[4495-13] © 2001 SPIE - The International Society for Optical Engineering
Proceedings of SPIE, SPIE-The International Society for Optical Engineering,
"Instruments, Methods, and Missions for Astrobiology IV".
29 July - 3 August 2001, San Diego, California, USA
Rock Varnish As A Habitat For Extant Life On Mars
Barry E. DiGregorio
Cardiff Centre for Astrobiology, Cardiff, UK
[email protected]
ABSTRACT
Many of the rocks on the surface of Mars that have been imaged by the Viking and Mars Pathfinder
Landers display dark shiny surface coatings resembling Mn-rich terrestrial rock varnish. On our planet,
these thin (5 um – 1 mm) coatings can be the result of a combination of various weathering processes
combined with microbial precipitation of mineral oxides over a wide variety of geographical locations but
most commonly in those with arid and semi-arid conditions. Terrestrial Mn-rich rock varnish is produced
by a wide variety of microorganisms including epilithic and edolithic cyanobacteria, bacteria and
microcolonial fungi. As these microorganisms absorb trace amounts of Mn and Fe from atmospheric dust,
rain and fog, they slowly precipitate “reddish” iron and “brown to black” manganese oxides as well as
magnetite particles. These microbial communities then produce secretions that cement the Mn/Fe mix
together with clay particles in a process involving time periods of perhaps thousands of years for a thin 5
um layer. Mn-rich rock varnish has been found to form on the surfaces of undisturbed desert fragments and
even sand grains. Both Mn and Fe would serve as a UV shield for any microflora residing beneath and
within the layers of varnish thus protecting against high UV irradiation, dissication, and widely varying
temperature extremes. Recent research on rock varnish has led to the discovery that some microbial
communities that produce dark ferromanganese varnishes also precipitate biogenic magnetite. In view
recent independent evidence put forth by D. McKay and E.I. Friedmann et al for indigenous biogenic
magnetite-chains in ALH 84001 along with meteorological models showing the possibility for small
quantities of liquid water on the surface of Mars in combination with data obtained from the Viking LR
experiment 27 years ago, recommendations are made to elucidate on whether or not the shiny dark-coatings
covering some Martian rocks have been produced by living or extinct microbial communities.
Keywords: rock varnish, desert varnish, fog desert, endolithic, stromatolites, rock coatings, Mars
Pathfinder, Viking Lander, cyanobacteria, microcolonial fungi
INTRODUCTION:
ORIGIN, CHARACTERISTICS AND OCCURRENCE OF TERRESTRIAL ROCK
VARNISH
Prior to the oxygen levels required to form a sufficient solar UV ozone shield in Earth’s atmosphere during
the Proterozoic Eon 2.5 to 0.55 Ga ago it is thought that Archean stromatolite and microbial mat forming
communities were limited to shallow water environments (1). The early Archean atmosphere was largely
CO2 but gradually acquired free oxygen that was released through the photosynthetic activities of algae and
cyanobacteria. The banded iron formations are considered strong evidence that the oxygen levels were
extremely low during the Proterozoic, otherwise these iron deposits would have never made it to the oceans
unoxidized. However, as oxygen levels in the atmosphere gradually began to accumulate it would have
poisoned the same anaerobic microbial communities producing it had it not combined with iron deposits on
land to form the well known red beds 1.8 billion years ago. This would allow evolution time to develop the
eukaryotic cell and aerobic metabolism.
The emergence of life on the barren rocky landscape in the form of simple microbial communities is often
estimated to be during the Ordovician Period 490 m.y. ago (2). This suggests that the amount of oxygen
necessary to photo-dissociate into O3 and produce a biologically protective ozone layer would not have
allowed the development of life on land until the late Ordovician. However, it was noted by Olson and
Pierson that ferric iron strongly absorbs UV in the harmful wavelengths of 220-270 nm (3). These authors
also noted that ferrous iron absorbs harmful UV too, but less strongly. The oldest known sedimentary
rocks are from the Ishua Superacrustals and date from 3.8 Ga years ago that contain the banded iron
formations in which 30% of the iron is ferric. Beverly K. Pierson et al demonstrated that relatively low
concentrations of ferric iron (0.034 to 0.34 ppt) could act as an effective ozone shield in sediments 1 to 3
mm thick (4). However, Sagan et al reported in 1968 that a terrestrial microorganisms brought to Mars by
spacecraft could become sequestered in ferric oxide dust particles on Mars that would shield them from
impinging solar UV and then would be distributed by winds across the planet (5). Sagan suggested
terrestrial organisms protected in this manner might contaminate the surface of Mars over a period of time.
These examples serve to illustrate the possibility that microbial incursions onto dry land may have begun
much sooner than the establishment of a substantial ozone layer in the late Ordovician, if the
microorganisms were protected by ferric or ferrous iron dust.
According to the theory of succession of life on Earth, before vascular plants could evolve and establish
themselves onto dry land, microbial communities such as lichens, and edolithic algae would have had to
populate the rocky surface and interiors of rocks. These microbes would then secrete organic acids and
enzymes that over a period of hundreds to thousands of years would corrode or oxidize the rock
components breaking the rock down to a mix of minerals and organic matter. Thus, the first soils appeared.
However, it may well be that manganese precipitating fungi and bacteria protected from solar UV by low
concentrations of ferric iron were the first land inhabitants. They would have been distributed globally from
sea spray and dust carried by strong winds. Eventually the dust would settle out of the atmosphere onto
rock surfaces, and the fungi and bacteria would begin precipitating Mn slowly building thinly protective
layers (5 um – 1 mm) or microstromatolite structures called “rock varnish”.
Rock varnish consists of Fe and Mn oxides 20-30%, clay minerals 60%, and oxides of other trace elements.
The varnish layers have a laminated texture alternating from dark to light colored laminae that strongly
resemble stromatolites (6). Krinsley and Dorn et al in 1990 reported that rock varnish layers are porous and
permeable and can contain cracks that allow fluid to percolate into them (7). The sedimentation rate for a
Mn and Fe-rich rock varnish coating can take over a thousand years or more for just a few microns (8). For
this reason, rocks on which rock varnish is found must remain undisturbed for thousands or hundreds of
thousands of years for a complete coating to form. This makes rock varnish an attractive paleobiomarker
because as the individual layers form they trap atmospheric dust and organic materials. Dorn (9) found that
as desert sands sweep across rocks covered with rock varnish, that the varnish can be eroded away in part
or completely. If the wind and sand abrasion fail to remove all the varnish new varnish will begin to grow.
This would also make rock varnish a useful tool or indicator for past aeolian activity. Rock varnish has
been found on a variety of rocks such as coarse-grained granites, limestones, sandstones, and quartzites.
However rocks such as limestone rarely can support thick rock varnish coatings because they erode more
quickly than the rock varnish can form.
The Mn and Fe minerals of rock varnish are not indigenous to the rock substrate itself and the fact that the
underlying rock is devoid of any Mn or Fe demonstrate these minerals are brought in by winds, rain, dew or
fog. Dorn and Oberlander also made the important distinction that dark biologically oxidized Mn-rich rock
varnish only occurs on rocks with a slightly acidic or neutral pH while non-biological mechanisms for
manganese oxidation operate strictly in highly alkaline environments where the pH is over 9 (10). Dark
Mn- rich rock varnish does not occur below the soil line although orange colored Fe varnish often does.
This peculiar difference in coloration would in part lead to the discovery that microorganisms are involved
in the Mn-rich varnish process. It should also be noted that terrestrial inorganic coatings such as silica glaze
interfingered with rock varnish produce a more lustrous look.
The term “rock varnish” has replaced previous terminology such as “desert varnish”, “desert patina”,
“weathering crust”, desert lacquer” and “desert rind” because it is now recognized to occur in almost all
terrestrial environments, but most commonly in arid and semi-arid climates. Mn-rich rock varnish has also
been documented in the cold dry desert of Antarctica (11).
The first scientific data supporting that rock varnish hinted at a biological origin dates back to White in
1924 (12) and then Laudermilk in 1931 (13). The formation of rock varnish begins with small botryoidal
nucleation centers of Mn-oxidizing bacteria, fungi, or algae that bloom and grow outward to cover the
exposed rock surfaces. In extensive laboratory studies from rock varnish samples gathered from several
regions around the world Krumbein and Jens in 1981 were able to culture 17 strains of manganese and iron
precipitating fungi and 28 strains of bacteria. All these microflora shared in their ability to precipitate
manganese oxide. Furthermore, Krumbein and Jens found endolithic fungi and cyanobacteria had bored
pits in the rock substrate below layers of Mn-rich rock varnishes. These same researchers documented that
exposed desert pavements also accumulate the same Mn-rich rock varnish coatings precipitated by
microorganisms (14).
Other interesting investigations by El-Baz and Prestal presented to the 11th Lunar and Planetary Science
Conference in 1980 revealed that sand grains from the Southwestern desert of Egypt were coated with
reddish to dark brown oxide coatings, indicating rock varnish on individual sand grains (15).
Raymond et al in 1992 compared the Mn-rich layers of rock varnish to microstromatolites (Figure 1) and
described bacteria, fungi and organic matter within the layers (16) (Figure 2). Dorn and Oberlander also
previously described the active precipitation and concentration of Mn within rock varnish coatings (17).
There are generally two classifications of rock varnish 1) Rock varnish found in semi-arid to arid
environments in weakly acidic or weakly alkaline soil conditions that have Mn- and Fe-rich coatings 2)
Fe-rich but Mn-poor rock varnish. The ratio of Fe to Mn determine the color of the varnish, with Mn-rich
varnishes giving a brown, black or grey (lead color) while the Fe-rich/Mn-poor variety display orange and
reddish coloration’s. The shinny effect observed on rocks coated with Mn-rich rock varnishes are produced
by the thickness of the coating and varying concentrations of Fe, Mn, and clay minerals (18).
One of the most recent important observations regarding rock varnish was the detection of magnetite
components within various layers. Arvidson et al participated in the 1997 Lavic Lake (Mojave Desert)
Field tests using the Rocky 7 prototype Mars Rover which carried a Iron-nuclear Magnetic Resonance
(57Fe-NMR) Spectrometer (19). The 57Fe-NMR instrument was developed by Soon Sam Kim of the Jet
Propulsion Laboratory and while using it in-situ discovered magnetite in rock varnish. Independent work
by Rocco Mancinelli of NASA Ames Research Center along with colleagues Melisa White and Christine
Sawyer presented evidence at the May 7, 1997 annual meeting for the American Society of Microbiology
that they found magnetite-producing bacteria in samples of Death Valley rock varnish. They determined
this by growing the bacteria from the rock varnish scrapings under laboratory conditions (20).
E.I. Friedmann et al demonstrated the presence of magnetite crystal chains, previously considered the
missing evidence for biological magnetite in the Martian meteorite ALH 84001 (21). The mineralogical
analysis of five other SNC Martian meteorites has revealed an average Mn oxide concentration of 0.48%
relative to the 0.1 concentration of Mn found in Earth’s crust. Analysis of the soils at the Mars Pathfinder
landing site by the Alpha Proton X-ray Spectrometer showed FeO/MnO oxides were 29.2% by weight.
These intriguing figures beg the following questions: 1) Do Martian rocks and soils have coatings of
biologically produced rock varnish on them? 2) Are the environmental conditions on Mars today capable of
supporting extant microbial communities needed to produce rock varnish?
FOG DESERTS
Lying west of the Andes Mountains in northern Chile is the Atacama Desert, one of the driest deserts in the
world. Average precipitation here is only 1 cm/yr. from winter fogs and dew. These winter fogs frequently
blanket the coastal deserts and act as an effective block to solar UV radiation. They are produced when cold
north-flowing ocean currents that dominate along the coastlines of Chile and Peru prevent winds from
picking up enough moisture to produce rain. Daily temperatures can vary between 0 degrees C to 25
degrees C.
To the untrained eye, the Atacama Desert appears as a stark lifeless expanse of sand, dust and rocks. At the
microscopic level however, a wide range of living microorganisms such as bacteria, actinomycetes, fungi
and algae can be found thriving in a variety of environments: Endedaphic microorganisms are found in
crusts on the soil surface, hypolithic microbes survive on stones, while lythophytes such as chasmolithic
algae live inside rock fissures and endolithic algae penetrate into the pore spaces of rocks. Dusky-brown to
black ferromanganese coatings produced by Mn-fixing bacteria drape and sometimes completely obscure
the rock surfaces and desert pavement of the Atacama.
At times the condensate from winter fogs is sufficient enough to moisten soil to depths of a millimeter or
two. Also, because of the relative high humidity near the coastal range, biologically significant quantities of
water are available as dew from the June to August winter nights. Although the dew formed during the
night is largely evaporated at sunrise, the near-surface soil moisture can remain relatively high during the
cold winter months. During periods of fogs solar UV can be significantly attenuated.
Given that terrestrial fog and dew alone can sustain large populations of microorganisms in desert soil and
rocks make the extensive low lying fogs on the planet Mars intriguing areas to search for similar microbial
desert inhabitants including those that produce rock varnish.
Pollack et al first reported on Martian ground fogs at both Viking 1 and 2 landing sites that were separated
by 4,000 km in different geographical locations (22). Their observation period ran from mid-July 1976 to
February of 1977 as Mars was changing seasons from early summer to midfall. It was concluded at the
time that ground fogs composed of water ice particles formed before sunrise and quickly dissipated during
the late morning. The fogs were present at both landing sites during the length of their observation period.
In 1997 Moore et al noted that the Viking Lander 2 footpad collector head temperatures reached 273
degrees K and above along with atmospheric pressures exceeding 7 millibars (23). Moore and his
colleagues first commented that it was possible that brief periods of liquid water (wetting events) might
exist during times when conditions at the landing site exceeded the triple point of water. Rock and soil
temperature measurements were made using the Viking Orbiter IRTM instrument from 1976-1979 by
Principal Investigator Terry Z. Martin. He stated that some surface IRTM temperature data from the
equatorial regions reached highs of 300 -310 degrees K (24).
In 1995 Hannu Savijarvi presented a model of diurnal moisture cycle and soil properties at the Viking 1
landing site based on nighttime fogging and frosting of the Viking cameras (25). Savijarvi then calculated
that cold nighttime temperatures led to saturation of water vapor and surface fog with humidities at 100%
during the night and dropping off below 1% in the afternoon. However, the issue of a diurnal moisture
cycle would not be fully addressed again until 1998 when Levin and Levin first published a model for the
diurnal presence of biologically significant quantities of liquid water over large areas on Mars (26).
Since then others have constructed similar moisture cycle models with the most striking being published by
Lobitz et al that combined both Viking pressure, temperature and Mars Global Surveyor MOLA data to
suggest liquid water would be stable over the northern lowlands equatorward to about 40 degrees (27). The
authors cited that the southwestern portion of Utopia Planitia could have liquid water as much as 34% of
the Martian year if it were present. This model supports that low laying fogs observed from the Viking
orbiters in regions near the valley networks such as Labyrinthus Noctis, might produce seasonal wetting
events due to fog. The Mariner 9 and Viking orbiter images also confirmed fogs forming in the southern
hemisphere, especially in the Hellas and Argyre basins, Solis Planum, Sinus Sabeus, and in numerous low
laying craters (28).
These observations call into question the possibility that summer Martian ground fogs can wet rocks and
moisten soils similar as fogs do in the Atacama Desert.
ARE MARTIAN ROCKS COVERED BY ROCK VARNISH?
The first mention of rock stains or patinas on Mars, came from Alan B. Binder et al who described the
geology of the Viking 1 landing site as having partly stained outcrops, rock fragments, and finer material
(29). He and his colleagues attributed this staining to chemical weathering analogous to terrestrial deserts.
In 1979 E.L. Strickland III noted that the surfaces and sides of many rocks at both Viking landing sites
were identical in color to the duricrust. Strickland then went on to state the colors on most of the rocks at
both Viking 1 and 2 landing sites suggested deposition by environmental processes, rather than
mineralogical variations within the underlying rock (30).
In 1996 prior to the Mars Pathfinder mission to Mars, E.A. Guinness et al showed that coated rocks at the
Viking landing sites had high reflectance in the forward scattering direction (specular reflection) similar to
varnished rock surfaces found in terrestrial deserts (Figures 3 & 4). This abstract also revealed that rocks at
both Viking 1 and 2 landing sites appeared to show dark shiny coatings under different sun angles, but most
commonly after sunrise (31). They concluded that the rock coatings on Mars might make the measurement
of underlying rock chemistry difficult for Mars Pathfinder APXS.
Numerous attempts have been made using remote sensing in the near infrared with instruments on the
Marniner 9 and Viking Orbiters to try and obtain information on the mineralogy of Martian rocks and
soils. However, interference by the spectral signatures of atmospheric dust, aerosols, CO2 molecules and
water vapor have all made this work extremely difficult. The Thermal Emission Spectrometer aboard Mars
Global Surveyor has fared no better. According to J.G. Ward et al thermal infrared spectral signatures of
desert varnish and silica-glaze cooling rinds are so similar, that if it is on Mars it requires a different
method of detection to differentiate between them (32).
Because of the dust coatings found to be covering all the rocks at the Mars Pathfinder landing site in 1997,
chemical analysis by the APXS and near-infrared reflectance measurements were reduced significantly.
Kraft et al suggest in their paper that data about the coatings covering the MPF rocks seem to be similar to
terrestrial rock varnish (33). Multispectral analysis of the rocks at Mars Pathfinder as reported by McSween
et al were essentially non-conclusive (34). In a 1999 article written for NEW SCIENTIST, McSween stated
that rocks at the MPF site were so difficult to analyze because of the coatings covering the rocks, that he
suggested on a future Mars landers a mechanism designed to scrape-off dust and rock coatings to expose
the underlying rock surfaces for examination (35).
Because of its orbit and position as the fourth planet in the solar system, the incident solar radiation
reaching Mars is approximately 44% less than the Earth. This makes the atmospheric density and pressure
of the near-surface environment of Mars comparable to the terrestrial atmosphere at 30,000 meters
(100,000 feet) (36). However, since Mars’ orbit is elliptical this can vary by a factor of two. Therefore
obtaining data about the ultraviolet (UV) flux at the surface of Mars is crucial to understanding how any
life might be able to survive there. As mentioned previously in the introduction of this paper, the current
terrestrial biosphere is shielded by the ozone (O3) layer which filters out the Hartley bands in the 200-300
nm range and the Huggins bands from 300-360 nm. Since Mars has a variable ozone abundance of only
2% that of Earth, it receives a higher flux of UV. How much UV reaches the surface unattenuated is a
question that needs to be addressed over a period of different Martian seasons and conditions with future
spacecraft landers. It is well known that CO2 is an excellent absorber of UV in the wavelengths < 204 nm
and provides an effective shield below ~ 190 nm. Since Mars has < 50 times more CO2 than the terrestrial
atmosphere that can change dramatically by 25% with the onset of winter, further studies on the variability
of UV intensity will need to be conducted (37).
Dust suspended in the Martian atmosphere would also absorb and scatter UV effectively, however,
observations made by Philip James and Steven Lee et al using the Hubble Space Telescope 1995-1997 have
reported that the dust can settle out of the atmosphere nearly completely at times (38). Clouds, low laying
fogs, and snow all would act to temporarily attenuate UV as well. Thus it is conceivable that an ongoing
combination of these meteorological events could filter out biologically harmful solar UV for significant
periods of time so that well-adapted Martian microorganisms could take advantage of times of low UV
flux to carry out their life processes.
Also, just as it does on Earth, any biologically produced Mn-rich and Fe rock varnish on Mars would offer
UV protection to the microflora residing within and beneath the varnished layers (39). During these periods
of low UV flux perhaps due to persistent dust, clouds, and fogs, Martian microbes would then add new
layers to the outer portions of the rock varnish.
Evidence returned by the Apollo 12 crew from the surface of the Moon revealed that a species of
Streptococcus mitis survived on a layer of insulted foam inside the Surveyor III lunar probe which
remained on the Moon a period of 2 ½ years (40). Mileikowsky et al have calculated that the maximum
time a Martian meteorite ejected from the surface of Mars could reside in the vacuum of space and still
maintain viable spores is 100,000 years (41) due to ionizing radiation. If this estimate is accurate, then
endolithic spores sequestered within Martian rock varnish could become viable anytime conditions would
allow.
CONCLUSIONS AND RECOMMENDATIONS
It is completely plausible that life developed on Mars roughly the same time period as Earth, or perhaps
even before. The early exchange of planetary debris during the late bombardment period would have
allowed viable microbial spores from the Earth to reach Mars, and vise versa. If Mars had the liquid water
necessary to create the extensive dry tributary and drainage systems observed today, then there is no reason
to think that Martian life would not have flourished and quickly evolved. However, perhaps due to a
catastrophic impact(s) event(s) that dissipated its thicker atmosphere and abundant liquid water millions or
billions of years ago, life on Mars would have had to struggle and find every possible niche to survive.
Without pools of liquid water to sequester them, Martian life forms would have to adapt to conditions of
dry land and derive their water requirements directly from atmospheric sources. This would mean living
under dust layers in the soil, within rocks, and constructing biogenic rock coatings. The Labeled Release
(LR) experiment on both the Viking 1 and 2 landers conducted nine soil tests on the surface of Mars in two
locations separated by thousands of kilometers. The LR used a variety of experimental test conditions that
would lead the Principal Investigator to conclude active metabolism of microorganisms on Mars (42).
Surprisingly, little has been done to date by the scientific community to elucidate on this extremely
important matter.
Since rock coatings have been observed at both Viking landing sites and the Mars Pathfinder landing site, it
is reasonable to assume that rock coatings on Mars are a globally ubiquitous phenomenon. Given the
importance that the discovery of biologically produced rock varnish on Mars would have on astrobiology,
immediate measures should be taken to plan spacecraft landers with appropriately equipped instruments
capable of rendering a conclusion.
The next opportunity to examine the rock coatings on Mars is with the Beagle 2 lander scheduled to touch
down in December of 2003. Beagle 2 will land in Isidis Planitia a sedimentary basin 10 degrees north of the
equator. Fortunately, Beagle 2 is equipped with several key scientific instruments (flown for the first time
on any Mars mission) necessary to shed light on the issue of rock varnish, they are: 1) XRD spectrometer
2) Mossbauer spectrometer 3) microscope 4) Gas Analysis Package 5) and rock corer (43).
A typical scenario to examine the rock coatings on Mars for extant or extinct microorganisms would be the
following:
A) Select several target rocks with dark shinny coatings on them and use the coring device to extract rock
coatings from shallow samples.
B) Examine the core samples by microscope and seek any morphology of laminations indicative of
microstromatolites (rock varnish). Also observe cross sections for evidence of the size and shape for
bacteria (cigar shaped and rounded forms) and fragments of cell-wall casts .
C) Look for evidence of organic material entrapped within the individual laminae with GAP.
D) Try and determine the Mn-Fe ratio of the samples by XRD.
E) Use the Beagle 2 Mossbauer spectrometer to determine oxidation sate of minerals in the rock varnish
samples.
One other important issue to consider is that many of the rocks on Mars may actually be meteorites and
therefore their analysis would not any shed light on the mineralogy of Mars. However, even meteorite
specimens on Mars might be coated with indigenous rock varnish and would be a reliable source of
information regarding indigenous microorganisms and windblown sediments on Mars.
ACKNOWLEDGEMENTS
The author is grateful to Dr. Ronald J. Dorn of the Arizona State University Geography Department in
Tempe, Arizona for useful suggestions and images.
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FIGURES
Figure 1 Backscattered electron microscope sequence of Mn-concentrating bacteria from rock
varnish samples found covering basalt from the Ashikule Basin in Tibet. The basalts in this region
are dominated by silica glaze with pockets of Mn-rich varnish. Note layered stromatolite appearance
of the sample. Dark and light layers represent variations in the Mn/Fe ratio indicating altering
conditions of deposition.
Figure 2 Rock varnish from Kitt Peak, Arizona shown in two different perspectives: left is secondary
electron microscopy, right: backscattered electrons. Both images show botryoidal
micromorphologies associated with Mn-oxidizing microorganisms.
Figure 3 Early morning front-lighted view from the Viking 1 lander showing dark coatings on rocks.
Compare same scene with afternoon lighting in figure 4.
Figure 4 Same Viking 1 lander scene and viewing angle as Figure 3 but only under afternoon lighting
conditions.