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Register a new userOn the chemical composition of Titans dry lakebed evaporites
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Titan, the main satellite of Saturn, has an active cycle of methane in its troposphere. Among other evidence for a mechanism of evaporation at work on the ground, dry lakebeds have been discovered. Recent Cassini infrared observations of these empty lakes have revealed a surface composition poor in water ice compared to that of the surrounding terrains --- suggesting the existence of organic evaporites deposits. The chemical composition of these possible evaporites is unknown. In this paper, we study evaporite composition using a model that treats both organic solids dissolution and solvent evaporation. Our results suggest the possibility of large abundances of butane and acetylene in the lake evaporites. However, due to uncertainties of the employed theory, these determinations have to be confirmed by laboratory experiments.
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Numerous geological features that could be evaporitic in origin have been identified on the surface of Titan. Although they seem to be water-ice poor, their main properties -chemical composition, thickness, stratification- are essentially unknown. In this paper, which follows on a previous one focusing on the surface composition (Cordier et al., 2013), we provide some answers to these questions derived from a new model. This model, based on the up-to-date thermodynamic theory known as PC-SAFT, has been validated with available laboratory measurements and specifically developed for our purpose. 1-D models confirm the possibility of an acetylene and/or butane enriched central layer of evaporitic deposit. The estimated thickness of this acetylene-butane layer could explain the strong RADAR brightness of the evaporites. The 2-D computations indicate an accumulation of poorly soluble species at the deposits margin. Among these species, HCN or aerosols similar to tholins could play a dominant role. Our model predicts the existence of chemically trimodal bathtub rings which is consistent with what it is observed at the south polar lake Ontario Lacus. This work also provides plausible explanations to the lack of evaporites in the south polar region and to the high radar reflectivity of dry lakebeds.
Aims. Our goal is to determine the molecular composition of the circumstellar disk around AB Aurigae (hereafter, AB Aur). AB Aur is a prototypical Herbig Ae star and the understanding of its disk chemistry is of paramount importance to understand the chemical evolution of the gas in warm disks. Methods. We used the IRAM 30-m telescope to perform a sensitive search for molecular lines in AB Aur as part of the IRAM Large program ASAI (A Chemical Survey of Sun-like Star-forming Regions). These data were complemented with interferometric observations of the HCO+ 1-0 and C17O 1-0 lines using the IRAM Plateau de Bure Interferometer (PdBI). Single-dish and interferometric data were used to constrain chemical models. Results. Throughout the survey, several lines of CO and its isotopologues, HCO+, H2CO, HCN, CN and CS, were detected. In addition, we detected the SO 54-33 and 56-45 lines, confirming the previous tentative detection. Comparing to other T Tauris and Herbig Ae disks, AB Aur presents low HCN 3-2/HCO+ 3-2 and CN 2-1/HCN 3-2 line intensity ratios, similar to other transition disks. AB Aur is the only protoplanetary disk detected in SO thus far. Conclusions. We modeled the line profiles using a chemical model and a radiative transfer 3D code. Our model assumes a flared disk in hydrostatic equilibrium. The best agreement with observations was obtained for a disk with a mass of 0.01 Msun , Rin=110 AU, Rout=550 AU, a surface density radial index of 1.5 and an inclination of 27 deg. The intensities and line profiles were reproduced within a factor of 2 for most lines. This agreement is reasonable taking into account the simplicity of our model that neglects any structure within the disk. However, the HCN 3-2 and CN 2-1 line intensities were predicted more intense by a factor of >10. We discuss several scenarios to explain this discrepancy.
The Kuiper Belt of our solar system is a source of short-period comets that may have delivered water and other volatiles to Earth and the other terrestrial planets. However, the distribution of water and other volatiles in extrasolar planetary systems is largely unknown. We report the discovery of an accretion of a Kuiper-Belt-Object analog onto the atmosphere of the white dwarf WD 1425+540. The heavy elements C, N, O, Mg, Si, S, Ca, Fe, and Ni are detected, with nitrogen observed for the first time in extrasolar planetary debris. The nitrogen mass fraction is $sim$ 2%, comparable to that in comet Halley and higher than in any other known solar system object. The lower limit to the accreted mass is $sim$ 10$^{22}$ g, which is about one hundred thousand times the typical mass of a short-period comet. In addition, WD 1425+540 has a wide binary companion, which could facilitate perturbing a Kuiper-Belt-Object analog into the white dwarfs tidal radius. This finding shows that analogs to objects in our Kuiper Belt exist around other stars and could be responsible for the delivery of volatiles to terrestrial planets beyond
The inner regions of protoplanetary discs (from $sim$ 0.1 to 10 au) are the expected birthplace of planets, especially telluric. In those high temperature regions, solids can experience cyclical annealing, vaporisation and recondensation. Hot and warm dusty grains emits mostly in the infrared domain, notably in N-band (8 to 13~$mu$m). Studying their fine chemistry through mid-infrared spectro-interferometry with the new VLTI instrument MATISSE, which can spatially resolve these regions, requires detailed dust chemistry models. Using radiative transfer, we derived infrared spectra of a fiducial static protoplanetary disc model with different inner disc ($< 1$ au) dust compositions. The latter were derived from condensation sequences computed at LTE for three initial $C/O$ ratios: subsolar ($C/O=0.4$), solar ($C/O=0.54$), and supersolar ($C/O=1$). The three scenarios return very different N-band spectra, especially when considering the presence of sub-micron-sized dust grains. MATISSE should be able to detect these differences and trace the associated sub-au-scale radial changes. We propose a first interpretation of N-band `inner-disc spectra obtained with the former VLTI instrument MIDI on three Herbig stars (HD142527, HD144432, HD163296) and one T Tauri star (AS209). Notably, we could associate a supersolar (`carbon-rich) composition for HD142527 and a subsolar (`oxygen-rich) one for HD1444432. We show that the inner disc mineralogy can be very specific and not related to the dust composition derived from spatially unresolved mid-infrared spectroscopy. We highlight the need for including more complex chemistry when interpreting solid-state spectroscopic observations of the inner regions of discs, and for considering dynamical aspects for future studies.
We report the detection of a spectral signature observed at 682 cm$^{-1}$ by the Cassini Composite Infrared Spectrometer (CIRS) in nadir and limb geometry observations of Titans southern stratospheric polar region in the middle of southern fall, while stratospheric temperatures are the coldest since the beginning of the Cassini mission. The 682 cm$^{-1}$ signature, which is only observed below an altitude of 300-km, is at least partly attributed to the benzene (C$_6$H$_6$) ice $ u_{4}$ C-H bending mode. While we first observed it in CIRS nadir spectra of the southern polar region in early 2013, we focus here on the study of nadir data acquired in May 2013, which have a more favorable observation geometry. We derived the C$_6$H$_6$ ice mass mixing ratio in 5{deg}S latitude bins from the south pole to 65{deg}S and infer the C$_6$H$_6$ cloud top altitude to be located deeper with increasing distance from the pole. We additionally analyzed limb data acquired in March 2015, which were the first limb dataset available after the May 2013 nadir observation, in order to infer a vertical profile of its mass mixing ratio in the 0.1 - 1 mbar region (250 - 170 km). We derive an upper limit of $sim$1.5 $mu$m for the equivalent radius of pure C$_6$H$_6$ ice particles from the shape of the observed emission band. Several other unidentified signatures are observed near 687 and 702 cm$^{-1}$ and possibly 695 cm$^{-1}$, which could also be due to ice spectral signatures as they are observed in the deep stratosphere at pressure levels similar to the C$_6$H$_6$ ice ones. We could not reproduce these signatures with pure nitrile ice (HCN, HC$_3$N,CH$_3$CN, C$_2$H$_5$CN and C$_2$N$_2$) spectra available in the literature except the 695 cm$^{-1}$ feature that could possibly be due to C$_2$H$_3$CN ice.
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