Titan global climate model: A new 3-dimensional version of the IPSL Titan GCM

We have developed a new 3-dimensional climate model for Titan’s atmosphere, using the physics of the IPSL Titan 2-dimensional climate model with the current version of the LMDZ General Circulation Model dynamical core. Microphysics and photochemistry are still computed as zonal averages. This GCM covers altitudes from surface to 500 km altitude, with barotropic waves now being resolved and the diurnal cycle included. The boundary layer scheme has been changed, yielding a strong improvement in the tropospheric zonal wind profile modeled at Huygens descent position and season. The potential temperature profile is fairly consistent with Huygens observations in the lowest 10 km. The latitudinal profile of the near-surface temperature is close to observed values. The minimum of zonal wind observed by the Huygens probe just above the tropopause is also present in these simulations, and its origin is discussed by comparing solar heating and dynamical transport of energy. The stratospheric temperature and wind fields are consistent with our previous works. Compared to observations, the zonal wind peak is too weak (around 120 m/s) and too low (around 200 km). The temperature structures appear to be compressed in altitude, and depart strongly from observations in the upper stratosphere. These discrepancies are correlated, and most probably related to the altitude of the haze production. The model produces a detached haze layer located more than 150 km lower than observed by the Cassini instruments. This low production altitude is due to the current position of the GCM upper boundary. However, the temporal behaviour of the detached haze layer in the model may explain the seasonal differences observed between Cassini and Voyager 1. The waves present in the GCM are analyzed, together with their respective roles in the angular momentum budget. Though the role of the mean meridional circulation in momentum transport is similar to previous work, and the transport by barotropic waves is clearly seen in the stratosphere, a significant part of the transport at high latitudes is done all year long through low-frequency tropospheric waves that may be baroclinic waves.     

The planetary fourier spectrometer (PFS) onboard the European Venus Express mission

The planetary fourier spectrometer (PFS) for the Venus Express mission is an infrared spectrometer optimized for atmospheric studies. This instrument has a short wavelength (SW) channel that covers the spectral range from 1700 to 11400 cm⁻¹ (0.9–5.5 μm) and a long wavelength (LW) channel that covers 250–1700 cm⁻¹ (5.5–45 μm). Both channels have a uniform spectral resolution of 1.3 cm⁻¹. The instrument field of view FOV is about 1.6 ° (FWHM) for the short wavelength channel and 2.8 ° for the LW channel which corresponds to a spatial resolution of 7 and 12 km when Venus is observed from an altitude of 250 km. PFS can provide unique data necessary to improve our knowledge not only of the atmospheric properties but also surface properties (temperature) and the surface-atmosphere interaction (volcanic activity).PFS works primarily around the pericentre of the orbit, only occasionally observing Venus from larger distances. Each measurements takes 4.5 s, with a repetition time of 11.5 s. By working roughly 1.5 h around pericentre, a total of 460 measurements per orbit will be acquired plus 60 for calibrations. PFS is able to take measurements at all local times, enabling the retrieval of atmospheric vertical temperature profiles on both the day and the night side.The PFS measures a host of atmospheric and surface phenomena on Venus. These include the:(1) thermal surface flux at several wavelengths near 1 μm, with concurrent constraints on surface temperature and emissivity (indicative of composition); (2) the abundances of several highly-diagnostic trace molecular species; (3) atmospheric temperatures from 55 to 100 km altitude; (4) cloud opacities and cloud-tracked winds in the lower-level cloud layers near 50-km altitudes; (5) cloud top pressures of the uppermost haze/cloud region near 70–80 km altitude; and (6) oxygen airglow near the 100 km level. All of these will be observed repeatedly during the 500-day nominal mission of Venus Express to yield an increased understanding of meteorological, dynamical, photochemical, and thermo-chemical processes in the Venus atmosphere. Additionally, PFS will search for and characterize current volcanic activity through spatial and temporal anomalies in both the surface thermal flux and the abundances of volcanic trace species in the lower atmosphere.Measurement of the 15 μm CO₂ band is very important. Its profile gives, by means of a complex temperature profile retrieval technique, the vertical pressure-temperature relation, basis of the global atmospheric study.PFS is made of four modules called O, E, P and S being, respectively, the interferometer and proximity electronics, the digital control unit, the power supply and the pointing device.     

Vertical structure of the Venus cloud top from the VeRa and VIRTIS observations onboard Venus Express

We investigate the Venus cloud top structure by joint analysis of the data from Visual and Thermal Infrared Imaging Spectrometer (VIRTIS) and the atmospheric temperature sounding by the Radio Science experiment (VeRa) onboard Venus Express. The cloud top altitude and aerosol scale height are derived by fitting VIRTIS spectra at 4–5 μm with temperature profiles taken from the VeRa radio occultation. Our study shows gradual descent of the cloud top from 67.2 ± 1.9 km in low latitudes to 62.8 ± 4.1 km at the pole and decrease of the aerosol scale height from 3.8 ± 1.6 km to 1.7 ± 2.4 km. These changes correlate with the mesospheric temperature field. In the cold collar and high latitudes the cloud top position remarkably coincides with the sharp minima in temperature inversions suggesting importance of radiative cooling in their maintenance. This behaviour is consistent with the earlier observations. Spectral trend of the cloud top altitude derived from a comparison with the earlier observations in 1.6–27 μm wavelength range is qualitatively consistent with sulphuric acid composition of the upper cloud and suggests that particle size increases from equator to pole.     

First ever in situ observations of Venus’ polar upper atmosphere density using the tracking data of the Venus Express Atmospheric Drag Experiment (VExADE)

On its highly elliptical 24 h orbit around Venus, the Venus Express (VEX) spacecraft briefly reaches a periapsis altitude of nominally 250 km. Recently, however, dedicated and intense radio tracking campaigns have taken place in August 2008, October 2009, February and April 2010, for which the periapsis altitude was lowered to the 186–176 km altitude range in order to be able to probe the upper atmosphere of Venus above the North Pole for the first time ever in situ. As the spacecraft experiences atmospheric drag, its trajectory is measurably perturbed during the periapsis pass, allowing us to infer total atmospheric mass density at the periapsis altitude. A Precise Orbit Determination (POD) of the VEX motion is performed through an iterative least-squares fitting process to the Doppler tracking data, acquired by the VEX radioscience experiment (VeRa). The drag acceleration is modelled using an initial atmospheric density model (VTS3 model, Hedin, A.E., Niemann, H.B., Kasprzak, W.T., Seiff, A. [1983]. J. Geophys. Res. 88, 73–83). A scale factor of the drag acceleration is estimated for each periapsis pass, which scales Hedin’s density model in order to best fit the radio tracking data. Reliable density scale factors have been obtained for 10 passes mainly from the second (October 2009) and third (April 2010) VExADE campaigns, which indicate a lower density by a factor of about 1.8 than Hedin’s model predicts. These first ever in situ polar density measurements at solar minimum have allowed us to construct a diffusive equilibrium density model for Venus’ thermosphere, constrained in the lower thermosphere primarily by SPICAV-SOIR measurements and above 175 km by the VExADE drag measurements (Müller-Wodarg et al., in preparation). The preliminary results of the VExADE campaigns show that it is possible to obtain with the POD technique reliable estimates of Venus’ upper atmosphere densities at an altitude of around 175 km. Future VExADE campaigns will benefit from the planned further lowering of VEX pericenter altitude to below 170 km.     

Electron conductivity and density profiles derived from the mutual impedance probe measurements performed during the descent of Huygens through the atmosphere of Titan

During the descent of the Huygens probe through the atmosphere of Titan, on January 14th, 2005, the permittivity, waves and altimetry (PWA) subsystem, a component of the Huygens atmospheric structure instrument (HASI), detected an ionized layer at altitudes around 63 km with two different instruments, the relaxation probe (RP) and the mutual impedance probe (MIP). A very detailed analysis of both data sets is required, in order to correct for environmental effects and compare the two independent estimates of the electrical conductivity. The present work is dedicated to the MIP data analysis. New laboratory tests have been performed to validate or improve the available calibration results. Temperature effects have been included and numerical models of the MIP sensors and electric circuitry have been developed to take into account the proximity of the Huygens probe body. The effect of the vertical motion of the vessel in the ionized atmosphere is estimated in both analytical and numerical ways. The peculiar performance of the instrument in the altitude range 100–140 km is scrutinized. The existence of a prominent ionized layer, and of enhancements in the conductivity and electron density profiles at 63 km, are discussed in the light of previous theoretical predictions.     

Small-scale temperature fluctuations seen by the VeRa Radio Science Experiment on Venus Express

The Venus Express Radio Science Experiment VeRa retrieves atmospheric profiles in the mesosphere and troposphere of Venus in the approximate altitude range of 40–90 km. A data set of more than 500 profiles was retrieved between the orbit insertion of Venus Express in 2006 and the end of occultation season No. 11 in July 2011. The atmospheric profiles cover a wide range of latitudes and local times, enabling us to study the dependence of vertical small-scale temperature perturbations on local time and latitude.Temperature fluctuations with vertical wavelengths of 4 km or less are extracted from the measured temperature profiles in order to study small-scale gravity waves. Significant wave amplitudes are found in the stable atmosphere above the tropopause at roughly 60 km as compared with the only shallow temperature perturbations in the nearly adiabatic region of the adjacent middle cloud layer, below.Gravity wave activity shows a strong latitudinal dependence with the smallest wave amplitudes located in the low-latitude range, and an increase of wave activity with increasing latitude in both hemispheres; the greatest wave activity is found in the high-northern latitude range in the vicinity of Ishtar Terra, the highest topographical feature on Venus.We find evidence for a local time dependence of gravity wave activity in the low latitude range within ±30° of the equator. Gravity wave amplitudes are at their maximum beginning at noon and continuing into the early afternoon, indicating that convection in the lower atmosphere is a possible wave source.The comparison of the measured vertical wave structures with standard linear-wave theory allows us to derive rough estimates of the wave intrinsic frequency and horizontal wavelengths, assuming that the observed wave structures are the result of pure internal gravity waves. Horizontal wavelengths of the waves at 65 km altitude are on the order of ≈300–450 km with horizontal phase speeds of roughly 5–10 m/s.     

Titan atmosphere profiles from Huygens engineering (temperature and acceleration) sensors

Data from several Huygens probe housekeeping sensors (an engineering accelerometer and housekeeping temperature sensors) are studied to determine how effectively such nonideal instruments may characterize the density or temperature structure of the atmosphere. While only confirming the results of the dedicated atmospheric structure instrument, this exercise is of relevance to possible future missions to various bodies which might not be equipped with such science-grade sensors able to accurately profile the atmosphere top-to-bottom. It is found that for typical engineering accelerometers with 8-bit resolution, the atmosphere density for ∼4 scale heights above the peak deceleration altitude may be recovered. If, as with Huygens, the peak deceleration exceeds the range of the accelerometers, recovery of an additional scale height or so below the peak is still possible, but relies on accurate total velocity knowledge. Engineering temperature sensors can, with care, be analyzed to recover the temperature structure of at least the lowest ∼30 km of Titan's atmosphere. Fortunately, in the case of Huygens, data from the surface after landing were available to constrain models of heat leaks which offset the observed temperature from that of the ambient air during descent; data from before and during the entry phase on other missions would be similarly useful. When corrections are made for the estimated heat transfer processes, the atmospheric temperature can be recovered to within about 3 K.     

Radiative transfer analyses of Titan’s tropical atmosphere

Titan’s optical and near-IR spectra result primarily from the scattering of sunlight by haze and its absorption by methane. With a column abundance of 92 km amagat (11 times that of Earth), Titan’s atmosphere is optically thick and only ～10% of the incident solar radiation reaches the surface, compared to 57% on Earth. Such a formidable atmosphere obstructs investigations of the moon’s lower troposphere and surface, which are highly sensitive to the radiative transfer treatment of methane absorption and haze scattering. The absorption and scattering characteristics of Titan’s atmosphere have been constrained by the Huygens Probe Descent Imager/Spectral Radiometer (DISR) experiment for conditions at the probe landing site (Tomasko, M.G., Bézard, B., Doose, L., Engel, S., Karkoschka, E. [2008a]. Planet. Space Sci. 56, 624–247; Tomasko, M.G. et al. [2008b]. Planet. Space Sci. 56, 669–707). Cassini’s Visual and Infrared Mapping Spectrometer (VIMS) data indicate that the rest of the atmosphere (except for the polar regions) can be understood with small perturbations in the high haze structure determined at the landing site (Penteado, P.F., Griffith, C.A., Tomasko, M.G., Engel, S., See, C., Doose, L., Baines, K.H., Brown, R.H., Buratti, B.J., Clark, R., Nicholson, P., Sotin, C. [2010]. Icarus 206, 352–365). However the in situ measurements were analyzed with a doubling and adding radiative transfer calculation that differs considerably from the discrete ordinates codes used to interpret remote data from Cassini and ground-based measurements. In addition, the calibration of the VIMS data with respect to the DISR data has not yet been tested. Here, VIMS data of the probe landing site are analyzed with the DISR radiative transfer method and the faster discrete ordinates radiative transfer calculation; both models are consistent (to within 0.3%) and reproduce the scattering and absorption characteristics derived from in situ measurements. Constraints on the atmospheric opacity at wavelengths outside those measured by DISR, that is from 1.6 to 5.0 μm, are derived using clouds as diffuse reflectors in order to derive Titan’s surface albedo to within a few percent error and cloud altitudes to within 5 km error. VIMS spectra of Titan at 2.6–3.2 μm indicate not only spectral features due to CH₄ and CH₃D (Rannou, P., Cours, T., Le Mouélic, S., Rodriguez, S., Sotin, C., Drossart, P., Brown, R. [2010]. Icarus 208, 850–867), but also a fairly uniform absorption of unknown source, equivalent to the effects of a darkening of the haze to a single scattering albedo of 0.63 ± 0.05. Titan’s 4.8 μm spectrum point to a haze optical depth of 0.2 at that wavelength. Cloud spectra at 2 μm indicate that the far wings of the Voigt profile extend 460 cm^?1 from methane line centers. This paper releases the doubling and adding radiative transfer code developed by the DISR team, so that future studies of Titan’s atmosphere and surface are consistent with the findings by the Huygens Probe. We derive the surface albedo at eight spectral regions of the 8 × 12 km² area surrounding the Huygens landing site. Within the 0.4–1.6 μm spectral region our surface albedos match DISR measurements, indicating that DISR and VIMS measurements are consistently calibrated. These values together with albedos at longer 1.9–5.0 μm wavelengths, not sampled by DISR, resemble a dark version of the spectrum of Ganymede’s icy leading hemisphere. The eight surface albedos of the landing site are consistent with, but not deterministic of, exposed water ice with dark impurities.     

Circulation of the Venus upper mesosphere/lower thermosphere: Doppler wind measurements from 2001–2009 inferior conjunction,sub-millimeter CO absorption line observations

Sub-millimeter ¹²CO (346 GHz) and ¹³CO (330 GHz) line absorptions, formed within the mesospheric to lower thermospheric altitude (70–120 km) region of the Venus atmosphere, have been mapped across the nightside disk of Venus during 2001–2009 inferior conjunctions, employing the James Clerk Maxwell Telescope (JCMT). Radiative transfer analysis of these thermal line absorptions supports temperature and CO mixing profile retrievals, as described in a companion paper (Clancy et al., 2012). Here, we consider the analysis of the sharp line absorption cores of these CO spectra in terms of accurate Doppler wind profile measurements at 95–115 km altitudes versus local time (～8 pm–4 am) and latitude (～60N–60S). These Doppler wind measurements support determinations of the nightside zonal and subsolar-to-antisolar (SSAS) circulation components over a variety of timescales. The average behavior fitted from 21 retrieved maps of ¹²CO Doppler winds (obtained over hourly, daily, weekly, and interannual intervals) indicates stronger average zonal (85 m/s retrograde) versus SSAS (65 m/s) circulation at the 1 μbar pressure (108–110 km altitude) level. However, the absolute and relative magnitudes of these circulation components exhibit extreme variability over daily to weekly timescales. Furthermore, the individual Doppler wind measurements within each nightside mapping observation generally show significant deviations (20–50 m/s, averaged over 5000 km horizontal scales) from the simple zonal/SSAS solution, with distinct local time and latitudinal characters that are also time variable. These large scale residual circulations contribute 30–70% of the observed nightside Doppler winds at any given time, and may be most responsible for global variations in nightside lower thermospheric trace composition and temperatures, as coincidentally retrieved CO abundance and temperature distributions do not correlate with solution retrograde zonal and SSAS winds (see companion paper, Clancy et al., 2012). Limited comparisons of these nightside submillimeter results with dayside infrared Doppler wind measurements suggest distinct dayside versus nightside circulations, in terms of zonal winds in particular. Combined ¹²CO and ¹³CO Doppler wind mapping observations obtained since 2004 indicate that the average zonal and SSAS wind components increase by 50–100% between altitudes of 100 and 115 km. If gravity waves originating from the cloud levels are responsible for the extension of zonal winds into the thermosphere (Alexander, M.J. [1992]. Geophys. Res. Lett. 19, 2207–2210), such waves deposit substantial momentum (i.e., break) in the lower nightside thermosphere.     

Vertical profiling of SO2 and SO above Venus’ clouds by SPICAV/SOIR solar occultations

New measurements of sulfur dioxide (SO₂) and monoxide (SO) in the atmosphere of Venus by SPICAV/SOIR instrument onboard Venus Express orbiter provide ample statistics to study the behavior of these gases above Venus’ clouds. The instrument (a set of three spectrometers) is capable to sound atmospheric structure above the clouds in several observation modes (nadir, solar and stellar occultations) either in the UV or in the near IR spectral ranges. We present the results from solar occultations in the absorption ranges of SO₂ (190–230 nm, and at 4 μm) and SO (190–230 nm). The dioxide was detected by the SOIR spectrometer at the altitudes of 65–80 km in the IR and by the SPICAV spectrometer at 85–105 km in the UV. The monoxide’s absorption was measured only by SPICAV at 85–105 km. We analyzed 39 sessions of solar occultation, where boresights of both spectrometers are oriented identically, to provide complete vertical profiling of SO₂ of the Venus’ mesosphere (65–105 km). Here we report the first firm detection and measurements of two SO₂ layers. In the lower layer SO₂ mixing ratio is within 0.02–0.5 ppmv. The upper layer, also conceivable from microwave measurements by Sandor et al. (Sandor, B.J., Todd Clancy, R., Moriarty-Schieven, G., Mills, F.P. [2010]. Icarus 208, 49–60) is characterized by SO₂ increasing with the altitude from 0.05 to 2 ppmv, and the [SO₂]/[SO] ratio varying from 1 to 5. The presence of the high-altitude SO_x species could be explained by H₂SO₄ photodissociation under somewhat warmer temperature conditions in Venus mesosphere. At 90–100 km the content of the sulfur dioxide correlates with temperature increasing from 0.1 ppmv at 165–170 K to 0.5–1 ppmv at 190–192 K. It supports the hypothesis of SO₂ production by the evaporation of H₂SO₄ from droplets and its subsequent photolysis at around 100 km.     

Vertical pressure profile of Titan—observations of the PPI/HASI instrument

The Huygens entry probe descended through the atmosphere of Titan and provided an excellent set of observations of the atmosphere and the surface of Titan. During the 150-min descent the Huygens Atmospheric Structure Instrument (HASI) observed a comprehensive set of variables, including pressure, temperature, density and atmospheric electricity. The atmospheric pressure profile was recorded by the Pressure Profile Instrument (PPI), provided by Finnish Meteorological Institute (FMI). The instrument started measurements at an altitude of 150 km, and produced about 28 bits of data per second. Data were also obtained through the time of 31 min beyond the time of surface impact. The first-order scientific analysis of the PPI results has been performed. The observations together with hydrostatic assumption and in combination with other measurements have provided the first atmospheric pressure profile and the surface pressure (of approximately ) for Titan's atmosphere. To carry out the pressure profile reconstruction we developed a real gas formulation, which is applicable also for other Titan atmospheric investigations. The altitude versus time speed of the descent was calculated and the results were compared with the direct altitude observations by the radar altimeter during the last 40 km of the descent. The fit was excellent demonstrating the high-quality level of the PPI observations as well as the utilized investigation methods.     

Analytic theory of Titan’s Schumann resonance: Constraints on ionospheric conductivity and buried water ocean

This study presents an approximate model for the atypical Schumann resonance in Titan’s atmosphere that accounts for the observations of electromagnetic waves and the measurements of atmospheric conductivity performed with the Huygens Atmospheric Structure and Permittivity, Wave and Altimetry (HASI–PWA) instrumentation during the descent of the Huygens Probe through Titan’s atmosphere in January 2005. After many years of thorough analyses of the collected data, several arguments enable us to claim that the Extremely Low Frequency (ELF) wave observed at around 36 Hz displays all the characteristics of the second harmonic of a Schumann resonance. On Earth, this phenomenon is well known to be triggered by lightning activity. Given the lack of evidence of any thunderstorm activity on Titan, we proposed in early works a model based on an alternative powering mechanism involving the electric current sheets induced in Titan’s ionosphere by the Saturn’s magnetospheric plasma flow. The present study is a further step in improving the initial model and corroborating our preliminary assessments. We first develop an analytic theory of the guided modes that appear to be the most suitable for sustaining Schumann resonances in Titan’s atmosphere. We then introduce the characteristics of the Huygens electric field measurements in the equations, in order to constrain the physical parameters of the resonating cavity. The latter is assumed to be made of different structures distributed between an upper boundary, presumably made of a succession of thin ionized layers of stratospheric aerosols spread up to 150 km and a lower quasi-perfect conductive surface hidden beneath the non-conductive ground. The inner reflecting boundary is proposed to be a buried water–ammonia ocean lying at a likely depth of 55–80 km below a dielectric icy crust. Such estimate is found to comply with models suggesting that the internal heat could be transferred upwards by thermal conduction of the crust, while convective processes cannot be ruled out.     

Descent motions of the Huygens probe as measured by the Surface Science Package (SSP): Turbulent evidence for a cloud layer

The Huygens probe underwent vigorous short-period motions during its parachute descent through the atmosphere of Saturn's moon Titan in January 2005, at least some of which were excited by the Titan environment. Several sensors in the Huygens Surface Science Package (SSP) detect these motions, indicating the transition to the smaller stabilizer parachute, the changing probe spin rate, aerodynamic buffeting, and pendulum motions. Notably, in an altitude range of about 20–30 km where methane drops will freeze, the frequency content and statistical kurtosis of the tilt data indicate excitation by turbulent air motions like those observed in freezing clouds on Earth, supporting the suggestion of Tokano et al. [Tokano, T., McKay, C.P., Neubauer, F.M., Atreya, S.K., Ferri, F., Fulchignoni, M., Niemann, H.B. (2006a). Methane drizzle on Titan. Nature 442, 432–435] that the probe passed through such a cloud layer. Motions are weak below 20 km, suggesting a quiescent lower atmosphere with turbulent fluctuations of nominally <0.15 m/s (to within a factor of ∼2) but more violent motions in the upper troposphere may have been excited by turbulent winds with amplitudes of 1–2 m/s. Descent in part of the stratosphere (150–120 km) was smooth despite strong ambient wind (∼100 m/s), but known anomalies in the probe spin prevent investigation of turbulence in the known wind-shear layer from 60 to 100 km.     

Thermospheric/mesospheric temperatures on Venus: Results from ground-based high-resolution spectroscopy of CO2 in 1990/1991 and comparison to results from 2009 and between other techniques

We report temperatures in Venus’ upper mesosphere/lower thermosphere, deduced from reanalyzing very high resolution infrared spectroscopy of CO₂ emission lines acquired in 1990 and 1991. Kinetic temperatures at ～110 km altitude (0.15 Pa) are derived from the Doppler width of fully-resolved single line profiles measured near 10.4 μm wavelength using the NASA GSFC Infrared Heterodyne Spectrometer (IRHS) at the NASA IRTF on Mauna Kea, HI, close to Venus inferior conjunction and two Venus solstices. Measured temperatures range from ～200 to 240 K with uncertainty typically less than 10 K. Temperatures retrieved from similar measurement in 2009 using the Cologne Tuneable Heterodyne Infrared Spectrometer (THIS) at the NOAO McMath Telescope at Kitt Peak, AZ are 10–20 K lower. Temperatures retrieved more recently from the SOIR instrument on Venus EXpress are consistent with these results when the geometry of observation is accounted for. It is difficult to compare ground-based sub-mm retrievals extrapolated to 110 km due to their much larger field of view, which includes the night side regions not accessible to infrared heterodyne observations. Temperature variability appears to be high on day-to-day as well as longer timescales. Observed short term and long term variability may be attributed to atmospheric dynamics, diurnal variability and changes over solar activity and seasons. The Venus International Reference Atmosphere (VIRA) model predicts cooler temperatures at the sampled altitudes in the lower thermosphere/upper mesosphere and is not consistent with these measurements.     

Dynamical properties of the Venus mesosphere from the radio-occultation experiment VeRa onboard Venus Express

The dynamics of Venus’ mesosphere (60–100 km altitude) was investigated using data acquired by the radio-occultation experiment VeRa on board Venus Express. VeRa provides vertical profiles of density, temperature and pressure between 40 and 90 km of altitude with a vertical resolution of few hundred meters of both the Northern and Southern hemisphere. Pressure and temperature vertical profiles were used to derive zonal winds by applying an approximation of the Navier–Stokes equation, the cyclostrophic balance, which applies well on slowly rotating planets with fast zonal winds, like Venus and Titan. The main features of the retrieved winds are a midlatitude jet with a maximum speed up to 140 ± 15 m s^?1 which extends between 20°S and 50°S latitude at 70 km altitude and a decrease of wind speed with increasing height above the jet. Cyclostrophic winds show satisfactory agreement with the cloud-tracked winds derived from the Venus Monitoring Camera (VMC/VEx) UV images, although a disagreement is observed at the equator and near the pole due to the breakdown of the cyclostrophic approximation. Knowledge of both temperature and wind fields allowed us to study the stability of the atmosphere with respect to convection and turbulence. The Richardson number Ri was evaluated from zonal field of measured temperatures and thermal winds. The atmosphere is characterised by a low value of Richardson number from ～45 km up to ～60 km altitude at all latitudes that corresponds to the lower and middle cloud layer indicating an almost adiabatic atmosphere. A high value of Richardson number was found in the region of the midlatitude jet indicating a highly stable atmosphere. The necessary condition for barotropic instability was verified: it is satisfied on the poleward side of the midlatitude jet, indicating the possible presence of wave instability.     

Direct wind measurements from November 2007 in Venus’ upper atmosphere using ground-based heterodyne spectroscopy of CO2 at 10 μm wavelength

Between November 23 and 28, 2007, the Cologne Tuneable Heterodyne Infrared Spectrometer THIS was installed at the McMath-Pierce Solar Telescope (Kitt Peak, Arizona, USA) to determine zonal wind velocities and to estimate the subsolar-to-antisolar flow. We investigate dynamics in the upper atmosphere of Venus by measuring the Doppler shift of fully-resolved non-LTE CO₂ emission lines at 959.3917 cm^?1 (10.423 μm), which probe a narrow altitude region in Venus’ atmosphere around 110 ± 10 km (～1 μbar). The results show no significant zonal wind velocity at the equator. An increase with latitude up to 43 ± 13 m/s at a latitude of 33°N was observed. This confirms the deduction of a minor influence of Venus superrotation at an altitude of 110 km from previous measurements in May 2007 (Sornig et al., 2008). The specific observing geometry enables estimating the maximum cross terminator velocity of the subsolar-to-antisolar flow at 72 ± 47 m/s.     

Titan’s aerosol and stratospheric ice opacities between 18 and 500 μm: Vertical and spectral characteristics from Cassini CIRS

Vertical distributions and spectral characteristics of Titan’s photochemical aerosol and stratospheric ices are determined between 20 and 560 cm^?1 (500–18 μm) from the Cassini Composite Infrared Spectrometer (CIRS). Results are obtained for latitudes of 15°N, 15°S, and 58°S, where accurate temperature profiles can be independently determined.In addition, estimates of aerosol and ice abundances at 62°N relative to those at 15°S are derived. Aerosol abundances are comparable at the two latitudes, but stratospheric ices are ～3 times more abundant at 62°N than at 15°S. Generally, nitrile ice clouds (probably HCN and HC₃N), as inferred from a composite emission feature at ～160 cm^?1, appear to be located over a narrow altitude range in the stratosphere centered at ～90 km. Although most abundant at high northern latitudes, these nitrile ice clouds extend down through low latitudes and into mid southern latitudes, at least as far as 58°S.There is some evidence of a second ice cloud layer at ～60 km altitude at 58°S associated with an emission feature at ～80 cm^?1. We speculate that the identify of this cloud may be due to C₂H₆ ice, which in the vapor phase is the most abundant hydrocarbon (next to CH₄) in the stratosphere of Titan.Unlike the highly restricted range of altitudes (50–100 km) associated with organic condensate clouds, Titan’s photochemical aerosol appears to be well-mixed from the surface to the top of the stratosphere near an altitude of 300 km, and the spectral shape does not appear to change between 15°N and 58°S latitude. The ratio of aerosol-to-gas scale heights range from 1.3–2.4 at about 160 km to 1.1–1.4 at 300 km, although there is considerable variability with latitude. The aerosol exhibits a very broad emission feature peaking at ～140 cm^?1. Due to its extreme breadth and low wavenumber, we speculate that this feature may be caused by low-energy vibrations of two-dimensional lattice structures of large molecules. Examples of such molecules include polycyclic aromatic hydrocarbons (PAHs) and nitrogenated aromatics.Finally, volume extinction coefficients Nχ_E derived from 15°S CIRS data at a wavelength of λ = 62.5 μm are compared with those derived from the 10°S Huygens Descent Imager/Spectral Radiometer (DISR) data at 1.583 μm. This comparison yields volume extinction coefficient ratios Nχ_E(1.583 μm)/Nχ_E(62.5 μm) of roughly 70 and 20, respectively, for Titan’s aerosol and stratospheric ices. The inferred particle cross-section ratios χ_E(1.583 μm)/χ_E(62.5 μm) appear to be consistent with sub-micron size aerosol particles, and effective radii of only a few microns for stratospheric ice cloud particles.     

The abundance and vertical distribution of the unknown ultraviolet absorber in the venusian atmosphere from analysis of Venus Monitoring Camera images

Observations of Venus using the ultraviolet filter of the Venus Monitoring Camera (VMC) on ESA’s Venus Express Spacecraft (VEX) provide the best opportunity for study of the spatial and temporal distribution of the venusian unknown ultraviolet absorber since the Pioneer Venus (PV) mission. We compare the results of two sets of 125 radiative transfer models of the upper atmosphere of Venus to each pixel in a subset of VMC UV channel images. We use a quantitative best fit criterion based upon the notion that the distribution of the unknown absorber should be independent of the illumination and observing geometry. We use the product of the cosines of the incidence and emission angles and search for absorber distributions that are uncorrelated with this geometric parameter, finding that two models can describe the vertical distribution of the unknown absorber. One model is a well-mixed vertical profile above a pressure level of roughly 120 mb (～63 km). This is consistent with the altitude of photochemical formation of sulfuric acid. The second model describes it as a thin layer of pure UV absorber at a pressure level roughly around 24 mb (～71 km) and this altitude is consistent with the top of upper cloud deck. We find that the average abundance of unknown absorber in the equatorial region is 0.21 ± 0.04 optical depth and it decreases in the polar region to 0.08 ± 0.05 optical depth at 365 nm.     

Water vapor near the cloud tops of Venus from Venus Express/VIRTIS dayside data

Observations of the dayside of Venus performed by the high spectral resolution channel (–H) of the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on board the ESA Venus Express mission have been used to measure the altitude of the cloud tops and the water vapor abundance around this level with a spatial resolution ranging from 100 to 10 km. CO₂ and H₂O bands between 2.48 and 2.60 μm are analyzed to determine the cloud top altitude and water vapor abundance near this level. At low latitudes (±40°) mean water vapor abundance is equal to 3 ± 1 ppm and the corresponding cloud top altitude at 2.5 μm is equal to 69.5 ± 2 km. Poleward from middle latitudes the cloud top altitude gradually decreases down to 64 km, while the average H₂O abundance reaches its maximum of 5 ppm at 80° of latitude with a large scatter from 1 to 15 ppm. The calculated mass percentage of the sulfuric acid solution in cloud droplets of mode 2 (～1 μm) particles is in the range 75–83%, being in even more narrow interval of 80–83% in low latitudes. No systematic correlation of the dark UV markings with the cloud top altitude or water vapor has been observed.     

The Huygens Probe Descent Trajectory Working Group: Organizational framework,goals, and implementation

Cassini/Huygens, a flagship mission to explore the rings, atmosphere, magnetic field, and moons that make up the Saturn system, is a joint endeavor of the National Aeronautics and Space Administration, the European Space Agency, and Agenzia Spaziale Italiana. Comprising two spacecraft—a Saturn orbiter built by NASA and a Titan entry/descent probe built by the European Space Agency—Cassini/Huygens was launched in October 1997. The Huygens probe parachuted to the surface of Titan in January 2005. During the descent, six science instruments provided in situ measurements of Titan's atmosphere, clouds, and winds, and photographed Titan's surface. To correctly interpret and correlate results from the probe science experiments, and to provide a reference set of data for ground-truth calibration of orbiter remote sensing measurements, an accurate reconstruction of the probe entry and descent trajectory and surface landing location is necessary. The Huygens Descent Trajectory Working Group was chartered in 1996 as a subgroup of the Huygens Science Working Team to develop and implement an organizational framework and retrieval methodologies for the probe descent trajectory reconstruction from the entry altitude of 1270 km to the surface using navigation data, and engineering and science data acquired by the instruments on the Huygens Probe. This paper presents an overview of the Descent Trajectory Working Group, including the history, rationale, goals and objectives, organizational framework, rules and procedures, and implementation.