A comparison of measurements of O2(1delta) airglow from SPICAM instrument on Mars Express with calculations from GM3
GM3 is a multiscale global Mars general circulation model with a vertical domain from the surface to about 160 km. It has a water cycle that includes covered ice cap and regolith as well as bulk ice clouds. The model also includes a COx, Ox, and HOx chemistry of the Martian atmosphere, with a focus on airglow as a means of evaluating the chemistry and dynamics of the model. In this case we focus on the O2(1delta) airglow and its sources, photolysis of O3 and O self recombination together with its sinks of emission and quenching. We find large values of O2(1delta) from O recombination in the polar night with the descent of O rich air from the thermosphere. Our model simulation of O2(1delta) is compared with available measurement from the Spectroscopy for Investigation of Characteristics of the Atmosphere of Mars (SPICAM) instrument onboard the Mars Express orbiter.
Simulations of the Martian Airglow and the Recovery of Temperature Profiles From Limb Airglow Observations
Although Mars is our neighbour, the distance between our planets is such that most observation of Mars is done remotely. This limits the amount and types of observations possible. It is thus imperative to develop new techniques to retrieve more and new types of information from observational data. In this work a new method for retrieving temperature and hydroxyl volume emission rates from the Martian atmosphere will be presented. A non linear optimal estimation global fitting method is used to retrieve the profiles from simulated nocturnal airglow measurements. The simulated data is for a limb viewing instrument. The profiles are determined by minimizing the difference between the generated spectrum and the simulated measurements using the Marquardt Levenberg iteration method. The method fits all data from a full simulated scan of the atmosphere rather than one level at a time which greatly reduces the effect of atmospheric interference and instrument noise on the results. A two stage fitting method is needed to retrieve both the volume emission rate and temperature profiles as the fitting method cannot resolve the two simultaneously. Since the retrieval of the hydroxyl emission rates has little dependence on the accuracy of the temperature profile, it is possible to fit first the spectrum for the volume emission rate using an estimated temperature profile. These retrieved rates can then be used to refine the estimate of the temperature profile by using the same fitting method to retrieve temperatures. The two stage fitting method allows the algorithm to be very flexible as it does not require an atmospheric model to retrieve the profiles. This flexibility could be exploited to apply this methodology to the Venetian atmosphere where nighttime hydroxyl airglow has already been detected1. 1G. Piccioni et al. First detection of hydroxyl in the atmosphere of Venus. Astronomy and Astrophysics 483 (3) L29-L33 (2008).
Minor gaseous constituents and aerosols of the Venus mesosphere measured by SPICAV/SOIR on board Venus Express
The SOIR instrument performs solar occultation measurements in the IR region (2.2 - 4.3 μm) at a resolution of 0.12 cm-1, the highest on board Venus Express. It combines an echelle spectrometer and an AOTF (Acousto-Optical Tunable Filter) for the order selection. A brief description of the instrument and its in- flight measured performances will be presented. The wavelength range probed by SOIR allows a detailed chemical inventory of the Venus atmosphere above the cloud layer (65 to 150 km) with an emphasis on vertical distribution of the gases. In particular, measurements of HDO, H2O, HCl, HF, CO and CO2 vertical profiles have been routinely performed. Temperature retrieval will be described and tentative results presented. Aerosols extinction profiles are also simultaneously retrieved from the SOIR spectra. Some results for selected orbits will be investigated and discussed. It will be shown that size distribution can be addressed but only when considering all three channels of the SPICAV/SOIR instrument.
Studying the Mars atmosphere using a SOIR Instrument
SOIR (Solar Occultation InfraRed spectrometer) is currently part of the SPICAV/SOIR instrument on board the Venus Express orbiter (VEX). SOIR, an Echelle infrared spectrometer using an acousto-optic tunable filter (AOTF) for the order selection, is probing the atmosphere by solar occultation, operating between 2.2 and 4.3 μm, with a resolution of 0.15 cm-1. This spectral range is suitable for the detection of several key components of planetary atmospheres, including H2O and its isotopologue HDO, CH4 and other trace species. The SOIR instrument was designed to have a minimum of moving parts, to be light and compact in order to fit on top of the SPICAV instrument. The AOTF allows a narrow range of wavelengths to pass, according to the radio frequency applied to the TeO2 crystal; this selects the order. The advantage of the AOTF is that different orders can be observed quickly and easily during one occultation. To obtain a compact optical scheme, a Littrow configuration was implemented in which the usual collimating and imaging lenses are merged into a single off-axis parabolic mirror. The light is diffracted on the echelle grating, where orders overlap and addition occurs, and finally is recorded by the detector. The detector is 320x256 pixels and is cooled to 88K during an occultation measurement, to maximise the signal to noise ratio. SOIR on VEX has been in orbit around Venus since April 2006, allowing us to characterise the instrument and study its performance. These data have allowed the engineering team to devise several instrumental improvements. The next step in further improving the readiness for Martian atmospheric studies comes in close collaboration with the Mars Atmospheric Modelling group at BIRA-IASB. A General Circulation Model is used to simulate the Martian atmosphere. Currently work is underway with SPICAM data to verify the GCM inputs and outputs. Later the GCM output will be used as feedback for instrumental design of both an improved version of SOIR and the UVIS instrument for the ExoMars mission. We will show Mars data as could be observed by a SOIR instrument to demonstrate what SOIR would be capable of in Mars orbit.
Modelling of the Chemistry of Sulfur Oxides in the Middle Atmosphere of Venus
Venus' middle atmosphere (∼ 60-110 km) is a dynamic region in which photochemistry dominates and the time scales for chemical loss and transport are roughly comparable for many species. It is also a region where it has been difficult to observe the abundances of species that play important roles in two of the dominant chemical cycles on Venus. The CO2 cycle comprises photodissociation of CO2 to produce CO and O, transport of some CO and O to the night side, production of O2 from 2O+M→O2+M on the day and night sides, and production of CO2 from CO and O2. The sulfur oxidation cycle comprises oxidation of SO2 to form H2SO4, condensation, subsidence of some particles to the lower atmosphere, evaporation, and thermal decomposition or photodissociation to produce SO2 and H2O. Recent mesospheric observations have provided clear evidence of diurnal variability in the abundances of sulfur oxides. Observed SO has its peak abundance on the day side and observed SO2 has its peak abundance on the night side (Sandor et al, 2008). We have used global average model calculations (Pernice et al, 2004; Mills and Allen, 2007) to derive approximate analytic expressions for [SO], [SO2], and [SO]/[SO2] on the day and night sides. The results are generally consistent across a broad range of atmospheric oxidation states (Mills and Allen, 2007). Our model results and the key uncertainties will be discussed. A related topic is the identity of the UV-blue absorber that is responsible for the absorption observed in the upper cloud layer (∼ 60-70 km) at 320-500 nm. One proposed suggestion is S2O (Hapke and Graham, 1985; Na and Esposito, 1997). Our model results for S2O and their implications will be discussed and compared with previous work.
The Sensitivity of Venus' Oxygen Budget to Venus' Mesospheric Temperature
The dominant chemical cycle in Venus' mesosphere above the clouds (70-110 km altitude) is the CO2 cycle. The primary steps of this cycle are photodissociation of CO2 to produce CO and O on the day side, transport of CO and O from the day side to the night side, formation of O2 on the day and night sides, and production of CO2 from CO and O2. Many photochemical models have attempted to identify the mechanisms by which CO2 is produced, but none has satisfactorily reproduced the observational upper limit on the O2 abundance (Trauger and Lunine 1983, Krasnopolsky 2006). In these models (Yung and DeMore 1982, Krasnopolsky and Parshev 1983, Pernice et al 2004) the assumed mesospheric vertical pressure and temperature profiles were derived from Pioneer Venus data. However, recent SPICAV observations (Bertaux et al. 2007) indicate mesospheric temperatures at 110 km may be up to 50 K warmer than the standard values adopted from the Pioneer Venus data. The CO2 cross section is sensitive to temperature, so an increase in temperature in the upper part of the mesosphere will increase photodissociation in the upper part of the mesosphere and decrease photodissociation at lower altitudes. These changes should, in turn, affect the abundances and vertical profiles of CO, O2 and O. We have developed a simplified version of the Caltech/JPL photochemical model (Allen et al. 1981) which limits the mesospheric chemistry solely to carbon and oxygen species. Using temperature dependent CO2 cross-section data in this model, we will investigate the impact of temperature on the vertical profile of CO2 photodissociation and the calculated abundances of CO, O, and O2. Two sets of temperature dependent CO2 cross section data a) Lewis and Carver 1983, and b) Yoshino et al. 1996; Parkinson et al 2003 will be utilized in this study. The sensitivity of the model results to differences between the two sets of cross section measurements will be quantified.
Night OH in the mesosphere of Venus and Earth: A comparative planetology perspective.
Satellite measurements of the terrestrial nightside mesosphere from the MLS/Aura MLS instrument show a layer of OH near 82 km. This layer confirms earlier measurements by ground-based UVFTS. The MLS and UVFTS observations measure OH in the lowest vibrational state and are distinct, but related chemically, from vibrationally-excited emission from the OH Meinel bands in the near infrared. The Caltech 1-D KINETICS model has been extended to include vibrational dependence of OH reactions and shows good agreement with MLS OH data and with observations of the Meinel bands . The model shows a chemical lifetime of HOx that increases from less than a day at 80 km to over a month at 87 km. Above this altitude transport processes become an im-portant part of HOx chemistry. The model predicts that ground state OH represents 99% of the total OH up to 84 km. Similarly, Venus airglow emissions detected at wavelengths of 1.40 to 1.49 and 2.6 to 3.14 μm in limb observations by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on the Venus Express spacecraft are attributed to the OH Meinel band transitions as well . The integrated emission rates for the OH Meinel bands were measured to be 100±40 and 880±90 kR respectively, both peaking at an altitude of 96±2 km near midnight local time for the considered orbit. We use the same Caltech 1-D KINETICS model to model these observations for Venus as was used for the Earth  and discuss the conclusions from a comparative planetology perspec-tive, highlighting the similarities and differences between Venus and Earth. References:  Pickett H. M., Read W. G., Lee K. K. and Yung Y. L. (2006) GRL, 33, L19808.  Piccioni G., Drossart P., Zasova L., Migliorini A., Gérard J.-C., Mills F. P., Shakun A., Garcia Munoz A., Ignatiev N., Grassi D., Cottini V., Taylor F. W., Erard S., and the VIRTIS-Venus Express Technical Team (2008) A and A., 483, L29-L33.
Recent Results on the Titan Ionosphere from Cassini Radio Occultations
Previous Cassini radio occultations, which provided measurements of the vertical electron density profiles in the Titan ionosphere, were conducted on March 26 and May 28, 2006, and March 19 and May 20, 2007 ( Kliore,A.J., et al. (2008), J. Geophys. Res., 113, A09317, doi:10.1029/2007JA012965) The 3006 observations probed the Titan ionosphere at low-to mid-Southern latitudes, while the 2007 measurements were made at North and South polar latitudes. In all cases the main ionization peak was observed near 1,200 km. altitude. The peak density of the low-latitude observations ranged from about 1,400 /cc near the dawn terminator to 1,800 /cc on the dusk side. At polar latitudes, the peak densities from the March 19 observations were about 1,300 /cc, but 2,800 /cc in the May 19 measurements,which also showed large lower peaks (1,200 -2,900 /cc) at about 500 km. During the ongoing Cassini Extended Mission, two more occultations on Nov. 5, 2008 and April 4, 3009 provided data at northern m-d-latitudes and the equatorial region, as well as diverse magnetospheric ram angles different from those of the previous measurements, which elucidates the role of the Saturn magnetosphere in producing the Titan ionosphere. This work was performed at the jet Propulsion Laboratory, California Institute of Technology, and at the University of Michigan under NASA contracts.