Stability field and phase transition pathways of hydrous ferric sulfates in the temperature range 50 °C to 5 °C: Implication for martian ferric sulfates

We report the results from a systematic laboratory investigation on the fundamental properties of hydrous ferric sulfates. The study involves 150 experiments with duration of over 4 years on the stability field and phase transition pathways under Mars relevant environmental conditions for five ferric sulfates: ferricopiapite [Fe_4.67(SO₄)₆(OH)₂·20H₂O], kornelite [Fe₂(SO₄)₃·7H₂O], a crystalline and an amorphous pentahydrated ferric sulfate [Fe₂(SO₄)₃·5H₂O], and rhomboclase [FeH(SO₄)₂·4H₂O]. During the processes of phase transitions, we observed the phenomena that reflect fundamental properties of these species and the occurrence of other common hydrous ferric sulfates, e.g. paracoquimbite [Fe₂(SO₄)₃·9H₂O]. Based on the results of this set of experiments, we have drown the boundaries of deliquescence zone of five hydrous ferric sulfates and estimated the regions of their stability field in temperature (T) – relative humidity (RH) space. Furthermore, we selected the experimental parameters for a next step investigation, which is to determine the location of the phase boundary between two solid ferric sulfates, kornelite [Fe₂(SO₄)₃·7H₂O] and pentahydrated ferric sulfate [Fe₂(SO₄)₃·5H₂O]. The experimental observations in ferricopiapite dehydration processes were used to interpret the observed spectral change of Fe-sulfate-rich subsurface soils on Mars after their exposure by the Spirit rover to current martian atmospheric conditions.     

Intramolecular carbon isotopic analysis of acetic acid by direct injection of aqueous solution

We report an improved method for determining the intramolecular carbon isotopic composition of acetate using direct injection of aqueous samples. The system builds upon prior work that established pyrolytic conditions for online analysis and represents a significant advance in that it requires minimal preparation for samples containing as little as 1 mM sodium acetate in aqueous solution. The technique is applicable for analysis of oilfield brines, culture samples, biological samples and natural porewaters. We demonstrate its accuracy by use of a stable isotope dilution series. We also show that addition of a base and cryogenic preconcentration may induce an isotopic effect on the carboxyl carbon. This isotopic fractionation does not appear to extend to the measured methyl carbon isotope value although it can significantly alter the measured isotopic composition of the whole molecule. Our preconcentration experiments demonstrate that the method is suitable for carbon isotopic measurements of acetate methyl carbon in natural samples at concentrations as low as 90 μM, considerably broadening potential applications.     

Early-Type Stars

Away from the young disk, several classes of early type stars are found. They include (i) the old, metal-poor blue horizontal branch stars of the halo and the metal-poor tail of the thick disk; (ii) metal-rich young A stars in a rapidly rotating subsystem but with a much higher velocity dispersion than the A stars of the young disk, and (iii) a newly discovered class of metal-poor young main sequence A stars in a subsystem of intermediate galactic rotation (V_rot ≈ 120 km s⁻¹). The existence and kinematics of these various classes of early type stars provide insight into the formation of the metal-poor stellar halo of the Galaxy and into the continuing accretion events suffered by our Galaxy. This revised version was published online in July 2006 with corrections to the Cover Date.     

Autoregressive estimation of complex eigenfrequencies in low frequency seismic spectra

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The fact that a seismogram can be represented as an autoregressive (AR) time series and the use of Prony's method enable us to obtain the complex frequencies from the poles of the corresponding AR process. A frequency-domain formulation which employs a time-domain tapering technique is devised so that spectral peaks can be analysed individually, or in small groups. Statistical analysis of the estimators and several examples from two recent earthquakes illustrate the method.     

A photoautotrophic source for lycopane in marine water columns

Suspended particulate matter and recent sediments from diverse oceanic sites have been investigated for their contents of lycopane. Lycopane was present in all samples, including both oxic and anoxic water column and sediments. The highest concentrations in the water column were found in surface waters of the central Pacific gyre (1.5 ng/L) and in the anoxic waters of the Cariaco Trench (1.1 ng/L) and the Black Sea (0.3 ng/L). Vertical concentration profiles suggest that lycopane is probably algal in origin. Moreover, biogeochemical conditions in anoxic zones apparently result in a secondary production of lycopane from an as yet unidentified precursor. Compound-specific carbon isotopic analyses have been carried out on lycopane from water column and sediment samples. Isotopic compositions of lycopane range between -23.6% and -32.9% and are consistent with a photoautotrophic origin. We postulate that some lycopane is produced in surface waters of the ocean, while additional lycopane is produced in anoxic zones by anaerobic microbial action on an algal precursor.