Exobiology and the solar system: The CASSINI mission to Titan

The recent Voyager mission and the simulation experiments in the laboratory suggest that a complex nitrogen-organic chemistry is occuring at the periphery of Titan. Thus, this satel lite of Saturn appears as a privileged place in the solar system for the study of extraterrestrial organic chemistry which can be considered as part of Exobiology.Projects of space mission relating to Titan are already under investigation, in particular with the CASSINI proposal. The CASSINI project is a combination of a Saturn orbiter and a Titan probe mission. Such a mission would allow the first study in situ of a complex extraterrestrial organic chemistry in atmospheric phase.     

NASA's Exobiology Program

The goal of NASA's Exobiology Progam is to understand the origin, evolution, and distribution of life, and life-related molecules, on Earth and throughout the universe. Emphasis is focused on determining how the rate and direction of these processes were affected by the chemical and physical environment of the evolving planet, as well as by planetary, solar, and astrophysical phenomena. This is accomplished by a multi-disciplinary program of research conducted by over 60 principal investigators in both NASA and university laboratories. Major program thrusts are in the following research areas: biogenic elements; chemical evolution; origin of life; organic geochemistry; evolution of higher life forms; solar system exploration; and the search for extraterrestrial intelligence (SETT).     

The Perseus Exobiology Mission on MIR Behaviour of Amino Acids and Peptides in Earth Orbit

Leucine, -methyl leucine and two peptides were exposed tospace conditions on board the MIR station during the Perseus-Exobiology mission. This long duration space mission was aimed at testing the delivery of prebiotic building blocks. Duringthis mission, two amino acids (leucine and -methyl leucine) and two peptides (leucine-diketopiperazine and trileucine thioethylester) were exposed in Earth orbit for threemonths. Basalt, clay and meteorite powder were also mixed with the samples in order to simulate the effects of potential meteorite protection. Analysis of the material after the flight did not reveal any racemization or polymerisation but did provideinformation regarding photochemical pathways for the degradationof leucine and of the tripeptide. Amino acids appeared to be moresensitive to UV radiation than peptides, the cyclic dipeptide being found to be as particularly resistant. Meteorite powder which exhibits the highest absorption in Vacuum UltraViolet (VUV)afforded the best protection to the organic molecules whereasmontmorillonite clay, almost transparent in VUV, was the leastefficient. By varying the thickness of the meteorite, we found that the threshold for efficient protection against radiation was about 5 m. The possible exogenous origin of biologicalbuilding blocks is discussed with respect to the stability to themolecules and the nature of the associated minerals.     

Corona chemistry in Titan.

The atmosphere of Titan is constantly bombarded by galactic cosmic rays and Saturnian magnetospheric electrons causing the formation of free electrons and primary ions, which are then stabilized by ion cluster formation and charging of aerosols. These charged particles accumulate in drops in cloud regions of the troposphere. Their abundance can substantially increase by friction, fragmentation or collisions during convective activity. Charge separation occurs with help of convection and gravitational settling leading to development of electric fields within the cloud and between the cloud and the ground. Neutralization of these charged particles leads to corona discharges which are characterized by low current densities. We have therefore, experimentally studied the corona discharge of a simulated Titan's atmosphere (10% methane and 2% argon in nitrogen) at 500 Torr and 298 K by GC-FTIR-MS techniques. The main products have been identified as hydrocarbons (ethane, ethyne, ethene, propane, propene+propyne, cyclopropane, butane, 2-methylpropane, 2-methylpropene, n-butane, 2-butene, 2,2-dimethylpropane, 2-methylbutane, 2-methylbutene, n-pentane, 2,2-dimethylbutane, 2-methylpentane, 3-methylpentane, n-hexane, 2,2-dimethylhexane, 2,2-dimethylpentane, 2,2,3-trimethylbutane, 2,3-dimethylpentane and n-heptane), nitriles (hydrogen cyanide, cyanogen, ethanenitrile, propanenitrile, 2-methylpropanenitrile and butanenitrile) and a highly branched hydrocarbon deposit. We present the trends of hydrocarbons and nitriles formation as a function of discharge time in an ample interval and have derived their initial yields of formation. The results clearly demonstrate that a complex organic chemistry can be initiated by corona processes in the lower atmosphere. Although photochemistry and charged particle chemistry occurring in the stratosphere can account for many of the observed hydrocarbon species in Titan, the predicted abundance of ethene is too low by a factor of 10 to 40. While some ethene will be produced by charged-particle chemistry, the production of ethene by corona processes and its subsequent diffusion into the stratosphere appears to be an adequate source. Because little UV penetrates to the lower atmosphere to destroy the molecules formed there, the corona-produced species may be long-lived and contribute significantly to the composition of the lower atmosphere and surface.     

Titan cells confer protection from phagocytosis in Cryptococcus neoformans infections

The human fungal pathogen Cryptococcus neoformans produces an enlarged "titan" cell morphology when exposed to the host pulmonary environment. Titan cells exhibit traits that promote survival in the host. Previous studies showed that titan cells are not phagocytosed and that increased titan cell production in the lungs results in reduced phagocytosis of cryptococcal cells by host immune cells. Here, the effect of titan cell production on host-pathogen interactions during early stages of pulmonary cryptococcosis was explored. The relationship between titan cell production and phagocytosis was found to be nonlinear; moderate increases in titan cell production resulted in profound decreases in phagocytosis, with significant differences occurring within the first 24 h of the infection. Not only were titan cells themselves protected from phagocytosis, but titan cell formation also conferred protection from phagocytosis to normal-size cryptococcal cells. Large particles introduced into the lungs were not phagocytosed, suggesting the large size of titan cells protects against phagocytosis. The presence of large particles was unable to protect smaller particles from phagocytosis, revealing that titan cell size alone is not sufficient to provide the observed cross-protection of normal-size cryptococcal cells. These data suggest that titan cells play a critical role in establishment of the pulmonary infection by promoting the survival of the entire population of cryptococcal cells.     

Fluorometric assay for polyamines in urine and tissues using electrophoresis on Titan III cellulose acetate

A simple, rapid assay method for polyamines (putrescine, spermidine, and spermine) in urine and tissues using electrophoresis on Titan III cellulose acetate was developed. In this procedure, polyamines are preliminarily extracted from a hydrolysate of urine or from supernatants of tissue homogenates by use of a Bio-Rex 70 minicolumn. After electrophoretic separation, polyamines are fluorometrically detected by the reaction with o-phthalaldehyde and 2-mercaptoethanol. Six extracts and two external standards of polyamines can be separated and detected in 11 min on a cellulose acetate strip. This method permits the determination of polyamines in a range of 0.1 mM (25 pmol) to 1.0 mM (250 pmol).     

On the thermogenesis of the Titan arum (Amorphophallus titanum)

The Titan arum (Araceae) produces the largest bloom of all flowering plants. Its flowering period of two days is divided into a female flowering phase in the first night and a male flowering phase in the second night. Recently, we have documented thermogenesis in the spadix of the Titan arum during the female flowering phase. Here, we document a second thermogenic phase in which the male florets are heated during the male flowering phase. Obviously the two nocturnal thermogenic phases are linked with the two flowering periods. These observations now allow a more detailed understanding of the flowering behavior of the Titan arum.Key words: Amorphophallus titanum, araceae, thermogenesis, infrared thermography, pollinationThe Titan arum (Amorphophallus titanum) is one of the most spectacular flowering plants. It has drawn the attention of botanists and naturalists since its discovery in 1878. However, the species has been rare in cultivation and flowering events in botanical gardens were even rarer. Besides, flowering events observed in the plant''s natural habitat are very few.^1,2 Therefore, the knowledge on A. titanum that depended on exact observations and scientific experiments remained very limited. It was only in the late 90s when a monograph on the species, containing anatomical details and some first experimental hypotheses, has been published.¹The Botanical Gardens of the University of Bonn (Germany) have been cultivating Amorphophallus titanum for more that 70 years and obtained 14 flowerings between 1937 and 2009. These regular flowering events have been the prerequisite to study A. titanum in detail. Consequently the data and hypotheses in the monograph mentioned above were gained mainly from the A. titanum plants in the Botanical Gardens Bonn. As a result of three flowering events in 2006, we have recently documented for the first time, that the inflorescence undergoes thermogenesis in which the central column (spadix) heats up to 36°C. Meanwhile four additional flowering events yielded additional insights into the flowering behavior of A. titanum.The inflorescence of A. titanum consists of a thickened unbranched inflorescence axis bearing hundreds of small female and male florets which are spatially separated (Fig. 1A). The inflorescence axis is extended into an appendix (spadix) and enveloped by a large bract referred to as spathe. The spathe enclosing the florets forms the floral chamber. Since the whole inflorescence functions as a single unit in pollination is often referred to as a bloom or “flower”. A. titanum has two timely separated flowering phases: a female flowering phase during the first evening and night after opening of the spathe and a male flowering phase in the following night.Open in a separate windowFigure 1(A) Flash-light photograph of an Amorphophallus titanum flowering zones showing part of the appendix, the male florets and the female florets. (B) Thermographic image taken during the male flowering phase. The male florets are heated to the maximum temperature of 35.9°C whereas the other parts of the plant have largely ambient air temperature of 26°C.Thermogenesis plays an important role in the pollination ecology of Araceae^3,4 and therefore occurs in many genera.^4–7 Similarly in A. titanum, we have reported the thermogenic spadix during the female flowering phase. Based on our observations of now six inflorescences, the heat production is determined and begins around 20 h, the temperature maximum of 36–38°C being reached around midnight. The duration of heat production differs between individual plants but usually stops between 2 h and 4 h in the morning. The spathe begins to close the next day in the early morning hours or in the forenoon. The opening and closing of the spathe seems to be influenced by the hours of daylight but these might be different in European countries from the plants native habitat in the tropical rainforests of Sumatra. The flowering events in Bonn usually take place in summer, the spathe opens during a daytime when it is still very bright and is fully opened when the daylight is decreasing while it is already dark then in the tropics.Some authors have observed heating of the male florets prior to appendix heating in other Araceae species.^4,5,8,9 In A. titanum however, we could not find an evidence for this, as stated in our previous article. But a question that still remained open is: when exactly the male flowering phase begins and if there might be thermogenic activity during the male flowering phase. To study this, the male florets were made visible by removing a part of the spathe of two flowering A. titanum and observed right after opening of the spathe. The beginning of the male flowering phase is easily to determine since the pollen is shed in well visible string-like structures.The pollen dissemination began in the evening around 17:20 h. Thereupon we filmed the male florets with a thermographic camera (Flexcam, GORATEC) taking an image every five minutes. The male florets were clearly thermogenic reaching a temperature maximum of 35.9°C between 18:40 h and 20:00 h (Fig. 1B). They slowly cooled down to ambient air temperature (ca. 26°C) around midnight. To test whether the temperature in the floral chamber around the male florets increases while they are heated, we recorded the temperature within the spathe of three intact inflorescences with data loggers (Tinytag, Gemini Data Loggers). However, no warming within the chamber in comparison with ambient air temperature could be measured, so the heated florets seem not to affect the temperature within the floral chamber.The flowering behavior of A. titanum is summarised in Figure 2. The carrion-like odor and the thermogenic spadix attract pollinators in the female flowering phase, during the first evening and night of the flowering period. Heating of the male florets occurs when no more odor is produced and hence no olfactorical attraction of the pollinators can take place. As a consequence, there must be only one attraction time period which is more or less restricted to the female flowering phase and to the nighttime where pollinators can be successfully attracted. The pollinators, although not exactly known,¹⁰ hence must be active only in these evening hours and at night. Once attracted, the pollinators stay inside the inflorescence and most likely use it as mating site or as a place to stay during the following day. It has already been hypothesised that Araceae inflorescences forming floral chambers may offer mating sites or places to rest for insects and rather keep their pollinators inside the floral chamber instead of a second attraction phase.^{11, 12} Numerous insects inside a A. titanum inflorescence have indeed been observed in its natural habitat,¹³ although the author did not explain these observations, it provides evidence for our hypothesis that pollinators spend some time inside the inflorescence.Open in a separate windowFigure 2Scheme of the flowering behavior of Amorphophallus titanum over its two-days flowering period. The scheme is idealised but represents observation of seven A. titanum inflorescences that all behave highly similar. Deviations in the time when opening and closing of the spathe begin in individual plants are indicated with a dashed line.The male florets are heated while pollen is released. There is evidence that at least some insects are able to percept IR light and it has been hypothesised that infrared radiation itself could be an attractant for insects, most likely to locate food sources.¹⁵ Floral heat may also be a direct reward for pollinators, helping them increasing their body temperature and thus faster reaching their activity level.^11,14 Both may also apply to A. titanum—the insects that have spent the day within the flower chamber may use the heated surface of the male florets to warm themselves up and by this collect pollen or feed on pollen. Still, a verification of these hypotheses could only come from field observations.To draw a conclusion, the new observations reported here now allow us a good understanding of the flowering behavior of A. titanum. Its two thermogenic phases are clearly linked with the two flowering phases and the plant''s complex interaction with its pollinators.     

Selective Staining of Magnesium by Titan Yellow Applied to Incinerated Tissue Sections

Tissue sections were microincinerated with a Bunsen burner, allowed to cool, and coated with an 0.2% aqueous solution of titan yellow. Upon addition of 2 N NaOH, sites of magnesium deposits exhibited a flame red color, which persisted as long as the alkalinity of the mounting medium was preserved. Tests with calcium salts dissolved in serum were negative.     

Formation of organic compounds from simulated Titan atmosphere: perspectives of the Cassini mission.

Gas mixtures of methane and nitrogen were subjected to proton irradiation (PI), gamma irradiation (GI), UV irradiation (UV) or spark discharges (SD), and the products were analyzed to compare possible energy sources for synthesis of organics in Titan. SD mainly gave unsaturated hydrocarbons, while PI gave saturated hydrocarbons. N-containing organics were detected in PI, GI and SD, but not in UV. The formers yielded amino acids after acid-hydrolysis of solid phase products (tholin). Comparison of the present results with those by Cassini-Huygens [correction of Heygens] mission will make it possible to prove major energy sources for organic synthesis in Titan atmosphere.