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Register a new userRetrieval of Water Vapor Column Abundance and Aerosol Properties from ChemCam Passive Sky Spectroscopy
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Added by
Timothy McConnochie
Publication date
2017
fields
Physics
and research's language is
English
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We derive water vapor column abundances and aerosol properties from Mars Science Laboratory (MSL) ChemCam passive mode observations of scattered sky light. Each ChemCam passive sky observation acquires spectra at two different elevation angles. We fit these spectra with a discrete-ordinates multiple scattering radiative transfer model, using the correlated-k approximation for gas absorption bands. The retrieval proceeds by first fitting the continuum of the ratio of the two elevation angles to solve for aerosol properties, and then fitting the continuum-removed ratio to solve for gas abundances. The final step of the retrieval makes use of the observed CO2 absorptions and the known CO2 abundance to correct the retrieved water vapor abundance for the effects of the vertical distribution of scattering aerosols and to derive an aerosol scale height parameter. The ChemCam-retrieved water abundances show, with only a few exceptions, the same seasonal behavior and the same timing of seasonal minima and maxima as the TES, CRISM, and REMS-H data sets that we compare them to. However ChemCam-retrieved water abundances are generally lower than zonal and regional scale from-orbit water vapor data, while at the same time being significantly larger than pre-dawn REMS-H abundances. Pending further analysis of REMS-H volume mixing ratio uncertainties, the differences between ChemCam and REMS-H pre-dawn mixing ratios appear to be much too large to be explained by large scale circulations and thus they tend to support the hypothesis of substantial diurnal interactions of water vapor with the surface. Our preliminary aerosol results, meanwhile, show the expected seasonal pattern in dust particle size but also indicate a surprising inter-annual increase in water-ice cloud opacities.
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Transmission spectroscopy provides a powerful probe of the atmospheric properties of transiting exoplanets. To date, studies of exoplanets in transit have focused on inferring their atmospheric properties such as chemical compositions, cloud/haze properties, and temperature structures. However, surface inhomogeneities in the host stars of exoplanets in the form of cool spots and hot faculae can in principle imprint signatures on the observed planetary transit spectrum. Here we present Aura, a new retrieval paradigm for inferring both planetary and stellar properties from a transmission spectrum. We apply our retrieval framework to a sample of hot giant exoplanets to determine the significance of stellar heterogeneity and clouds/hazes in their spectra. The retrieval analyses distinguish four groups of planets. First, the spectra of WASP-6b and WASP-39b are best characterised by imprints of stellar heterogeneity and hazes and/or clouds. HD 209458b and HAT-P-12b comprise the second group for which there is weak evidence for stellar heterogeneity and a high significance of hazes and/or clouds. The third group constitutes HAT-P-1b and WASP-31b and shows weak evidence against stellar heterogeneity but weak to substantial indications of clouds/hazes. The fourth group -- WASP-19b, WASP-17b, and WASP-12b -- is fit best by molecular and alkali absorbers with H$_2$ scattering without evidence for stellar heterogeneity and weak to no evidence for clouds/hazes. Our retrieval methodology paves the way to simultaneous information on the star and planet from higher resolution spectra using future facilities such as the James Webb Space Telescope and large ground-based facilities.
Dust is the main driver of Mars atmospheric variability. The determination of Martian dust aerosol properties is of high relevance for radiative modelling and calculating its weather forcing. In particular, the light scattering behaviour at intermediate and large scattering angles can provide valuable information regarding the airborne dust particle shape. The angular distribution of sky brightness observed by the Mars Science Laboratory engineering cameras (Navcam and Hazcam) is used here to characterise the atmospheric dust single scattering phase function and to constrain the shape of the particles. An iterative radiative transfer based retrieval method was implemented in order to determine the aerosol modelling parameters which best reproduce the observed sky radiance as a function of the scattering angle in the solar almucantar plane. The aerosol models considered in this study for retrieving dust radiative properties were an analytical three term Double Henyey-Greenstein phase function, T-matrix calculations for cylindrical particles with different diameter-to-length aspect ratios and experimental phase functions from laboratory measurements of several Martian dust analogue samples. Results of this study returned mean DHG phase function parameter values in line with Wolff et al. (2009). Although differences were observed during the low opacity aphelion season (lower forward scattering values, presence of a peak in the backward region) compared to the rest of the year, no clear evidences of seasonal behaviour or interannual variability were derived. The obtained average D/L aspect ratios for T-matrix calculated cylindrical particles were 0.70{pm}0.20 and 1.90{pm}0.20, and the best fitting Martian dust analogue corresponded to the basalt sample.
We present the detection and modeling of more than 70 far-IR pure rotational lines of water vapor, including the 18O and 17O isotopologues, towards Orion KL. Observations were performed with the Long Wavelength Spectrometer Fabry-Perot (LWS/FP; R~6800-9700) on board the Infrared Space Observatory (ISO) between ~43 and ~197 um. The water line profiles evolve from P-Cygni type profiles (even for the H2O18 lines) to pure emission at wavelengths above ~100 um. We find that most of the water emission/absorption arises from an extended flow of gas expanding at 25+-5 kms^-1. Non-local radiative transfer models show that much of the water excitation and line profile formation is driven by the dust continuum emission. The derived beam averaged water abundance is 2-3x10^-5. The inferred gas temperature Tk=80-100 K suggests that: (i) water could have been formed in the plateau by gas phase neutral-neutral reactions with activation barriers if the gas was previously heated (e.g. by shocks) to >500 K and/or (ii) H2O formation in the outflow is dominated by in-situ evaporation of grain water-ice mantles and/or (iii) H2O was formed in the innermost and warmer regions (e.g. the hot core) and was swept up in ~1000 yr, the dynamical timescale of the outflow.
Oxygen is the most common element after hydrogen and helium in Jupiters atmosphere, and may have been the primary condensable (as water ice) in the protoplanetary disk. Prior to the Juno mission, in situ measurements of Jupiters water abundance were obtained from the Galileo Probe, which dropped into a meteorologically anomalous site. The findings of the Galileo Probe were inconclusive because the concentration of water was still increasing when the probe died. Here, we initially report on the water abundance in the equatorial region, from 0 to 4 degrees north latitude, based on 1.25 to 22 GHz data from Juno Microwave radiometer probing approximately 0.7 to 30 bars pressure. Because Juno discovered the deep atmosphere to be surprisingly variable as a function of latitude, it remains to confirm whether the equatorial abundance represents Jupiters global water abundance. The water abundance at the equatorial region is inferred to be $2.5_{-1.6}^{+2.2}times10^3$ ppm, or $2.7_{-1.7}^{+2.4}$ times the protosolar oxygen elemental ratio to H (1$sigma$ uncertainties). If reflective of the global water abundance, the result suggests that the planetesimals formed Jupiter are unlikely to be water-rich clathrate hydrates.
An increasing number of potentially habitable terrestrial planets and planet candidates are found by ongoing planet search programs. The search for atmospheric signatures to establish planetary habitability and the presence of life might be possible in the future. We want to quantify the accuracy of retrieved atmospheric parameters which might be obtained from infrared emission spectroscopy. We use synthetic observations of hypothetical habitable planets, constructed with a parametrized atmosphere model, a high-resolution radiative transfer model and a simplified noise model. Classic statistical tools such as chi2 statistics and least-square fits were used to analyze the simulated observations. When adopting the design of currently planned or proposed exoplanet characterization missions, we find that emission spectroscopy could provide weak limits on surface conditions of terrestrial planets, hence their potential habitability. However, these mission designs are unlikely to allow to characterize the composition of the atmosphere of a habitable planet, even though CO2 is detected. Upon increasing the signal-to-noise ratios by about a factor of 2-5 (depending on spectral resolution) compared to current mission designs, the CO2 content could be characterized to within two orders of magnitude. The detection of the O3 biosignature remains marginal. The atmospheric temperature structure could not be constrained. Therefore, a full atmospheric characterization seems to be beyond the capabilities of such missions when using only emission spectroscopy during secondary eclipse or target visits. Other methods such as transmission spectroscopy or orbital photometry are probably needed in order to give additional constraints and break degeneracies. (abridged)
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