Error analysis for retrieval of Venus' IR surface emissivity from VIRTIS/VEX measurements

Venus' surface emissivity data in the infrared can serve to explore the planet's geology. The only global data with high spectral, spatial, and temporal resolution and coverage at present is supplied by nightside emission measurements acquired by the Visible and InfraRed Thermal Imaging Spectrometer VIRTIS-M-IR (1.0-5.1 μm) aboard ESA's Venus Express. A radiative transfer simulation and a retrieval algorithm can be used to determine surface emissivity in the nightside spectral transparency windows located at 1.02, 1.10, and 1.18 μm. To obtain satisfactory fits to measured spectra, the retrieval pipeline also determines auxiliary parameters describing cloud properties from a certain spectral range. But spectral information content is limited, and emissivity is difficult to retrieve due to strong interferences from other parameters. Based on a selection of representative synthetic VIRTIS-M-IR spectra in the range 1.0-2.3 μm, this paper investigates emissivity retrieval errors that can be caused by interferences of atmospheric and surface parameters, by measurement noise, and by a priori data, and which retrieval pipeline leads to minimal errors. Retrieval of emissivity from a single spectrum is shown to fail due to extremely large errors, although the fits to the reference spectra are very good. Neglecting geologic activity, it is suggested to apply a multi-spectrum retrieval technique to retrieve emissivity relative to an initial value as a parameter that is common to several measured spectra that cover the same surface bin. Retrieved emissivity maps of targets with limited extension (a few thousand km) are then additively renormalized to remove spatially large scale deviations from the true emissivity map that are due to spatially slowly varying interfering parameters. Corresponding multi-spectrum retrieval errors are estimated by a statistical scaling of the single-spectrum retrieval errors and are listed for 25 measurement repetitions. For the best of the studied retrieval pipelines, temporally varying interfering atmospheric parameters (cloud parameters, minor gas abundances) contribute errors in the order of 3%-10% of the true emissivity, depending on the surface window, the reference spectrum, and assuming statistical independence of the parameters. Temporally constant interfering parameters that spatially vary on a scale of 100 km (surface elevation, interfering emissivities) add 9%-16%. Measurement noise with a standard deviation of 10e-4 W/(m2 sr μm) leads to additional 1%-4%. Reasonable modifications of a priori mean values have negligible impacts. Retrieved maps are most reliable at 1.02 μm. There is an overall tendency for better results for cases with small cloud opacity, high surface elevation, high emissivity, and small observation angle, but this depends on the emissivity window, retrieval pipeline, and measurement repetition number. Calibration, preprocessing, and simulation errors can lead to additional errors. Based on the presented results, a subsequent paper will discuss emissivity data retrieval for a selected surface target

Multi-spectrum retrieval of Venus IR surface emissivity maps from VIRTIS/VEX nightside measurements at Themis Regio

Surface emissivity maps in the infrared can contribute to explore Venus’ geology. Nightside radiance spectra at Themis Regio acquired by the IR mapping channel of the Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS-M-IR) aboard Venus EXpress (VEX) are used to derive emissivity data from the three accessible spectral surface windows at 1.02, 1.10, and 1.18 μm. The measured spectra are simulated by applying a full radiative transfer model. Neglecting geologic activity, a multi-spectrum retrieval algorithm is utilized to determine the emissivity maps of the surface target as parameter vectors that are common to many spectrally resolved images that cover this target. Absolute emissivity values are difficult to obtain due to strong interferences from other parameters. The true emissivity mean of the target cannot be retrieved, nor can the emissivity mean of a retrieved map be strictly preset. The retrieved map can exhibit trends with latitude and topography that are probably artificial. Once the trends have been removed in a post-processing step, it can be observed that the magnitude of the resulting spatial emissivity fluctuations around their mean value increases with increasing mean value. A linear transformation is applied that converts the de-trended map to exhibit a defined emissivity mean value called reference emissivity, here 0.5, yielding the ‘renormalized emissivity map’ with accordingly transformed fluctuations. It is verified that renormalized emissivity maps are largely independent of the emissivity mean before renormalization, of modifications to interfering atmospheric, surface, and instrumental parameters, and of selected details of the retrieval pipeline and data calibration and preprocessing. Extremely large emissivity retrieval errors due to imperfect or unconsidered forward model parameters are effectively avoided. If the absolute emissivity at a given bin of the target were known, the absolute emissivity map of the entire target could be computed according to the mentioned transformation, assuming absent true trends with latitude and topography. Until then, the renormalized emissivities are interpreted as spatial variations relative to the reference emissivity. They represent an important step toward the retrieval of absolute emissivities. Renormalized emissivity maps of Themis Regio at the three surface windows are determined from 64 measurement repetitions. Retrieval errors are estimated by a statistical evaluation of maps derived from various disjoint selections of spectra and using different assumptions on the interfering parameters. Double standard deviation errors for the three surface windows amount to 3%, 8%, and 4%, respectively, allowing geologic interpretation. A comparison to results from an earlier error analysis based on synthetic spectra shows that unconsidered time variations of interfering atmospheric parameters are a major error source. Spatial variations of the 1.02 μm surface emissivity of 20% that correspond to the difference between unweathered granitic and basaltic rocks would be easily detectable, but such variations are ruled out for the studied target area. Emissivity anomalies of up to 8% are detected at both 1.02 and 1.18 μm. At present sensitivity, no anomalies are identified at 1.10 μm, but anomalies exceeding the determined error level can be excluded. With single standard deviation significance, all three maps show interesting spatial emissivity variations

Lower atmosphere minor gas abundances as retrieved from Venus Express VIRTIS-M-IR data at 2.3 µm

Minor gas abundances in the lower atmosphere of Venus׳ southern hemisphere are investigated using spectroscopic nightside measurements recorded by the Visible and InfraRed Thermal Imaging Spectrometer aboard ESA’s Venus Express mission in the moderate spectral resolution infrared mapping channel (VIRTIS-M-IR, 1–5 µm, FWHM=17 nm). The entire usable data archive is utilized including only radiation spectra sampled at long detector exposure times (≥3.3 s) during eight Venus solar days between April 2006 and October 2008. Combined radiative transfer and retrieval techniques are applied for a simultaneous determination of total cloud opacity and H2O, CO, and OCS abundances from the 2.3 µm atmospheric transparency window that sounds the altitude range between about 30 and 45 km. A wavelength-dependent CO2 opacity correction is considered. Zonal averages of CO abundances at 35 km increase by about 35% from (22.9±0.8) ppmv at equatorial latitudes to (31.0±2.1) ppmv at 65 °S and then decrease to (29.4±2.4) ppmv at 80 °S The±figures refer to the statistical variability of retrieved abundances. In accordance with earlier results, the observed latitudinal variation of tropospheric CO is consistent with a Hadley cell-like circulation. Dawn side CO abundances at high latitudes are slightly smaller than dusk side values by about 7%. The latitudinal distribution of OCS at 35 km is anticorrelated with that of CO, ranging from about (1.15±0.2) ppmv at 65 °S to (1.60±0.2) ppmv at low latitudes (poleward decrease of 28%). Zonal averages of H2O abundances near 35 km slightly decrease toward the South Pole by about 10%, and the hemispheric average is (32.0±1.3) ppmv. A significant local time dependence of OCS and H2O is not observed. Detailed analyses of individual spectrum retrieval errors for different atmospheric models reveal that CO abundance results are reliable (error 4–7%), while H2O and OCS results have lower confidence (errors 30–47% and 41–86%, respectively). SO2 abundances cannot reliably be retrieved from VIRTIS-M-IR spectra

Surface emissivity retrieval from VIRTIS/VEX data in the Quetzalpetlatl quadrangle on Venus based on the new MSR multi-spectrum retrieval technique

Surface emissivity is difficult and error-prone to retrieve from VIRTIS measurements of Venus’ nightside. A detailed radiative transfer forward model simulation is used to generate synthetic spectra for given atmospheric and surface parameters. The new MSR multi-spectrum retrieval technique is applied to retrieve atmospheric and surface parameters that allow the synthetic spectra to fit the measurements. The incorporation of expected spatial-temporal correlations between parameters describing a selection of contiguous measurements leads to much more reliable parameters, as does the retrieval of surface emissivity of a surface bin as a parameter that is common to measurements that repeatedly cover that bin, thereby neglecting geologic activity. The method is applied to Quetzalpetlatl quadrangle including the Lada Terra rise and the Quetzalpetlatl corona. This area combines corona-dominated rises, rifted volcanic rises, and large coronae structures. Retrieved emissivity at 1.02 μm is related to regional geologic units

The second Venus flyby of BepiColombo mission reveals stable atmosphere over decades

Studies of the Venusian mesosphere provide important information about the current state of the entire Venusian atmosphere. This includes information about the dense cloud structure, its vertical thermal profile, temperature fields, and the resulting dynamical and meteorological processes that contribute to a deeper understanding of the climatologically different evolutionary paths of Earth and Venus. However, the last measurements were acquired in 1983 during Venera-15 mission. In this paper, results of mid-infrared spectral measurements of the Venusian atmosphere are presented. Here we show Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) measurements of the Venusian atmosphere during the second flyby of BepiColombo mission on its way to Mercury. Our Venus measurements provide reliable retrievals of mesospheric temperature profiles and cloud parameters between 60 and 75 km altitude, although MERTIS was only designed to operate in Mercury environment. Our results are in good agreement with the Venera-15 mission findings. This indicates the stability of the Venusian atmosphere on time scales of decades

Sensitivity of net thermal flux to the abundance of trace gases in the lower atmosphere of Venus

We calculated the net thermal flux in the atmosphere of Venus from the surface to 100 km altitude. Our atmospheric model was carefully constructed especially for altitudes below the clouds (2 absorption data. It includes updated collision-induced absorptions in the -1, 1200-1500 cm-1, and 2650-3130 cm-1 wave number ranges. We studied sensitivity of the net thermal flux below the clouds on the abundances of trace gases that were varied within the range reported by observations. Our results reveal a considerable effect of trace gases on radiative budget. We successfully simulate net thermal flux profiles measured in situ by the Night and North probes of Pioneer Venus using 20-50 ppmv H2O, suggesting that the high H2O abundance of 200 ppmv derived in the earlier analysis is not required. Our sensitivity study shows that the trace gases SO2, H2O, and OCS are effective thermal agents, while CO and HCl influences are rather weak. We suggest that the influence of the former three gases should be taken into account to estimate the net radiative energy in the deep atmosphere

Analyses of spectroscopic and atmospheric parameter influences on radiative heating and cooling rates in the middle and lower atmosphere of Venus

Radiative fluxes and temperature change rates in the middle and lower atmosphere of Venus (0-100 km) are calculated over the broad spectral range 0.125- 1000 μm applying a radiative transfer model. Responses of these quantities to both spectroscopic model parameter changes and atmospheric parameter variations are examined in great detail. A new model for the unknown UV absorber is proposed. The calculated radiative cooling/heating rates are very reliable at altitudes below 95/85 km at fixed atmospheric conditions with maximum uncertainties of about 0.25 K/day. Heating uncertainties may reach 3-5 K/day at 100 km. Cooling rates strongly respond to variations of atmospheric thermal structure, while heating rates are less sensitive. Except for episodic SO2 boosts, the influence of mesospheric minor gas abundance variations is rather small, but variations of cloud mode parameters may significantly alter radiative temperature change rates up to 50% in Venus’ lower mesosphere and upper troposphere

Retrieval of surface emissivity from VIRTIS/VEX radiation measurements over the Northern hemisphere of Venus

The radiation measurements of VIRTIS-M-IR (1-5 micron) on Venus Express provide a valuable database for systematic studies of the atmosphere and surface of Venus. The high variability of nightside atmospheric and surface emission window radiances with respect to cloud opacity and surface elevation is modeled and discussed in comparison with measurements performed over the Northern hemisphere. The radiative transfer simulations for a quantitative evaluation of atmospheric and surface parameters use the DISORT code and include absorption, emission, and multiple scattering by atmospheric gaseous and particulate constituents. Look-up tables of quasi-monochromatic absorption cross-sections of gaseous constituents are calculated on the basis of a line-by-line procedure that makes use of appropriate spectral line databases. Microphysical parameters of the four-mode H2SO4 clouds are calculated from Mie theory. Deep atmospheric window continuum absorption is estimated from simultaneous retrievals of many spectra that were recorded for a high variety of atmospheric and surface conditions. Due to the conservative scattering behavior of the clouds below 1.5 micron, radiance ratios of the emission windows, which are located between 1.10 and 1.35 micron, and the most topography sensitive window at 1.02 micron may serve to de-cloud the measurement data and yield a quick-look tool to extract the surface elevation. While the ratio-based VIRTIS topography is in good agreement with Magellan topography for most swaths, some differences occur in localized areas. Possible origins of such anomalies are discussed. They can be due to local surface or near surface temperature fluctuations, local unstable atmospheric dynamics, or still unknown spectral cloud features. Some of the predicted anomaly candidates were found to originate from measurement or calibration uncertainties, but some other point to variations of surface emissivity. Such variations are due to changes in the chemical composition (mineralogy) and/or surface texture. They are therefore important indicators of the nature of the surface material. Radiance retrievals along a number of complete Northern orbits reveal systematic trends towards lower values of highland surface emissivity compared with surrounding lowlands. This result is in accordance with increased Magellan radar reflectivities at high altitudes and supports the hypothesis of older highland regions with felsic rock components

Atmospheric thermal structure and cloud features in the southern hemisphere of Venus as retrieved from VIRTIS/VEX radiation measurements

Thermal structure and cloud features in the atmosphere of Venus are investigated using spectroscopic nightside measurements recorded by the Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS) aboard ESA’s Venus Express mission in the moderate resolution infrared mapping channel (M-IR, 1–5 μm). New methodical approaches and retrieval results for the northern hemisphere have been recently described by Haus et al. (Haus, R., Kappel, D., Arnold, G. [2013]. Planet. Space Sci. 89, 77–101. http://dx.doi.org/10.1016/j.pss.2013.09.020). Now, southern hemisphere maps of mesospheric temperature and cloud parameter fields are presented that cover variations with altitude, latitude, local time, and mission time. Measurements from the entire usable data archive are utilized comprising radiation spectra recorded during eight Venus solar days between April 2006 and October 2008. Zonal averages of retrieved temperature altitude profiles in both hemispheres are very similar and give evidence of global N–S axial symmetry of atmospheric temperature structure. Cold collar and warmer polar vortex regions exhibit the strongest temperature variability with standard deviations up to 8.5 K at 75°S and 63 km altitude compared with about 1.0 K at low and mid latitudes above 75 km. The mesospheric temperature field strongly depends on local time. At altitudes above about 75 km, the atmosphere is warmer in the second half of night, while the dawn side at lower altitudes is usually colder than the dusk side by about 8 K. Local minimum temperature of 220 K occurs at 03:00 h local time at 65 km and 60°S. Temperature standard deviation at polar latitudes is particularly large near midnight. Temperature variability with solar longitude is forced by solar thermal tides with a dominating diurnal component. The influence of observed cloud parameter changes on retrieved mesospheric zonal average temperature structure is moderate and does not exceed 2–3 K at altitudes between 60 and 75 km. The mesospheric thermal structure was essentially stable with Julian date between 2006 and 2008. Global N–S axial symmetry is also observed in cloud structures. Cloud top altitude at 1 μm slowly decreases from 71 km at the equator to 70 km at 45–50° and rapidly drops poleward of 50°. It reaches 61 km over both poles. Average particle size in the vertical cloud column increases from mid latitudes toward the poles and also toward the equator resulting in minimum and maximum zonal average cloud opacities of about 32 and 42 and a planetary average of 36.5 at 1 μm. Zonal averages of cloud features are similar at different solar days, but variations with local time are very complex and inseparably associated with the superrotation of the clouds

Construction of global maps of atmospheric and surface features of Venus based on new retrieval methods

The exploration of Venus in the context of comparative planetology and solar system research is an important key to understand crucial aspects of planetary evolution, geology, and climate. Sufficient information can only be gained by applying a long-term remote sensing observation strategy. Early missions to Venus established some basic information about atmospheric and surface features, but only since ESA’s Venus Express (VEX) mission is orbiting the planet, the first global database for systematic atmospheric and surface studies became available. It brings Venus back into the focus of exploration of the terrestrial planets after a period of more than 20 years. The Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on Venus Express, after six years in a polar Venus orbit, provided an enormous amount of new data and a four-dimensional picture of the planet (2D imaging + spectral dimension + temporal variations). The spectral dimension permits a sounding at different levels of the atmosphere from the ground up to the thermosphere. The planned work focuses on the investigation of temperature fields, cloud composition and altitude distribution, and trace gas concentrations in the atmosphere of Venus. Studies will be mainly performed on the nightside of the planet where the narrow atmospheric window emissions are not obscured by the more intense solar radiation reflected by the clouds. The resulting multi-dimensional maps of atmospheric state parameters will be used to calculate atmospheric net fluxes, heating and cooling rates, and the radiative energy balance of the middle and lower atmosphere of Venus, and to produce required input data for global circulation models. The quantification and elimination of atmospheric impact factors on surface emissivity retrievals are additional important components of this work. The construction of emissivity maps and specification of local emissivity variations will allow of acquire clues on different soil compositions that enable statements about the geologic development of the planet. Recently by the authors newly developed and verified radiative transfer models and special algorithms, which simultaneously use information from different atmospheric windows for each individual spectrum (multiwindow application), can be improved to a large extent by adaptation of new multi-spectrum retrieval techniques (multi-spectrum application) and by the utilization of all available a priori information on surface and atmospheric parameters. In combination with new developments for sophisticated data calibration and pre-processing of VIRTIS-M-IR data this will seriously enhance the accuracy of retrieved atmospheric and surface parameters. The paper will discuss the capability of the new multi-spectrum retrieval technique as well as the main scientific objectives of the planned work on global atmospheric and surface features of Venus