Spectra Of Venus Research Articles

Limits to dynamic range in GHz-THz single-dish planetary spectra

Abstract When bright solar-system objects are observed by GHz-THz regime telescopes, off-axis signals bounce around locally and re-enter the signal path with a time delay, causing sinusoidal ripples in output spectra. Ripples that are unstable over time are challenging to remove. A typical detection limit for planetary spectral lines is a fraction ∼10−3 of continuum signal, restricting searches for minor atmospheric trace-gases. Modern wideband spectra of Venus demonstrate a plethora of effects, at three example telescopes spanning nearly a factor-of-50 in frequency. Characterisation of instrumental effects as families of pure sine-waves via Fourier-analysis is shown to improve dynamic-range by factors of a few. An example upper limit on sulphuric acid (H2SO4) vapour in Venus’ mesosphere, from fully-automated data-cleaning of a 3.5 GHz band containing 10 line components, goes as deep as the best previously-published limit. The most challenging cases are searches for single lines of width comparable to ripple periods. Traditional polynomial-fitting approaches can be deployed to test for false positives, to demonstrate robustness at a level of zero ”fake lines” in >1000 comparisons. Fourier-based data-cleaning avoids subjectivity and can be fully automated, and synthetic spectra can be injected before processing to test to what degree signals are lost in cleaning. An ideal robustness strategy is mitigation at the data-acquisition stage, e.g. using slow drifts in target-velocity with respect to the telescope to isolate a planetary line from a quasi-static instrumental ripple pattern.

Fiber Fabry-Perot astrophotonic correlation spectroscopy for remote gas identification and radial velocity measurements.

We present a novel, to the best of our knowledge, remote gas detection and identification technique based on correlation spectroscopy with a piezoelectric tunable fiber-optic Fabry-Perot filter. We show that the spectral correlation amplitude between the filter transmission window and gas absorption features is related to the gas absorption optical depth, and that different gases can be distinguished from one another using their correlation signal phase. Using a previously captured telluric-corrected high-resolution near-infrared spectrum of Venus, we show that the radial velocity of Venus can be extracted from the phase of higher order harmonic lock-in signals. This correlation spectroscopy technique has applications in the detection and radial velocity determination of weak spectral features in astronomy and remote sensing. We experimentally demonstrate a remote CO2 detection system using a lock-in amplifier, fiber-optic Fabry-Perot filter, and single channel avalanche photodiode.

Claimed Detection of PH3 in the Clouds of Venus Is Consistent with Mesospheric SO2

Abstract The observation of a 266.94 GHz feature in the Venus spectrum has been attributed to phosphine (PH3) in the Venus clouds, suggesting unexpected geological, chemical, or even biological processes. Since both PH3 and sulfur dioxide (SO2) are spectrally active near 266.94 GHz, the contribution to this line from SO2 must be determined before it can be attributed, in whole or part, to PH3. An undetected SO2 reference line, interpreted as an unexpectedly low SO2 abundance, suggested that the 266.94 GHz feature could be attributed primarily to PH3. However, the low SO2 and the inference that PH3 was in the cloud deck posed an apparent contradiction. Here we use a radiative transfer model to analyze the PH3 discovery, and explore the detectability of different vertical distributions of PH3 and SO2. We find that the 266.94 GHz line does not originate in the clouds, but above 80 km in the Venus mesosphere. This level of line formation is inconsistent with chemical modeling that assumes generation of PH3 in the Venus clouds. Given the extremely short chemical lifetime of PH3 in the Venus mesosphere, an implausibly high source flux would be needed to maintain the observed value of 20 ± 10 ppb. We find that typical Venus SO2 vertical distributions and abundances fit the JCMT 266.94 GHz feature, and the resulting SO2 reference line at 267.54 GHz would have remained undetectable in the ALMA data due to line dilution. We conclude that nominal mesospheric SO2 is a more plausible explanation for the JCMT and ALMA data than PH3.

The statistical reliability of 267-GHz JCMT observations of Venus: no significant evidence for phosphine absorption

ABSTRACT In the light of the recent announcement of the discovery of the potential biosignature phosphine in the atmosphere of Venus, I present an independent reanalysis of the original James Clerk Maxwell Telescope (JCMT) data to assess the statistical reliability of the detection. Two line detection methods are explored: low-order polynomial fits and higher order multiple polynomial fits. A non-parametric bootstrap analysis reveals that neither line detection method is able to recover a statistically significant detection. Similar to the results of other reanalyses of ALMA(Atacama Large Millimetre Array) Venus spectra, the polynomial fitting process results in false positive detections in the JCMT spectrum. There is thus no significant evidence for phosphine absorption in the JCMT Venus spectra.

The Largest Electron Differential Energy Flux Observed at Mars by the Mars Express Spacecraft, 2004-2016.

The goal of this paper is to understand the processes by which solar wind electrons are energized in the Martian magnetosphere and how this compares to processes at Venus and Earth. Each is unique in the source of its magnetic field topology and how this influences electron energization. To achieve this goal, 24 million spectra spanning 13 years have been examined using the electron spectrometer from the Mars Express spacecraft between about 12,000 km and about 250 km altitude, and from all latitudes and local times. The top 10 largest differential energy flux at energies above the differential energy flux peak have been found: seven spectra from the magnetosheath near noon, three from the dark tail (the largest two from the middle and ionospheric edge of the magnetosheath). Spectral comparisons show a decade range in the peak of the electron distributions; however, all distributions show a similar energy maximum dictated by solar wind/planet interaction. Similarly derived, the largest Venus spectrum occurred near the magnetosheath bow shock and had the same shape as the most intense Mars inner magnetosheath spectrum. The Mars and Venus dayside spectra compared to the Mars nightside spectrum that included an enhanced optical signal attributed to discrete “auroral” precipitation show a similar shape. These spectra are also compared to a selected auroral zone electron spectra from the Earth. The Mars and Venus results suggest that there is no more energy needed to generate electrons forming the nightside precipitation than is gained during the solar wind/planet interaction.

Venus Upper Clouds and the UV Absorber From MESSENGER/MASCS Observations

AbstractOne of the most intriguing, long‐standing questions regarding Venus's atmosphere is the origin and distribution of the unknown UV absorber, responsible for the absorption band detected at the near‐UV and blue range of Venus's spectrum. In this work, we use data collected by Mercury Atmospheric and Surface Composition Spectrometer (MASCS) spectrograph on board the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission during its second Venus flyby in June 2007 to address this issue. Spectra range from 0.3 μm to 1.5 μm including some gaseous H2O and CO2 bands, as well as part of the SO2 absorption band and the core of the UV absorption. We used the NEMESIS radiative transfer code and retrieval suite to investigate the vertical distribution of particles in the equatorial atmosphere and to retrieve the imaginary refractive indices of the UV absorber, assumed to be well mixed with Venus's small mode 1 particles. The results show a homogeneous equatorial atmosphere, with cloud tops (height for unity optical depth) at 75 ± 2 km above surface. The UV absorption is found to be centered at 0.34 ± 0.03 μm with a full width at half maximum of 0.14 ± 0.01 μm. Our values are compared with previous candidates for the UV aerosol absorber, among which disulfur oxide (S2O) and dioxide disulfur (S2O2) provide the best agreement with our results.

Extreme ultraviolet spectra of Venusian airglow observed by EXCEED

Extreme ultraviolet (EUV) spectra of Venus in the wavelength range 520−1480 Å with 3−4 Å resolutions were obtained in March 2014 by an EUV imaging spectrometer EXCEED (Extreme Ultraviolet Spectroscope for Exospheric Dynamics) on the HISAKI spacecraft. Due to its high sensitivity and long exposure time, many new emission lines and bands were identified. Already known emissions such as the O II 834 Å, O I 989 Å, HILy−β1026 Å, and the C I 1277 Å lines (Broadfoot et al., 1974; Bertaux et al., 1980; Feldman et al., 2000) are also detected in the EXCEED spectrum. In addition, N2 band systems such as the Lyman-Birge-Hopfield (a1Πg−X1Σg+) (2, 0), (2, 1), (3, 1), (3, 2) and (5, 3) bands, the Birge-Hopfield (b1Πu−X1Σg+) (1, 3) band, and the Carroll-Yoshino (c4′1Σu+−X1Σg+) (0, 0) and (0, 1) bands together are identified for the first time in the Venusian airglow. We also identified the CO Hopfield-Birge (B1Σ+−X1Σ+) (1, 0) band in addition to the already known (0, 0) band, and the CO Hopfield-Birge (C1Σ+−X1Σ+) (0, 1), (0, 2) bands in addition to the already known (0, 0) band (Feldman et al., 2000; Gérard et al., 2011).

Search for HBr and bromine photochemistry on Venus

HBr (1–0) R2 2605.8/6.2 cm−1, the strongest line of the strongest band of HBr, was observed when searching for this species on Venus. The observation was conducted using the NASA IRTF and a high-resolution long-slit spectrograph CSHELL with resolving power of 4 ×104. 101 spectra of Venus were analyzed, and the retrieved HBr abundances varied from -8 to+ 5 ppb. Their mean value is -1.2 ppb, standard deviation is 2.5 ppb, and uncertainty of the mean is 0.25 ppb. The negative value presumes a systematic error, and the estimated upper limit of the HBr mixing ratio at the cloud tops of Venus is ∼1 ppb. From the simultaneously retrieved CO2 abundances, this corresponds to an altitude of 78km for the uniform distribution of HBr. A simplified version of the bromine photochemistry is included into the photochemical model (Krasnopolsky 2012, Icarus 218, 230–246). Photolysis of HBr and its reactions with O and H deplete the HBr mixing ratio at 70–80km relative to that below 60km by a factor of ≈300. Reanalysis of the observational data with the calculated profile of HBr gives an upper limit of 20–70 ppb for HBr below 60km and the aerosol optical depth of 0.7 at 70km and 3.84µm.The bromine chemistry may be effective on Venus even under the observed upper limit. However, if a Cl/Br ratio in the Venus atmosphere is similar to that in the Solar System, then HBr is ≈1 ppb in the lower atmosphere and the bromine chemistry is insignificant. Thermodynamic calculations based on the chemical kinetic model (Krasnopolsky 2013, Icarus 225, 570–580) predict HBr as a major bromine species in the lower atmosphere.

The CO2 absorption spectrum in the 2.3 µm transparency window by high sensitivity CRDS: (II) Self-absorption continuum

The CO2 absorption continuum near 2.3µm is determined for a series of sub atmospheric pressures (250–750Torr) by high sensitivity Cavity Ring Down Spectroscopy. An experimental procedure consisting in injecting successively a gas flow of CO2 and synthetic air, keeping constant the gas pressure in the CRDS cell, has been developed. This procedure insures a high stability of the spectra baseline by avoiding changes of the optical alignment due to pressure changes.The CO2 continuum was obtained as the difference between the CO2 absorption coefficient and a local lines simulation using a Voigt profile truncated at ±25cm−1. Following the results of the preceding analysis of the CO2 rovibrational lines (Vasilchenko S et al. J Quant Spectrosc Radiat Transfer 10.1016/j.jqsrt.2016.07.002, a CO2 line list with intensities obtained by variational calculations and empirical line positions was preferred to the HITRAN line list. A quadratic pressure dependence of the absorption continuum is observed, with an average binary absorption coefficient increasing from 2 to 4×10−8cm−1 amagat−2 between 4320 and 4380cm−1.The obtained continuum is found in good agreement with a previous measurement using much higher densities (20 amagat) and a low resolution grating spectrograph and is consistent with values currently used in the analysis of Venus spectra.

High sensitivity Cavity Ring Down spectroscopy of carbon dioxide in the 1.19–1.26 µm region

A Cavity Ring Down spectrometer using a fiber-connected External Cavity Diode Laser (ECDL) as light source has been developed to access the 1.26–1.19μm region. We present here the first application of this newly developed CW-CRDS spectrometer to the study of the highly sensitive spectrum of natural carbon dioxide between 7909 and 8370cm−1. The spectrum is dominated by the ν1+3ν3 dyad at 8192.55 and 8293.95cm−1 which forms the low energy border of the 1.2µm transparency window of importance for planetary applications. The achieved sensitivity (noise equivalent absorption, αmin, in the 10−10–10−11cm−1 range) allowed detection of numerous new transitions with intensity values down to 10−30cm/molecule, in particular hot bands reaching upper states with energy up to about 10,600cm−1.More than 3400 transitions belonging to the six major isotopologues of ‘natural’ carbon dioxide were assigned using the predictions of effective Hamiltonian (EH) models. A total of 2027, 442, 548, 303, 92 and 13 transitions belonging to 37, 9, 8, 4, 2 and 1 bands were rovibrationnally assigned for 12C16O2, 13C16O2, 16O12C18O, 16O12C17O, 16O13C18O and 16O13C17O, respectively. For comparison, only 14 12C16O2 absorption bands were previously known in the region (mostly from Venus spectra). Intensity values range between 2.1×10−30 and 2.4×10−24cm/molecule. All the identified bands correspond to the ΔP=11 series of transitions, where P=2V1+V2+3V3 is the polyad number (Vi are vibrational quantum numbers). The overall agreement of the EH predicted and measured line positions is very good (for instance, average and rms deviations of 0.64×10−3 and 3.8×10−3cm−1 for 12C16O2, respectively).The band-by-band analysis provided accurate spectroscopic parameters of 57 bands from a fit of the measured line positions. The global fits of the obtained intensity values of the ΔP=11 series of transitions were used to refine the corresponding set of effective dipole moment (EDM) parameters of the six studied isotopologues.The obtained results will help to improve the spectral line parameters of carbon dioxide in the most currently used spectroscopic databases. In particular, the calculated band intensities of a few weak hot bands are now in good agreement with the observations.

LINE PARAMETERS OF THE 782 nm BAND OF CO2

The 782 nm band of CO{sub 2}, in a transparent window of Earth's atmosphere, was the first CO{sub 2} band observed 80 yr ago in the spectra of Venus. The band is very weak and therefore not saturated by the thick atmosphere of Venus, but its spectral parameters are still very limited due to the difficulty of detecting it in the laboratory. It is the highest overtone (ν{sub 1} + 5ν{sub 3}) of CO{sub 2} given in widely used spectroscopy databases such as HITRAN and GEISA. In the present work, the band is studied using a cavity ring-down spectrometer with ultra-high sensitivity as well as high precision. The positions of 55 lines in the band were determined with an absolute accuracy of 3 × 10{sup –5} cm{sup –1}, two orders of magnitude better than previous studies. The line intensities, self-induced pressure broadening coefficients, and the shift coefficients were also derived from the recorded spectra. The obtained spectral parameters can be applied to model the spectra of the CO{sub 2}-rich atmospheres of planets like Venus and Mars.

Venus observations from ENVISAT–SCIAMACHY: Measurements and modeling

This work shows the capability of observing Venus with a sensor originally designed for Earth remote sensing. SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY), onboard ENVISAT, successfully observed visible and near-infrared spectra from the Venusian atmosphere. The Venus spectra were simulated using a line-by-line radiative transfer model. The single scattering approximation was applied in order to consider the effects of an approximately 20km-thick haze layer above the main cloud deck, which was considered as a reflecting cloud located in the upper atmosphere of the planet. CO2 absorption lines could be distinguished in both observed and simulated spectra and a good agreement between them was also found.

Assignment and rotational analysis of new absorption bands of carbon dioxide isotopologues in Venus spectra

We present absorption bands of carbon dioxide isotopologues, detected by the Solar Occultation for the Infrared Range (SOIR) instrument on board the Venus Express Satellite. The SOIR instrument combines an echelle spectrometer and an Acousto-Optical Tunable Filter (AOTF) for order selection. It performs solar occultation measurements in the Venus atmosphere in the IR region (2.2–4.3μm), at a resolution of 0.12–0.18cm−1. The wavelength range probed by SOIR allows a detailed chemical inventory of the Venus atmosphere above the cloud layer (65–150km) to be made with emphasis on the vertical distributions of gases. Thanks to the SOIR spectral resolution, a new CO2 absorption band was identified: the 21101–01101 band of 16O12C18O with R branch up to J=31. Two other previously reported bands were observed dispelling any doubts about their identifications: the 20001–00001 band of 16O13C18O [Villanueva G, et al. J Quant Spectrosc Radiat Transfer 2008;109:883–894] and the 01111–00001 band of 16O12C18O [Villanueva G, et al. J Quant Spectrosc Radiat Transfer 2008;109:883–894 and Wilquet V, et al. J Quant Spectrosc Radiat Transfer 2008;109:895–905]. These bands were analyzed, and spectroscopic constants characterizing them were obtained. The rotational assignment of the 20001–00001 band was corrected. The present measurements are compared with data available in the HITRAN database.

Observation of DCl and upper limit to NH3 on Venus

To search for DCl in the Venus atmosphere, a spectrum near the D35Cl (1–0) R4 line at 2141.54cm−1 was observed using the CSHELL spectrograph at NASA IRTF. Least square fitting to the spectrum by a synthetic spectrum results in a DCl mixing ratio of 17.8±6.8ppb. Comparing to the HCl abundance of 400±30ppb (Krasnopolsky [2010a] Icarus, 208, 314–322), the DCl/HCl ratio is equal to 280±110 times the terrestrial D/H=1.56×10−4. This ratio is similar to that of HDO/H2O=240±25 times the terrestrial HDO/H2O from the VEX/SOIR occultations at 70–110km. Photochemistry in the Venus mesosphere converts H from HCl to that in H2O with a rate of 1.9×109cm−2s−1 (Krasnopolsky [2012] Icarus, 218, 230–246). The conversion involves photolysis of HCl; therefore, the photochemistry tends to enrich D/H in HCl and deplete in H2O. Formation of the sulfuric acid clouds may affect HDO/H2O as well. The enriched HCl moves down by mixing to the lower atmosphere where thermodynamic equilibriums for H2 and HCl near the surface correspond to D/H=0.71 and 0.74 times that in H2O, respectively. Time to establish these equilibriums is estimated at ∼3years and comparable to the mixing time in the lower atmosphere. Therefore, the enriched HCl from the mesosphere gives D back to H2O near the surface. Comparison of chemical and mixing times favors a constant HDO/H2O up to ∼100km and DCl/HCl equal to D/H in H2O times 0.74.Ammonia is an abundant form of nitrogen in the reducing environments. Thermodynamic equilibriums with N2 and NO near the surface of Venus give its mixing ratio of 10−14 and 6×10−7, respectively. A spectrum of Venus near the NH3 line at 4481.11cm−1 was observed at NASA IRTF and resulted in a two-sigma upper limit of 6ppb for NH3 above the Venus clouds. This is an improvement of the previous upper limit by a factor of 5. If ammonia exists at the ppb level or less in the lower atmosphere, it quickly dissociates in the mesosphere and weakly affects its photochemistry.

Transmission spectrum of Venus as a transiting exoplanet

On 5-6 June 2012, Venus will be transiting the Sun for the last time before 2117. This event is an unique opportunity to assess the feasibility of the atmospheric characterisation of Earth-size exoplanets near the habitable zone with the transmission spectroscopy technique and provide an invaluable proxy for the atmosphere of such a planet. In this letter, we provide a theoretical transmission spectrum of the atmosphere of Venus that could be tested with spectroscopic observations during the 2012 transit. This is done using radiative transfer across Venus' atmosphere, with inputs from in-situ missions such as Venus Express and theoretical models. The transmission spectrum covers a range of 0.1-5 {\mu}m and probes the limb between 70 and 150 km in altitude. It is dominated in UV by carbon dioxide absorption producing a broad transit signal of ~20 ppm as seen from Earth, and from 0.2 to 2.7 {\mu}m by Mie extinction (~5 ppm at 0.8 {\mu}m) caused by droplets of sulfuric acid composing an upper haze layer above the main deck of clouds. These features are not expected for a terrestrial exoplanet and could help discriminating an Earth-like habitable world from a cytherean planet.

Venus transit 2004: Illustrating the capability of exoplanet transmission spectroscopy

The transit of Venus in 2004 offered the rare possibility to remotely sense a well-known planetary atmosphere using ground-based observations for absorption spectroscopy. Transmission spectra of Venus' atmosphere were obtained in the near infrared using the Vacuum Tower Telescope (VTT) in Tenerife. Since the instrument was designed to measure the very bright photosphere of the Sun, extracting Venus' atmosphere was challenging. CO_2 absorption lines could be identified in the upper Venus atmosphere. Moreover, the relative abundance of the three most abundant CO_2 isotopologues could be determined. The observations resolved Venus' limb, showing Doppler-shifted absorption lines that are probably caused by high-altitude winds. This paper illustrates the ability of ground-based measurements to examine atmospheric constituents of a terrestrial planet atmosphere which might be applied in future to terrestrial extrasolar planets.

Quantification of middle and lower cloud variability and mesoscale dynamics from Venus Express/VIRTIS observations at 1.74 μm

We present an analysis of VIRTIS-M-IR observations of 1.74 μm emission from the nightside of Venus. The 1.74 μm window in the near infrared spectrum of Venus is an ideal proxy for investigating the evolution of middle and lower cloud deck opacity of Venus because it exhibits good signal to noise due to its brightness, good contrast between bright and dark regions, and few additional sources of extinction beside the clouds themselves. We have analyzed the data from the first 407 orbits (equivalent to 407 Earth days) of the Venus Express mission to determine the magnitude of variability in the 1.74 μm radiance. We have also performed an analysis of the evolution of individual features over a span of roughly 5–6 h on two successive orbits of Venus Express. We find that the overall 1.74 μm brightness of Venus has been increasing through the first 407 days of the mission, indicating a gradual diminishing of the cloud coverage and/or thickness, and that the lower latitudes exhibited more variability and more brightening than higher latitudes. We find that individual features evolve with a time scale of about 30 h, consistent with our previous analysis. Analysis of the evolution and motion of the clouds can be used to estimate the mesoscale dynamics within the clouds of Venus. We find that advection alone cannot explain the observed evolution of the features. The measured vorticity and divergence in the vicinity of the features are consistent with evolution under the influence of significant vertical motions likely driven by a radiative dynamical feedback. We measure a zonal wind speed of around 65 m/s, and a meridional wind speed around 2.5 m/s by tracking the motion of the central region of the features. But we also find that the measured wind speeds depend strongly on the points chosen for the wind speed analysis.

High sensitivity CW-cavity ring down spectroscopy of 12CO 2 near 1.35 μm (I): line positions

The absorption spectrum of carbon dioxide in natural isotopic abundance has been investigated by CW-cavity ring down spectroscopy with a new setup based on fibred distributed feedback (DFB) laser diodes. By using a series of 25 DFB lasers, the CO 2 spectrum was recorded in the 7123–7793 cm −1 region with a typical sensitivity of 3×10 −10 cm −1. A 2125 transitions with intensities as low as 1×10 −29 cm/molecule were detected and assigned to the 1 2C 16O 2, 1 6O 1 2C 1 7O and 1 6O 1 2C 1 8O isotopologues. For comparison, only 357 of them were previously reported from Venus spectra and 344 transitions were included in the 2004 version of the HITRAN database. The band by band analysis has led to the determination of the rovibrational parameters of 28, 2 and 6 bands for the 1 2C 16O 2, 1 6O 1 2C 1 7O and 1 6O 1 2C 1 8O isotopologue, respectively. While the uncertainty on the experimental line positions is on the order of 5×10 −4 cm −1, the average deviation from the 1 2C 16O 2 calculated values provided by the most recent version of the carbon dioxide spectroscopic databank (CDSD) is −2.8×10 −3 cm −1 with an root mean square ( rms) deviation of 3.5×10 −3 cm −1. Maximum deviations in the order of 0.02 and 0.12 cm −1 were evidenced for some bands of the 1 6O 1 2C 1 7O and 1 6O 1 2C 1 8O minor isotopologues. The obtained results improve significantly the previous measurements from Venus spectra and will be valuable to refine the sets of effective Hamiltonian parameters used to generate the CDSD database.

Line parameters for the 01111–00001 band of 12C 16O 18O from SOIR measurements of the Venus atmosphere

CO 2 is the major constituent of the atmosphere of Venus. Absorption lines due to its 12C 16O 18O isotopologue have been observed for the first time in Venus spectra in the 2930–3015 cm −1 spectral region, where the HITRAN database does not contain any line from this isotopologue. The measurements were performed by the SOIR instrument, which is part of the SPICAV/SOIR instrument on board the Venus Express mission of ESA. SOIR measured the atmospheric transmission of the upper atmosphere of Venus ( z>70 km) by performing a solar occultation experiment using the atmosphere as a gigantic absorption cell. The identification of this newly observed band was first made recently from Mars atmosphere observations by US colleagues. We have made independent theoretical calculations of the positions of the lines of this new 01111–00001 absorption band, which coincide perfectly with the positions of the observed lines. Assuming an oxygen isotopic ratio similar to the one measured previously in the lower atmosphere of Venus, the line strengths of each observed line are deduced and listed.

A correlated- k model of radiative transfer in the near-infrared windows of Venus

We present a correlated- k-based model for generating synthetic spectra in the near-infrared window regions, from 1.0 to 2.5 μm, emitted from the deep atmosphere of Venus on the nightside. This approach is applicable for use with any near-infrared instrument, ground-based and space-borne, for analysis of the thermal emissions in this spectral range. We also approach this work with the view of using the model, in conjunction with a retrieval algorithm, to retrieve minor species from the Venus Express/VIRTIS instrument. An existing radiative-transfer model was adapted for Venusian conditions to deal with the prevailing high pressures and temperatures and other conditions. A comprehensive four-modal cloud structure model based on Pollack et al. [Near-infrared light from venus’ nightside: a spectroscopic analysis. Icarus 1993;103:1–42], using refractive indices for a 75% H 2SO 425% H 2O mixture from Palmer and Williams [Optical constants of sulfuric acid; application to the clouds of Venus? Appl Opt 1975;14(1):208–19], was also implemented. We then utilized a Mie scattering algorithm to account for the multiple scattering effect between cloud and haze layers that occur in the Venusian atmosphere. The correlated- k model is shown to produce good agreement with ground-based spectra of Venus in the near infrared, and to match the output from a line-by-line radiative-transfer model to better than 10%.