Ground-based Infrared Spectroscopy Research Articles

Wave signature in the Venus dayside cloud layer at 58–64 km observed by ground-based infrared spectroscopy

We performed infrared spectroscopic measurements of the Venus’ dayside to find out atmospheric wave structures at 58–64km in the cloud layer by quantifying carbon dioxide absorption. This is the first investigation of atmospheric waves at this altitude region. This effective absorption altitude lies in between that of 70km by the ultraviolet measurements and of 50km by the infrared thermal emission measurements. The apparent rotation periods of wave-like signature found are 3.5, 4.9 and 8.4days, respectively, in May 2007, November 2007 and August 2010 assuming dominance of wavenumber one structure. Those apparent periods may be interpreted as superposition of the mean zonal flow and waves such as the Kelvin and the Rossby.

FORMATION OF ORGANIC MOLECULES AND WATER IN WARM DISK ATMOSPHERES

Observations from Spitzer and ground-based infrared spectroscopy reveal significant diversity in the molecular emission from the inner few AU of T Tauri disks. We explore theoretically the possible origin of this diversity by expanding on our earlier thermal-chemical model of disk atmospheres. We consider how variations in grain settling, X-ray irradiation, accretion-related mechanical heating, and the oxygen-to-carbon ratio can affect the thermal and chemical properties of the atmosphere at 0.25-40 AU. We find that these model parameters can account for many properties of the detected molecular emission. The column density of the warm (200-2000K) molecular atmosphere is sensitive to grain settling and the efficiency of accretion-related heating, which may account, at least in part, for the large range in molecular emission fluxes that have been observed. The dependence of the atmospheric properties on the model parameters may also help to explain trends that have been reported in the literature between molecular emission strength and mid-infrared color, stellar accretion rate, and disk mass. We discuss whether some of the differences between our model results and the observations (e.g., for water) indicate a role for vertical transport and freeze-out in the disk midplane. We also discuss how planetesimal formation in the outer disk (beyond the snowline) may imprint a chemical signature on the inner few AU of the disk and speculate on possible observational tracers of this process.

Spatially-resolved high-resolution spectroscopy of Venus 2. Variations of HDO, OCS, and SO 2 at the cloud tops

While CO, HCl, and HF, that were considered in the first part of this work, have distinct absorption lines in high-resolution spectra and were detected four decades ago, the lines of HDO, OCS, and SO 2 are either very weak or blended by the telluric lines and have not been observed previously by ground-based infrared spectroscopy at the Venus cloud tops. The H 2O abundance above the Venus clouds is typically below the detection limit of ground-based IR spectroscopy. However, the large D/ H ratio on Venus facilitates observations of HDO. Converted to H 2O with D/ H ≈ 200, our observations at 2722 cm −1 in the Venus afternoon show a H 2O mixing ratio of ∼1.2 ppm at latitudes between ±40° increasing to ±60° by a factor of 2. The observations in the early morning reveal the H 2O mixing ratio that is almost constant at 2.9 ppm within latitudes of ±75°. The measured H 2O mixing ratios refer to 74 km. The observed increase in H 2O is explained by the lack of photochemical production of sulfuric acid in the night time. The recent observations at the P-branch of OCS at 4094 cm −1 confirm our detection of OCS. Four distributions of OCS along the disk of Venus at various latitudes and local times have been retrieved. Both regular and irregular components are present in the variations of OCS. The observed OCS mixing ratio at 65 km varies from ∼0.3 to 9 ppb with the mean value of ∼3 ppb. The OCS scale height is retrieved from the observed limb darkening and varies from 1 to 4 km with a mean value of half the atmospheric scale height. SO 2 at the cloud tops has been detected for the first time by means of ground-based infrared spectroscopy. The SO 2 lines look irregular in the observed spectra at 2476 cm −1. The SO 2 abundances are retrieved by fitting by synthetic spectra, and two methods have been applied to determine uncertainties and detection limits in this fitting. The retrieved mean SO 2 mixing ratio of 350 ± 50 ppb at 72 km favors a significant increase in SO 2 above the clouds since the period of 1980–1995 that was observed by the SOIR occultations at Venus Express. Scale heights of OCS and SO 2 may be similar, and the SO 2/OCS ratio is ∼500 and may be rather stable at 65–70 km under varying conditions on Venus.

DO WE REALLY KNOW THE DUST? SYSTEMATICS AND UNCERTAINTIES OF THE MID-INFRARED SPECTRAL ANALYSIS METHODS

The spectral region around 10 micrometer, showing prominent dust emission bands, is commonly used to derive the chemical composition of protoplanetary dust. Different analysis methods have been proposed for this purpose, but so far, no comparative test has been performed to test their validity. We calculated model spectra of disk models with different geometries and central sources, using a 2D radiative transfer code. These spectra were then fitted in a blind test using four different spectral decomposition methods. We studied the effect of disk structure (flared vs. flat), inclination angle, size of the inner disk hole and stellar luminosity on the fitted chemical composition. Our results show that the derived dust compositions by all methods deviate systematically from the real chemical composition. Out of the four tested spectral decomposition methods, our new two-layer temperature distribution method, differs the least from the input dust composition and the results show the weakest systematic effects. The reason for the deviations of the results given by other methods lies in their simplifying assumptions (e.g. single average grain temperature or one component continuum). We also tested the influence of different noise levels on the results of the spectral decomposition methods. We find that, for ground-based observations (8-13 micrometer) the expected uncertainty in the value of the crystallinity is about 11% for a signal-to-noise ratio of 100, while for space-based observations (7-17 micrometer) the uncertainty is about 5%. On the basis of our results, we propose a recipe for the analysis and interpretation of dust spectroscopy data in the mid-infrared which should be especially valuable for analysing Spitzer data and ground-based infrared spectroscopy data in the 10 micrometer window.

Hydrogen peroxide on Mars: evidence for spatial and seasonal variations

Hydrogen peroxide (H 2O 2) has been suggested as a possible oxidizer of the martian surface. Photochemical models predict a mean column density in the range of 10 15–10 16 cm −2. However, a stringent upper limit of the H 2O 2 abundance on Mars (9×10 14 cm −2) was derived in February 2001 from ground-based infrared spectroscopy, at a time corresponding to a maximum water vapor abundance in the northern summer (30 pr. μm, Ls=112°). Here we report the detection of H 2O 2 on Mars in June 2003, and its mapping over the martian disk using the same technique, during the southern spring ( Ls=206°) when the global water vapor abundance was ∼10 pr. μm. The spatial distribution of H 2O 2 shows a maximum in the morning around the sub-solar latitude. The mean H 2O 2 column density (6×10 15 cm −2) is significantly greater than our previous upper limit, pointing to seasonal variations. Our new result is globally consistent with the predictions of photochemical models, and also with submillimeter ground-based measurements obtained in September 2003 ( Ls=254°), averaged over the martian disk (Clancy et al., 2004, Icarus 168, 116–121).

A determination of albedos and surface temperatures on Mars from ground-based infrared spectroscopy

Surface temperatures and albedos of the Martian disk have been measured from ground-based infrared spectroscopy, between September and December 1988. As a result, the measured surface temperatures seem to be higher than the Viking temperatures measured in 1977, and closer to the theoretical temperatures calculated from the Viking 1976 data (primary mission). This suggests that the Martian dust activity in 1988, at the time of opposition, was probably less intense than in 1977.