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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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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