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Astronomie - Mars-Meteorit Strukturen: Optimismus für Außerirdisches Leben?

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1.03.2014

This scanning electron microscope image shows speroidal features embedded in a layer of iddingsite, a mineral formed by action of water, in a meteorite that came from Mars.

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A meteorite from Mars has been studied up-close and scientists have detected tiny structures that could be interpreted as having a biological origin.

This moment of déjà vu is brought to you by a new paper published in the February issue of Astrobiology where a team of scientists from NASA’s Johnson Space Center in Houston, Texas, and the Jet Propulsion Laboratory in Pasadena, Calif., describe the results of work on a 14 kilogram (30 pound) meteorite called Yamato 000593 (Y000593). The meteorite sample contains strong evidence that Mars used to be a lot wetter than it is now, but the researchers also report on the discovery of evidence for “biological processes” that occurred on the Red Planet hundreds of millions of years ago.

Although this sounds exciting, there will likely be some skepticism, but the researchers appear to have foreseen the media circus that “Mars life” always inspires and refused to appear overly excited of some pretty fascinating evidence for ancient microbial life.

In 1996, President Clinton made a high profile announcement on national television that evidence for life had been discovered by NASA scientists inside another Martian meteorite called Allan Hills 84001 (ALH84001). The discovery focused around scanning electron microscope images of the microscopic detail of ALH84001. The team, led by David McKay of Johnson Space Center, identified “biogenic structures” inside the meteorite that was theorized to be formed by indigenous life on Mars.

The controversial media storm surrounding that 1996 announcement stirred a backlash that threw McKay’s team’s findings into doubt. However, McKay’s team defended the work after ruling out terrestrial contamination and other factors that may have created the nanometer-sized worm-like structures. McKay also worked on the Y000593 study until his death in February 2013.

This new work focuses around a meteorite that was discovered in the Yamato Glacier, Antarctica, by a Japanese Antarctic Research Expedition in 2000.

Analysis of the meteorite shows that it formed on the surface of Mars 1.3 billion years ago from a lava flow. Then, around 12 million years ago, a powerful impact event shattered the region, blasting quantities of Martian crust, containing any hypothetical lifeforms (and evidence thereof), into space. These chunks of Mars rock then traveled through interplanetary space until one of the samples, Y000593, encountered Earth and fell to the surface as a meteorite, falling on Antarctica some 50,000 years ago.

There are many known samples of Mars crust that have fallen to Earth as meteorites and are considered incredibly valuable scientific specimens that can be used as time capsules into Mars’ geologic past. These meteorites are nature’s ‘sample return missions,’ no spaceship required.

“While robotic missions to Mars continue to shed light on the planet’s history, the only samples from Mars available for study on Earth are Martian meteorites,” said lead author Lauren White, of NASA’s Jet Propulsion Laboratory, in a news release. “On Earth, we can utilize multiple analytical techniques to take a more in-depth look into meteorites and shed light on the history of Mars. These samples offer clues to the past habitability of this planet. As more Martian meteorites are discovered, continued research focusing on these samples collectively will offer deeper insight into attributes which are indigenous to ancient Mars. Furthermore, as these meteorite studies are compared to present day robotic observations on Mars, the mysteries of the planet’s seemingly wetter past will be revealed.”

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This scanning electron microscope image of a polished thin section of a meteorite from Mars shows tunnels and curved microtunnels.

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In their research, the scientists describe features associated with Martian clay deposits — micro-tunnels thread throughout the Y000593 sample. When compared with terrestrial samples, the Martian shapes appear to closely resemble “bio-alteration textures” in basaltic glasses. This basically means that this Mars meteorite contains microscopic features that resemble mineral formations created by bacteria on Earth.

Another factor is the discovery of nanometer to micrometer-sized spherules sandwiched between the layers of rock in the meteorite. These spherules are distinct from the minerals inside the rock and are rich in carbon, another sign that they may have been formed through biological interactions inside the rocky material.

The First Rule of “Mars Life”: Don’t Talk About “Mars Life”

Is this proof of Martian bacteria munching through Mars rock? Sadly, that’s one conclusion that cannot be made from this study and the researchers are very cautious not to write the word “life” at any point in their publication — it’s replaced by technical terms like “biogenic origins” and “biotic activity.”

“We cannot exclude the possibility that the carbon-rich regions in both sets of features may be the product of abiotic mechanisms,” the scientists write in their paper. ‘Abiotic’ means mechanisms that are not caused by microbial life, such as some chemical reaction in the rock’s geology. “However, textural and compositional similarities to features in terrestrial samples, which have been interpreted as biogenic, imply the intriguing possibility that the martian features were formed by biotic activity.”

Their caution has been applauded by other astrobiologists. “(The authors) have done well not to cry wolf and to scientifically speculate on the tubules’ origins, accepting that, as of yet, they do not know whether they are of biological origin or not,” said Louisa Preston of the U.K.’s Open University.

“This is no smoking gun,” said White. “We can never eliminate the possibility of (terrestrial) contamination in any meteorite. But these features are nonetheless interesting and show that further studies of these meteorites should continue.”

Since the 1996 ALH84001 controversy, many other researchers have come forward with meteorite studies that appear to show evidence for life on Mars and other interplanetary locations, but most have been published in sketchy journals with little to no peer review process, which serves to blur valuable research being carried out by astrobiologists. Therefore, skepticism for any Mars life study is often high.

So, until we can detect and analyze DNA of extraterrestrial origin or have the ability to return pristine samples from Mars, work like this will be filed under “fascinating but not conclusive” in the profound hunt for life beyond Earth.

Quelle: D-News

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Update: 2.03.2014

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Putative Indigenous Carbon-Bearing Alteration Features in Martian Meteorite Yamato 000593

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Abstract
We report the first observation of indigenous carbonaceous matter in the martian meteorite Yamato 000593. The
carbonaceous phases are heterogeneously distributed within secondary iddingsite alteration veins and present in
a range of morphologies including areas composed of carbon-rich spheroidal assemblages encased in multiple
layers of iddingsite. We also observed microtubular features emanating from iddingsite veins penetrating into
the host olivine comparable in shape to those interpreted to have formed by bioerosion in terrestrial basalts. Key
Words: Meteorite—Yamato 000593—Mars—Carbon. Astrobiology 14, 170–181.
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1. Introduction
The martian meteorites represent samples of the martian
regolith that were ejected from the planet as a consequence
of bolide impact events that have occurred over the
last 20 million years (Nyquist et al., 2001). Radiometric
crystallization ages determined for individual meteorites indicate
they formed across much of the martian geological
record, from as recently as 165 million years ago (Shergotty)
to as long as 4.1 billion years ago (ALH84001) (Nyquist
et al., 2001). Martian meteorites are principally mafic or ultramafic.
Nearly all those that have been studied in microscopic
detail, however, exhibit evidence of at least transient
interaction with aqueous fluids, such as the presence of secondary
mineral assemblages and evaporitic deposits present
within veins and internal fracture surfaces. Several main
mineral phases associated with these secondary alteration
features include iddingsite, carbonates, sulfates, halite, and
various clay minerals (Gooding and Wentworth, 1987;
Gooding et al., 1988, 1991; Wentworth and Gooding, 1988a,
1988b, 1990, 1991; Wentworth and McKay, 1999; Greenwood,
2000; Wentworth et al., 2001; Treiman, 2005). Other
minor phases include sulfides (Treiman et al., 1993; Wentworth
et al., 1998; Treiman, 2005), ferric oxides (Treiman
et al., 1993; Treiman, 2005) and ferrihydrite (Treiman and
Gooding, 1991; Treiman, 2005). While the exact physical
environment under which these minerals formed and the
composition of the alteration fluid are not well constrained, a
martian origin can be established based on isotopic grounds
(Imae et al., 2002) and through microstratigraphic relationships
between the secondary phases and the fusion crust (e.g.,
Wentworth and Gooding, 1990; Treiman and Goodrich,
2002). The lack of extensive alteration of primary minerals
such as olivine suggests secondary mineral formation during
evaporation of brine-like solutions (Gooding and Wentworth,
1991; Bridges and Grady, 2000; Bridges et al., 2001) at low
temperatures ( < 150C) and over relatively short periods of
time (Wentworth et al., 2005). The abundance of secondary
minerals in different meteorites ranges from *0.2 to
2 vol %, being highest in the nakhlites (e.g., Nakhla and
Lafayette) at*2 vol % (Bridges and Grady, 1999, 2000) and
in ALH84001 at *1 vol % (Mittlefehldt, 1994).
Yamato 000593 (henceforth Y000593) was discovered in
2000 at the Yamato Glacier in Antarctica by the Japanese
Antarctic Research Expedition. Based on oxygen isotope
data (Imae et al., 2002) and mineralogy (Mikouchi et al.,
2002, 2003; Imae et al., 2003a, 2003b; Misawa et al., 2003a,
2003b), it belongs to the nakhlite subgroup of the martian
meteorites. The meteorite represents a fragment of a larger
fall and has been paired with the Yamato 000749 and Yamato
000802 meteorites (Misawa et al., 2003a). Together,
these meteorites compose a whole rock mass of *15 kg
(Meyers, 2003; Misawa et al., 2003a). Mineralogically,
Y000593 is an unbrecciated igneous rock consisting mainly
of coarse-grained crystals of augite and olivine with minor
plagioclase, pyrrhotite, apatite, fayalite, tridymite, and
magnetite (Imae et al., 2003b; Mikouchi et al., 2003).
Evidence of interaction with aqueous fluids is substantiated
by carbonate phases and clay-rich iddingsite veins containing
amorphous silica-rich material, possibly a gel or opalinelike
phase, also present in the matrix (Spencer et al., 2008a,
2008b). Because iddingsite alteration veins in Y000593 are
truncated at the fusion crust, it appears likely that they
formed prior to atmospheric entry and hence have a martian
origin (Treiman and Goodrich, 2002). Additional evidence
of interaction of Y000593 with aqueous fluids includes the
presence of microtubular features or microtunnels emanating
from iddingsite veins and penetrating into the surrounding
olivine crystals. Because olivine is a reactive mineral,
exposure to aqueous fluids results in the epitactic and/or
topotactic nucleation of iddingsite within wedge-shaped etch
channels, which results in a typical lamellar structure observed
in partly altered olivine grains (Eggleton, 1984;
Smith et al., 1987). The observed microtunnels, however,
display curved, undulating shapes consistent with bioalteration
textures observed in basaltic glasses (Fisk et al.,
1998, 2006; Furnes et al., 2001; Preston et al., 2011).
Previous reports describe tunnel features and their associated
mineralogy in Nakhla. However, no investigation has
yet been reported for similar features in Y000593 and
compared to Nakhla. For the first time, we describe features
called microtunnels associated with iddingsite veins in
Y000593. Additionally, we also report the presence of indigenous
organic matter occurring as heterogeneous carbonrich
areas in iddingsite veins and carbon-bearing spheroidal
features interleaved between layers of iddingsite.
2. Methods
Optical identification of secondary phases in a 25.4mm
round polished thin section of Y000593 was performed by
using a Nikon 120 microscope equipped with a digital camera.
Series of images were taken over a range of focal distances and
combined with ImageJ1 software to give extended depth-offield
images through the depth of the section. After optical
imaging, a conductive carbon surface coating <1nm thick
was applied to the thin section to enable chemical characterization
with a JEOL 6340F field emission scanning electron
microscope (FE-SEM) equipped with an IXRF System energydispersive
X-ray spectrometer (EDS) that allows detection of
light elements including carbon. A range of acceleration
voltages and beam currents were used to determine the optimum
conditions for imaging and analysis. The optimum voltage
and current for backscatter, secondary electron, and lower
secondary imaging were determined to be 15kV at 10 lA.
Individual (point) spectra were collected by using 6.5, 10, and
15kV with an analysis spot size ranging from *0.1 to 1 lm.
A polished petrographic thin section of LEW87051, an
Antarctic achondrite recovered from the ice by a US team in
1987 (Mason, 1989), was prepared under identical conditions
to that of Y000593 to serve as a control. While the
provenance of LEW87051 is uncertain (Warren and Kallemeyn,
1990), the oxygen isotopic ratios exclude a martian
origin. This meteorite is classified as an angrite2 composed
predominantly of large magnesium-containing olivine
grains (*Fo80) embedded within a fine-grained groundmass
of euhedral laths of anorthite intergrown with clinopyroxene
and magnesium-poor olivine crystals (up to
*Fo100) (McKay et al., 1990). Its terrestrial residence time
based on 14C radiochronology is estimated to be *50
thousand years (Eugster et al., 1991), which is nearly
identical to that for Y000593 (Nishiizumi and Hillegonds,
2004). LEW87051 weighs *0.6 g (Eugster et al., 1991),
only *0.004% of that of Y000593 at *13.7 kg. The difference
in weights between the two meteorites and consequently
greater surface area to volume of LEW87051
implies effects of terrestrial contamination3 or weathering4
are likely to be more pronounced in LEW87051. While not
recovered from the same blue ice field, LEW87051 provides
a useful means to evaluate whether our observations
of Y000593 could be a product of its residence time in
Antarctica.
A chip of Y000593 (from allocated split Y000593,80)
*2mm in size was attached to a 12.7mm aluminum pin
mount with conductive carbon tape before being sputtercoated
with a thin ( < 1 nm) layer of platinum to enable
charge dissipation during FE-SEM/EDS analysis. Imaging
was performed at 15 kV, while EDS analyses were performed
at 6.5 and 10 kV; the lower kilovolt value allows for
improved detection of light elements (Z < 9).
3. Results
Optical images of the Y000593 thin section show an
extensive network of brown/orange iddingsite veins hundreds
of microns in length that cross-cut fractured olivine
crystals. Extending out approximately perpendicular from a
large fraction of the veins are iddingsite-filled tunnels typically
‡ 0.5 lm in width and tens of microns in length
(Fig. 1). In addition to the large-width ( ‡ 0.5 lm) tunnels,
approximately a third of the veins show threadlike microtunnels
of iddingsite that range from *100 to 200 nm in
width and *<1 to 4 lm in length (Figs. 1 and 2). The
majority of microtunnels display curved and/or sigmodal
(S-shaped) morphologies (Fig. 2). EDS spectra show that the
host olivine contains approximately equimolar *Si:Fe,
while the adjacent iddingsite is strongly iron-enriched with
respect to silicon (Fig. 3). A layer of silica *2 lm in width
rims a portion of the iddingsite vein (Fig. 1). EDS spectrum
of silica shows Si > Fe compared to that for both olivine and
iddingsite (Fig. 3). Two representative EDS spectra of iddingsite
show its heterogeneous nature at the submicron
scale; one contains major carbon, calcium, and manganese
consistent with the presence of mixed cation carbonate
(Fig. 3), while the other contains major carbon with little to
no calcium or manganese, indicating the carbon may be
independent of carbonate.
Y000593 chips revealed close-packed spheroidal structures
encased within and surrounded by multiple layers of
iddingsite-like compositions (Fig. 4). Structures ranged in
diameter from *100 to 500 nm and are enriched in carbon
relative to the underlying host mineral phases and surrounding
iddingsite (Fig. 4).
LEW87051 was characterized by identical techniques and
instrumentation used for Y000593. Minor rusty or oxidized
phases are present, presumably as a consequence of Antarctic
weathering, while LEW87051 showed no evidence of
iddingsite or silica alteration phases. Furthermore, there
were no observations of microtunnels extending from fractures
and veins (Fig. 5).
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FIG. 1. (A) Low-magnification scanning electron microscope/backscattered electron (SEM/BSE) view of a polished thin
section of Y000593. Expanded views of regions enclosed by green and red boxes are shown in (B) and (C), respectively.
Running diagonally across the image from top to bottom is a dark epoxy-filled vein crossing fractured crystals of olivine and
augite (gray). (B) Optical image of region enclosed by green box in (A), showing characteristic red-brown iddingsite
alteration of the silicates at the vein interface. (C) SEM/BSE view of the region enclosed by the red box in (A), the detailed
structure of the alteration edges including tunnels and microtunnels. The four circles indicate the locations that the point
EDS spectra in Fig. 3 were obtained from. The blue inset box indicates the location of the high-magnification region shown
in Fig. 2.
FIG. 3. SEM/EDS spectra of select regions shown by
green circles in Fig. 1. (A) Olivine host matrix with a significant
Fe peak indicating an intermediate composition
between fayalite and forsterite end members. (B) Silica-rich
band along the olivine fracture surface showing depletion of
Fe and enrichment of Al compared to the adjacent host
olivine. (C) Iddingsite Regions 1 and 2 illustrating the significant
heterogeneity in carbon abundance and cation variability
of the iddingsite at the micron scale. Note that the
use of epoxy-embedding medium for thin-sample preparation
makes it difficult to wholly exclude the contribution of
epoxy to observed carbon abundances; this was minimized
by acquiring spectra from alteration regions that were contiguous
at the scale of the analysis spot and showed no
observable surface fracturing or porosity.
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4. Discussion
Terrestrial iddingsite was first described in 1893 as a
mixture of hydrous non-aluminous silicates of iron, magnesia,
and soda pseudomorphous with olivine phenocrysts
having a bright orange-yellow color (Lawson, 1893). Its
composition and mineralogy has remained imprecise, since
it lacks a definite chemical composition and therefore cannot
be regarded as a simple submicroscopic intergrowth of
two or more well-characterized minerals. It is best defined
as a complex mineral assemblage5 where it can be envisioned
to represent the continuous transformation of an ironbearing
olivine crystal as it passes through various stages of
structural and chemical change in the presence of liquid
water at both elevated (300–450 K; Tomasson and Kristmannsdottir,
1972) and lower temperatures ( < 300 K) given
sufficiently long time frames (Zolensky et al., 1988). Typical
iddingsite veins show regular etching in the form of
lamellar fissures formed during the opening of fractures and
cleavage planes in olivine; these fissures display a pattern of
wedge-shaped or sawtooth borders resulting from the crystallographically
controlled dissolution of the olivine crystal
structure (Eggleton, 1984; Smith et al., 1987). Smectite
produced by weathering of olivine contains aluminum and
potassium introduced from another source.
Iddingsite in the nakhlites has long been interpreted as
pre-terrestrial (Wentworth and Gooding, 1989; Gooding
et al., 1991; Romanek et al., 1998; Treiman, 2005) and
provided the first laboratory evidence for indigenous liquid
water and a martian hydrosphere (Wentworth and Gooding,
1988a, 1989; Gooding et al., 1991; Karlsson et al., 1992;
Treiman et al., 1993; Romanek et al., 1998). Isotopic dating
suggests the iddingsite formation in Nakhla iddingsite occurred
*600 million years ago (Swindle et al., 2000), either
in a single event or through episodic aqueous exposure over
a short time interval (Swindle and Olson, 2004). Iddingsite
veins in Y000593 appear to be consistent in both composition
and mineralogy to veins in other nakhlites (Wentworth
and McKay, 1999; Treiman, 2005; Fisk et al., 2006;
McKay et al., 2006). Observations from Y000593 show
microtunnels radiating from the edges of iddingsite vein into
host olivine (Figs. 1 and 2). Unlike typical sawtooth etch
fissures in the alteration zone, undulating microtunnels
display curved and/or sigmoid-shaped threadlike morphologies
similar in size and shape to those in terrestrial silicates
interpreted as being formed by bioweathering processes
(Fisk et al., 1998, 2006; Furnes et al., 2004; Fisk and
McLoughlin, 2013). Microtunnels may have initially been
hollow but now are filled with iddingsite. It is noteworthy
that similar microtunnels emanating from iddingsite veins
have also been described within Nakhla (Gooding et al.,
1991; Fisk et al., 2006). Features are also consistent with a
recently reported library of bioerosion textures in terrestrial
basalts (Fisk and McLoughlin, 2013).
The surface of Mars has received a significant contribution
of abiotic organic matter derived from exogenous
sources (Bland and Smith, 2000) and through planetary
processes (Chang, 1993). However, current robotic in loco
parentis investigations of martian surface regolith have yet
to provide any definitive evidence for carbonaceous matter.
Nevertheless, it is worth remembering that only the Viking 1
and 2 lander missions carried molecular analysis packages
specifically designed to detect organic matter (Biemann
et al., 1976, 1977; Biemann, 1979). The failure of the Viking
landers to conclusively identify the presence of any
simple organic species in surface soils (Biemann et al.,
1976, 1977; Biemann, 1979) is now generally, although not
necessarily satisfactorily, explained as the result of UV
photocatalytic oxidation combined with an unusual and
highly oxidizing surface soil chemistry (Oro and Holzer,
1979; Zent and McKay, 1992; Yen et al., 2000). The 2012
Mars Science Laboratory Curiosity has a suite of analytical
tools associated with the Sample Analysis at Mars (SAM)
instrument, which can measure the nature of organic matter
within the near-surface of Mars. To date, the results from the
SAM analyses show that indigenous organic content on
Mars is yet to be determined (Leshin et al., 2013; Ming
et al., 2013).
In contrast, many of the martian meteorites contain detectable
organic compounds with measured abundances
typically between 10 and 200 ppm (Wright et al., 1986;
Grady et al., 1994; Romanek et al., 1994; Bada et al., 1998;
Jull et al., 2000; Sephton et al., 2002; Steele et al., 2012).
The problem is that each studied martian meteorite also
contains some terrestrial organic compounds or contaminants,
so the challenge is to discriminate the terrestrial organics
from those purported to be martian. Spatial
association (McKay et al., 2011) and isotopic analyses ( Jull
et al., 2000) have been used previously to establish a martian
heritage for some of these organics. For example, in a
recent report by McKay et al. (2011), regions of carbon-rich
matter in Nakhla were encased within iddingsite and salt
crystals and interpreted as having a martian origin. Investigations
by Jull et al. (2000) of Nakhla in which d13C
and d14C isotope analysis procedures were used discovered
that a significant amount (70–80%) of both the acid-soluble
and insoluble carbonaceous matter in this meteorite was
indigenous. Sephton et al. (2000, 2002) have subsequently
reported the presence of indigenous high-molecular-weight
organic matter in Nakhla by using solvent extraction in
combination with flash-pyrolysis gas chromatography–mass
spectrometry; they observed a suite of isotopically distinct
(d13C) aromatic and Cn-alkyl aromatic hydrocarbons.
The Y000593 iddingsite assemblage displays chemical
and mineral heterogeneity at the submicron scale (Fig. 3C;
Iddingsite Regions 1 and 2), consistent with previous
observations of martian iddingsite (e.g., Wentworth and
Gooding, 1988b, 1989; Gooding et al., 1991; Treiman,
2005). Iddingsite spectrum of Region 1 (Fig. 3C; also see
Fig. 1) shows Fe ‡ Si with minor calcium, magnesium,
manganese, and carbon, indicating the presence of abundant
iron oxides and mixed cation carbonate. By comparison, an
iddingsite spectrum of Region 2 (Fig. 3C; also see Fig. 1)
shows Si > Fe and major carbon with very minor calcium
and magnesium, indicating the presence of few iron oxides
and little, if any, carbonate. The presence of major carbon
and lack of corresponding cations is consistent with the
occurrence of organic matter embedded in iddingsite.
EDS analysis of the polished thin section of Y000593 revealed
some carbon-rich areas heterogeneously distributed
throughout the iddingsite veins. These carbon-rich areas do
not appear to be spatially associated with specific morphological
or mineralogical features.
In contrast, analysis of Y000593 chips revealed submicrometer-
sized spheroids enriched in carbon relative to
the underlying host olivine and some regions of iddingsite
(Fig. 4). Preliminary selected area electron diffraction
analysis of the underlying layer revealed only silicate
compositions; therefore, the carbon enrichment is not likely
bound to carbonate. These structures were observed embedded
in between multiple layers of iddingsite (Fig. 4); and
the spatial association of carbon-rich areas and spheroids
with the iddingsite indicates these features formed either
prior to, or contemporaneously with, the iddingsite in which
they are encapsulated. This suggests that, like the iddingsite,
they also formed on Mars. We note that indigenous organic
matter in martian samples has been arguably confirmed from
multiple meteorites by numerous independent research
groups ( Jull et al., 1999, 2000; Sephton et al., 2000, 2002),
with the most recent report in 2011 describing carbon-rich
features spatially associated with iddingsite and salt crystals
in the martian meteorite Nakhla (McKay et al., 2011).
Martian meteorites Y000593 and Nakhla have experienced
strikingly different environmental conditions after
their impact with Earth. The former was collected as a find
in Antarctica after a *50-thousand-year residence time and
the latter as a sighted fall in Egypt recovered and quickly
placed in a museum after its fall. The observation that these
two meteorites exhibit similar microtunnel features despite
the very different terrestrial landing environments and postrecovery
histories strongly argues that the microtunnels are
not the result of terrestrial contamination and instead were
formed during an aqueous alteration on Mars. For example,
while some contact with wet ground cannot be excluded for
Nakhla, its terrestrial exposure history was many orders of
magnitude less than that of Y000593. While Antarctic meteorites
undergo some terrestrial weathering, non-martian Antarctic
meteorite LEW87051 showed no evidence of iddingsite
or silica alteration phases, nor were any of the microtunnels
observed extending from fractures and veins (Fig. 5A). Again,
the presence of these microtunnels and iddingsite veins in
Y000593 (Fig. 5B) and their absence in LEW87051, coupled
with the presence of similar iddingsite veins and microtunnels
in Nakhla, is strong evidence that they were not formed
during terrestrial weathering but were formed on Mars.
This is the first report of purported indigenous carbonaceous
matter in martian meteorite Y000593. This matter is
embedded within spherules encased in layers of iddingsite
compositions and embedded within iddingsite veins presumably
formed by the action of liquid aqueous fluids on
Mars (Gooding et al., 1991). Both the spherules and microtunnel
features formed either prior to, or contemporaneously
with, iddingsite and hence have a martian origin. It
is possible the carbonaceous matter has an abiotic origin or
origins derived from exogenous (cometary/asteroidal/interplanetary
dust) sources (Flynn and McKay, 1989, 1990;
Flynn, 1993, 1996) and/or through planetary process including
magmatic and impact-generated gases (Zolotov and
Shock, 1998, 1999). Alternatively, the spherules and associated
carbonaceous matter may have biogenic origins because
spherulitic features are similar in both size (*0.1–0.5
lm) and shape to known terrestrial fossilized microbes reported
in the range of 0.13–0.55 lm (Folk and Chafetz,
2000; Brigmon et al., 2008). The presence and distribution
of carbon-rich areas with tunnel erosion patterns in iddingsite
imply this matter is relatively insoluble, consistent with
the geopolymer kerogen (Kim et al., 2006). While detailed
analyses of carbonaceous matter are outside the scope of this
paper, given abundant sample amounts, future analysis in
which techniques more destructive to the sample are used
(e.g., mass spectrometry) may provide deeper insight into
the nature of the carbon.
The Y000593 microtunnels are remarkably similar in
morphology to bioerosion textures found in terrestrial Fe-
Mg silicates described by Fisk et al. (2006), Preston et al.
(2011), and Furnes et al. (2004). The microtunnels are inconsistent
with alteration channels produced by the crystallographically
controlled abiotic dissolution of olivine
(Eggleton, 1984; Smith et al., 1987). Fisk et al. (2006)
suggested common features of biotic alteration include the
following: tunnels that emerge from a glass or mineral
surface that has been in contact with water, a host mineral or
glass replaced with hydrous minerals, dark brown to black
boundary between the glass and fracture-filling clay, uniform
tunnel size and shape in a single sample, uniform
tunnel diameter along the length of the individual tunnel,
and localized, nonuniform distribution of tunnels along
fractures or mineral edges. We suggest that the microtunnels
in Y000593 display all the aforementioned characteristics.
Previous studies of terrestrial glass and olivine samples
interpreted the presence of tunnels and microtunnels to be of
biogenic origin (Fisk et al., 1998, 2006; Furnes et al., 2004;
Preston et al., 2011). This interpretation is greatly
strengthened by the presence of DNA in some of these
terrestrial features (Fisk et al., 1998, 2006). Similar DNA
fluorescence experiments performed by Fisk et al. (2006) on
microtunnels in Nakhla did not detect DNA. As noted earlier,
however, the nakhlites appear to have been subjected to
aqueous alteration during a period as old as 600 million
years ago. Therefore, if martian DNA were introduced into
the tunnel structure of nakhlites at that time, its instability
during weathering and aging would likely preclude its survival
in present-day samples.
Previous studies reveal that a combination of SEM and
EDS can be used to differentiate between mineralized carbonate
and organic matter by analyzing composition and
texture (Toporski et al., 2000; Toporski and Steele, 2007;
McKay et al., 2011). Specifically, the composition of carbonate
requires the presence of cations (e.g., Mg, Ca, Mn, or
Fe) to balance the CO2 3 anion. Therefore, the relative
abundances of the cations can be detected by EDS and should
correlate to carbon in carbonaceous material. If no cations are
detected by EDS to correlate with carbon, one may consider
the possibility of this material existing in the organic in
phase. Furthermore, observed textures such as stringlike
membranous material, tubular features, honeycomblike
networks of material are inconsistent with typical mineralcarbonate
structures and, in correlation with compositional
observations, may indicate organic material. However, we
note that when both phases are intermixed, such as may be
the case in Y000593, differentiating between these phases is
complex. Due to the often limited sample size for meteoritic
and lunar materials, SEM and EDS are well-established
nondestructive analytical methods that are frequently utilized
to characterize extraterrestrial samples (Heiken et al., 1972;
Wentworth and McKay, 1987; Toporski et al., 2000; McKay
et al., 2011; Ross et al., 2011; Ruzicka et al., 2012).
5. Summary and Conclusions
The martian meteorite Y000593 contains two distinctive
sets of features associated with the martian-derived iddingsite.
The tunnel and microtunnel structures are typically
found in olivine along the margins of mineralogically complex
iddingsite veins. These microtunnels contain areas of
enhanced carbon abundance that, in some cases, are not associated
with common carbonate cations and therefore are
interpreted as carbonaceous material, perhaps similar to
kerogen. The second set of features consists of nanometer- to
micrometer-sized spherules sandwiched between layers of
indigenous iddingsite and distinct from carbonate and the
underlying silicate layer. Similar spherules have also been
described in Nakhla (Gibson et al., 2001). EDS spectra of the
Y000593 spherules show that they are significantly enriched
in carbon compared to the nearby surrounding iddingsite
layers. A striking observation is that these two sets of features
in Y000593, recovered from Antarctica after about
*50-thousand-year residence time, are similar to features
found in Nakhla, an observed fall collected shortly after
landing. We cannot exclude the possibility that the carbonrich
regions in both sets of features may be the product of
abiotic mechanisms; however, textural and compositional
similarities to features in terrestrial samples, which have
been interpreted as biogenic, imply the intriguing possibility
that the martian features were formed by biotic activity.
Acknowledgments
We gratefully acknowledge the allocation of Yamato
000593 by the Polar Institute of Japan. This research was
conducted at NASA Johnson Space Center while the first
author (L. White) was under contract with Jacobs Engineering
(Jacobs Engineering, ESCG, 2400 Bay Area Blvd,
Houston, TX). Part of this work was also conducted at Jet
Propulsion Laboratory, California, Institute of Technology,
under contract with National Aeronautics and Space Administration.
This manuscript is dedicated to our colleague
David S. McKay, who died on February 19, 2013. Dave’s
guidance and perception of the important features within
martian materials will long be remembered.
Quelle: ASTROBIOLOGY



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