Effect of gravity on the velocity of a crystallization front

The crystal growth from a liquid can be studied by methods of non-equilibrium thermodynamics of a discontinuity surface. These methods are used to derive the dependence of the crystalization velocity on transport and surface phenomena across and along an interface solid-liquid and on the acceleration of gravity.     

Measurements of the 12C/13C Ratio in Planetary Nebulae and Implications for Stellar Evolution

We present the results of a study aimed at determining the 12C/13C ratio in two samples of planetary nebulae (PNe) by means of mm-wave observations of 12CO and 13CO. The first group includes six PNe which have been observed in the 3He+ hyperfine transition; the other group consists of 23 nebulae with rich molecular envelopes. We have determined the isotopic ratio in 14 objects and the results indicate a range of values between 9 and 23. In particular, three PNe have ratios well below the value predicted by standard evolutionary models ( 20), indicating that some extra-mixing process has occurred in these stars. We briefly discuss the implications of our results for standard and nonstandard stellar nucleosynthesis.     

Tírez lake as a terrestrial analog of Europa

Tírez Lake (La Mancha, central Spain) is proposed as a terrestrial analogue of Europa's ocean. The proposal is based on the comparison of the hydrogeochemistry of Tírez Lake with the geochemical features of the alteration mineralogy of meteoritic precursors and with Galileo's Near Infrared Mapping Spectrometer data on Europa's surface. To validate the astrobiological potential of Tírez Lake as an analog of Europa, different hydrogeochemical, mineral, and microbial analyses were performed. Experimental and theoretical modeling helped to understand the crystallization pathways that may occur in Europa's crust. Calculations about the oxidation state of the hypothetical Europan ocean were estimated to support the sulfate-rich neutral liquid model as the origin of Europa's observed hydrated minerals and to facilitate their comparison with Tírez's hydrogeochemistry. Hydrogeochemical and mineralogical analyses showed that Tírez waters corresponded to Mg-Na-SO(4)-Cl brines with epsomite, hexahydrite, and halite as end members. A preliminary microbial ecology characterization identified two different microbial domains: a photosynthetically sustained community represented by planktonic/benthonic forms and microbial mat communities, and a subsurficial anaerobic realm in which chemolithotrophy predominates. Fluorescence in situ hybridization has been used to characterize the prokaryotic diversity of the system. The subsurficial community seemed to be dominated by sulfate-reducing bacteria and methanogens. Frozen Tírez brines were analyzed by Fourier-transform infrared techniques providing spectra similar to those reported previously using pure components and to the Galileo spectral data. Calorimetric measurements of Tírez brines showed pathways and phase metastability for magnesium sulfate and sodium chloride crystallization that may aid in understanding the processes involved in the formation of Europa's icy crust. The use of fluorescence hybridization techniques for microbial detection and characterization in hypersaline environments makes this methodology strongly advisable for future Europa astrobiological missions.     

Ozone concentrations and ultraviolet fluxes on Earth-like planets around other stars

Coupled radiative-convective/photochemical modeling was performed for Earth-like planets orbiting different types of stars (the Sun as a G2V, an F2V, and a K2V star). O(2) concentrations between 1 and 10(-5) times the present atmospheric level (PAL) were simulated. The results were used to calculate visible/near-IR and thermal-IR spectra, along with surface UV fluxes and relative dose rates for erythema and DNA damage. For the spectral resolution and sensitivity currently planned for the first generation of terrestrial planet detection and characterization missions, we find that O(2) should be observable remotely in the visible for atmospheres containing at least 10(-2) PAL of O(2). O(3) should be visible in the thermal-IR for atmospheres containing at least 10(-3) PAL of O(2). CH(4) is not expected to be observable in 1 PAL O(2) atmospheres like that of modern Earth, but it might be observable at thermal-IR wavelengths in "mid-Proterozoic-type" atmospheres containing approximately 10(-1) PAL of O(2). Thus, the simultaneous detection of both O(3) and CH(4) - considered to be a reliable indication of life - is within the realm of possibility. High-O(2) planets orbiting K2V and F2V stars are both better protected from surface UV radiation than is modern Earth. For the F2V case the high intrinsic UV luminosity of the star is more than offset by the much thicker ozone layer. At O(2) levels below approximately 10(-2) PAL, planets around all three types of stars are subject to high surface UV fluxes, with the F2V planet exhibiting the most biologically dangerous radiation environment. Thus, while advanced life is theoretically possible on high-O(2) planets around F stars, it is not obvious that it would evolve as it did on Earth.     

Observational Overview of Young Intermediate-Mass Objects: Herbig Ae/Be Stars

Recent discoveries of planets orbiting several nearby solar-mass stars have focussed renewed attention on the frequency and evolution of planetary systems and their evolutionary precursors around both solar and intermediate (2 M/M 8) mass stars. As a result of a wealth of new observations at all wavelengths of the circumstellar material around the nearest of the young intermediate-mass stars, the so-called Herbig Ae/Be (HAeBe) stars, we are beginning to see how these systems are similar to the solar mass objects, and how they differ. A review of the recent literature is presented, including the evolutionary status of the stars, binary frequency and the star forming environment, the morphology of the circumstellar material, including the first direct detections of disks in Keplerian rotation around these objects, and mass loss and infall phenomena. Prospects for advances in this research area as a result of advances in instrumentation are reviewed.     

Comparative metagenomics of two microbial mats at cuatro ciénegas basin I: ancient lessons on how to cope with an environment under severe nutrient stress

Abstract The Cuatro Ciénegas Basin (CCB) is an oasis in the desert of Mexico characterized by low phosphorus availability and by its great diversity of microbial mats. We compared the metagenomes of two aquatic microbial mats from the CCB with different nutrient limitations. We observed that the red mat was P-limited and dominated by Pseudomonas, while the green mat was N-limited and had higher species richness, with Proteobacteria and Cyanobacteria as the most abundant phyla. From their gene content, we deduced that both mats were very metabolically diverse despite their use of different strategies to cope with their respective environments. The red mat was found to be mostly heterotrophic, while the green mat was more autotrophic. The red mat had a higher number of transporters in general, including transporters of cellobiose and osmoprotectants. We suggest that generalists with plastic genomes dominate the red mat, while specialists with minimal genomes dominate the green mat. Nutrient limitation was a common scenario on the early planet; despite this, biogeochemical cycles were performed, and as a result the planet changed. The metagenomes of microbial mats from the CCB show the different strategies a community can use to cope with oligotrophy and persist. Key Words: Microbial mats-Metagenomics-Metabolism. Astrobiology 12, 648-658.     

Bacterial communities and the nitrogen cycle in the gypsum soils of cuatro ciénegas basin, coahuila: a Mars analogue

Abstract The OMEGA/Mars Express hyperspectral imager identified gypsum at several sites on Mars in 2005. These minerals constitute a direct record of past aqueous activity and are important with regard to the search of extraterrestrial life. Gale Crater was chosen as Mars Science Laboratory Curiosity's landing site because it is rich in gypsum, as are some desert soils of the Cuatro Ciénegas Basin (CCB) (Chihuahuan Desert, Mexico). The gypsum of the CCB, which is overlain by minimal carbonate deposits, was the product of magmatic activity that occurred under the Tethys Sea. To examine this Mars analogue, we retrieved gypsum-rich soil samples from two contrasting sites with different humidity in the CCB. To characterize the site, we obtained nutrient data and analyzed the genes related to the N cycle (nifH, nirS, and nirK) and the bacterial community composition by using 16S rRNA clone libraries. As expected, the soil content for almost all measured forms of carbon, nitrogen, and phosphorus were higher at the more humid site than at the drier site. What was unexpected is the presence of a rich and divergent community at both sites, with higher taxonomic diversity at the humid site and almost no taxonomic overlap. Our results suggest that the gypsum-rich soils of the CCB host a unique microbial ecosystem that includes novel microbial assemblies. Key Words: Cuatro Ciénegas Basin-Gale Crater-Gypsum soil microbial diversity-Molecular ecology-Nitrogen cycle. Astrobiology 12, 699-709.     

Oriented platform for the VEGA-probe to the Halley comet,position detection,working program

Two deep-space probes of the Venera type for the encounter with Halley's comet are being prepared in the Intercosmos program. 150 kg of scientific equipment will be onboard each sonde, including two TV cameras and two spectrometers. These instruments need precise orientation during the fly-by of the comet. For this purpose a stabilised platform is being developed in Czechoslovakia which will be installed onboard both probes and will be able to point to the target with precision 5 minutes of arc and maximum angular velocity 1°/second. A system of position detectors and the working program during the start and the fly-by are described.     

Dynamic and figure parameters of Venus and Mars

Parameters of the best-fitting tri-axial ellipsoids representing external equipotential surfaces of Venus and Mars have been determined from satellite data. The dynamic consequence of the equatorial flattening of Venus has been discussed from the point of view of the s.c. synodic resonance rotation. The major gravitational anomalies of Venus have been interpreted, space locations and magnitudes of anomalous masses determined and their contribution to the second zonal Stokes' constant in the gravitational potential computed. The conclusions were done: The figure of the aphroditoid is strange even if there is a relatively small polar flattening; an equatorial “disc” of Venus is enormous. Recent space data do not support hypothesis that the Earth controls the spin of Venus.