are composed of laminated or homogeneous sand.
Some apparent structure in the pipes could be the result of
fluidized grain movement from liquids flowing up the pipes
creating a fabric by rearrangement of the grains. Other cylindrical mega pipe features measure over 100 m in height and
70 m in diameter (Fig. 6D); the biggest recorded such sandstone pipes in the terrestrial continental stratigraphic record.
Many of the megapipes currently weather out as massive
positive features, surrounded by ring fractures and faults that
may have resulted from downward relaxation after the initial upward injection (Figs. 7A and 7B). The massive pipes
commonly have slightly different, stronger cementation than
the surrounding stratified host rock, and can physically stand
out as cylindrical pipes, or can be recognized even aerially
by the different color, texture, and weathering pattern. The
bleached appearance of some sandstone pipes and/or their
contacts show that they have acted as pathways for the reducing fluids responsible for the mobilization of the iron. Some
of the knobs and pipes have different textural characteristics (e.g., grain size or packing) and could act as preferential
pathways for mineralizing fluids that could precipitate iron
oxides (Figs. 12A and 12B). However, pipes can also contain other types of mineral cements (e.g., calcite or quartz)
(Fig. 6). In many cases, some of the cement appears to have
been dissolved in the bleaching process, creating some secondary porosity and small vugs along with the removal of
the iron oxide grain coatings (e.g., Surdam et al., 1993).
In other cases, the bleaching appears to have occurred before the current cement was formed. This bleaching could
have increased the porosity and allowed for more cement
to be emplaced, making the sediment stronger. In the case
of the pipes, it may be that the “fluidization” process rearranges the grains in a way that makes them more cohesive.
The bleaching and cementation of these observed syndeformational structures indicate that their formation occurred
prior to the phase of reducing fluid flow. These structures
can be attributed to several conditions within the Jurassic desert environments that likely include dune loading of
water-saturated, poorly consolidated substrates, and some
external triggers, such as mudflows, earthquakes, or bolide
impact (Chapman, 1989; Alvarez et al., 1998; Netoff, 2002;
Chan et al., 2002). These conditions could produce upward
fluid injection into loose overlying dune sand. These mega
sandstone pipe injections have proportions not yet recognized among recent analogues of earthquake induced liquefaction structures. Another unusual feature of southern Utah
is large potholes and/or weathering pits that commonly occur on flat, exposed sandstone surfaces (Figs. 8A and 8B).
The host rock is typically eolian sandstone that may range
from Permian through Jurassic in age. Some potholes are
small and shallow, yet others are tens of meters wide, steep
sided, and several tens of meters in depth. The formation
of the potholes is somewhat enigmatic. Some potholes appear to be related to accumulated water and may host a
variety of prokaryotic and eukaryotic organisms (Chan et
Diagenetic hematite deposits in Utah and on Mars
301
Fig. 4. Examples of iron oxide cementation in sediments in southern Utah. (A), (B), (C) show hematite-cemented, vertical cylindrical pipes in a paleotopographic high area of the Jurassic Navajo Sandstone, Dubinky Well area, Utah (GPS coordinates 38◦ 42 N, 109◦ 54 W. (A) Cross-section through a hematite
cemented pipe cutting the bedding of the host sediment. US quarter dollar for scale. (B) Top view of hematite cemented pipe showing shadow effect of
hematite stain that has shifted with fluid flow direction (towards the top and top right). (C) Clustered hematite cemented pipes weather out and concretions
are accumulated on the ground surface. (D) Ferricrete layer of hematite-cemented coarse-grained sandstone (at an unconformity) at the top of the Jurassic
Navajo Sandstone, Dubinky Well area, Utah (GPS coordinates 38◦ 44 N, 109◦ 56 W). (E) A zone of hematite-cemented sandstone in the bleached Jurassic
Navajo Sandstone, Grand Staircase Escalante National Monument (GPS coordinates 37◦ 52 N, 111◦ 27 W). The hematite cementation follows laterally along
stratigraphically controlled permeable surfaces.
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Fig. 5. Examples of large accumulations of weathered out hematite-cemented concretions. (A) Arrow marked “a” points at bleached beds with tar sand. Arrow
“b” indicate part of an accumulation of hematite-cemented erosional residue from vertical cylindrical pipes in a paleotopographic high area of the Jurassic
Navajo Sandstone. This stratigraphic view covering several tens of meters, Dubinky Well area, Utah (GPS coordinates 38◦ 41 N, 109◦ 54 W). (B) Concentration
(foreground) of spherical hematite-cemented concretions accumulated on a flat weathered surface, Snow Canyon, Utah (GPS coordinates 37◦ 13 N, 113◦ 39 W).
The orange-colored Jurassic Navajo Sandstone comprises all the exposed rocks. (C) Close up of accumulation of weathered out hematite concretions from
Navajo Sandstone, Grand Staircase Escalante National Monument. (D) Photomicrograph (under plane polarized light) of concretionary Navajo Sandstone,
Escalante area, Utah. Image is about 1 mm across. Hematite cement crystals precipitate radially out from grains into pore space.
al., 2001b). Others are giant weathering pits that appear
to be a function of wind abrasion (Netoff et al., 1995;
Netoff and Shroba, 1997).
2. Method
Hynek et al. (2002) performed a detailed mapping of the
part of Meridiani Planum where the hematite deposits are lo-
cated. They sorted out the stratigraphic relationship between
different units and gave examples of surface features typical
of each unit. In this study we focus on surface features in the
martian hematite regions of Meridiani Planum, and Aram
Chaos, which appear to be analogous to the geological and
geomorphological features in the area of the Utah hematite
deposits. As Aram Chaos was not included in the study by
Hynek et al. (2002) we used the map by Christensen et al.
(2001) for the location of the hematite-rich unit in that area.
Diagenetic hematite deposits in Utah and on Mars
303
Fig. 6. Comparison between knob-formation in Utah and in Meridiani Planum, Mars. (A) and (B) Bleached, fluidized, cylindrical sandstone pipes in the
Jurassic Carmel Formation, near Paria and Lake Powell, Utah (GPS coordinates 37◦ 02 N, 111◦ 49 W). These cylindrical pipes with massive interiors are more
resistant to weathering than the surrounding eolian sandstone host rock. There is also common bleaching in the host rock surrounding the pipe (B). (C) A large
cluster of positively weathered-out mega sandstone pipes in the Jurassic Entrada Sandstone of Lake Powell, Utah. The conical weathered form just left of
center (GPS coordinates approximately 37◦ 11 N, 111◦ 13 W) is about 30 m high. The horizontal panning distance of the photograph is about 300 m. Aerial
photograph courtesy of Dennis Netoff. (D) One large mega sandstone pipe in the Jurassic Entrada Sandstone, close to the location of (C). This positively
weathered-out plug/neck has local relief of about 70 m (GPS coordinates 37◦ 11 N, 111◦ 3 W). Photograph courtesy of Dennis Netoff. (E) Image from layered
deposit at Meridiani Planum showing knobs revealed by removal of surrounding host material. Original image resolution 3.07 m/pixel. Arrow “a” points at
knob just released from the layered material. Arrow “b” show cylindrical topographic dome interpreted to be knob not yet exposed. Arrow “c” show how the
knobs maintain their distinct appearance long after the host material is removed. This apparent resistance to erosion is visible also in the evenly distributed
knobs in (F) (original image resolution 6.11 m/pixel).
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Fig. 7. Examples of high-albedo ring features in Utah ((A) and (B)) and Meridiani Planum, Mars (C). (A) Fluidized mega sandstone pipes and associated
ring faults and ring edge bleaching in the Jurassic Entrada Sandstone, Lake Powell, Utah (near GPS coordinates 37◦ 05 N, 111◦ 18 W). Horizontal field of
view (slightly oblique) across is approximately 30 m. Center cylindrical plug ∼ 12 m across, with outermost cylindrical ring ∼ 22 m across. (B) Ring fault
(associated with sandstone pipe) in Jurassic Carmel Formation with edge bleaching, near Paria and Lake Powell, Utah (GPS coordinates 37◦ 02 N, 111◦ 49 W).
(C) Distinct high-albedo ring surrounding a depression (crater and/or vent?) in Meridiani Planum (original image resolution 3.00 m/pixel).
Iron can be a very sensitive indicator of fluid flow that can
produce coloration patterns (reflecting mineral composition
and abundance) on mm scales up to km scales. Hematite ce-
mentation of strata can cause structures on the same scales
that may be further enhanced by later erosion of weaker,
non-cemented, confining material. These patterns and struc-
Diagenetic hematite deposits in Utah and on Mars
305
Fig. 8. Bleached layers and erosional pits in southern Utah and Meridiani Planum, Mars. (A) Arrow point at top of a fluidized mega sandstone pipe (approximately stretching between the arrow and the large pit to the lower right of the arrow) measuring over 50 m across in exposures of the Middle Jurassic Entrada
Sandstone, Lake Powell, Utah (GPS coordinates 37◦ 05 N and 111◦ 18 W). Superimposed large weathering pits are several meters across and up to 15 m deep.
Field view in this oblique photo covers ∼ 580 × 800 m. Aerial photograph courtesy of Dennis Netoff. (B) Deeply carved, steep-sided, cylindrical weathering
pits (some filled with collected rainwater at the bottom) in the Jurassic Navajo Sandstone, Moab, Utah (GPS coordinates 38◦ 35 N, 109◦ 32 W). (C) Eroded
domes of high-albedo (from mineral precipitation or bleaching?) layered deposits in Meridiani Planum. Original image resolution 1.52 m/pixel. Arrow “a”
points at a row of pits that appear to be strata bound (wind erosion pits?). Arrow “b” points at an example of pit with what seems to be low-albedo ejecta
(impact crater?).
tures can be seen visually in the field as well as with remote
sensing, as applied in this study. We have studied all 156
MOC images from mission phases dating from August 2001
to January 2002 and February 2002 to July 2002 within
the sectors 18◦ W–24◦ W/5◦ N–0◦ N (Aram Chaos) and
5◦ E–10◦ W/5◦ N–10◦ S (Meridiani Planum). One selected
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J. Ormö et al. / Icarus 171 (2004) 295–316
image from an older phase was also used. The studied sectors and location of MOC images presented in this work are
outlined in Fig. 2. The study of the MOC images focused
on conspicuous geological and geomorphological features,
and the relations between the units (based on Hynek et al.,
2002) in which the landforms occur. The features included
knobs, high-albedo rings, bright (high-albedo) rim craters,
bright (high-albedo) layers, and possible mega vents (suggested by Hynek et al., 2002 to be vestiges of hydrothermal
systems). These features occurred in almost all the mapped
images (see Fig. 2). In the rare case that several different
features occur in the same image, the most common feature was mapped. The MOC image interpretation has been
combined with fieldwork by team members at the sites in
southern Utah (Fig. 1). The mineral products and geochemical conditions necessary to mobilize iron from a hypothetical
Martian basaltic shergottite volcanoclastic sediment were investigated using the Geochemist Workbench (Bethke, 1998).
We present a model for the formation of the hematite deposits on Mars based on Mars’ geological and environmental
constraints, and the apparent link between the geological and
geomorphological features, the hydrological system, and the
hematite accumulations in Utah. Our study emphasizes the
landforms and fluid flow pathways connected to the mechanisms for the formation of hematite concretions in any sedimentary setting (e.g., Utah and Mars), where fluids can move
through permeable sediments with adequate chemistry for
iron mobilization.
3. Comparison between surface features in southern
Utah and in the Meridiani Planum and Aram Chaos
3.1. Bright rim craters and high albedo rings
The mapping of Meridiani Planum and Aram Chaos sectors shows that knobs, “mega vents,” and bright rim craters
may occur in the same image, although rarely. Bright rim
craters and rings occur more frequently within the same image. Only bright rim craters occur in both the etched unit and
the hematite-rich plains unit mapped by Hynek et al. (2002),
however with a certain preference to the hematite-rich unit
and the cratered unit to the immediate south (Fig. 2). High
albedo wind-streaks from the bright parts of the crater rims
indicate that the bright appearance of the rims is not a consequence of illumination. This bright appearance of the crater
rims can be the consequence of fluid flow causing either
bleaching or mineral precipitation along zones of increased
porosity, possibly due to impact related fractures (Fig. 9).
The bright rims can also be due to uplift and exposure of high
albedo material. In particular, this appears to be the case for
the Eagle Crater, studied by the Opportunity rover. Ground
truth from more craters in Meridiani Planum is needed before it can be established whether the bright rims have only
one or several causes.
High albedo rings commonly occur in connection to the
bright rim craters. However, the ring features, in contrast
to bright rim craters, are only present within the hematiterich unit where they dominate the surface features visible in
MOC images. The hematite-rich unit in Meridiani Planum
and Aram Chaos appears as a flat, smooth plain with small
mesas and underlying brighter terrain that may indicate that
the surface is exhumed (Hynek et al., 2002). Hynek et al.
(2002) date the hematite rich unit to Late Noachian to Early
Hesperian. Kelsey et al. (2000) and Hartmann et al. (2001)
suggested many of the depressions linked to bright (sometimes dark) ring pattern to be vestiges of old “fossil” craters,
which gives a much higher age to the unit. However, Hynek
et al. (2002) show that the supposed fossil craters have both a
non-random spatial, as well as, size distribution inconsistent
with the occurrence of impact craters. They also point out
that a higher age (high crater density) of the unit does not
comply with the stratigraphic relationship with surrounding
units. Based on this they conclude that it is unlikely that the
high albedo rings are individual fossil craters.
The connection between high albedo rings and fossil
craters is evoked by the occurrence in some of the most
apparent rings by smooth, sub-circular depressions. Indeed,
some of the most distinct rings occur as bright rims around
bowl-shaped, crater-like depressions (Fig. 7C). However,
with the transition into more frequent rings, the link to
crater-like depressions becomes less obvious (Fig. 9). It appears that the rings remain, but the “craters” disappear. The
rings also lose their circular outline and begin to overlap
each other in a percolative pattern typical for mineral precipitation and/or mobilization from seeping fluids [cf. percolation theory, which considers the connectedness of a phase
or multiple phases across a microstructure (Stauffer and
Aharony, 1992). This theory is commonly used to simulate
the percolation of substances (e.g., calcium hydroxide) in a
hydrating cement paste system (e.g., Bentz et al., 1998)].
It is doubtful that crater rims of different sizes would remain as distinct albedo features on a level surface, while
the crater depressions have been completely subdued. This
is in support of the notion by Hynek et al. (2002) that the
rings may not be directly related to impact craters, but instead related to subsurface fluids. However, the transition
from bright crater rims into diffuse ring patterns may suggest a link between the high-albedo rings and impact craters,
although not a morphological one. If the depressions are
buried impact craters, the localized increase in fracturing
could provide conduits for later groundwater flow. On Mars,
the most common cause for fracturing of the upper crust is
cosmic impacts. Instead of the linear fracture patterns most
common on Earth, zones of intense fracturing on Mars has
a more patchy or ring-shaped appearance. In our model for
the high-albedo ring formation on Mars, we continue on the
suggestion by Hynek et al. (2002) that the depressions represent vents for fluids, and we suggest the rings to be zones
of possible alteration, such as bleaching and/or precipitation
of, most often, high albedo minerals (e.g., carbonates, sul-
Diagenetic hematite deposits in Utah and on Mars
307
Fig. 9. Model for the formation of high-albedo rings in the hematite rich plains unit on Mars based on the process forming the Utah analogues in Fig. 7, and
the apparent transition from bright rim craters to rings (1)–(3). As rings may overlap, alteration may “bleed” out and cause some blending and coalescing
of altered areas (bleaching and/or mineral precipitation). Original image resolution of image E1001273: 3.04 m/pixel, image E1203255: 3.00 m/pixel, and
image E1103488: 3.04 m/pixel.
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J. Ormö et al. / Icarus 171 (2004) 295–316
Fig. 10. Example of concentric structure similar to structures mapped as possible vestiges of hydrothermal systems by Hynek et al. (2002). In this study
mapped as “mega vents.” Original image resolution 3.05 m/pixel.
fates) (Figs. 7 and 9). It may be that the rings are surface
expressions of pipes still buried within the sediments, as an
alternative, or maybe companion to the buried-crater-model
presented in Fig. 9. As a consequence, the etched zone may
have had rings before, but after erosion the rings remain as
knobs, “mega vents,” and pitted bright layers. Large concentric structures, suggested by Hynek et al. (2002) to be
vestiges of large hydrothermal systems, occur at some locations in the etched unit (“mega vents” Fig. 10).
Bright rim craters do not occur in the Utah sandstone
examples so it is difficult to make a direct comparison in
this case. However, structurally controlled, ring-shaped fault
or fracture bleaching occurs in connection to the sand intrusions, although on much smaller scales than on Mars
(Fig. 7). Of interest is the strong connection between the altered rings and ring-shaped fractures around the intrusions
acting as pathways for fluids through less permeable layers.
Sometimes more strongly resistant altered material such as
pipes and rings in Utah stand out in positive relief. This is
also noticed at some of the martian ring features (Fig. 7C)
and high albedo layers (Fig. 8C). It is clear, however, that
at the Utah site, bright material exists both as a consequence
of bleaching and as mineral precipitation (e.g., carbonate cements, or evaporitic sabkha deposits). The same can be valid
for Mars.
3.2. Bright layers and ridges
More extensive high-albedo features are confined within
the areas mapped as etched unit by Hynek et al. (2002),
which they defined to be in a stratigraphically lower position than the hematite rich unit. These high-albedo features
commonly appear as layers, with sharp scarps, as well as
concentric rings possibly due to erosion of layered domes
(Fig. 8C). The bright layers are typically pierced by numerous, similar-sized, dark, pits. However, not all of these dark
pits are circular and randomly distributed, criteria to support an impact origin. Several appear in clusters, adopt very
Diagenetic hematite deposits in Utah and on Mars
elongated shapes, and most importantly, seem to be strata
bound (“a” in Fig. 8C). If impact craters, the high density
would suggest a high age of these bright layers. However, the
rather uniform size distribution, the apparent distinct appearance, and most of all that they follow certain layers, entirely
exclude an impact origin. Whereas low albedo ejecta patterns are visible around a few such pits (“b” in Fig. 8C),
most of the other similar sized pits completely lack visible
ejecta. These pits show a most striking resemblance to the
wind-erosion pits in southern Utah (Figs. 8A and 8B). The
observation that the bright layers in Meridiani Planum often
appear to stand out in relief at the same time as they seem
subject to localized wind erosion is an indication for a different response to erosion than the adjacent material.
Figures 11A–11C show examples of bleaching linked
to faults and fractures in southern Utah. Bleaching of the
Kaibab Uplift (Fig. 11A) occurs along the eastern fault,
as well as in a large adjacent area extending over 30
km, whereas along smaller faults, fractures, and joints the
bleached zone may be limited to a few meters or less
(Figs. 11B and 11C). High-albedo material also occurs
around faults in Meridiani Planum (Fig. 11D), in this particular case along ring faults and radial faults related to a large
impact crater. When the bleached fractures are viewed from
a birds-eye view in the Utah examples, the albedo difference to the surrounding unbleached material can be striking
(Fig. 11C). In some cases, deformed zones can stand out in
positive relief, which is also observed in some locations in
the martian etched terrain (Fig. 11E). Hynek et al. (2002)
suggested some ridges in the Meridiani Planum to be areas where groundwater caused cementations along vertical
joints. Most often, such ridges have a high albedo and can
occur with transition into material resembling the bright layers, possibly as erosional remnants of wider bleached or
cemented zones around joints (Fig. 11E).
3.3. Knobs, domes, and circular mesas
Knobs of sizes ranging from the limit of the resolution
of the MOC images to a couple of hundred meters are common in the etched unit and within large craters in the cratered
units of the studied areas on Mars (Fig. 2). There are similarities in both the circular form (compare Figs. 6 and 12),
as well as the distribution and spacing, although the Mars
features may be an order of magnitude larger than the majority of the Utah knobs. It is clear from the relationship
between the knobs and the layered deposits in Figs. 6E and
12C that the knobs have formed within the layered material
before becoming exposed. This is an analogue with the formation of the knobby topography in Utah (Figs. 6 and 12). It
is also clear that the knobs remain as distinct morphological
features for long periods of time after exposure, sometimes
occurring in lines, but most often in a random distribution
(Fig. 6F).
On Mars, however, no knobs have so far been observed
within the hematite-rich plains unit. Figure 13 possibly
309
shows the hematite-rich plains unit onlapping on the knobby
etched unit, which also has been suggested by Hynek et
al. (2002). Some knobs are protruding through the thinning
plains unit. Alternatively, scattered mounds and buttes exposed in bright outcrops underlying hematite deposits are
similar to fumarolic mounds formed by vapor escape and
differential cementation in terrestrial ignimbrite deposits
(Chapman and Tanaka, 2002). Fumarolic pipes form above
the welded zone in the unwelded overlying ash; the loose ash
is often eroded away by wind, exposing the fumarolic knobs.
4. Discussion
4.1. Geological and geomorphological features
Diagenetic concretionary iron-oxide precipitation is common in the terrestrial sedimentary record (e.g., Liesegang
banding). What makes southern Utah a useful analog for
comparison with the martian hematite-rich areas is the that
it is applicable to different geological settings. The extent of the hematite accumulations and the link between
the deposits and certain geological features (bright layers,
knobs, rings, etc.) seem to be characteristics shared with the
hematite-rich areas on Mars. However, on Mars, the direct
link between the geological features and the hematite deposits is more difficult to establish than in Utah. For instance,
there is very little hematite (weak or no TES anomaly) within
the etched unit. Whereas in Utah, the hematite accumulates
in close proximity to the rocks that they weathered out from.
In contrast, the areas that have been “etched” on Mars do not
have a high concentration of hematite. Instead the hematite
is located in the overlying smooth and flat plains unit characterized by the high albedo rings. If the Utah model would
be directly applicable to Mars, we would expect hematite
to have accumulated near the bleached layers, knobs and
possible mega vents. However, in our model the hematite
accumulations in the plains unit is erosional after the plains
unit was “etched” as well (Fig. 14).
Therefore, we seek a model that can explain why there is
no hematite accumulation as erosional remnants within the
etched unit. One explanation can be that there was a difference in composition between the plains unit with the rings
and the layered deposits in which the knobs of the etched
unit have formed.
In Fig. 14 we have applied a strict Utah model (including knob and ring-forming intrusions) to Mars. The fluids
are driven by differences in confining pressure between permeable near-surface layers, where precipitation occurs, and
a deeper located fluid reservoir.
Assuming that fluid-flow related alteration is responsible
for the formation of bright layers in the etched unit, still the
amount of iron in the material must have been too low to generate MGS-TES detectable accumulations of hematite. However, if the fluid flow continues, or is reinitiated after a de-
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Fig. 11. Examples of bleaching along faults and fractures in southern Utah and possible analogues in Meridiani Planum, Mars. (A) Landsat 7 ETM + bands 7,
4, 2 image of Kaibab fault and bleached Navajo Sandstone cliffs. Original image resolution 30 m/pixel, image is 18 km across (B) Bleaching (shown at arrow)
of the “Roberts Rift” fracture near Moab, Utah (GPS coordinates 38◦ 35 N, 109◦ 42 (W). (C) Bleached fault trace in the Jurassic Salt Wash Formation. Salt
Wash fault system southeast of Green River near Moab, Utah (GPS coordinates 38◦ 49 N, 110◦ 01 W). (D) Bright material (from mineral precipitation and/or
bleaching?) along ring faults (“a”) and radial faults (“b”) at large impact crater in Meridiani Planum, Mars. Original image resolution 4.56 m/pixel. (E) Bright
ridges and adjacent material in etched unit Meridiani Planum. Similar features were suggested by Hynek et al. (2002) to be possible cemented fractures. The
high albedo may be due to alteration such as bleaching in analogue to the Utah examples. Original image resolution 6.10 m/pixel.
Diagenetic hematite deposits in Utah and on Mars
311
Fig. 12. Examples of large knobs and knob formation in southern Utah and Meridiani Planum, Mars. (A) Preferentially hematite-cemented sandstone pipes
(towers) encased in fine-grained, mudstone-dominated host rock in the Jurassic Carmel Formation (also known as the Dewey Bridge Member of the Jurassic Entrada Formation). This cluster of resistant weathered towers, with individual heights up to 10+ m tall, Dubinky Well area, Utah (GPS coordinates
38◦ 43 N, 109◦ 55 W). (B) A topographic high called Mollies Nipple is a circular outcrop of dense, concentrated iron-cemented Jurassic Navajo Sandstone.
The iron-cemented center top is resistant to weathering, and much of the surrounding area of the knob is paved with weathered pieces of the iron-cemented
sandstone that help cap and protect the softer surrounding bleached white Navajo Sandstone. The photograph width is approximately 1 km. Location in Grand
Staircase National Monument (GPS coordinates 37◦ 27 N, 112◦ 05 W). ((C) and (C )) Exposure of large knobs from layered deposits in the etched unit, Meridiani Planum. Arrow “a” shows knob (cemented pipe?) still embedded in host material, “b,” “c,” and “d” show close up of exposed knobs similar to Mollies
Nipple, Utah. Original image resolution 3.06 m/pixel. ((D) and (D )) Circular knobs, some with depression on top, exposed by erosion in Meridiani Planum
etched unit. Original image resolution 3.06 m/pixel.
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J. Ormö et al. / Icarus 171 (2004) 295–316
position of a unit with much higher iron content (Fig. 14C),
then iron can be part of the process and hematite be precipitated in contact with oxidizing fluids. Fluids capable of
transporting iron in a reduced state can pass up through less
permeable layers in places where impacts or intrusions have
generated fractures. Alteration rings (mostly high albedo)
form at the surface or in permeable layers at the same time
as hematite is precipitated at oxidizing fronts. There is evidence that the hematite-rich plains unit covered larger areas,
but has been extensively eroded (Hynek et al., 2002). This
erosion may have caused accumulation of the precipitated
hematite in the same fashion as what has been suggested for
the hematite deposits in Aram Chaos (Catling and Moore,
2003). This is supported by the recent results from the Opportunity landing site where hematite concretions seem to
weather out from a sedimentary host rock to form extensive beds (Squyres and the Athena Science Team, 2004a;
Christensen et al., 2004).
4.2. Iron geochemistry
There are other factors that are important to the model:
the source for the iron, the fluid acting as transporting agent,
and the nature and form of the precipitated hematite. A strict
Utah model involves first destabilization of detrital ferrous
minerals, then oxidation of ferrous iron in detrital ferrous
minerals to form the initial red coloration. The oxidized iron
is then mobilized by chemical reduction bleaching the red
sandstones, transported in reduced form and fixed by chemical oxidation. The martian example may include some or all
of these steps. Critical steps in this process are: (1) destabilization of source iron, (2) mobilization of the iron, and (3)
oxidation to fix the iron in hematite concretions.
4.3. Geochemical simulation
Fig. 13. Image illustrating the stratigraphic relation between the knobby
etched unit (E) and the plains unit (P2). P2 is overlapping unit E indicating
a younger age. Original image resolution 5.78 m/pixel.
On Mars, the composition of the layered deposits that
may provide the source of the iron for the hematite deposits
appears to be basaltic ( 80%) (Christensen et al., 2001) or
possibly with a more andesitic composition (Bandfield et al.,
2000). The preliminary results from the Opportunity landing
site crater (Eagle Crater) support a basaltic composition at
this location (Squyres and the Athena Science Team, 2004a;
Christensen et al., 2004). No evidence has been detected for
coarse grained carbonates or crystalline silica (Christensen
et al., 2004). In fact, an apparent andesitic composition may
be due to a silica-rich weathering rind (Wyatt and McSween,
2002). If the iron source is an ferric oxide, such as a oxidized glassy basalt (Minitti et al., 2004), reduction of the
iron would be required for transport. However, if the iron
source is already reduced (ferrous), then acidic fluids would
be sufficient for transport (Fernández-Remolar et al., 2004).
Basaltic shergottite has been suggested to be similar to the
crustal composition of Mars based on spectroscopy of martian meteorites and the surface mineralogy detected by remote sensing (e.g., Singer aud McSween, 1993). Therefore,
Diagenetic hematite deposits in Utah and on Mars
313
Fig. 14. Model based on the Utah analogue for the formation of the conspicuous geological and geomorphological features and hematite deposits in Meridiani
Planum and Aram Chaos, Mars. Gray shades represent different content of iron and levels of bleaching and/or precipitation of high albedo minerals (e.g.,
carbonates, sulfates).
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J. Ormö et al. / Icarus 171 (2004) 295–316
we use shergottite as a model composition of the parent
material from which iron was derived for the hematite precipitation. Shergottite contains iron in the ferrous state. Because the iron is already reduced (as ferrous iron = Fe2+ ,
vs. ferric = Fe3+ ) interaction with reducing fluids is not
required to mobilize the iron. Rather, some chemical mechanism, such as acidity, that would destabilize the host minerals can put ferrous iron from these basaltic minerals into
solution to effectively “bleach” the host rock.
In the martian atmosphere, CO2 is the most common gas
(Owen, 1992) and a possible source of acidity in liquid water
on Mars. Acid from elevated CO2 (carbonic acid) pressure
mobilizes iron and also increases the solubility of carbonates. Another potential iron mobilizing fluid is sulfuric acid,
based on the possible jarosite identified at the Opportunity
landing site (Squyres and the Athena Science Team, 2004b).
Our modeling stresses the role of pH differences in the
mobilization of the iron. We choose to use CO2 as it is
a plausible source for acidic solutions on Mars, and as it
has been previously used in modeling the release (mobilization) of iron from ferromagnesian minerals on Mars (Marion
et al., 2003). The initial Marion et al. (2003) stage of a
four-stage model is carbonic acid weathering of primary
ferromagnesian minerals to form an initial magnesium-ironbicarbonate-rich solution. With subsequent oxidation of the
iron, HFO (hydrous ferric oxide) concretions would precipitate. The minor amounts of martian carbonates is a result of
surface acidification and/or sediment burial (Marion et al.,
2003).
Our geochemical modeling of basaltic shergottite reacting at 100 ◦ C with CO2 (gas) dissolved in water indicates
that the reactions involved in the removal of the ferrous iron
from these minerals results in the precipitation of minerals
such as talc, siderite, magnesite, dolomite, and quartz. Thus,
these minerals are related to iron mobilization and not the
later iron oxide precipitation, and would therefore not necessarily be expected in the same outcrop as the hematite concretions, which can explain the apparent absence of carbonate precipitates at the Opportunity landing site. In addition,
future alteration could lead to the absence of this mineral assemblage. The occurrence of high albedo material in areas
where we suggest iron mobilization to have occurred (e.g.,
vents with high albedo rings, bright material adjacent to fracture zones) can be an indication for localized existence of
such mineral precipitates. Hence, it would be most valuable
with ground-based data from, for instance, the high albedo
ring features.
We choose to present a model for the system at 100 ◦ C
because this is a normal upper end temperature for diagenetic burial conditions. In our model for Mars, we suggest
the hematite precipitation to occur in buried sediments, and
not at the surface. However, to allow different scenarios, we
also studied the effect of temperature change in the interval
0–100 ◦ C. Increasing the temperature moves the transition
for solubility from a pH of about 1.5 up to 3. Hence, slightly
less acidic fluids are needed to mobilize iron at warmer tem-
peratures. There is no sharp boundary between diagenesis
and hydrothermal alteration.
If the Fe2+ saturated fluid reached an oxidizing environment, hematite would precipitate. Therefore, the chemical
bleaching mechanisms for the Utah and Mars systems may
be quite different (Mars = acidity, Utah = reducing). The
mobilization of iron may occur in more than one stage of
oxidation, dissolution, and reduction such as the terrestrial
example. Ferrous silicates may be incorporated into a mafic
material. Destabilization of the ferrous silicates with acidity
would release the iron, which might be oxidized grain coatings, as in the Utah case. Chemical reduction or acidity could
mobilize the iron in the ferrous state. Iron oxides (in the ferric state) are then concentrated similar to nodules observed
in the terrestrial examples.
Precipitates initially start as polynuclear amorphous
HFO (hydrous ferric oxide), that upon maturation goes
to metastable goethite, and then hematite. It is likely that
hematite grows and becomes coarser-grained through Ostwald ripening (e.g., Ostwald, 1896; Giege et al., 1996). Ostwald ripening is the process by which larger particles grow at
the expense of smaller ones due to the higher solubility of the
smaller particles with more surface area, and due to molecular diffusion through the continuous phase. Given the right
fluid chemical conditions, sufficient pore volumes, and the
right host rock, gray hematite could be generated in layered
deposits with some different erosional patterns (possibly cemented joints). These occurrences would be consistent with
our study of geological and geomorphological features in
Meridiani Planum and Aram Chaos, having less inconsistencies than previous primary and secondary hematite origins
that have been proposed to date. Diagenetic precipitation of
hematite through interactions of reducing (or acidic) and oxidizing fluids does not require hot/thermal temperatures.
4.4. Implications
Our model is consistent with the geological setting of the
Meridiani Planum area. Meridiani Planum lacks obvious geomorphologic features indicative of thermal events such as
volcanic or tectonic sites. This area is characterized by layered deposits, with the hematite occurring in a single surface
layer. Furthermore, there are other Earth analogs of the Utah
type that are based on similar groundwater processes, showing that the Utah model for hematite deposition is common
in the terrestrial stratigraphic record.
Although acidic brines will likely suffice for the Utah
model in a martian setting, organic influence, hydrocarbons
(i.e., methane) or biomediation would help accelerate and
facilitate chemical reactions. Gas-related (methane) structures, although highly speculative, have been proposed on
Mars (Ori et al., 2000). Methane does not necessarily originate from biological processes. However, if methane was
really involved in the formation of the hematite deposit on
Mars, astrobiological implications of the hematite formation
should be investigated in the future.
Diagenetic hematite deposits in Utah and on Mars
5. Summary
In the quest for understanding the recently discovered
hematite on Mars, much of the past literature has focused
on Earth analogs that were either hydrothermal in origin, a
process of ground water leaching through soils, or layered
lake or oceanic precipitates similar to banded iron formations. There is yet another important Earth analog that could
be applicable to understanding large-scale and localized distribution of hematite on Mars. Excellent examples and superb exposures of chemical bleaching and iron oxide precipitation on a variety of scales occur in sedimentary strata of
southern Utah. The distribution of iron oxides in these areas
is controlled by the presence of preferential fluid flow pathways related to variations in porosity and permeability. The
distribution can be strataform, or cross-cutting, and deposits
can be further concentrated by erosion and preferential accumulation of these resistant mineralogies. There are several geological and geomorphological features in the Utah
hematite areas that are linked to the process of iron accumulation. Similar features appear to occur at the hematite-rich
areas in Meridiani Planum and Aram Chaos. The striking
similarities call for further detailed study and it is expected
that the new discoveries with the ongoing Mars mission
will shed considerable light on refining our understanding
of these important Earth and planetary processes.
Acknowledgments
The work by Jens Ormö was partially supported by the
Spanish Ministry for Science and Technology (Reference
AYA2003-01203) and the Spanish Ramón y Cajal program.
The work by Goro Komatsu was supported by funding from
the Italian Space Agency. The authors acknowledge the use
of Mars Orbiter Camera images processed by Malin Space
Science Systems that are available at http://www.msss.com/
moc_gallery/. Acknowledgment is made to the donors of the
American Chemical Society Petroleum Research Fund and
Grand Staircase Escalante National Monument for partial
support of this research (to Chan and Parry). We thank Dennis Netoff for use of his photographs and his input on Utah
wind-erosion and weathering pits and mega sandstone pipe
features. We are grateful to Mary Chapman and an anonymous reviewer whose thoughtful and constructive comments
helped to improve the paper.
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