Amino acid photostability on the Martian surface

Abstract— In the framework of international planetary exploration programs, several space missions are planned to search for organics and bio‐signatures on Mars. Previous attempts have not detected any organic compounds in the Martian regolith. It is therefore critical to investigate the processes that may affect organic molecules on and below the planet's surface. Laboratory simulations can provide useful data about the reaction pathways of organic material at Mars' surface. We have studied the stability of amino acid thin films against ultraviolet (UV) irradiation and use those data to predict the survival time of these compounds on and in the Martian regolith. We show that thin films of glycine and D‐alanine are expected to have half‐lives of 22 ± 5 hr and of 3 ± 1 hr, respectively, when irradiated with Mars‐like UV flux levels. Modelling shows that the half‐lives of the amino acids are extended to the order of 10⁷ years when embedded in regolith. These data suggest that subsurface sampling must be a key component of future missions to Mars dedicated to organic detection.     

In Situ Biological Contamination Studies of the Moon: Implications for Planetary Protection and Life Detection Missions

NASA and ESA have outlined visions for solar system exploration that will include a series of lunar robotic precursor missions to prepare for, and support a human return to the Moon, and future human exploration of Mars and other destinations, including possibly asteroids. One of the guiding principles for exploration is to pursue compelling scientific questions about the origin and evolution of life. The search for life on objects such as Mars will require careful operations, and that all systems be sufficiently cleaned and sterilized prior to launch to ensure that the scientific integrity of extraterrestrial samples is not jeopardized by terrestrial organic contamination. Under the Committee on Space Research’s (COSPAR’s) current planetary protection policy for the Moon, no sterilization procedures are required for outbound lunar spacecraft, nor is there a different planetary protection category for human missions, although preliminary COSPAR policy guidelines for human missions to Mars have been developed. Future in situ investigations of a variety of locations on the Moon by highly sensitive instruments designed to search for biologically derived organic compounds would help assess the contamination of the Moon by lunar spacecraft. These studies could also provide valuable “ground truth” data for Mars sample return missions and help define planetary protection requirements for future Mars bound spacecraft carrying life detection experiments. In addition, studies of the impact of terrestrial contamination of the lunar surface by the Apollo astronauts could provide valuable data to help refine future Mars surface exploration plans for a human mission to Mars.     

The provenance,formation, and implications of reduced carbon phases in Martian meteorites

This review is intended to summarize the current observations of reduced carbon in Martian meteorites, differentiating between terrestrial contamination and carbon that is indigenous to Mars. Indeed, the identification of Martian organic matter is among the highest priority targets for robotic spacecraft missions in the next decade, including the Mars Science Laboratory and Mars 2020. Organic carbon compounds are essential building blocks of terrestrial life, so the occurrence and origin (biotic or abiotic) of organic compounds on Mars is of great significance; however, not all forms of reduced carbon are conducive to biological systems. This paper discusses the significance of reduced organic carbon (including methane) in Martian geological and astrobiological systems. Specifically, it summarizes current thinking on the nature, sources, and sinks of Martian organic carbon, a key component to Martian habitability. Based on this compilation, reduced organic carbon on Mars, including detections of methane in the Martian atmosphere, is best described through a combination of abiotic organic synthesis on Mars and infall of extraterrestrial carbonaceous material. Although conclusive signs of Martian life have yet to be revealed, we have developed a strategy for life detection on Mars that can be utilized in future life‐detection studies.     

Experimental determination of photostability and fluorescence‐based detection of PAHs on the Martian surface

Abstract– Even in the absence of any biosphere on Mars, organic molecules, including polycyclic aromatic hydrocarbons (PAHs), are expected on its surface due to delivery by comets and meteorites of extraterrestrial organics synthesized by astrochemistry, or perhaps in situ synthesis in ancient prebiotic chemistry. Any organic compounds exposed to the unfiltered solar ultraviolet spectrum or oxidizing surface conditions would have been readily destroyed, but discoverable caches of Martian organics may remain shielded in the subsurface or within surface rocks. We have studied the stability of three representative polycyclic aromatic hydrocarbons (PAHs) in a Mars chamber, emulating the ultraviolet spectrum of unfiltered sunlight under temperature and pressure conditions of the Martian surface. Fluorescence spectroscopy is used as a sensitive indicator of remaining PAH concentration for laboratory quantification of molecular degradation rates once exposed on the Martian surface. Fluorescence‐based instrumentation has also been proposed as an effective surveying method for prebiotic organics on the Martian surface. We find the representative PAHs, anthracene, pyrene, and perylene, to have persistence half‐lives once exposed on the Martian surface of between 25 and 60 h of noontime summer UV irradiation, as measured by fluorescence at their peak excitation wavelength. This equates to between 4 and 9.6 sols when the diurnal cycle of UV light intensity on the Martian surface is taken into account, giving a substantial window of opportunity for detection of organic fluorescence before photodegradation. This study thus supports the use of fluorescence‐based instrumentation for surveying recently exposed material (such as from cores or drill tailings) for native Martian organic molecules in rover missions.     

Why exobiology on Mars?

Processing of organic molecules by liquid water was probably an essential requirement towards the emergence of terrestrial primitive life. According to Oparin's hypothesis, organic building blocks required for early life were produced from simple organic molecules formed in a primitive reducing atmosphere. Geochemists favour now a less reducing atmosphere dominated by carbon dioxide. In such an atmosphere, very few building blocks are formed. Import of extraterrestrial organic molecules may represent an alternative supply. Experimental support for such an alternative scenario is examined in comets, meteorites and micrometeorites. The early histories of Mars and Earth clearly show similarities. Liquid water was once stable on the surface of Mars attesting the presence of an atmosphere capable of decelerating C-rich micro-meteorites. Therefore, primitive life may have developed on Mars, as well. Liquid water disappeared from the surface of Mars very early, about 3.8 Ga ago. The Viking missions did not find, at the surface of the Martian soil, any organic molecules or clear-cut evidence for microbial activities such as photosynthesis, respiration or nutrition. The results can be explained referring to an active photochemistry of Martian soil driven by the high influx of solar UV. These experiments do not exclude the existence of organic molecules and fossils of micro-organisms which developed on early Mars until liquid water disappeared. Mars may store below its surface some well preserved clues of a still hypothetical primitive life.     

Amino acid analyses of R and CK chondrites

Exogenous delivery of amino acids and other organic molecules to planetary surfaces may have played an important role in the origins of life on Earth and other solar system bodies. Previous studies have revealed the presence of indigenous amino acids in a wide range of carbon‐rich meteorites, with the abundances and structural distributions differing significantly depending on parent body mineralogy and alteration conditions. Here we report on the amino acid abundances of seven type 3–6 CK chondrites and two Rumuruti (R) chondrites. Amino acid measurements were made on hot water extracts from these meteorites by ultrahigh‐performance liquid chromatography with fluorescence detection and time‐of‐flight mass spectrometry. Of the nine meteorites analyzed, four were depleted in amino acids, and one had experienced significant amino acid contamination by terrestrial biology. The remaining four, comprised of two R and two CK chondrites, contained low levels of amino acids that were predominantly the straight chain, amino‐terminal (n‐ω‐amino) acids β‐alanine, and γ‐amino‐n‐butyric acid. This amino acid distribution is similar to what we reported previously for thermally altered ureilites and CV and CO chondrites, and these n‐ω‐amino acids appear to be indigenous to the meteorites and not the result of terrestrial contamination. The amino acids may have been formed by Fischer–Tropsch‐type reactions, although this hypothesis needs further testing.     

Water Ice on Comets and Satellites

This brief overview will cover some recent work on solar system ices. The focus is on the origin, physical properties, composition, and radiation-induced chemistry of ices dominated by water, with an emphasis on comets and water-rich ices on satellites such as Europa. Understanding the physical characteristics and chemistry of these ices is important for explaining observations such as the albedo and sublimation of ice from planetesimals and comets, the formation of molecules that may have led to life, and planning present and future space missions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermodynamic models of the distribution of life-related organic molecules in the interstellar medium

New equilibrium thermodynamic distribution models of organic substances important to life in giant molecular clouds are suggested. These models use the normal distribution law of the standard enthalpy and the free energy (Helmholtz energy) for various organic molecules, amino acids, and nucleic acid components. These models were used to forecast organic molecule resources in the interstellar medium of the observable universe. The resources of the main biological molecules were estimated from radio-astronomical data.     

Cometary organic chemistry: a review from observations,numerical and experimental simulations

This paper is a review dealing with the organic chemistry of comets. It describes how the chemical composition of comets can provide information about the chemistry of the interstellar medium, and the formation of the solar system. We discuss to what extent they could have brought to Earth the ingredients essential to the emergence of life: water and prebiotic compounds. We review all molecules which have been detected or tentatively detected in comets by remote sensing or in-situ observations, inputs of theoretical models, and all other organic species expected to be present from the results of experimental simulations. This compilation yields a list of more than a hundred molecules which can be used as a reference for the preparation of experiments developed for the Rosetta and Deep Space 4 cometary missions. We point out that further experiments are necessary to investigate the connections between the solid and gaseous phases of comets, especially studying the photodegradation of high molecular weight compounds which could be present in the nuclei.     

A propensity for n‐ω‐amino acids in thermally altered Antarctic meteorites

Abstract– Asteroids and their fragments have impacted the Earth for the last 4.5 Gyr. Carbonaceous meteorites are known to contain a wealth of indigenous organic molecules, including amino acids, which suggests that these meteorites could have been an important source of prebiotic organic material during the origins of life on Earth and possibly elsewhere. We report the detection of extraterrestrial amino acids in thermally altered type 3 CV and CO carbonaceous chondrites and ureilites recovered from Antarctica. The amino acid concentrations of the thirteen Antarctic meteorites ranged from 300 to 3200 parts‐per‐billion (ppb), generally much less abundant than in amino acid‐rich CI, CM, and CR carbonaceous chondrites that experienced much lower temperature aqueous alteration on their parent bodies. In contrast to low‐temperature aqueously altered meteorites that show complete structural diversity in amino acids formed predominantly by Strecker–cyanohydrin synthesis, the thermally altered meteorites studied here are dominated by small, straight‐chain, amine terminal (n‐ω‐amino) amino acids that are not consistent with Strecker formation. The carbon isotopic ratios of two extraterrestrial n‐ω‐amino acids measured in one of the CV chondrites (δ¹³C approximately ?25‰) are consistent with ¹³C‐depletions observed previously in hydrocarbons produced by Fischer‐Tropsch type reactions. The predominance of n‐ω‐amino acid isomers in thermally altered meteorites hints at cosmochemical mechanisms for the preferential formation and preservation of a small subset of the possible amino acids.     

Combining IR and X‐ray microtomography data sets: Application to Itokawa particles and to Paris meteorite

In the near future, a new generation of sample return missions (Hayabusa2, OSIRIS‐REx, MMX, etc.) will collect samples from small solar system bodies. To maximize the scientific outcome of laboratory studies and minimize the loss of precious extraterrestrial samples, an analytical sequence from less destructive to more destructive techniques needs to be established. In this work, we present a combined X‐ray and IR microtomography applied to five Itokawa particles and one fragment of the primitive carbonaceous chondrite Paris. We show that this analytical approach is able to provide a 3‐D physical and chemical characterization of individual extraterrestrial particles, using the measurement of their 3‐D structure and porosity, and the detection of mineral and organic phases, and their spatial co‐localization in 3‐D. We propose these techniques as an efficient first step in a multitechnique analytical sequence on microscopic samples collected by space missions.     

Ultraviolet photolysis of amino acids in a 100 K water ice matrix: Application to the outer Solar System bodies

We report the rates of decomposition by ultraviolet (UV) photolysis of four amino acids in millimeter-thick crystalline water ice matrices at 100 K to constrain the survivability of these important organic molecules within ice lying near the surfaces of outer Solar System bodies. We UV-irradiated crystalline ice samples containing known concentrations of the amino acids glycine, aspartic acid, glutamic acid, and phenylalanine, then we measured the surviving concentrations using high performance liquid chromatography (HPLC) with fluorescence detection. From these experiments, we determine photolytic decomposition rates and half-lives. The half-life varies linearly with the ice thickness for all acids studied here. For example, glycine is the most resistant to photolytic destruction with a half-life of 50, 12, and 3.7 h in 1.6, 0.28, and 0.14 mm thick ices, respectively. We explain this linear variation of half-life with thickness as a consequence of extinction, mostly due to scattering, within these macroscopically thick ice samples. Applied to low latitude surface ice on Jupiter's satellite Europa, this analysis indicates that the concentration of any of these amino acids within the top meter of similar ice will be halved within a ∼10 year timescale.     

Raman spectra of nitrogen-containing organic compounds obtained in high altitude sites using a portable spectrometer: Possible application for remote robotic Titan studies

Well-resolved Raman spectra of examples of nitrogen-containing compounds were detected using a portable Raman instrument (Ahura First Defender XL) outdoors at a low temperature of −15 °C at an altitude of 2860 m (Pitztall, Austria). The portable Raman spectrometer tested here is equipped with a 785 nm diode laser and fixed frontal probe. Solid forms of formamide, urea, 3-methylpyridine, aniline, indene, 1-(2-aminoethyl)piperazine, benzofuran and indoline were detected unambiguously under field high-mountain conditions. The main Raman features (strong, medium and partially weak bands) were observed at their correct wavenumber positions (spectral resolution 7-10 cm⁻¹) in the range 200-2000 cm⁻¹. The results obtained demonstrate the possibility of applying a miniaturised Raman spectrometer as key instrument for investigating the presence of nitrogen-containing organic compounds and biomolecules under low temperature field conditions. Within the payload designed by ESA and NASA for future missions focusing not only on Mars, Raman spectroscopy will be an important non-destructive analytical tool for the in-situ identification of both organic and inorganic compounds relevant to life detection on planetary surfaces or near sub-surfaces.     

Methane and related trace species on Mars: Origin,loss, implications for life,and habitability

One of the most puzzling aspects of Mars is that organics have not yet been found on the surface. The simplest of organic molecules, methane, was detected in the Martian atmosphere for the first time in 2003. The existence and behavior of methane on Mars is of great significance, as methane is a potential biomarker. In this paper we review our current understanding of possible sources and sinks of methane on Mars. We also investigate the role of other trace species in the maintenance and removal of methane from the atmosphere, as well as of other organic material from the surface. In particular, we examine the exogenous, hydrogeochemical—especially serpentinization—and biological sources, for supplying methane to Mars. We suggest that comets and meteorites are the least likely, whereas low-temperature serpentinization is the most plausible of all candidates to explain the methane observations. Nevertheless, it is premature to rule out the role of biology in producing methane on Mars, in view of available data. It is important to note that the loss of methane to surface must also be factored into any “source” scenarios for methane. Ordinary heterogeneous loss process to surface tends to be very slow. On the other hand, a reactive surface could potentially accelerate the destruction of methane. If correct, it would imply that a larger source of methane is present than currently estimated on the basis of photochemical loss alone. A reactive surface can also explain why no organic material has ever been detected on the Martian surface. The surface could become reactive if some oxidizer were present. We suggest that vast quantities of a powerful oxidant, hydrogen peroxide, can be produced in electrochemistry triggered by electrostatic fields generated in the Martian dust devils and dust storms, and in normal saltation process close to the surface. Finally, current observations are inadequate to prove or disprove the existence of life on Mars, now or in the past. The question of extraterrestrial life is a fundamental one, and it should be addressed meticulously on future missions to Mars. Measurements planned on the Mars Science Laboratory (MSL), especially carbon isotopes and chirality, will go a long way in meeting this goal. A brief overview of the MSL Mission and measurements relevant to the question of life and habitability of Mars is also presented in this paper.     

A search for amino acids and nucleobases in the Martian meteorite Roberts Massif 04262 using liquid chromatography‐mass spectrometry

The investigation into whether Mars contains signatures of past or present life is of great interest to science and society. Amino acids and nucleobases are compounds that are essential for all known life on Earth and are excellent target molecules in the search for potential Martian biomarkers or prebiotic chemistry. Martian meteorites represent the only samples from Mars that can be studied directly in the laboratory on Earth. Here, we analyzed the amino acid and nucleobase content of the shergottite Roberts Massif (RBT) 04262 using liquid chromatography‐mass spectrometry. We did not detect any nucleobases above our detection limit in formic acid extracts; however, we did measure a suite of protein and nonprotein amino acids in hot‐water extracts with high relative abundances of β‐alanine and γ‐amino‐n‐butyric acid. The presence of only low (to absent) levels of several proteinogenic amino acids and a lack of nucleobases suggest that this meteorite fragment is fairly uncontaminated with respect to these common biological compounds. The distribution of straight‐chained amine‐terminal n‐ω‐amino acids in RBT 04262 resembled those previously measured in thermally altered carbonaceous meteorites (Burton et al. 2012; Chan et al. 2012). A carbon isotope ratio of ?24‰ ± 6‰ for β‐alanine in RBT 04262 is in the range of reduced organic carbon previously measured in Martian meteorites (Steele et al. 2012). The presence of n‐ω‐amino acids may be due to a high temperature Fischer‐Tropsch‐type synthesis during igneous processing on Mars or impact ejection of the meteorites from Mars, but more experimental data are needed to support these hypotheses.     

The specific molecular identification of life experiment (SMILE)

We describe a compact, highly integrated instrument concept for the detection and identification of a wide range of molecules associated with extinct/extant life or potential life processes. The Specific Molecular Identification of Life Experiment (SMILE) will be sensitive to the presence of a range of target molecules using both electrical and optical transduction techniques, and incorporates molecular imprinted polymers in addition to traditional biological receptors such as antibodies. A number of versions of the concept are possible depending on available resources e.g. mass, volume, etc. The full concept utilises a novel imaging interferometer where a large number of molecular receptors are deposited on the measurement plane of an imaging interferometer and read out by an imaging detector, enabling multiple targets—biomarkers—within a sample to be measured simultaneously. The optics can also form the basis of an UV-NIR imaging Fourier spectrometer allowing basic mineralogy studies to be conducted using optical properties to assist in the determination of the geological context of the samples. By incorporating micro-fabricated transducer arrays, micro-fluidics and artificial molecular recognition systems, as well as recombinant antibody technology with appropriate integration methods, SMILE forms a compact and robust “Life Marker Chip” which has been proposed for future planetary missions including ESA's ExoMars mission, where the instrument offers the possibility of conducting a direct in situ search for signs of past or present biological activity on Mars. In addition to its role in planetary exploration, derivatives of SMILE have multiple terrestrial applications in fields such as forensic analysis and environmental monitoring.     

Cold curation of pristine astromaterials: Insights from the Tagish Lake meteorite

The curation and handling of volatile‐bearing astromaterials is of prime importance in current and future plans for sample return missions to targets containing organic compounds, ices, or other volatile components. We report on the specific curation constraints required for the preservation of the Tagish Lake meteorite, a C2 ungrouped chondrite that contains significant concentrations of organic matter, including compounds of prebiotic interest or volatile in character, and which was recovered from a frozen lake surface a few days after its fall. Here, we review the circumstances of the meteorite's handling, its complement of intrinsic and contaminant organic compounds, and an unusual reaction between some of the specimens and the Al foil in which they were enclosed. From our results, we derive the requirements for curation of the meteorite, and describe a specialized facility that enables its curation and handling. The Subzero Facility for Curation of Astromaterials consists of a purified Ar glove box enclosed within a freezer chamber, and enables investigations relevant to curation of samples at or below ?10 °C. We provide several recommendations based on insights obtained from the commissioning and initial use of the facility that are relevant to collection of freshly fallen meteorites, curation of volatile‐bearing meteorites and other astromaterials, and planning and implementation of curation plans for future sample return missions to volatile‐bearing targets.     

Interstellar Grains as Amino Acid Factories and the Origin of Life

Some two decades ago, Hoyle and Wickramasinghe (1976) proposed that the physical conditions inside dense molecular clouds favour the formation of amino acids and complex organic polymers. There now exists both astronomical and laboratory evidence supporting this idea. Recent millimeter array observations have discovered the amino acid glycine (NH₂CH₂COOH) in the gas phase of the dense star-forming cloud Sagittarius B2. These observations would pose serious problems for present-day theories of molecule formation in space because it is unlikely that glycline can form by the gas-phase reaction schemes normally considered for dense cloud chemistry. Several laboratory experiments suggest a new paradigm in which amino acids and other large organic molecules are chemically manufactured inside the bulk interior of icy grain mantles photoprocessed by direct and scattered ultraviolet starlight. Frequent chemical explosions of the processed mantles would eject large fragments of organic dust into the ambient cloud. Large dust fragments break up into smaller ones by sputtering and ultimately by photodissociation of individual molecules. Hence, a sizeable column density (N≈ 10¹⁰−10¹⁵ cm^-2) of amino acids would be present in the gaseous medium as a consequence of balancing the rate of supply from exploding mantles with the rate of molecule destruction. Exploding mantles can therefore solve the longstanding molecule desorption problem for interstellar dense cloud chemistry. A sizeable fraction of the organic dust population can survive destruction and seed primitive planetary systems throughout our galaxy with prebiological organic molecules needed for proteins and nucleic acids in living organisms. This possibility provides fresh grounds for a new version of the old panspermia hypothesis first introduced by Anaxagoras. It is shown that panspermia is more important than asteroid and cometary organic depositions onto primitive Earth. Furthermore, no appeal to Miller-Urey synthesis in a nonoxidizing atmosphere of primitive Earth is then needed to seed terrestrial life. This revised version was published online in July 2006 with corrections to the Cover Date.     

Concentration and variability of the AIB amino acid in polar micrometeorites: Implications for the exogenous delivery of amino acids to the primitive Earth

Abstract— Micrometeorites (MMs) are extraterrestrial particles ranging in size from 25 μm to 2 mm that survive atmospheric entry and are collected on the Earth's surface. They represent the largest mass flux (MF) of extraterrestrial material (30,000 ± 20,000 t/yr) to the present‐day Earth. Studies of large collections of MMs suggest that about 20% have not been heated to high temperatures and that they contain organic carbon. Since non‐protein amino acids have been found in some carbonaceous meteorites, they might also be found in unmelted MMs. However, previous searches for amino acids in MMs were inconclusive. We combined a new extraction method for amino acids with a highly sensitive analytical method to detect and quantitate amino acids in MMs collected at the South Pole. We found the non‐protein amino acid α‐amino isobutyric acid (AIB) in one of our samples. The non‐detection of this amino acid in the other samples analyzed suggests that there are amino acid‐containing and amino acid‐free MMs, with ?14% of the MMs containing AIB. Since the MF of MMs is much higher than that of carbonaceous chondrites (CMs), amino acids in these small particles would represent an important source of exogenous delivery of organic molecules. Therefore, the results are discussed on the basis of their implications for astrobiology.     

Extended survival of several organisms and amino acids under simulated martian surface conditions

Recent orbital and landed missions have provided substantial evidence for ancient liquid water on the martian surface as well as evidence of more recent sedimentary deposits formed by water and/or ice. These observations raise serious questions regarding an independent origin and evolution of life on Mars. Future missions seek to identify signs of extinct martian biota in the form of biomarkers or morphological characteristics, but the inherent danger of spacecraft-borne terrestrial life makes the possibility of forward contamination a serious threat not only to the life detection experiments, but also to any extant martian ecosystem. A variety of cold and desiccation-tolerant organisms were exposed to 40 days of simulated martian surface conditions while embedded within several centimeters of regolith simulant in order to ascertain the plausibility of such organisms’ survival as a function of environmental parameters and burial depth. Relevant amino acid biomarkers associated with terrestrial life were also analyzed in order to understand the feasibility of detecting chemical evidence for previous biological activity. Results indicate that stresses due to desiccation and oxidation were the primary deterrent to organism survival, and that the effects of UV-associated damage, diurnal temperature variations, and reactive atmospheric species were minimal. Organisms with resistance to desiccation and radiation environments showed increased levels of survival after the experiment compared to organisms characterized as psychrotolerant. Amino acid analysis indicated the presence of an oxidation mechanism that migrated downward through the samples during the course of the experiment and likely represents the formation of various oxidizing species at mineral surfaces as water vapor diffused through the regolith. Current sterilization protocols may specifically select for organisms best adapted to survival at the martian surface, namely species that show tolerance to radical-induced oxidative damage and low water activity environments. Additionally, any hypothetical martian ecosystems may have evolved similar physiological traits that allow sporadic metabolism during periods of increased water activity.