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PubMed2002

Microbial metal-ion reduction and Mars: extraterrestrial expectations?

Dissimilatory metal-ion-reducing bacteria (DMRB) can couple the reduction of a variety of different metal ions to cellular respiration and growth. The excitement of this metabolic group lies not only in the elucidation of a new type of metabolism, but also in the potential use of these abilities for the removal of toxic organics, and in their ability to reduce (and thus, detoxify) other toxic metals, such as U(VI) and Cr(VI). This review focuses on recent advances in the study of DMRB, including the use of external electron shuttles to enhance rates of metal reduction; genome sequencing and consequent genomic and proteomic analyses; new imaging approaches for high resolution analysis of both cells and chemical components; the demonstration of fractionation of stable isotopes of iron during iron reduction; and the elucidation of the types and patterns of secondary mineral formation during metal reduction. One of the secondary minerals is magnetite, the subject of intense controversy regarding the possibility of evidence for life from the Martian meteorite ALH84001. This review thus ends with a short consideration of the evidence for magnetic 'proof' of the existence of past life on Mars.

Bacteria↗
PubMed1998

Biomarkers as tracers for life on early earth and Mars.

Biomarkers in geological samples are products derived from biochemical (natural product) precursors by reductive and oxidative processes (e.g., cholestanes from cholesterol). Generally, lipids, pigments and biomembranes are preserved best over longer geological times and labile compounds such as amino acids, sugars, etc. are useful biomarkers for recent times. Thus, the detailed characterization of biomarker compositions permits the assessment of the major contributing species of extinct and/or extant life. In the case of the early Earth, work has progressed to elucidate molecular structure and carbon isotropic signals preserved in ancient sedimentary rocks. In addition, the combination of bacterial biochemistry with the organic geochemistry of contemporary and ancient hydrothermal ecosystems permits the modeling of the nature, behavior and preservation potential of primitive microbial communities. This approach uses combined molecular and isotopic analyses to characterize lipids produced by cultured bacteria (representative of ancient strains) and to test a variety of culture conditions which affect their biosynthesis. On considering Mars, the biomarkers from lipids and biopolymers would be expected to be preserved best if life flourished there during its early history (3.5-4 x 10(9) yr ago). Both oxidized and reduced products would be expected. This is based on the inferred occurrence of hydrothermal activity during that time with the concomitant preservation of biochemically-derived organic matter. Both known biomarkers (i.e., as elucidated for early terrestrial samples and for primitive terrestrial microbiota) and novel, potentially unknown compounds should be characterized.

Bacteria↗
PubMed1999

Did Viking discover life on Mars?

A major argument in the claim that life had been discovered during the Viking mission to Mars is that the results obtained in the Labeled Release (LR) experiment are analogous to those observed with terrestrial microorganisms. This assertion is critically examined and found to be implausible.

Amino Acids↗
PubMed2004

Importance of biologically active aurora-like ultraviolet emission: stochastic irradiation of Earth and Mars by flares and explosions.

Habitable planets will be subject to intense sources of ionizing radiation and fast particles from a variety of sources--from the host star to distant explosions--on a variety of timescales. Monte Carlo calculations of high-energy irradiation suggest that the surfaces of terrestrial-like planets with thick atmospheres (column densities greater than about 100 g cm(-2)) are well protected from directly incident X-rays and gamma-rays, but we find that sizeable fractions of incident ionizing radiation from astrophysical sources can be redistributed to biologically and chemically important ultraviolet wavelengths, a significant fraction of which can reach the surface. This redistribution is mediated by secondary electrons, resulting from Compton scattering and X-ray photoabsorption, the energies of which are low enough to excite and ionize atmospheric molecules and atoms, resulting in a rich aurora-like spectrum. We calculate the fraction of energy redistributed into biologically and chemically important wavelength regions for spectra characteristic of stellar flares and supernovae using a Monte-Carlo transport code and then estimate the fraction of this energy that is transmitted from the atmospheric altitudes of redistribution to the surface for a few illustrative cases. For atmospheric models corresponding to the Archean Earth, we assume no significant ultraviolet absorbers, only Rayleigh scattering, and find that the fraction of incident ionizing radiation that is received at the surface in the form of redistributed ultraviolet in the biologically relevant 200-320 nm region (UV-C and UV-B bands) can be up to 4%. On the present-day Earth with its ultraviolet ozone shield, this fraction is found to be 0.2%. Both values are many orders of magnitude higher than the fraction of direct ionizing radiation reaching the surface. This result implies that planetary organisms will be subject to mutationally significant, if intermittent, fluences of UV-B and harder radiation even in the presence of a narrow-band ultraviolet shield like ozone. We also calculate the surficial transmitted fraction of ionizing radiation and redistributed ultraviolet radiation for two illustrative evolving Mars atmospheres whose initial surface pressures were 1 bar. We discuss the frequency with which redistributed ultraviolet flux from parent star flares exceeds the parent star ultraviolet flux at the planetary surface. We find that the redistributed ultraviolet from parent star flares is probably a fairly rare intermittent event for habitable zone planets orbiting solar-type stars except when they are young, but should completely dominate the direct steady ultraviolet radiation from the parent star for planets orbiting all stars less massive than about 0.5 solar masses. Our results suggest that coding organisms on such planets (and on the early Earth) may evolve very differently than on contemporary Earth, with diversity and evolutionary rate controlled by a stochastically varying mutation rate and frequent hypermutation episodes.

Atmosphere↗
PubMed1999

Water on Mars: isotopic constraints on exchange between the atmosphere and surface.

Using a new measurement of the D/H fractionation efficiency and new estimates of the water loss, we calculate that Mars has the equivalent of a approximately 9 m global water layer in a reservoir that exchanges with the atmosphere. The measured D/H enrichment is about 5 times the terrestrial value, but without exchange, the atmosphere converges on an enrichment of 50 in about 0.5 Ma. Due to the large buffering reservoir and the rapid loss rate (10(-3) pr-micrometers yr-1), the small atmospheric reservoir, averaging 10 pr-micrometers, is unlikely to be in continuous isotopic equilibrium with the full 9 m exchangeable reservoir. Instead, it presumably equilibrates during periods of high obliquity; the atmospheric D/H ratio is expected to be enriched in between such periods. If isotopic exchange with a small (4 mm global layer) reservoir occurs under current conditions, it possible for the atmospheric D/H ratio to be within 10% of its long term equilibrium.

Atmosphere↗
PubMed1989

The case for planetary sample return missions. 2. History of Mars.

Principal science goals for exploration of Mars are to establish the chemical, isotopic, and physical state of Martian material, the nature of major surface-forming processes and their time scales, and the past and present biological potential of the planet. Many of those goals can only be met by detailed analyses of atmospheric gases and carefully selected samples of fresh rocks, weathered rocks, soils, sediments, and ices. The high-fidelity mineral separations, complex chemical treatments, and ultrasensitive instrument systems required for key measurements, as well as the need to adapt analytical strategies to unanticipated results, point to Earth-based laboratory analyses on returned Martian samples as the best means for meeting the stated objectives.

Chemical Phenomena↗
PubMed1995

A slightly more massive young Sun as an explanation for warm temperatures on early Mars.

The valley network channels on the heavily cratered ancient surface of Mars suggest the presence of liquid water approximately 3.8 Gyr ago. However, the implied warm climate is difficult to explain in the context of the standard solar model, even allowing for the maximum CO2 greenhouse heating. In this paper we investigate the astronomical and planetary implications of a nonstandard solar model in which the zero-age, main-sequence Sun had a mass of 1.05 +/- 0.02 M solar. The excess mass was subsequently lost in a solar wind during the first 1.2(-0.2, +0.4) Gyr of the Sun's main sequence phase. The implied mass-loss rate of 4(+3, -2) x 10(-11) M solar yr-1, or about 10(3)x that of the current Sun, may be detectable in several nearby young solar type stars.

Astronomy↗
PubMed1997

High-temperature life without photosynthesis as a model for Mars.

Discoveries in biology and developments in geochemistry over the past two decades have lead to a radical revision of concepts relating to the upper temperature at which life thrives, the genetic relationships among all life on Earth, links between organic and inorganic compounds in geologic processes, and the geochemical supply of metabolic energy. It is now apparent that given a source of geochemical energy, in the form of a mixture of compounds that is far from thermodynamic equilibrium, microorganisms can take advantage of the energy and thrive without the need for photosynthesis as a means of primary productivity. This means that life can exist in the subsurface of a planet such as Mars without necessarily exhibiting a surface expression. Theoretical calculations quantify the geochemically provided metabolic energy available to hyperthermophilic organisms in submarine hydrothermal systems on the Earth, and help to explain the enormous biological productivity of these systems. Efforts to place these models in the context of the early Earth reveal that substantial geochemical energy would have been available and that organic synthesis would have been thermodynamically favored as hydrothermal fluids mix with seawater.

Euryarchaeota↗
PubMed1990

A numerical simulation of climate changes during the obliquity cycle on Mars.

A one-dimensional seasonal energy balance climate model has been developed for the Martian surface and coupled to a model of CO2 distribution between atmosphere, regolith, and polar caps. This model takes into account the greenhouse warming of carbon dioxide, the meridional transport of heat, the CO2 condensation and sublimation cycle, and its adsorption in the regolith. The model takes into consideration the diurnal variation of solar irradiation, since it is shown that disregard of this effect yields temperatures too high by several degrees. The yearly-averaged temperatures calculated from this climate model at different obliquities are used to estimate the importance of CO2 exchanges between the regolith and atmosphere-cap systems during the obliquity cycle. For this purpose, the equation of thermal diffusion into the ground is solved for each latitude belt. The results differ substantially from those of previous studies, due in part to the consideration of the diurnal and seasonal variations of the solar irradiance. The model shows the importance of taking these short-period variations into account instead of using yearly-averaged quantities, due to the strong nonlinearity of the climate system on Mars. The roles of meridional heat transport and greenhouse warming are analyzed and shown to be important. For example, a permanent polar cap of carbon dioxide is destroyed by heat transport when the obliquity is high, while at low obliquity, high-pressure systems without permanent cap can exist if enough exchangeable carbon dioxide is available. Further, the results show the possible existence of hysteresis cycles in the formation and sublimation of permanent deposits during the course of the obliquity cycle.

Atmosphere↗
PubMed2001

Mars exploration.

An international flotilla of spacecraft are to be sent to Mars over the next decade in an effort to understand the planet's geology and climate history, and to determine whether some form of life ever started there. At least two spacecraft will be sent at each launch opportunity, and at times up to four spacecraft may be operating simultaneously at the planet.

Humans↗
PubMed2001

Evidence for recent climate change on Mars from the identification of youthful near-surface ground ice.

Ground ice in the crust and soil may be one of the largest reservoirs of water on Mars. Near-surface ground ice is predicted to be stable at latitudes higher than 40 degrees (ref. 4), where a number of geomorphologic features indicative of viscous creep and hence ground ice have been observed. Mid-latitude soils have also been implicated as a water-ice reservoir, the capacity of which is predicted to vary on a 100,000-year timescale owing to orbitally driven variations in climate. It is uncertain, however, whether near-surface ground ice currently exists at these latitudes, and how it is changing with time. Here we report observational evidence for a mid-latitude reservoir of near-surface water ice occupying the pore space of soils. The thickness of the ice-occupied soil reservoir (1-10 m) and its distribution in the 30 degrees to 60 degrees latitude bands indicate a reservoir of (1.5-6.0) x 104 km3, equivalent to a global layer of water 10-40 cm thick. We infer that the reservoir was created during the last phase of high orbital obliquity less than 100,000 years ago, and is now being diminished.

Climate↗
PubMed1991

Making Mars habitable.

Mars is believed to be lifeless, but it may be possible to transform it into a planet suitable for habitation by plants, and conceivably humans. The success of such an enterprise would depend on the abundance, distribution and form of materials on the planet that could provide carbon dioxide, water and nitrogen.

Atmosphere↗
PubMed1996

Experimental simulations of the photodecomposition of carbonates and sulphates on Mars.

There is indirect spectroscopic evidence for the presence of sulphates and carbonates on the martian surface, and such minerals are also found in SNC meteorites, which are thought to be of martian origin. But although carbonates are expected to be abundant in the martian regolith, attempts to detect them directly have been unsuccessful. Here we report laboratory studies of the decompostion of calcium carbonate and magnesium sulphate under ultraviolet irradiation, which mimic the conditions under which photodecomposition of surface minerals by solar ultraviolet light might occur on Mars. We find that, even for a low abundance of carbonate minerals in the martian regolith, the rate of CO2 release due to photodecomposition is higher than the rate of CO2 loss from the atmosphere by solar-wind-induced sputtering processes, making this process a potential net source of atmospheric CO2 over time. SO2 is also released from the sulphate, albeit more slowly. The rate of carbonate degradation is high enough to explain the apparent absence of these compounds at the martian surface.

Calcium Carbonate↗
PubMed2005

Raman spectroscopic analysis of cyanobacterial gypsum halotrophs and relevance for sulfate deposits on Mars.

The Raman spectra of cyanobacterial species, Gloecapsa and Nostoc, in clear gypsum crystals from the Haughton Crater, Devon Island, Canadian High Arctic, site of a meteorite impact during the Miocene some 23 Mya, have been recorded using several visible and near-infrared excitation wavelengths. The best spectra were obtained using a green wavelength at 514.5 nm and a confocal microscope with an image footprint of about 2 micro in diameter and 2 micro theoretical depth. Raman biosignatures for beta-carotene and scytonemin were obtained for one type of colony and parietin and beta-carotene for another; chlorophyll was detected in both types of colony. The different combination of these radiation protectant biomolecules suggests that the two cyanobacterial colonies, namely Nostoc and Gloecapsa, are adopting different survival strategies in the system. Confocal spectroscopic probing of the gypsum crystals exhibited sufficient discrimination for the identification of the biomolecules through the gypsum crystal, in simulation of the detection of extant or extinct halotrophs. This supports the viability of Raman spectroscopic techniques for incorporation as part of the instrumentation suite of a robotic lander for planetary surface exploration for the detection of organisms inside sulfate crystals from previous hydrothermal activity on Mars.

Calcium Sulfate↗
PubMed2003

Microbial characterization of the Mars Odyssey spacecraft and its encapsulation facility.

Microbial characterization of the Mars Odyssey spacecraft and the Kennedy Space Center Spacecraft Assembly and Encapsulation Facility II (SAEF-II) was carried out by both culture-based and molecular methods. The most dominant cultivable microbes were species of Bacillus, with comamonads, microbacteria and actinomycetales also represented. Several spore-forming isolates were resistant to gamma-radiation, UV, H2O2 and desiccation, and one Acinetobacter radioresistens isolate and several Aureobasidium, isolated directly from the spacecraft, survived various conditions. Sequences arising in clone libraries were fairly consistent between the spacecraft and facility; predominant genera included Variovorax, Ralstonia and Aquaspirillum. This study improves our understanding of the microbial community structure, diversity and survival capabilities of microbes in an encapsulation facility and physically associated with colocated spacecraft.

Actinomycetales↗
PubMed2001

Cave biosignature suites: microbes, minerals, and Mars.

Earth's subsurface offers one of the best possible sites to search for microbial life and the characteristic lithologies that life leaves behind. The subterrain may be equally valuable for astrobiology. Where surface conditions are particularly hostile, like on Mars, the subsurface may offer the only habitat for extant lifeforms and access to recognizable biosignatures. We have identified numerous unequivocally biogenic macroscopic, microscopic, and chemical/geochemical cave biosignatures. However, to be especially useful for astrobiology, we are looking for suites of characteristics. Ideally, "biosignature suites" should be both macroscopically and microscopically detectable, independently verifiable by nonmorphological means, and as independent as possible of specific details of life chemistries--demanding (and sometimes conflicting) criteria. Working in fragile, legally protected environments, we developed noninvasive and minimal impact techniques for life and biosignature detection/characterization analogous to Planetary Protection Protocols. Our difficult field conditions have shared limitations common to extraterrestrial robotic and human missions. Thus, the cave/subsurface astrobiology model addresses the most important goals from both scientific and operational points of view. We present details of cave biosignature suites involving manganese and iron oxides, calcite, and sulfur minerals. Suites include morphological fossils, mineral-coated filaments, living microbial mats and preserved biofabrics, 13C and 34S values consistent with microbial metabolism, genetic data, unusual elemental abundances and ratios, and crystallographic mineral forms.

Exobiology↗
PubMed2002

Fluid inclusion studies of chemosynthetic carbonates: strategy for seeking life on Mars.

Fluid inclusions in minerals hold the potential to provide important data on the chemistry of the ambient fluids during mineral precipitation. Especially interesting to astrobiologists are inclusions in low-temperature minerals that may have been precipitated in the presence of microorganisms. We demonstrate that it is possible to obtain data from inclusions in chemosynthetic carbonates that precipitated by the oxidation of organic carbon around methane-bearing seepages. Chemosynthetic carbonates have been identified as a target rock for astrobiological exploration. Other surficial rock types identified as targets for astrobiological exploration include hydrothermal deposits, speleothems, stromatolites, tufas, and evaporites, each of which can contain fluid inclusions. Fracture systems below impact craters would also contain precipitates of minerals with fluid inclusions. As fluid inclusions are sealed microchambers, they preserve fluids in regions where water is now absent, such as regions of the martian surface. Although most inclusions are < 5 microns, the possibility to obtain data from the fluids, including biosignatures and physical remains of life, underscores the advantages of technological advances in the study of fluid inclusions. The crushing of bulk samples could release inclusion waters for analysis, which could be undertaken in situ on Mars.

Carbonates↗
PubMed2002

Application of hyperspectral infrared analysis of hydrothermal alteration on Earth and Mars.

An integrated analysis of both airborne and field short-wave infrared hyperspectral measurements was used in conjunction with conventional field mapping techniques to map hydrothermal alteration in the central portion of the Mount Painter Inlier in the Flinders Ranges, South Australia. The airborne hyperspectral data show the spatial distribution of spectrally distinct minerals occurring as primary minerals and as weathering and alteration products. Field spectral measurements, taken with a portable infrared mineral analyzer spectrometer and supported by thin-section analyses, were used to verify the mineral maps and enhance the level of information obtainable from the airborne data. Hydrothermal alteration zones were identified and mapped separately from the background weathering signals. A main zone of alteration, coinciding with the Paralana Fault zone, was recognized, and found to contain kaolinite, muscovite, biotite, and K-feldspar. A small spectral variation associated with a ring-like feature around Mount Painter was tentatively determined to be halloysite and interpreted to represent a separate hydrothermal fluid and fluid source, and probably a separate system. The older parts of the alteration system are tentatively dated as Permo-Carboniferous. The remote sensing of alteration at Mount Painter confirms that hyperspectral imaging techniques can produce accurate mineralogical maps with significant details that can be used to identify and map hydrothermal activity. Application of hyperspectral surveys such as that conducted at Mount Painter would be likely to provide similar detail about putative hydrothermal deposits on Mars.

Earth, Planet↗