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ALH84001 Abstracts From LPSC
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Life (?) in Martian Meteorite ALH 84001:
A Preview of Presentations at the Upcoming
29th Lunar and Planetary Science Conference
http://cass.jsc.nasa.gov/lpi/meteorites/29thlpscabs.html
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The 29th Lunar and Planetary Conference (LPSC), March 16-20 1998, will
feature many talks, posters, and abstracts related to the martian meteorite
ALH 84001 and its possible evidence for ancient martian life. The conference
will be in Houston, Texas at the NASA Johnson Space Center (JSC) and the
Lunar and Planetary Institute (LPI).
At least 41 presentations of research on (or related to) ALH 84001 will be
given at the 29th LPSC. Abstracts of these works are listed below, with
short summaries of their contents. The abstracts are arranged with oral
presentations first, poster presentations next, and print-only last. Oral
presentations on ALH 84001 are scheduled for Friday morning (March 20). Most
of the relevant posters will be available for viewing Thursday evening
(March 19).
To read a full abstract, click on the abstract number, which will connect
you to the online LPSC abstract in .PDF format. To view the abstracts, you
need a pdf reader, which can be obtained from Adobe.
Papers cited within the summaries are listed after the abstracts. Prepared
by Allan H. Treiman, L.P.I., 1998.
Regular Session Talks
Friday, March 20, 1998
ALH 84001: DOES IT BUG YOU?
8:30 a.m. Room B
Chairs: R. P. Harvey & I. P. Wright
1944 McKay G.* Mikouchi T. Schwandt C. Lofgren G.
Fracture Fillings in ALH84001 Feldspathic Glass: Carbonate and
Silica
The carbonate globules in ALH 84001 contain putative evidence
of ancient martian life, and are commonly associated with
feldspathic glass. From the textures of these globules and
glasses, the authors infer that the geological history of ALH
84001 is more complex that reported before. The authors
discovered a fracture, partially filled with a zoned carbonate
'pancake', that extends across pyroxene and
feldspar-composition glass, itself consisting of K-rich
fragments in K-poor matrix. The fracture and carbonate pancake
cut across the fragments and matrix, suggesting that the glass
was brecciated (as by impact) before the fracture formed and
the carbonate was deposited. Also, a shock event after
carbonate deposition is required to leave the feldspar in its
current glassy state. In other areas of ALH 84001, McKay et al.
found that the feldspar-composition glass had been granulated
or fractured, with silica (?) among the granules and filling
the fractures. McKay et al. conclude that ALH 84001 must have
once been very permeable, and was penetrated by a range of
fluids which deposited the silica and the carbonate globules.
1786 Scott E.R.D.* Krot A.N.
Carbonates in Martian Meteorite ALH84001: Petrologic Evidence
for an Impact Origin
Many petrographic observations tend to support the theory that
the carbonates and most of the shock features in ALH 84001
formed at high temperature in a single impact event (Scott et
al., 1997). New observations here include the recognition: that
most carbonate 'disks' in fractures are thinner at their edges
than their centers; that micro-disks of magnesite (10-30µm
diam.) are common near carbonate disks; that the proportion of
carbonate in pyroxene is correlated with its degree of
fracturing; and that carbonate grains can be dispersed in
linear trains within the meteorite's crush zones. Scott and
Krot explain all these carbonate textures, along with their
previous observations, as having formed from carbonate melt (±
plagioclase melt) formed during a single impact shock event.
1451 Brearley A.J.*
Magnetite in ALH 84001: Product of the Decomposition of Ferroan
Carbonate
A biogenic origin has been suggested for sub-micron sized
magnetite grains in the carbonate globules of ALH 84001. Based
on SEM and TEM observations, Brearley suggests that these
magnetites formed by the thermal decomposition of iron-bearing
carbonate minerals. Fragments of carbonate globules in
feldspathic glass are rich in defect-free 100 nm magnetite
grains in cuboid, teardrop, and subrounded shapes. Many of
these magnetites are associated with void spaces in the host
carbonate. These magnetite grains were most abundant in
ankeritic magnesite-siderite carbonates (Cc22Mg44Sd34). Bradley
infers that the magnetites formed during the heating event, to
~900°C, that melted the feldspathic glass surrounding the
carbonate fragments. Magnetites are most common in the ferroan
carbonate, as it has the lowest thermal stability of all the
carbonate compositions. The void spaces represent the carbon
dioxide evolved during decomposition of the carbonate minerals,
and this decomposition could account for the differences in
oxygen isotope compositions between the iron-rich and iron-poor
carbonate minerals.
1347 Blake D.F.* Treiman A.H. Cady S. Nelson C. Krishnan K.
Characterization of Magnetite Within Carbonate in ALH84001
A biogenic origin has been suggested for sub-micron sized
magnetite grains in the carbonate globules of ALH 84001. Blake
et al. used TEM to examine magnetites in dark rims on a
carbonate globule and nearby in the carbonates, and suggest
that these magnetite grains formed by inorganic
solution-precipitation at low temperature. First, the authors
used Energy Electron Loss Spectroscopy (EELS) to confirm that
this magnetite is truly magnetite and not the similar iron
oxide maghemite. Magnetite grains are fairly common in the
ankeritic carbonate near the dark rims. These grains are
defect-free, and are commonly in epitaxial or near-epitaxial
orientations with the host carbonate mineral. Many of the
magnetites are associated with holes or void spaces in the
carbonate, and some holes do not contain magnetite or other
minerals. The authors interpret the magnetites as inorganic
precipitates from the fluid that formed the carbonates, not as
biogenic products.
1494 Thomas-Keprta K.L.* Bazylinski D.A. Golden D.C. Wentworth S.J.
Gibson E.K.Jr. McKay D.S.
Magnetite from ALH84001 Carbonate Globules: Evidence of
Biogenic Signatures?
McKay et al. (1996) suggested a biogenic origin for sub-micron
sized magnetite grains in the carbonate globules of ALH 84001.
The authors examined many of these magnetites by TEM, and found
that a majority are similar in size, shape and structure to
magnetite formed by magnetotactic bacteria. Thomas-Keprta et
al. extracted thousands of magnetite grains from globule
fragments by dissolving the host carbonate in acid. The
magnetites range from ~20 to ~100 nm in size. Most are cuboidal
or irregular, with a significant proportion being
parallelepipeds. Elongate (whisker) magnetite grains were rare
but present. The parallelepiped magnetite crystals are most
similar in shape and size to magnetite grains produced by
magnetotactic bacteria, and are not similar to known abiogenic
magnetite grains. These parallelepiped magnetites may reflect a
complex, partially biogenic, martian history.
1928 Foley C.N. Humayun M.* Davis A.M. Kagan O.
Chemical and SEM Studies of Mineral Assemblages Within ALH84001
This abstract, the first work from an extended study of ALH
84001, reports principally on sulfide minerals in
carbonate-rich "pancakes" along fractures surfaces. The
"pancakes" are abundant on some fracture surfaces and rare on
others, do not appear to be associated with plagioclase glass,
and commonly have sulfide-bearing rim zones. The authors have
recognized a new type of carbonate: "pancakes" that are rich in
Fe-sulfides (mineral species not reported), dark colored rather
than orange (as with most "pancakes"), and contain up to 8%
weight sulfur. Their Fe-sulfides occur as ~0.1µm (100 nm)
grains concentrated along cleavage traces in the carbonate
grains of the "pancakes." These small sulfide grains are
inferred to have been injected into the carbonate "pancakes" as
sulfide-rich melts formed during the shock event that formed
feldspar-composition glass in the meteorite.
1425 Jones J.H.* Schwandt C.S.
Experimental Investigations of the High-Temperature Stability
of Siderite: Implications for the Origin of ALH 84001
Carbonates
The temperature of formation of the carbonate globules in a
matter of dispute; here, the authors performed laboratory
experiments at high pressure and temperatures to explore the
stability of carbonate minerals like those in ALH 84001. Jones
and Schwandt mixed magnesite, dolomite, and siderite carbonates
to simulate the most common carbonate composition in ALH 84001,
Cc20Ms40Sd40, and reacted the mix at 500-1200°C and 10 kbar
pressure at an oxygen fugacity of ~ NNO. At 600°C, the siderite
had begun to decompose, and above 800°C it had completely
reacted with the containment capsule to form (Fe,Ni)O and CO2.
These results are comparable with those from the literature,
and suggest that sideritic carbonate minerals, as in ALH 84001,
are not likely to be produced above ~500°C.
1630 Treiman A.H.* Ionov D.A. Amundsen H.E.F. Bunch T. Blake D.F.
A Terrestrial Analog for Carbonates in ALH 84001:
Ankerite-Magnesite Carbonates in Mantle Xenoliths and Basalts
from Spitsbergen (Svalbard), Norway
It has been difficult to understand the origin of the carbonate
globules and pancakes in ALH 84001, in part because no similar
carbonates have been reported from the Earth, where their
history and geologic setting can be studied in detail. Here,
the authors report a comparable (not identical) occurrence of
zoned carbonate globules and pancakes in mantle xenoliths and
basalts from Spitsbergen Island, Norway. The Spitsbergen
globules are zoned from ankerite or siderite cores outward to
magnesite rims, with many fine zones of Fe-richer or poorer
carbonate. The globules are present in vesicles in the basalt
and in glass within the xenoliths, and also as replacements of
xenolith olivine. Pancakes are present along fractures in the
xenoliths and host basalt. The carbonates are older than the
silica and smectite clays with which they are found. Presence
of carbonate globules in vesicles and fractures in the host
basalt implies that they formed after the basalt solidified.
The temperature of globule formation is poorly constrained, and
it is not clear if the carbonate is derived from mantle
carbonates (present in the xenoliths) or from Earth surface
materials.
1787 Boctor N.Z.* Wang J. Alexander C.O.A. Hauri E. Bertka C.M. Fei
Y. Humayun M.
Petrology and Hydrogen and Sulfur Isotope Studies of Mineral
Phases in Martian Meteorite ALH84001
The authors measured relative abundances of the isotopes of
hydrogen and of sulfur as clues the interactions of volatiles
with ALH 84001. Hydrogen isotope values measured in carbonate
globules and phosphate mineral grains (whitlockite) are nearly
all "heavy" compared to Earth hydrogen, d D = +165 – +287 ‰.
This range is much lower ("lighter" hydrogen) than the current
martian atmosphere's d D ~ +5000 ‰, suggesting that ALH 84001
and the current martian atmosphere have had little interaction.
Two of the three sulfur isotope measurements on pyrite are
consistent with previous results, d 34S ~ +7 ‰; the third
analysis shows significantly "lighter" sulfur, d 34S ~ +2 ‰,
which is nearly identical to magmatic sulfur from other martian
meteorites. The range of sulfur isotope abundances is not
consistent with the actions of sulfate-reducing bacteria, but
is consistent with pyrite formation at temperatures much below
~1000°C.
1872 Farquhar J.* Thiemens M.H. Jackson T.
D 17O Measurements of Carbonate from ALH 84001: Implications
for Oxygen Cycling Between the Atmosphere-Hydrosphere and
Pedosphere of Mars
The authors' measurements of oxygen isotopes show that the
carbonates in ALH 84001 had a different source oxygen than did
the silicate minerals. The relative proportions of the three
oxygen isotopes (16O, 17O, 18O) change during normal chemical
processes, but the ratio 18O/16O must change twice as fast as
the ratio 17O/16O ratio. However, the martian meteorites have
relative ratios of 16O:17O:18O that are different from those on
the Earth, which means that the meteorites cannot be from the
Earth. This difference is given as the value D 17O with the
Earth defined as zero; the martian meteorites have D 17O =
+0.3‰. Water from the martian meteorites has D 17O = +0.8‰,
distinctly different from the bulk meteorites and implying that
water on Mars has not chemically equilibrated with the silicate
minerals of its crust (Karlsson et al., 1992; Romanek et al.,
in press). Here, the authors measured oxygen isotope abundances
of carbonates in ALH 84001, and found that they also had D 17O
= +0.8‰. This D 17O value for the carbonates suggests that they
formed at low temperature from water that had communicated with
the martian atmosphere. The source of the high D 17O in the
martian water may be photochemical reactions involving CO2 in
the atmosphere.
1205 Becker L.* Popp B. Rust T. Bada J.L.
The Origin of Organic Matter in the Martian Meteorite ALH84001
No Abstract Available
1812 Clemett S.J.* Dulay M.T. Gillette S. Chillier X.D.F. Mahajan
T.B. Zare R.N.
Are the Polycyclic Aromatic Hydrocarbons in ALH84001 of
Extraterrestrial Origin?: A Reevaluation
Polycyclic aromatic hydrocarbons (PAHs) are among the evidence
cited as possible traces of ancient martian life in ALH
84001(McKay et al., 1996). However, Becker et al. (1997)
claimed that these PAHs were not martian, but were terrestrial
contamination acquired by ALH 84001 in Antarctic. Here, Clemett
et al. dispute Becker's claims, and conclude that the PAHs are
indigenous to ALH 84001 and therefore martian. 1) Becker et al.
claimed that carbonate minerals act as scavengers for PAHs in
solution. The authors were unable to duplicate the scavenging
experiments of Becker et al., and cite flaws in their
experimental designs. 2) Non-martian meteorites from the same
area of Antarctica contain considerably lower abundances of
PAHs than does ALH 84001. 3) Clay-rich, carbonaceous
micrometeorites, which are expected to be most efficient at
scavenging PAHs, show variable amounts of PAHs, which vary from
micrometeorite to micrometeorite and unlike those in ALH 84001
(esp. in being abundantly alkylated). 4) Meltwater from
Antarctic ice contains a negligible PAH content, in contrast to
analyses by Becker et al. 5) New data shows that PAHs in ALH
84001 are depleted near its fusion crust and absent in the
fusion crust. This distribution is not consistent with
Antarctic contamination, but with indigenous martian PAHs. The
depletion near the fusion crust is ascribed to oxidation
(burning) during the meteorite's flight through the Earth's
atmosphere.
1263 Stephan T.* Rost D. Jessberger E.K. Greshake A.
Polycyclic Aromatic Hydrocarbons in ALH84001 Analyzed with
Time-of-Flight Secondary Ion Mass Spectrometry
Polycyclic aromatic hydrocarbons (PAHs) are among the evidence
cited as possible traces of ancient martian life in ALH
84001(McKay et al., 1996). Stephan et al. used time-of-flight
secondary-ion-mass-spectrometry (TOF-SIMS) to confirm that ALH
84001 contains significant quantities of PAHs, and that these
PAHs are principally unalkylated. The PAH mass spectra here is
very different from that in McKay et al. (1996) because of the
different analytical methods -- it is reasonable that both
analyzed the same mix of PAHs. The PAHs detected here are not
laboratory contaminants, as no PAHs were found in embedding
material or standards. Unlike the results of McKay et al.
(1996), the authors here found that the PAHs were evenly
distributed throughout ALH 84001, not closely associated with
the carbonate globules. So, the PAHs are not intimately related
to the carbonates, their nanofossil forms, or other proposed
signs of ancient martian life in ALH 84001.
1184 Jull A.J.T.* Beck J.W. Courtney C. Jeffrey D.A.
Carbon Isotopic Evidence for Terrestrial Organic Compounds
Found in Some Martian Meteorites
The authors measured the isotopic composition of carbon in ALH
84001: 12C (stable), 13C (stable), and 14C (radioactive). Most
of the organic (non-carbonate) carbon is terrestrial
contamination, but a small portion may be pre-terrestrial.
Carbon from martian meteorites tends to have much higher
13C/12C ratios (expressed as d 13C) than Earth carbon; young
Earth carbon has detectable 14C, while old and extraterrestrial
carbon contains almost no 14C. The authors extracted carbon
from samples of ALH 84001 by acid dissolution, and by heating
in oxygen at various temperatures. Carbon extracted at <400°C
is terrestrial contamination - it has significant 14C and low d
13C like terrestrial organics. Acid-soluble carbon (= most of
the carbon released during heating from 400 and 700°C), is from
carbonate globules - that carbon is not terrestrial as it has
no 14C and has high d 13C. A small part of the carbon is not
acid-soluble and is released above 400°C - it has no 14C and
has moderate d 13C. This last carbon, about 20% of the
non-carbonate (organic) carbon in ALH84001, may be
pre-terrestrial. Much of this data is available in Jull et al.
(1998).
Posters
Tuesday, March 17, 1998
POSTER SESSION I
7:30 P.M. Space Center Houston
1107 Gupta A. Freund F.
Fatty acids and glycolamide extracted from olivine single
crystals
While this abstract does not mention ALH 84001 specifically,
it suggests a way that that indigenous organic compounds in
ALH 84001 could have formed without life. When a crystal
grows from a magma, it incorporates small quantities of H2O,
CO2, and nitrogen that were dissolved in the magma. When the
crystal cools, these chemical could react with each other
and form short- or long-chain aliphatic hydrocarbons, with
or without functional groups containing oxygen (e.g.,
organic acids) or nitrogen (e.g., amines). The authors
tested this hypothesis by extracting organic material from
olivine crystals from an Earth basalt, after careful
cleaning to avoid contamination. The olivine contained ppm
levels of these organic compounds, including long-chain
aliphatic hydrocarbons (mostly saturated), fatty acids, and
glycolamide (C2H5NO2). These hydrocarbons could conceivably
be non-biological precursors to the PAHs in ALH 84001.
Thursday, March 19, 1998
POSTER SESSION II
7:30 P.M. Space Center Houston
LIFE ON MARS AND EARTH
1505 Allen C. C. Graham C. R. Combie J. Albert F. G. Steele A.
McKay D. S.
Biological Signatures in Carbonates: Yellowstone National
Park
As part of a long-term study of possible terrestrial analogs
to biogenic features in ALH 84001, the authors are studying
mineral deposits and microorganisms in a hot spring from
Yellowstone National Park, U.S.A. Aragonite (CaCO3)
deposits, water (~70°C, pH ~ 6.4), and bacterial filaments
were collected. Most of the bacteria were Thermothrix sp.
The aragonite needles were arranged in distinctive
"streamer" fabrics, which are characteristic of bacterial
precipitation. No intact cells were found in the aragonite
samples, but incubated aragonite samples did yield a few
microbes of unknown affinity. The aragonite is rich in
biofilm, organic mucus, which contains acid-resistant
spherules of 100-200 nm diameter. The spherules are similar
to nanobacteria (?), but their actual nature is not yet
known.
1509 Allen C. C. McKay D. S.
Biomarkers in Thermal Spring Carbonates: Implications for
Mars
As part of a long-term study of potential signatures of
biological activity (biomarkers) in planetary samples,
especially from Mars, the authors focus here on thermal
spring carbonates. Many types of microbes thrive in
carbonate hot spring environments; these include
filamentous, spherical (coccoidal), and rod-like (bacillar)
shapes. Bacteria themselves decompose rapidly and are rarely
preserved in carbonates. Bacterial organic matter is also
rare in carbonate deposits that formed at >30°C. Biofilms
(polysaccharide slimes) are preserved through desiccation,
and they are readily mineralized. "Nanofossil" spheres of
100-200 nm (0.1 - 0.2 µm) are abundant in the deposits of
some springs; larger spheres of 300-500 nm (0.3 - 0.5µm) are
common in others.
1506 Allen C. C. Taunton A. E. Taylor M. R. McKay D. S.
Microbes in Carbonate Thermal Springs: Hot Springs National
Park
As part of a long-term study of possible terrestrial analogs
to biogenic features in ALH 84001, the authors are studying
mineral deposits and microorganisms in a hot spring from Hot
Springs National Park, U.S.A. The Hot Springs, which are
~65°C at a nearly neutral pH of ~ 7.3, precipitate aragonite
and calcite at water surfaces. Bacteria are common in
underground waters from the springs, including 1-2 µm rods,
and filaments of 0.1 µm diameter and up to 6 µm long. The
waters also deposit orange films of amorphous Fe-Si-O
material which is associated with a distinctive biota:
spherical bacteria 5 - 15 µm diameter, rod-shaped bacteria
0.5 - 1 µm long, and spherical shapes < 1 µm diameter. The
carbonates are precipitated abiogenically, and experiments
reported here also show that the orange films can form
without biological action.
ALH 84001
1332 Cooney T.F. Scott E.R.D. Krot A.N. Sharma S.K. Yamaguchi A.
Confocal Raman Microprobe and IR Reflectance Study of Minerals
in the Martian Meteorite ALH84001
As an aid to understanding the geological history of ALH
84001, the authors investigated some aspects of its
mineralogy. Silica (SiO2) is an uncommon accessory phase.
Raman spectroscopy showed that most of the silica is glassy
(amorphous) with a little crystalline quartz mixed in. The
Raman spectrum of the silica glass suggests shock pressures
above 31 GPa. The authors found two distinct phosphate phases
in ALH 84001: chlorapatite and merrillite. This is the first
sure report of chlorapatite, based on infrared (IR)
reflectance and Raman spectroscopy. There is no hint of water
(hydroxyl) in the chlorapatite. ALH 84001 also contains the
anhydrous phosphate merrillite, not whitlockite as has been
commonly reported. Finally, the authors obtained Raman spectra
of the cores of carbonate globules, average composition
(Ca.19Mg.47Fe.34)CO3. The Raman spectrum was consistent with a
single carbonate mineral, implying that these carbonates are
metastable single phases and not ankerite and siderite
interlayered (exsolved) at the submicron level.
1196 Treiman A H. Treado P.
Martian Maskelynite? Raman Spectra of Plagioclase –
Composition Glasses from ALH 84001, EETA79001, and ALHA77005
It has been unclear if plagioclase - composition glasses in
ALH 84001 were originally maskelynite, a shock glass formed
without melting, or are melt glasses from cooled liquids. The
authors obtained Raman spectra of crystalline plagioclase and
plagioclase-composition glasses to resolve this question.
Raman spectra of plagioclase crystals and from maskelynite
from the Manicouagan impact structure (Earth) all show sharp
Raman emission lines consistent with crystalline material.
Melt glasses from ALHA77005 show only broad Raman emissions.
However, the martian meteorite EETA79001 contains classic
maskelynite (shaped like plagioclase crystals, with chemical
zoning and twins preserved as differences in refractive index)
that has only broad Raman emissions, as is characteristic of
melt glass. Plagioclase-composition glass in ALH 84001 also
has only broad Raman emissions, and so is likely a melt glass.
The concept of 'maskelynite' may not be useful in
spectroscopic or micro-structural studies.
1830 Greenwood J.P. McSween H.Y.Jr.
Origin of Stoichiometric Feldspathic Glasses in ALH84001 by
Mixing of Plagioclase and Orthoclase During Multiple Shock
Events
In ALH 84001, all that remains of original feldspars and
silica is feldspathic glass. The authors investigated reports
that the glasses are non-stoichiometric (they lost or gained
some elements during melting), using electron microprobe
methods. Greenwood and McSween found that the glasses were
nearly all stoichiometric, representing mixtures of
plagioclase feldspar and orthoclase (actually, Or55Ab45 alkali
feldspar: ed.) composition. There are four varieties of
feldspathic glass. 1) Maskelynite (?) in the shapes and
composition of igneous plagioclase. 2) Mobilized glass of
plagioclase composition in veins and as crack fillings. 3)
Glass with the composition of orthoclase feldspar (actually,
Or55Ab45 alkali feldspar: ed.). 4) Mixed glasses, representing
mixtures of plagioclase and orthoclase composition feldspars.
This variety of glasses cannot be explained by a single shock
event. At least two shocks are required - one to produce the
mixed glasses, and another to make the mobilized glass veins
that cross the mixed glasses.
1280 Shearer C.K. Adcock C.T.
The Relationship Between the Carbonate and Shock-produced
Glass in ALH 84001
Shearer and Adcock examined mineral textures in thin sections
of ALH 84001 to evaluate the hypothesis that its carbonate
globules and feldspathic glass formed as immiscible melts. The
authors find textural evidence that indicates otherwise --
that the carbonate globules were precipitated into fractures
at low temperature, and were subsequently fragmented and
disaggregated by feldspathic shock melts. Not only are the
carbonate "pancakes" deposited in open fractures, the authors
find evidence that the rounded carbonate globules were
deposited in open cavities. They infer that feldspathic glass
surrounding globules was emplaced after the globules formed.
Shearer and Adcock also find that carbonate globules were
detached from their substrates, shattered, and transported by
mobile feldspathic glass.
1934 Sears D.W.G. Kral T.A.
SEM Imaging of Martian and Lunar Meteorites and Implications
for Microfossils in Martian Meteorites
The authors examined five lunar meteorites, recovered from
Antarctica, to see if the putative bacterial shapes in ALH
84001 might also occur without intervention of life. The lunar
meteorites contain rare objects that are similar to the
putative bacterial shapes. Sears and Kral examined original
and fracture surfaces of anorthositic and basaltic lunar
meteorites, using the same procedures as did McKay et al.
(1996). Many kinds of sub-micron objects were found, including
ovoid and elongate shapes similar in size and shape those in
reported from ALH 84001. Sears and Kral did not determine the
true nature of all these objects, but their presence in lunar
meteorites suggests that they are not characteristic of
biological activity on Mars.
1362 Westall F. Gobbi P. Gerneke D. Mazzotti G.
Microstructures in the Carbonate Globules of Martian Meteorite
ALH84001: Preliminary Results of a High Resolution SEM Study
In order to understand the "fossil bacteria" structures found
in ALH 84001, the authors are examining more fragments of the
meteorite with a high-resolution field-emission gun SEM.
Samples were uncoated and also coated with a very thin layer
of C-Pt. No unambiguous bacteria or bacterial fossils were
found. Surfaces of carbonate minerals are decorated with fine
raised lamellae, as were described by Bradley et al. (1997).
Many surfaces are also coated with 50-100 nm spherules of
amorphous silica, such as commonly are found in hydrothermal
(hot spring) deposits on Earth. The carbonate minerals contain
rounded to sub-angular inclusions to 130 nm long which are
unidentified at present. These inclusions could be
mineralogical or biological in origin.
1496 Morris P.A. Allen C.C. Gibson E.K.Jr. McKay D.S. Thomas-Keprta
K.
Reexamination of the Warrawoona Group Fossils (Towers
Formation, Western Australia, 3.3 to 3.5 Ga): Analogs of Mars
Meteorite Fossils?
As part of a long-term study of potential signatures of
biological activity (biomarkers) in planetary samples, the
authors are studying bacterial fossils in cherts of the
Warrawoona Group (Australia). The Warrawoona carbonaceous
cherts formed ~3.4 billion years ago, and contain some of the
Earth’s oldest fossils. The cherts contain a variety of
filament shapes of presumed biologic origin. Most striking are
filaments ~2 µm wide and over 100µm long, which are associated
with spherical objects (cocci?) ~2 µm diameter. Also present
are spheroidal objects in three groups: 26-33 µm diameter with
convoluted surfaces; 16-18 µm diameter with smoother surfaces;
and 5-12 µm with depressions and protuberances. All these
objects are mineralized (filaments by silica; spheres mostly
by iron oxides and carbonates). These objects are similar to
accepted fossil forms from the Warrawoona, and confirm the
value of SEM analysis (as was done with ALH 84001) in
recognizing mineralized fossils of bacteria.
1156 Flynn G.J. Keller L.P. Miller M. A. Jacobsen C. Wirick S.
Organic Compounds Associated with Carbonate Globules and Rims
in the ALH 84001 Meteorite
Several varieties of organic materials are present in ALH
84001, including terrestrial contaminants and possible
indigenous (Martian) material. The authors examined
ultramicrotomed thin sections with Synchrotron X-ray
Transmission Microscopy (SXTM) and infrared (IR) absorption
spectroscopy at the National Synchrotron Light Source
(Brookhaven NY). The analytical spot size for these methods is
~50 nm, 1000 times smaller than that available to McKay et al.
(1996). SXTM data showed that the carbonate globules contain
organic carbon (C-C, C=C, C-O and C-H bonds), that the carbon
compounds vary somewhat within the globules, and that the
organic carbon in the globule rims is different from that in
their cores. IR spectra showed that the organics of the
globule interiors is rich in -CH3 groups (characteristic of
small organic molecules), while organics in the globule rims
are dominated by -CH2- groups (characteristic of aliphatic
chain hydrocarbons). These results seem inconsistent with the
simplest model of terrestrial contamination -- infiltration
and evaporation of a hydrocarbon-bearing water.
1195 Treiman A.H.
The History of ALH 84001 Revised: Multiple Shock Events
Understanding the origin of possible biological feature in ALH
84001 depends on understanding the meteorite's history, before
and after formation of the possible traces of ancient martian
life. Here, Treiman proposes that a complex history of 4 or 5
shock impact events is needed to explain the mineral textures
and compositions of ALH 84001. The first impact granulated
portions of the meteorite, and a second impact turned some of
the feldspars in the meteorite to glass. Then followed
deposition of the carbonate globules with their putative
evidence of ancient martian life. After carbonate formation,
one impact shock is required to produce the feldspathic
glasses that cut across and disrupt the globules. A second
impact is required by some paleomagnetic evidence, and a third
could represent ejection from Mars. This complex history is
consistent with martian geology -- land surfaces as old as ALH
84001 are heavily cratered. As for discussions about putative
traces of ancient life in the meteorite, these traces must
have been modified by at least two shock events. It is
possible that elongate magnetites did in fact grow by vapor
deposition at high temperature, but that growth could have
been in an impact event after formation of the carbonate
globules. This work will be published as Treiman (1998).
1825 Mathew K.J. Marti K.
Nitrogen and Noble Gas Isotopic Signatures in Bulk ALH84001
with Carbonates
Isotope abundance ratios of nitrogen and noble gasses are
distinctive tracers for martian atmosphere in the martian
meteorites. The authors measured isotope abundances in
nitrogen and noble gases that were emitted from ALH 84001 as
it was heated. Most of the nitrogen is relatively depleted in
the heavy nitrogen isotope, 15N, compared to Earth standard --
the nitrogen has d 15N < 0‰. This "light" nitrogen may be from
the interior of Mars. In contrast, nitrogen released while the
carbonates decompose is "heavier," d 15N up to +7‰. This is
much "lighter" than the current martian atmosphere, which has
d 15N = +620‰, meaning that the carbonates have not reacted
with the current martian atmosphere. Isotope abundances of
xenon are like those reported earlier. Their sample of ALH
84001 was relatively rich in 38Ar, which is produced in
interplanetary space as cosmic rays hit calcium atoms. The
differences in 38Ar abundances in different samples suggest
that the published 16,000 year exposure duration for ALH 84001
may be an oversimplification of a complex history.
1705 Barlow N.G.
Status Report on the Search for Source Craters of ALH84001
Last year, the author reported on a search for suitable source
craters for ALH 84001, based on global imagery of Mars from
the Viking orbiters. Now, the search has been extended to
include circular craters larger than 20 km diameter and
elliptical craters larger than 10 km diameter. The craters
must be minimally degraded and be on ancient (Noachian)
surfaces, near ancient surfaces, or on older crater ejecta
that could likely contain ancient materials. Two suitable
elliptical craters were reported last year. At this point, six
more potential source craters on ejecta blankets or near the
highlands-lowlands boundaries have been identified. It is
estimated that approximately 600 craters in the ancient
martian highlands may also be suitable as potential source
craters for ALH 84001.
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1894 Bada J.L. Glavin D.P. McDonald G.D. Becker L.
Amino Acids in the ALH84001 Martian Meteorite
Short review of their recent Science paper (Bada et al.,
1998), which showed that ALH 84001 contains amino acids that
are terrestrial contamination.
1422 Bell M.S. Hörz F. Reid A.
Characterization of Experimental Shock Effects in Calcite and
Dolomite by X-Ray Diffraction
Several published histories of ALH 84001 have its carbonate
globules formed from carbonate shock-melts. The authors here
experimentally shocked common carbonate minerals to pressures
of 60 GPa. The minerals were deformed strongly, and showed
broadened X-ray diffraction peaks and some peak position
shifts. No evidence is given that these carbonate minerals
either decarbonated or melted at 60 GPa.
1757 Bradley J. P. McSween H. Y. Harvey R. P.
Mechanisms of Formation of Magnetite in Martian Meteorite
ALH84001
It has been argued that sub-micron magnetite grains in the
carbonate globules of ALH 84001 are the products of biological
activity. The authors here suggest that the magnetites formed
via several non-biological mechanisms. Their earlier work
showed that many elongate magnetite grains (whiskers) grew via
screw dislocations -- a common mechanism of inorganic crystal
growth -- and that some elongate magnetite crystals are
oriented in specific directions with respect to their host
carbonate, which implies that the magnetite crystals grew in
place on the carbonate minerals (Bradley et al., 1996, 1997).
Other magnetite whiskers lack screw dislocations, and
magnetite is also abundant as parallelepipeds, plates, blades,
and ribbons. The wide range of magnetite morphologies and
structures means that they grew by many different mechanisms,
which suggests that growth conditions (e.g., degree of
supersaturation) and not biology controlled growth of these
magnetite grains. The authors find it most reasonable that the
magnetite crystals grew from a vapor phase at high
temperature. A vapor phase seems required to form
whisker-shaped magnetite grains, and the other magnetite
morphologies are known to form during vapor phase growth.
1452 Brearley A. J.
Microstructures of Feldspathic Glass in ALH 84001 and Evidence
for Post Carbonate Formation Shock Melting
To evaluate the hypothesis that ALH 84001’s carbonate globules
contain traces of ancient martian life, it is important to
understand their formation and history. Using SEM and TEM
methods, Brearley studied the carbonate globules in relation
to the feldspathic glass that commonly surrounds them. In
places, carbonate globules have been disrupted to irregular
fragments embedded in feldspathic glass. Boundaries between
carbonate and glass are 50-100 nm wide zones that contain
elements characteristic of both carbonate and feldspathic
glass; this suggest that glass has replaced or reacted with
the carbonates. Away from the carbonates, the feldspathic
glass consists of rounded domains of feldspar-like
compositions separated by films of silica-rich compositions.
This texture looks like partial replacement of feldspar by
silica, as occurs in vapor-rich volcanic environments on
Earth. Some of the silica is crystalline, possibly the
high-pressure silica mineral stishovite. These and other
observations suggest that: the carbonate globules were
deposited into heterogeneous, non-stoichiometric material
(possibly from fluid alteration); that the carbonates were
deposited by a fluid (not as a melt); and that feldspathic
glass was melted and mobilized after the carbonates formed.
1433 Gibson E.K.Jr., McKay D.S., Thomas-Keprta K.
Exobiological Features Within ALH84001: Current Observations
Authors of the original hypothesis that ALH 84001 contains
traces of ancient martian life respond to recent criticisms
and review available data. Bada et al. (1998) and Jull et al.
(1998) both found that ALH 84001 contains a significant mass
of terrestrial organic material, presumably contamination from
its time in Antarctica. The authors point out that their
hypothesis did not include amino acids as martian biomarkers,
only PAH organic compounds. Further, they note that
terrestrial contamination and alteration is nearly ubiquitous
in Antarctic meteorites, and that these should contain
terrestrial 14C. Gibson et al. again emphasize the need to
examine many potential biomarkers, notably biofilms (which
they have reported in ALH 84001).
1569 Protheroe W.J.Jr., Stirling J.
Cathodoluminescence Study of Fragments of the Martian
Meteorite ALH 84001
Feldspathic glasses and pyroxene in ALH 84001 emit visible
light when bombarded by electrons in an electron microscope
(i.e., they cathodoluminesce). One grain of feldspathic glass
appeared cross-hatched in emitted cathodoluminescent light.
The cause of this pattern is not known.
1754 Shearer C. K. Adcock C. T.
The Composition and Distribution of Feldspathic Shock Glass in
ALH 84001
Feldspathic glass in ALH 84001 is commonly associated with
carbonate globules and may hold clues to the geologic history
of the meteorite. In the glass compositions and structures,
Shearer and Adcock see vestiges of a hydrothermal alteration
event early in the history of the meteorite. Chemical
compositions of the glass range from nearly stoichiometric
plagioclase, to silica-enriched, to potassium-rich (K-rich),
as reported in other studies. The K-rich glasses are also
silica rich (i.e., are depleted in Al). The K-rich glasses are
commonly found with broken carbonate globules and in fractures
and cavities near olivine grains in the orthopyroxene. The
plagioclase-composition glasses, however, tend to enclose the
carbonate globules. The plagioclase glasses are, however,
richer in sodium than typical martian igneous plagioclase,
which suggests that their feldspar compositions have been
altered. The authors suggest that the increased sodium content
arose during low-temperature aqueous alteration before shock;
silica was deposited in ALH 84001 at the same time, leading to
the silica-rich glasses.
1281 Shearer C.K. & Adcock C.T.
The Origin of Olivine in Martian Meteorite ALH 84001. The
Distribution of Olivine
Small quantities of the mineral olivine is present in ALH
84001, but its origin is not clear. It might be related to
formation of the carbonate globules, or might not. To help
resolve this uncertainty, the authors investigated the spatial
distribution of olivine in ALH 84001. The olivine does not
occur randomly or in pyroxene cores as might be expected if it
were igneous. Rather, the olivine is present as clusters of
grains in orthopyroxene adjacent to fractures containing
disrupted carbonate globules or K-rich feldspathic glass.
Shearer and Adcock infer that the olivine is not strictly
magmatic, nor is it related to formation of the carbonate
globules. They offer two hypotheses: 1) the olivine was
originally magmatic, but was re-distributed during shock
events, or 2) the olivine formed during shock metamorphism by
reaction of orthopyroxene, CO2, and feldspathic glass.
1286 Shearer C.K. & Leshin L.A.
The Origin of Olivine in Martian Meteorite ALH 84001. The
Oxygen Isotopic Systematics of the Olivine
Small quantities of the mineral olivine is present in ALH
84001, but its origin is not clear. It might be related to
formation of the carbonate globules, or might not. To help
resolve this uncertainty, the authors analyzed the oxygen
isotope composition of the olivine and adjacent minerals. The
oxygen isotope composition of the olivine ranges from d 18O =
+4.3-5.3‰, identical within analytical uncertainty. The
olivine d 18O is essentially identical to that of the host
orthopyroxene, which implies that the oxygen in both minerals
equilibrated with each other at high temperature. The
carbonates in ALH 84001, however, have a huge range of d 18O,
from about +5 to +25‰, which make it unlikely that the olivine
and the carbonates formed in the same event.
1489 Thomas-Keprta K.L. McKay D.S. Wentworth S.J. Stevens T.O.
Taunton A.E. Allen C.C. Gibson E.K.Jr. Romanek C.S.
Mineralization of Bacteria in Terrestrial Basaltic
Environments: Comparison with Possible Life Forms in Martian
Meteorite ALH84001
The possible martian bacteria shapes reported in ALH 84001 are
apparently fossilized, i.e. mineralized or replaced by
inorganic constituents. Little is known about mineralization
of bacteria in igneous rocks, so the authors grew bacteria on
basalt and examined the resultant samples with SEM and TEM
methods. The samples inoculated with bacteria contained hollow
bacteria-shaped objects 1-2.5 µm long (slightly less in
diameter) that consisted mostly of ferrihydrite (a ferric iron
hydroxide) and contained no cellular structures or
ultrastructures. These are likely to be mineralized bacteria,
whether replaced by ferrihydrite or originally coated by
ferrihydrite is not known. Other biogenic features included
tubular forms (0.3 - 2.4 µm long), commonly with an appendage
and commonly embedded in biofilm. The authors also observed
filaments of distinctive morphology, some attached to tubular
bacteria, and some unattached and composed of ferrihydrite.
These filaments are interpreted as bacterial appendages. These
bacterial fossil forms are similar in size and shape to forms
reported from ALH 84001, which are interpreted as biogenic.
1793 Wentworth S.J. Thomas-Keprta K.L. Taunton A.E. Velbel M.A.
McKay D.S.
Possible Weathering Features in ALH84001
Using SEM and TEM methods, the authors investigated
mineralogical and structural effects of low-temperature
weathering in order to document Antarctic effects and search
for martian features. Weathering minerals that definitely
formed on Earth include Ca-sulfate, Mg-sulfate, amorphous(?)
silica, and common salt. Weathering minerals of unknown origin
(not on the fusion crust) include blade-like to rhombohedral
crystals of a Mg-carbonate that may be hydrated (e.g.,
nesquehonite or hydromagnesite). Smectite clay, identified by
TEM, is present as half-micron-sized patches. Wispy and
fibrous coatings on some grains may also be clays. Surfaces of
carbonate and pyroxene grains have pits or rounded surfaces
that are unlike reported features from terrestrial weathering,
and may represent the effects of weathering on Mars.
1594 Wright I. P. Grady M. M. Pillinger C. T.
On the 14C and Amino Acids in Martian Meteorites
Recent results on amino acid and 14C analyses of ALH 84001
(Bada et al., 1998; Jull et al., 1998) have shown that ALH
84001 is significantly contaminated by terrestrial organic
matter. Here, the authors show that the data and their
interpretation is somewhat ambiguous, and does not necessarily
refute the hypothesis that ALH 84001 contains traces from
ancient martian life. Bada et al. (1998) seem to demonstrate
that all the amino acids in ALH 84001 are terrestrial
contamination. Yet, the authors here point out that the
abundances of D and L forms of the amino acid alanine seem to
require more than one source of amino acids. Also, ALH 84001
contains so much amino acids that each half-gram sample that
Bada analyzed would have had to scavenge all the amino acids
from ~13 liters of Antarctic ice (~50,000 liters through the
whole meteorite). This amount of water flowing through ALH
84001 should have produced some clay minerals and should have
eliminated all vestiges of martian hydrogen -- neither of
which is observed. The author's comments on 14C mostly concern
the martian meteorite EETA79001, and are not directly relevant
to ALH 84001.
References:
Bada, J.L., Glavin D.P., McDonald G.D., and Becker L. (1998) A search for
endogenous amino acids in martian meteorite ALH84001. Science 279, 362-365.
Becker L., Glavin D.P., and Bada J.L. (1997) Polycyclic aromatic
hydrocarbons (PAHs) in Antarctic Martian meteorites, carbonaceous
chondrites, and polar ice. Geochim. Cosmochim. Acta 61, 475-481.
Bradley J.P., Harvey R.P., and McSween H.Y.Jr. (1996) Magnetite whiskers and
platelets in ALH 84001 Martian meteorite: Evidence of vapor phase growth.
Geochim. Cosmochim. Acta 60, 5149-5155.
Bradley J.P., McSween H.Y.Jr., and Harvey R.P. (1997) Epitaxial growth of
single-domain magnetite in martian meteorite ALH84001 (abstract). Meteor.
Planet. Sci. 32, A20.
Jull A.J.T., Courtney C., Jeffrey D.A., and Beck J.W. (1998) Isotopic
evidence for a terrestrial source of organic compounds found in Martian
meteorites Allan Hills 84001 and Elephant Moraine 79001. Science 279,
366-369.
McKay D.S. Gibson E.K.Jr., Thomas-Keprta K.L., Vali H. , Romanek C.S.,
Clemett S.J., Chillier X.D.F., Maechling C.R., and Zare R.N. (1996) Search
for past life on Mars: Possible relic biogenic activity in martian meteorite
ALH 84001. Science 273, 924-930.
Scott E.R.D., Yamaguchi A., and Krot A.N. (1997) Petrological evidence for
shock melting of carbonates in the martian meteorite ALH 84001. Nature 387,
377-379.
Treiman A.H. (1998) The history of ALH 84001 revised: Multiple shock events.
Meteor. Planet. Sci. 33, in press.