Gas-Phase Reactions in Extraterrestrial Environments: Laboratory Investigations by Crossed Molecular Beams

We have investigated gas-phase reactions of N(²D) with the most abundant hydrocarbons in the atmosphere of Titan by the crossed molecular beam technique. In all cases, molecular products containing a novel CN bond are formed, thus suggesting possible routes of formation of gas-phase nitriles in the atmosphere of Titan and primordial Earth. The same approach has been recently extended to the study of radical–radical reactions, such as the reaction of atomic oxygen with the CH₃ and C₃H₅ radicals. Products other than those already considered in the modeling of planetary atmospheres and interstellar medium have been identified. Presented at: National Workshop On Astrobiology: Search For Life In The Solar System, Capri, Italy, 26 to 28 October, 2005.     

The atmosphere of Titan

Summary The discovery that Titan had an atmosphere was made by the identification of methane in the satellite's spectrum in 1944. But the abundance of this gas and the identification of other major constituents required the 1980 encounter by the Voyager 1 spacecraft. in the intervening years, traces of C₂H₂, C₂H₄, C₂H₆ and CH₃D had been posited to interpret emission bands in Titan's IR spectrum. The Voyager infrared Spectrometer confirmed that these gases were present and added seven more. The atmosphere is now known to be composed primarily of molecular nitrogen. But the derived mean molecular weight suggests the presence of a significant amount of some heavier gas, most probably argon. It is shown that this argon must be primordial, and that one can understand the evolution of Titan's atmosphere in terms of degassing of a mixed hydrate dominated by CH₄, N₂ and³⁶Ar. This model satisfactorily explains the absence of neon and makes no special requirements on the satellite's surface temperature. The organic chemistry taking place on Titan today invites comparision with chemical evolution on the primitive Earth prior to the origin of life.Adapted in commemoration of the many contributions of Harold Urey to the study of planetary atmospheres from an article in press in J Planet Sci (1982)     

Organic syntheses from CH4−N2 atmospheres: Implications for Titan

Numerous experiments have already been performed, simulating the evolution of gaseous mixtures containing CH₄ when submitted to energy flux. From their results, it appears that a variety of organic compounds, including unsaturated hydrocarbons and nitriles such as HCN, can be synthesized into noticeable amounts from CH₄–N₂ mixtures. In particular, systematic studies of the influence of the composition of the mixture on the nature and amount of synthesized compounds show that organic volatile nitriles, and particularly cyanoacetylene and cyanogen, are formed only in media rich in nitrogen. Those nitriles have been identified very recently in the atmosphere of Titan, and thus, data from such laboratory experiments may provide important indirect information on the organic chemistry occuring at the periphery of this satellite of Saturn. However, during these experiments, there is a continuous formation and accumulation of molecular hydrogen, which does not occur in the atmosphere of Titan, because of H₂ escape. In order to reassess the data already available from this type of laboratory studies, experiments on CH₄–N₂ atmospheres, with and without H2 escape, have been recently performed. The influence of this parameter on the chemical evolution of the atmosphere and on the nature and relative quantities of organic compounds has been studied.After reviewing these experiments, implications of the obtained results on the organic chemistry at the periphery of Titan are discussed.Paper presented at the 6th College Park Colloquium, October 1981.     

Exobiology on Titan

With a dense N₂-CH₄ atmosphere rich in organics, both in gas and aerosol phases, and with the possible presence of hydrocarbons oceans on its surface, Titan, the largest satellite of Saturn, appears as a natural laboratory to study chemical evolution toward complex organic systems, in a planetary environment and over a long time scale. Thanks to many analogies with planet Earth, it provides a unique way to look at the various physical and chemical processes, and their couplings which may have been involved in terrestrial prebiotic chemistry. Indeed, analogies with the Earth have a limit since Titan's temperatures are much lower than on the Earth and since liquid water is totally absent. However, from that aspect, Titan also serves as a reference laboratory worth studying — indirectly — the role of liquid water in exobiology. The Cassini-Huygens mission currently developed by NASA and ESA will send an orbiter around Saturn and Titan and a probe in Titan's atmosphere. This mission which will be launched in 1997 for an expected arrival in 2004, offers a unique opportunity to study in detail extra-terrestrial, not life-controled, organic processes, and consequently it will have significant implications in the fields of exobiology and the origins of life.This paper is dedicated to the memory of Cyril PONNAMPERUMA who largely contributed to the development of the scientific fields of Chemical Evolution and Exobiology.     

Formation of amino acids from models of Titan and more oxidized atmospheres

Protein and non-protein amino acids were synthesized following hydrolysis of products obtained by high frequency discharge techniques applied to model atmospheres consisting of N₂ as a nitrogen source together with CH₄ and/or CO₂ as a carbon source. Highest yields were obtained in the absence of CO₂ and from mixtures rich in CH₄. Amino acids would indeed be expected on the frozen surface of Titan with its CH₄–N₂ atmosphere.Paper presented at the 6th College Park Colloquium, October 1981.     

Review and Latest Results of Laboratory Investigations of Titan's Aerosols

Titan, the largest satellite of Saturn, has an atmosphere chiefly made up of N₂ and CH₄, and including many organics. This atmosphere also partly consists of hazes and aerosol particles which shroud the surface of this satellite, giving it a reddish appearance. The aerosols observed in Titan's atmosphere are thought to be synthesized at high altitudes (>300 km) and fall to the surface. Varying with temperature profiles, condensation phenomena take place in the lower atmosphere, about 100 km below. These solid particles, often called tholins, have been currently investigated for many years by laboratory scientists and physics modellers. This paper assesses past research and results in different fields (elemental composition, optical constants, pyrolysis, particle size), highlighting interests and questions aroused by these studies. It also presents the latest results and advances, and concludes with existing problems and future pathways.     

The lack of changes in nocturnal estuarine fish assemblages between new and full moon phases in Serpentine Creek,Queensland

Synopsis A series of 10 sites were sampled at new and full moon phases in a vertically homogenous estuary, Serpentine Creek, Queensland, Australia. Forty-five species (14,518 individuals) were caught and analysed using standardized Shannon & Gleason diversity indices, and total number of species and individuals. The coefficients of variation for these values were greater for times than for sites. There were no significant differences between 14 pairs of new and full moon phases for_t,_t,_t, and_t, or between mean number of individuals for the 16 most abundant species. If significant variations do exist within months, then experiments should be done between spring and neap tides as this study found no differences between the spring tides associated with new and full moon phases.     

CHEMICAL EVOLUTION ON TITAN: COMPARISONS TO THE PREBIOTIC EARTH

Models for the origin of Titan's atmosphere, the processing of the atmosphere and surface and its exobiological role are reviewed. Titan has gained widespread acceptance in the origin of life field as a model for the types of evolutionary processes that could have occurred on prebiotic Earth. Both Titan and Earth possess significant atmospheres ( 1 atm) composed mainly of molecular nitrogen with smaller amounts of more reactive species. Both of these atmospheres are processed primarily by solar ultraviolet light with high energy particles interactions contributing to a lesser extent. The products of these reactions condense or are dissolved in other atmospheric species (aerosols/clouds) and fall to the surface. There these products may have been further processed on Titan and the primitive Earth by impacting comets and meteorites. While the low temperatures on Titan ( 72–180 K) preclude the presence of permanent liquid water on the surface, it has been suggested that tectonic activity or impacts by meteors and comets could produce liquid water pools on the surface for thousands of years. Hydrolysis and oligomerization reactions in these pools might form chemicals of prebiological significance. Other direct comparisons between the conditions on present day Titan and those proposed for prebiotic Earth are also presented.     

Lunar influence on prey availability,diet shifts and niche overlap between Engraulidae larvae in tropical mangrove creeks

The influence of the lunar cycle on prey availability, diet shifts and overlap between larval Anchovia clupeoides and Cetengraulis edentulus was evaluated in mangrove creeks of the Goiana Estuary. Copepod eggs were highly abundant in the first and last quarter, at the full moon and zoea of Ucides cordatus (Ocypodidae) in the new moon. The Engraulidae larvae fed on microcrustaceans, algae and early planktonic stages of benthic organisms. The relative importance of prey varied according to prey availability in all moon phases. Larval diets were more even in the full and new moons, when the relative importance of calanoid copepods and zoeae of U. cordatus as food items increased (index of relative importance, >80% I_RI). Mangrove creeks were very important feeding grounds for engraulid larvae during spring tides. Larval diets were more diverse in the first and last‐quarter moon and included protozoeae of Caridean shrimp, larvae of Anomalocardia brasiliana (Veneridae), Isopoda, Gastropoda, ephippium of Daphnia sp. and nauplii of Cirripedia, Harpacticoidia and cyclopoid Copepoda. The last five items were not found in the creeks, suggesting feeding in the main channel. During neap tides, mangrove creeks were probably also used as refugia. These larvae are opportunistic and feed on highly available prey and both species feed on the same items, leading to high dietary overlap in all moon phases. The lunar cycle, which is related to the spring‐neap tidal cycle, was the major driver of quantitative and qualitative changes in feeding of engraulid larvae on a short time scale.     

Bioactivity of Amphitoxin,the Major Constituent of the Jamaican Sponge Amphimedon compressa

Fractionation of the MeOH/CH₂Cl₂ extract of the sponge Amphimedon compressa afforded the secondary metabolite amphitoxin ( 1 ), the structure of which was elucidated by interpretation of ¹H‐ and ¹³C‐NMR data. The crude extract and the fractions containing the metabolite 1 were assessed for ichthyotoxic and insecticidal activity towards Xiphophorus variatus (moon fish) and Cylas formicarius elegantulus (sweet potato weevil), respectively. In addition, the ability of 1 to cause mortality (toxicity and lethal effect) in the rodent Rattus norvegicus (Norway rat) was examined. Moderate insecticidal activity was observed, while the toxicity towards the moon fish was evidenced by the high mortality rates for all the fractions tested. In contrast, the rodent was not affected by the metabolite.     

Photochemistry of NH3, CH4 and PH3. Possible applications to the Jovian planets

Photolysis of NH₃ at 185 nm in the presence of a two-fold excess of CH₄ results in the loss of about 0.25 mole of CH₄ per mole of NH₃ decomposed (ΔCH₄/ΔNH₃). The loss arises from the abstraction of hydrogen atoms from CH₄ by photolytically generated hot hydrogen atoms, the presence of which is established by the constancy of ΔCH₄/ΔNH₃ between 298 and 156 K and by the quenching of the abstraction reaction when either H₂ or SF₆ is added. From the latter result, it can be concluded that NH₃ photolysis in the H₂-abundant atmosphere of Jupiter is not responsible for the presence of the carbon compounds observed there such as ethane, acetylene, and hydrogen cyanide, but may have had a role in the early atmosphere of Titan. Photolysis of PH₃ with a 206 nm light source gives P₂H₄, which in turn is converted to a red-brown solid (P₄?). The course of the photolysis is not changed appreciably when the temperature is lowered to 157 K except that the concentration of P₂H₄ increases. The presence of H₂ has no effect on the P₂H₄ yield. Photolysis of 9∶1 NH₃∶PH₃ gives a rate of decomposition of PH₃ that is comparable with that observed by the direct photolysis of PH₃. Comparable amounts of P₂H₄ and the red-brown solid are also observed. The mechanisms of these photochemical reactions together with their implications to the atmospheric chemistry of Jupiter are discussed. The structures of the compounds responsible for the wide array of colorse.g., brown, red and white, observed in the atmosphere of Jupiter have been the subject of extensive speculation. One theory suggests that these colors are due to organic materials formed by the action of either solar ultraviolet light or electric discharges on mixtures of CH₄, NH₃ and NH₄HS in the Jovian atmosphere (Ponnamperuma, 1976; Khareet al., 1978). An alternative hypothesis is that the colors are due to inorganic compounds resulting from the photolysis of NH₄HS and PH₃ (Lewis and Prinn, 1970; Prinn and Lewis, 1975). In this paper we will summarize our experiments which were designed to test some of these hypotheses.     

Dodging the moon: The moon effect on activity allocation of prey in the presence of predators

Animals can adapt their activity patterns throughout the circadian cycle. Prey may use moonlight as a predation risk cue and allocate their activity to lower risk periods. Here, we assessed moon transit influence on the activity allocation of nocturnal mammalian prey, in the presence of a predator (pumas, Puma concolor), during different moon phases, through camera trapping in Central Amazon. Thirty camera traps were installed 2 km apart from each other in each of our three study sites. Prey record distributions were assessed across the moon cycle, and their daily activity patterns were described in each moon phase. The record distributions of pacas (Cuniculus paca) (N = 262) and armadillos (Dasypus sp.) (N = 244) were concentrated in darker nights, contrasting with red brocket deer (Mazama americana) (N = 123) and pumas (N = 31), whose records were evenly distributed through the moon cycle. Yet, every prey was found to avoid the brighter times of the night regardless of the moon phase. These findings suggest prey can shift the temporal distribution of their activities under different moon phases when predators are present, perhaps in response to predation risk variations.     

Environmental correlates of Freckled Nightjar ( Caprimulgus tristigma ) activity in a seasonal, subtropical habitat

We examined environmental correlates of activity in the Freckled Nightjar (Caprimulgus tristigma), a nocturnal aerial insectivore that is resident year-round in the colder and drier parts of southern Africa. Specifically, we tested the prediction that air temperature (T _a), in addition to light availability, is a significant correlate of Freckled Nightjar activity. We found that aerial insect density varied by over three orders of magnitude (from <1.0 to 117.1 insects 1,000 m⁻³) during the course of the study, and was strongly and positively related to T _a. Nightjar activity was also significantly temperature-dependent, with a cessation of activity at T _a < 12°C. Consistent with previous studies, we found that activity levels were strongly related to ambient light, with markedly reduced activity levels on dark nights. The average number of Freckled Nightjars encountered per 14.6-km transect was 1.95 ± 2.44 nightjars transect⁻¹ during nights near full moon, but only 0.17 ± 0.61 nightjars transect⁻¹ around new moon. Our study reveals that caprimulgid activity can be significantly influenced by temperature as well as ambient light.     

Decrease in net photosynthesis caused by respiration

CO₂ exchange between air and leaf is conceived as a current along a resistor with a respiratory current, unaffected by CO₂ concentration, entering the resistor. The conclusion follows that a plant placed in an atmosphere free of CO₂ will increase the CO₂ concentration to the compensation concentration along a curve determined by the resistor and the volume of the atmosphere. This was verified. Also a photosynthesis rate calculated from the parameters of the observed curve agreed with an independent observation of photosynthesis in CO₂-free air. The decrease in net photosynthesis caused by respiration is, according to the model, the CO₂ compensation concentration divided by the concentration in the atmosphere.     

A global budget for atmospheric NH3

We provide an assessment of the global sources of NH₃ in the atmosphere, which indicates an annual flux of about 75 Tg of N as NH₃. The emissions from land are dominated by the release of NH₃ during the hydrolysis of urea from the urine of domestic animals (32 TgN/yr) and by emanations from soils in unmanaged ecosystems (10 TgN/yr) and from fertilized agricultural soils (9 TgN/yr). Emissions from the sea surface may approach 13 TgN/yr. The total annual source of NH₃ is in reasonable agreement with estimates of global NH ₄ ⁺ deposition from the atmosphere, the major fate of atmospheric NH₃. As an alkaline atmospheric species, NH₃ emitted to the atmosphere each year can neutralize only about 32% of the annual production of H⁺ in the atmosphere from natural and anthropogenic sources.     

Hydrolytic Enzyme Activities and Protein Pattern of Avocado Fruit Ripened in Air and in Low Oxygen, with and without Ethylene

The effect of 2.5% O₂ atmosphere with and without ethylene on the activities of hydrolytic enzymes associated with cell walls, and total protein profile during ripening of avocado fruits (Persea americana Mill., cv Hass) were investigated. The low 2.5% O₂ atmosphere prevented the rise in the activities of cellulase, polygalacturonase, and acid phosphatase in avocado fruits whose ripening was initiated with ethylene. Addition of 100 microliters per liter ethylene to low O₂ atmosphere did not alter these suppressive effects of 2.5% O₂. Furthermore, 2.5% O₂ atmosphere delayed the development of a number of polypeptides that appear during ripening of avocado fruits while at the same time new polypeptides accumulated. The composition of the extraction buffer and its pH greatly affected the recovery of cellulase activity and its total immunoreactive protein.     

Nitrogen fixation as evidence for the reducing nature of the early biosphere

Probably the first nitrogen fixers were anaerobic, non-photosynthetic, bacteria, i.e. fermenters. During the evolution of N₂ fixation they still needed nitrogen on the oxidation level of ammonia. Because of the complexities in structure and function of nitrogenase this evolution must have required a long time. The photosynthetic and later the respiring bacteria inherited the capacity for N₂ fixation from the fermenters, but the process did not change a great deal when it was taken over.Because of the long need for NH₃, which is unstable in a redoxneutral atmosphere, a long-persisting reducing atmosphere was needed. The transition to a redoxneutral atmosphere, dominated by CO₂, H₂O and N₂, cannot have been rapid, and the NH₃ in it was recycled. Probably the atmosphere contained for a long time, as was suggested by Urey but is often denied now, a great deal of methane as a reductant. The recycling occurred with participation of intermediates like cyanide, through energy input as UV radiation or as electric discharges. A stationary state was set up.The hypothesis is recalled that coloured, photosynthetic, NH₃ bacteria, analogous to coloured sulphur bacteria, may have existed, or may still exist, in reducing conditions. A few remarks are made about the origin of nitrification in the later, oxidizing atmosphere.     

Lunar influences in the diet habits and selectivity of larval clingfish Gobiesox marmoratus

The influence of the lunar cycle on the feeding habits, feeding success and selectivity was tested in situ on larval stages of the clingfish Gobiesox marmoratus (Gobiesocidae). Gobiesox marmoratus larvae and their prey were collected in the water column during the lunar cycle in austral spring 2015 and 2016, in shallow waters (<30 m depth) of El Quisco, central Chile. Feeding incidence was high (80–100%) throughout the moon cycle. The diet was composed of 32 prey items, being gastropod larvae and invertebrate eggs the most important. Prey composition varied among lunar phases with the lowest prey richness during new moon. During 2016, G. marmoratus larvae ingested the lowest number, but the largest prey during new moon. In spring 2015, there were no significant changes in the abundance of each prey taxon in the water column among moon phases, but in spring 2016 there were significant differences in the abundance of cypris and copepod nauplii, particularly between full and new moon. Feeding selectivity index showed that gastropod larvae were positively selected throughout the lunar cycle during spring 2015. In spring 2016, invertebrate eggs were selectively ingested in full moon and third quarter, but at a new moon G. marmoratus larvae selected gastropod larvae. Mean temperature of the water column and its vertical gradient and nocturnal cloud cover influences the feeding success of larval G. marmoratus. Lunar illumination favoured only an increase of richness of prey items. Therefore, nocturnal cloud cover precludes a greater influence of lunar illumination in the larval trophic ecology of this crypto-benthic fish.     

Photosynthetic Carbon Assimilation in a Shootless Orchid, Chiloschista usneoides (DON) LDL: A Variant on Crassulacean Acid Metabolism

Photosynthetic carbon assimilation in the roots of a shootless orchid Chiloschista usneoides (DON) LDL involves the synthesis and accumulation of malic acid from CO₂ in darkness. Malic acid is consumed in the light.

The roots do not possess stomata or any means of diurnally regulating the diffusive conductance of the pathway between the internal gas phase of the plant and the atmosphere. Regulation of internal CO₂ concentration near to atmospheric levels avoids a large net loss of CO₂ to the atmosphere during malic acid consumption in the light.

The water-absorbing function of the velamen conflicts with the photosynthetic function of the roots. Plants with water-saturated velamina do not acquire CO₂ from the atmosphere at night.

     

Kinetics of the Oxyhydrogen Reaction in the Presence and Absence of Carbon Dioxide in Scenedesmus obliquus

The oxyhydrogen reaction in the presence and absence of CO₂ was studied in H₂-adapted Scenedesmus obliquus by monitoring the initial rates of H₂, O₂, and ¹⁴CO₂ uptake and the effect of inhibitors on these rates with gas-sensing electrodes and isotopic techniques. In the presence of 0.02 atmosphere O₂, the pH₂ was varied from 0 to 1 atmosphere. Whereas the rate of O₂ uptake increased by only 30%, the rate of H₂ uptake increased severalfold over the range of pH₂ values. At 0.1 atmosphere H₂ and 0.02 atmosphere O₂, rates for H₂ and O₂ uptake were between 15 and 25 micromoles per milligram chlorophyll per hour. As the pH₂ was changed from 0 to 1 atmosphere, the quotient H₂:O₂ changed from 0 to roughly 2. This change may reflect the competition between H₂ and the endogenous respiratory electron donors. Respiration in the presence of glucose and acetate was also competitive with H₂ uptake. KCN inhibited equally respiration (O₂ uptake in the absence of H₂) and the oxyhydrogen reaction in the presence and absence of CO₂. The uncoupler carbonyl cyanide p-trifluoromethoxyphenylhydrazone accelerated the rate of respiration and the oxyhydrogen reaction to a similar extent. It was concluded that the oxyhydrogen reaction both in the presence and absence of CO₂ has properties in common with components of respiration and photosynthesis. Participation of these two processes in the oxyhydrogen reaction would require a closely linked shuttle between mitochondrion and chloroplast.