[meteorite-list] Carbonates in ALH 84001: Part of the Story of Water on Mars

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Jul 1 19:19:07 2004
Message-ID: <200407012144.OAA14035_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/July04/carbonatesALH84001.html

Carbonates in ALH 84001: Part of the Story of Water on Mars
Planetary Science Research Discoveries
July 1, 2004

--- The study of multi-generational carbonate assemblages in Martian
meteorite ALH 84001 reveals a complex history of crystal formation,
growth, and alteration.

Written by Catherine M. Corrigan
Smithsonian Institution, National Museum of Natural History

Carbonate-rich regions in ALH 84001 are complicated. There are familiar
forms of carbonate as well as fascinating textural forms previously
unreported including carbonate rosettes, planiform "slab" carbonates,
distinct "post-slab" magnesium carbonates (magnesite), and carbonates
interstitial to feldspathic glass and orthopyroxene. Slab carbonates
reveal portions of the carbonate growth sequence not seen in the
rosettes and suggest that initial nucleating compositions were rich in
calcium. They formed in two major stages. The first stage involved
growth of the rosettes and slab carbonates. This step was controlled by
the rate of crystal nucleation, how fast the ingredients were delivered
to the growing crystals, and how much fluid was available. Cosmochemists
call this type of growth "kinetically controlled." Next, an alteration
event formed the magnesite-siderite (iron carbonate) layers on the
exterior surfaces of the carbonate. Post-slab magnesite, intimately
associated with silica glass, is compositionally similar to the
magnesite in these secondary exterior layers, but represents a later
generation of carbonate growth. Formation of feldspathic glasses had
little or no thermal effect on carbonates, as indicated by the lack of
thermal decomposition or any compositional changes associated with
glass/carbonate contacts.

The carbonates tell an important story about water in the ancient crust
of Mars. The presence of numerous, distinct generations of carbonate
formation and relatively clear fracture chronology within carbonate
further suggest that interactions between ALH 84001 and the crustal
fluids of Mars were discontinuous and occurred only a few times over its
4.5 Ga history. The reactivation and remobilization of fluids (causing
events such as formation of magnesite-siderite-magnesite layers and
precipitation of post-slab magnesite) and the fracturing within the rock
were almost certainly driven by impacts. The evidence for punctuated,
impact-driven interaction between rocks and fluids supports scenarios
describing temporary hydrous environments as opposed to those including
large-scale, long-term hydrologic systems including oceans. Therefore,
unless ALH 84001 is a particularly rare sample that escaped intense
alteration, the hydrosphere of Mars may not have interacted with the
rocks as thoroughly as planetary geologists have inferred from the
presence of river networks and other features formed by flowing water.

Reference:

Corrigan C. M. and Harvey R. P. (2004) Multi-generational carbonate
assemblages in Martian meteorite Allan Hills 84001: Implications for
nucleation, growth and alteration. Meteoritics and Planetary Science, v.
39, p. 17-30.

------------------------------------------------------------------------

Water on Mars

A central goal of the Mars Exploration Program is to understand the
history of water on Mars. We need to know how much there is now and was
soon after the planet formed, how it cycles through the crust, where it
resides now, how much is in magmas produced in the mantle and how it
varies throughout the mantle, and how it has shaped the surface of the
planet. The quest to understand all about water on Mars is part of a
larger effort to determine if the planet was ever habitable enough for
life to have originated and evolved. (Detailed goals for the exploration
of Mars have been developed by the Mars Exploration Program Analysis
Group, MEPAG, and can be found at the MEPAG web site
<http://mepag.jpl.nasa.gov>. Link opens in a new window.)

Planetary geologists have identified numerous features that indicate
that vast amounts of water sculpted the Martian surface: valley
networks, huge outflow channels, layered sediments, and recent gullies.
There might even have been an ocean in the northern lowlands of Mars.
Recent observations by the Opportunity rover in Meridiani Planum show
that water reworked sediments and deposited a sequence of minerals as it
evaporated. Cosmochemists have calculated that the salty water that made
all these impressive features would have evaporated to produce vast
quantities of carbonate minerals. However, observations by the Thermal
Emission Spectrometer onboard the Mars Global Surveyor spacecraft
indicate that there is only a few percent of carbonate on the surface.
Martian meteorites contain a little carbonate, which studies show formed
on Mars, not after their arrival on Earth, but the amount is very small.
Clearly we are missing something important. Martian meteorite ALH 84001
has the most carbonate of any Martian meteorite, so it might hold the
key to understanding carbonate formation on Mars. The carbonates in ALH
84001 are an important part of the story of water on Mars.

layers in Holden Crater dendritic channels

LEFT: These layers of rock were probably deposited by water on Mars.
They look very similar to layers of sedimentary rock in many parts of
the world, such as the western United States. [More information on this
MOC image <http://www.msss.com/mars_images/moc/dec00_seds/holden/> from
Malin Space Science Systems.] RIGHT: The dendritic pattern of these fine
channels and their location on steep slopes suggest they are runoff
channels. [More information on this Viking image
<http://photojournal.jpl.nasa.gov/catalog/PIA00413> from NASA/JPL
Planetary Photojournal.]

------------------------------------------------------------------------

An Important but Confusing Meteorite ALH 84001

Following its identification in 1994 as a Martian meteorite by David
Mittlefehldt (Johnson Space Center, JSC), Allan Hills (ALH) 84001
(pictured on the right) was rapidly recognized as being a very important
sample of the Martian crust. Its fame exploded with the publication of a
paper in 1996 by David McKay and his colleagues from JSC, Stanford
University, and McGill University that said the meteorite contained
evidence for life on Mars. [See PSRD article Life on Mars?
<http://www.psrd.hawaii.edu/Oct96/LifeonMars.html>] This announcement
led to a huge number of studies designed to test this interpretation.
[See the PSRD archive of articles on Mars Life Issues
<http://www.psrd.hawaii.edu/Archive/Archive-MarsLife.html>.] Whether it
contains fossil life or not, ALH 84001 was affected by water-bearing
fluids while it was still home in the ancient crust of Mars. ALH 84001
is by far the oldest of the Martian meteorites (~4.5 Ga). ALH 84001 is a
cumulate orthopyroxenite, which means it is loaded with a mineral called
orthopyroxene. The orthopyroxene is accompanied by some chromite,
feldspathic glass, augite, apatite and olivine. It formed when a magma
invaded the juvenile Martian crust. As the magma crystallized,
orthopyroxene accumulated in the magma. Slow cooling in a big magma body
allowed time to make big crystals, so it is coarse grained. However, it
has a cataclastic texture, indicating that it has been exposed to a
series of impact shock events that partly demolished the original
igneous texture. This intense shock metamorphism has resulted in the
presence of crushed zones, or granular bands, that contain crushed
orthopyroxene, chromite, feldspathic glass, olivine and other phases,
including the all-important carbonates that Ralph Harvey (Case Western
Reserve University) and I studied.

Despite being one of the most studied rocks of all time, we do not
understand its complex history. Many distinct geologic stories have been
told for ALH 84001. Most agree on initial crystallization as part of a
slow-cooling underground magma chamber, and that the rock gained its
current, highly fractured state during several post-crystallization
impact events. The time between impacts is not known. The resultant
fractures provided conduits for the passage of fluids through the rock,
and allowed the development of secondary, non-igneous minerals within them.

The secondary minerals are of particular interest in ALH 84001 because
they offer physical and chemical clues to past Martian environments.
Carbonate <../PSRDglossary.html#carbonate> minerals are the dominant
secondary phase in ALH 84001, making up ~1% of the rock. They occur in a
variety of settings and textures, from interstitial crack fillings to
conspicuously zoned clusters, semi-circular in cross-section, which have
gotten the name "rosettes." ALH 84001 rosettes vary in composition
concentrically from Ca-rich near the center through dolomite-ankerite
(Mg-rich) to alternating magnesite-siderite-magnesite (MSM) layers at
the outer edges, the siderite layers of which often contain fine,
single-domain magnetite, a central part of the debate about evidence for
life in the meteorite.

The ages of the carbonates are difficult to determine, but careful work
by Grenville Turner (University of Manchester) and colleagues and Lars
Borg (University of New Mexico) and colleages indicate an age of 3.83 Ga
to ~4.04 Ga, coincident with the period of heavy bombardment in the
inner solar system prior to ~3.8 Ga. On the basis of their textural
setting in the rock and on their ages, it is safe to conclude that
carbonate formation clearly post-dates both initial igneous
crystallization and an initial episode of fracturing by impact.

Cosmochemists are still debating how the carbonates formed. Proposed
formation scenarios include low-temperature aqueous precipitation,
evaporative processes, high-temperature reactions, and impact-induced
melting. Recent experimental studies by D. C. Golden (Hernandez
Engineering Inc., Houston) and colleagues at JSC confirm that
low-temperature precipitation (150 oC) from a saturated fluid followed
by short-term heating can reproduce many of the carbonate features seen
in ALH 84001.

Uncertainty also lies in whether there were single or multiple
generations of carbonate formation and the role, if any, of alteration
after it had formed. Adrian Brearley (University of New Mexico)
suggested that some nanometer-scale mineralogy and textures in ALH 84001
result from thermal decomposition of pre-existing carbonate materials,
particularly siderite decomposition to magnetite. The experiments of D.
C. Golden and colleagues showed that subsequent heating of carbonates
formed in the laboratory (to 470 oC) was adequate to form magnetite
crystals. Alternatively, Kathie Thomas-Keprta (Lockheed Martin) and her
colleagues at JSC still believe that at least a subset of the tiny
crystals of magnetite in ALH 84001 were made by Martian microorganisms.

The carbonate minerals in ALH 84001 record part of the story about water
in the ancient Martian crust, but the complexity of the carbonates and
the drastically different interpretations of how they formed obscure the
story. Ralph Harvey and I tried to clear up the confusion by studying
several complex, carbonate-rich regions in ALH 84001. We examined forms
of carbonate familiar to cosmochemists who had studied ALH 84001, as
well as more complete exposures of carbonate growth sequences. We placed
our observations into context with previous work, offering insight into
the complicated story of carbonate formation in this unique and
important meteorite.

------------------------------------------------------------------------

What the Carbonates Look Like

The regions examined in this study revealed a variety of textural
relationships ranging from simple to complex, with carbonate,
feldspathic, and silica glasses being the most significant phases.
Carbonate occurs as the commonly described, photogenic rosettes, and in
three other forms seen previously but not described or classified in a
uniform way. These are discrete, layered packages here termed "slab"
carbonates, massive background fill ("post-slab magnesites") and
carbonate occurring "interstitial" to feldspathic glasses and
orthopyroxene.

carbonate structures in ALH 84001,303

Backscattered electron image of a region of secondary minerals in ALH
84001,303 (the ",303" indicates the subsample number). This image shows
carbonate rosettes (CR) to the left, feldspathic glass (Fs), slab
carbonates (Slab), interstitial carbonate (IC), and orthopyroxene (Opx).

Rosettes. Rosettes were found only in ALH 84001,303 (see image above).
These rosettes are identical to those described and analyzed in many
previous studies of ALH 84001, with semi-circular cross-sections and
distinct, consistent and concentric chemical zoning. This zoning
includes alternating magnesite-siderite-magnesite (MSM) layers on the
outer portions of the carbonate sequence.

carbonate structures in ALH 84001,302

Backscattered electron image of a region of secondary minerals in ALH
84001,302. Slab carbonates (Slab) are shown. Dark, Mg-rich carbonates
found between feldspathic glass and slabs are termed post-slab magnesite
(PSM) and are often mixed with silica glass (Si). Feldspathic glass
(Fs), interstitial carbonate (IC), and orthopyroxene (Opx) are also shown.

Slab. We identified a previously undescribed form of carbonate, which we
termed "slab" carbonate. It figures prominently in both of the regions
studied in detail. These slab carbonates are elongate packages that
conform to fracture faces and exhibit chemical zoning distinctly visible
in backscattered electron images taken with an electron microprobe,
duplicating zoning commonly seen in rosettes. (See images above.) This
zoning, however, is parallel to the faces of the slab (instead of
concentric around a central point) and exhibits a more complete chemical
zoning record with sharper contacts between compositional zones. Slab
carbonates include a thin, Ca-rich layer (bright in the electron images)
at one edge and the familiar magnesite-siderite-magnesite (MSM) layers
at the opposite edge, with three consistently distinct layers found
between. Slab carbonates are typically highly fractured, in a manner
indistinguishable from that seen in rosettes, with fractures generally
crossing all layers. (See image below.) Like rosettes, slabs show no
obvious preferential association with specific mineral species. They are
found in contact with orthopyroxene, feldspathic glass, and other
carbonates.

carbonate structures including

Backscattered electron image of ALH 84001,303 showing slab carbonate
(Slab), post-slab magnesites (PSM), silica glass (Si) and
magnesite-siderite-magnesite (MSM) rims. Orthopyroxene (Opx) is present
at the top of the image. Interstitial carbonates (IC) can be seen mixed
with both orthopyroxene and feldspathic glass (Fs).

Post-slab magnesite. The regions studied here contain almost pure
magnesite that is texturally distinct from carbonate in rosettes or
slabs. We refer to these carbonates as "post-slab magnesites" as we
believe they formed as a distinct generation, post-dating zoned slabs,
rosettes, and magnesite-siderite-magnesite (MSM) layers. Post-slab
magnesites occur as numerous blebs or grains, semi-circular in cross
section (see "PSM" in the images above). They do not appear to be the
outer edges of rosettes, and are smaller and more uniform in size. These
carbonates entrain small fragments of other minerals, including other
carbonates. Post-slab magnesites have a fracturing habit different from
the zoned carbonates, with fractures formed around individual blebs, and
rarely crossing through them.

In the regions we studied, we found post-slab magnesite in contact with
the high-Ca end of the slab carbonate (bright in the photographs). It
occurs with silica glass and mixed with fragments of other phases.
Silica glass tends to be associated with fractures surrounding post-slab
magnesite blebs, and occasionally occurs as larger irregular blebs (see
image below).

post-slab carbonates in ALH 84001,302

Backscattered electron image of post-slab magnesite (PSM) in ALH
84001,302 showing blebby texture of carbonate, silica glass (Si) rims
around black carbonate blebs, and larger blebs of silica glass. This
image was contrast enhanced to better show the mottled texture within
the post-slab magnesite region. Slab carbonate (Slab), orthopyroxene
(Opx), and feldspathic glass (Fs) are also labeled.

Interstitial carbonate. We treated carbonate interstitial to larger
domains of other minerals, particularly feldspathic glass and comminuted
orthopyroxene, as a separate category called interstitial carbonate (see
"IC" in the images above). Some carbonates appear to have been
mechanically entrained by feldspathic glass, which accounts for a
significant portion of the mineral inventory in the regions we studied.
Carbonates found in the interstices of orthopyroxene surrounding the
secondary mineral regions are similar in appearance to those found
entrained within feldspathic glasses. The carbonates in orthopyroxene
have been described previously.

------------------------------------------------------------------------

What the Carbonates are Made Of

I measured the composition of the carbonates using an electron
microprobe. The diagram below shows ~800 carbonate analyses from these
sections. The data are plotted on a triangular diagram, called a ternary
diagram. Each corner of the triangle represents the pure composition of
a mineral. All data are plotted as molar abundances of MgCO3, CaCO3 and
FeCO3. The distance from a corner gives the abundance of that chemical
component in a mixed mineral.

ternary diagram of all carbonates

Ternary diagram of major element compositions of all carbonates I
measured in ALH 84001,302 and ALH 84001,303 and individual textural groups.

My new analyses overlap and extend compositional ranges previously
reported for ALH 84001 carbonates. These new data show a much more
continuous compositional trend filling gaps seen in previous work,
including significant proportions of high-Ca carbonate that were seen
only sporadically in previous studies.

ternary diagrams of carbonates

Ternary diagrams of major element compositions of carbonate rosettes
(left) and slabs (right). Compositions from the interiors of rosettes
(more central to the ternary) are clearly distinguishable from those in
the magnesite-siderite-magnesite (MSM) rims (seen at the MgCO3 apex).
Compositions of slab carbonates span the entire range of compositions.

Compositions of slab carbonates differ from rosettes only in that they
exhibit a wider range of values, varying in a nearly continuous sequence
across the ternary compositional diagram (see diagrams above). The sharp
boundaries between layers seen in backscattered electron images also
correspond to measured compositional changes. Point analysis transects
were constructed approximately perpendicular to zoning across slab
carbonates in one of the samples studied and ALH 84001,302 revealing a
consistent sequence of compositional variation. This sequence suggests
that the compositional sequence seen in carbonate rosettes is a subset
of that seen in slab carbonates.

ternary diagram of carbonates

Ternary diagrams of major element compositions of post-slab magnesites
(left) and interstitial carbonates (right). As their name implies,
post-slab magnesites cluster in the Mg-carbonate apex of the diagram,
though they do reach farther toward intermediate compositions than do
the MSM rims seen in rosettes. Compositions of interstitial carbonates
span the entire range of compositions.

The massive, space-filling post-slab magnesites span a wider
compositional range than magnesites in the magnesite-siderite-magnesite
layers of rosettes, from nearly pure MgCO3 to intermediate compositions.
The interstitial carbonates are not chemically distinct. Their
compositions span nearly the entire range, though most are intermediate,
suggesting that they represent a combination of all observed carbonate
sources (see diagrams above).

------------------------------------------------------------------------

How the Carbonates Formed

The concentric zoning of the rosettes suggests that they formed in small
pockets of water by nucleating (forming a seed crystal) in one place and
growing from it to fill up the space until the water was used up. As the
rosettes grew, their formation changed the composition of the
surrounding water, causing them to become chemically zoned. They
nucleated on any other crystal (large orthopyroxenes, crushed
orthopyroxene, feldspar) and do not appear to have reacted with the
other minerals. This type of crystal growth producing zoned crystals is
called kinetically controlled--the growth is governed by the abundance
and speed of delivery of the raw materials to the growing carbonate
crystals.

The textures of microcrystalline slab carbonates are also consistent
with formation under kinetically controlled conditions. Slab carbonates
show visible zoning parallel to fracture surfaces, suggesting nucleation
from numerous, closely spaced points on a surface instead of from widely
spaced individual points. The identical zoning in rosettes and slabs
suggests that they formed during the same growth event; the difference
between the two forms relates to the amount of available space.
Essentially, when there is sufficient fluid volume (i.e, in larger
fractures) slabs will form, while rosettes will form when volume is
limited.

Slab carbonates are of particular value in understanding the sequence of
carbonate crystallization. Their semi-planar geometry offers an
advantage in that a random slice through a slab is more likely to
intersect the full range of compositions present in three dimensions
than is a slice through a rosette. Slab carbonates should thus provide a
more complete history of carbonate formation, exposing compositions
representative of early stages of formation (high-Ca layers) rarely seen
in rosettes.

------------------------------------------------------------------------

A Five-Step Sequence of Events

Our observations lead us to propose the history of carbonate formation
shown in the diagram below. Initial formation of the rock as a cumulate
orthopyroxenite was followed by impact events resulting in an initial
set of fractures within the rock. A carbonate growth stage occurred
next, during which rosettes and slab carbonates were precipitated into
the fractures from water super-saturated in carbonate components. We do
not know where the water came from, but it seems clear that the rock and
its surroundings were not saturated in water for a long time because of
the low abundance of carbonates and very limited alteration of the
original igneous minerals. Rosettes arose from isolated nucleation sites
in relatively small fractures, forming pancakes where perpendicular
growth was limited and more spheroidal shapes where space allowed. Slabs
nucleated in rare, larger fractures. Earliest formed carbonates were
Ca-rich, but crystallization progressed toward more Mg- and Fe-rich
compositions. Occasional recharge of fluids during carbonate growth
altered the cation concentrations resulting in the variable compositions
visible as zones in the backscattered electron images shown above.

Proposed 5-step formation sequence

The chemistry, mineralogy, and textures of carbonate lead us to favor a
thermal event as the mechanism for forming the
magnesite-siderite-magnesite (MSM) layers (as suggested by Adrian
Brearley of the University of New Mexico, and others) rather than a
dramatic change in the chemical composition of the fluid. We suggest
that this event caused the conversion of an exterior, Mg- and Fe-rich
carbonate composition into the residual MSM layers, which may have
involved replacement or dissolution and redeposition of carbonate
materials. The original carbonate growth trend from high to lower Ca
compositions (slabs) was replaced with carbonates of relatively constant
Ca and widely varying proportions of Fe and Mg (MSM). This event would
have occurred with the earliest slab carbonates still attached to
nucleation surfaces, as these innermost compositions appear to be
unaffected by alteration. In addition, MSM layers are found concentric
only around the exteriors of zoned carbonates, which would have required
free space between these surfaces and the fracture walls.

Textures suggest that post-slab magnesites represent a distinct
precipitation event that took place after the MSM sequences formed and
before feldspathic glass intruded. Most fractures crossing slab
carbonates and MSM layers do not cross into post-slab magnesites.
Entrainment of slab carbonate material into post-slab magnesites also
suggests that these magnesites formed after rosettes and slabs.
Post-slab magnesites are physically mixed with silica glasses,
suggesting that either the two phases were emplaced during the same
event or silica glass precipitated after post-slab magnesite.

Unlike MSM bands, post-slab magnesites are found in contact with the
oldest (earliest formed) slab surfaces. The Ca-rich edges of the slab
must have been detached from their original nucleation surfaces allowing
space for post-slab magnesite to precipitate. Chemically, MSM magnesites
and the post-slab magnesite are similar suggesting that they formed
either by similar processes or that they represent two stages of a
single event. The first decomposed existing carbonates and deposited the
MSM rims, while the second precipitated the chemically similar post-slab
magnesites and silica glass, filling in the trend with compositions lost
during the first event.

The final step involves the injection of feldspathic glass. There are
numerous occurrences of zoned carbonates and post-slab magnesite
entrained by feldspathic glass. In addition, there exist locations where
fractures transcend the boundaries between both types of carbonate but
do not cross into feldspathic glasses. These observations indicate that
feldspathic glass was the last phase to enter these fractures. The
bulbous texture and lobate contacts with post-slab magnesite suggest
that feldspathic glass was mobile and flowed into these fractures.
Feldspathic glass intrusion produced physical effects but did not seem
to cause significant chemical changes. Glass intrusion further widened
the fractures, entraining phases already present in fractures and
further peeling some carbonates from their nucleation sites. The
intrusion of feldspathic glass separated the slabs in ALH 84001,303, as
well as the attached post-slab magnesites.

Other investigators have suggested that injection of the feldspathic
glass caused heating and decarbonation of pre-existing carbonate. Our
observations contradict this interpretation. The feldspathic glass could
not have caused the heating because the sequence of events we deduced
from our study indicates that the glass formed last. It was not around
when the MSM layers formed. MSM sequences are present around the
exterior of zoned carbonates, but neither slabs nor rosettes are
consistently altered everywhere they are in contact with feldspathic
glass, contrary to what would be expected if feldspathic glass was
responsible for MSM formation. The variation in composition of
carbonates interstitial to feldspathic glass is strong evidence that
they are unaltered, mechanically entrained materials, as opposed to
post-intrusion precipitates. In addition, many occurrences of slab
carbonates are not visibly in contact with feldspathic glass, yet still
have MSM sequences.

Why did the injection of hot feldspathic glass not cause extensive
heating? Studies of glass rheology may provide a solution to this
paradox--it might not have been hot. Although most researchers studying
ALH 84001 assume that mobilization of feldspathic glass requires high
temperatures, experiments have shown that when silica glasses are
exposed to high static pressures (more than 10 times the pressure at the
surface of the Earth) their viscosity (resistance to flow) can drop many
orders of magnitude without significant temperature elevation .
Subsequent shear then easily deforms the glass with little thermal
consequence. Impact events generating 45-60 GPa of pressure provide more
than enough stress needed to reach this transition. As a result,
feldspathic glass can flow on the millimeter scale (as suggested by
textures seen in ALH 84001) in the absence of a thermal pulse. The
impact event(s) that mobilized the feldspathic glasses seen in the
regions studied here likely provided enough shear strength to allow the
glass to flow into the fractures at low temperatures.

------------------------------------------------------------------------

Mars/NASA Implications for the Ancient Martian Crust

The presence of numerous, distinct generations of carbonate formation
and relatively clear fracture chronology within carbonate show that
interactions between ALH 84001 and crustal fluids on Mars were
discontinuous and occurred only a few times over its 4.5 Ga history. The
reactivation and remobilization of fluids (causing events such as MSM
formation and precipitation of post-slab magnesite) and the fracturing
within the rock were almost certainly caused by impact. The evidence for
punctuated, impact-driven interaction between rocks and fluids supports
scenarios describing temporary hydrous environments as opposed to those
including large-scale, long-term hydrologic systems including oceans.
Therefore, unless ALH 84001 is a particularly rare, particularly
pristine sample, the hydrosphere of Mars may not have interacted with
the rocks as thoroughly as planetary geologists infer for Mars.
Geologists see clear evidence for not only river networks, but for
erosion of them. Such a warm, wet period could have pervasively altered
rocks in the ancient highlands, yet ALH 84001 was clearly not
significantly affected by a period in which Mars was warm and wet. This
could mean that ALH 84001 is just a lucky survivor. The inconsistency
between photogeological and rock data needs to be reconciled before we
understand the details of the history of water on Mars.

------------------------------------------------------------------------

ADDITIONAL RESOURCES

Borg, L. E., Connelly, J. N., Nyquist, L. E., Shih, C. Y., Wiesmann, H.
and Reese, Y. (1999) The age of the carbonates in Martian meteorite ALH
84001. Science, v. 286, p. 90-94.

Brearley, A. J. (1998) Magnetite in ALH 84001: Product of the
decomposition of ferroan carbonate. Lunar and Planetary Science
Conference XXIX (abstract 1451).

Corrigan, C. M. and Harvey, R. P. (2004) Multi-generational carbonate
assemblages in Martian meteorite Allan Hills 84001: Implications for
nucleation, growth and alteration. Meteoritics and Planetary Science, v.
39, p. 17-30.

Golden, D. C., Ming, D. W., Schwandt, C. S., Morris, R. V., Yang, V.,
and Lofgren, G. E. (2000) An experimental study on kinetically-driven
precipitation of calcium-magnesium-iron carbonates from solution:
Implications for low-temperature formation of carbonates in Martian
meteorite Allan Hills 84001. Meteoritics and Planetary Science, v. 35,
p.457-465.

Golden, D. C., Lauer, H. V., Jr., Lofgren, G. E., McKay, G. A., Ming, D.
W., Morris, R. V., Schwandt, C. S. and Socki, R. A. (2001) A simple
inorganic process for formation of carbonates, magnetite, and sulfides
in Martian meteorite ALH84001. American Mineralogist, v. 86, p. 370-375.

Mazurin, O. V., Startsev, Y. K., and Stoljar, S. V. (1982) Temperature
dependences of viscosity of glass-forming substances at constant fictive
temperatures. Journal of Non-Crystalline Solids, v. 52, p. 105-114.

Mittlefehldt, D. W. (1994) ALH 84001, a cumulate orthopyroxenite member
of the Martian meteorite clan. Meteoritics and Planetary Science, v. 29,
p. 214-221.

Rekhson, S. M., Heyes, D. M., Montrose, C. J., and Litovitz, T. A.
(1980) Comparison of viscoelastic behavior of glass with a Lennard-Jones
model system. Journal of Non-Crystalline Solids, v. 38-39, p. 403-408.

Taylor, G. J. (2000) Liquid Water on Mars: The Story from Meteorites.
Planetary Science Research Discoveries.
http://www.psrd.hawaii.edu/May00/wetMars.html.

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Received on Thu 01 Jul 2004 05:44:10 PM PDT