Combining visible and infrared radiometry and lidar data to test simulations in clear and ice cloud conditions

Measurements taken during the 2003 Pacific THORPEX Observing System Test (P-TOST) by the MODIS Airborne Simulator (MAS), the Scanning High-resolution Interferometer Sounder (S-HIS) and the Cloud Physics Lidar (CPL) are compared to simulations performed with a line-by-line and multiple scattering modeling methodology (LBLMS). Formerly used for infrared hyper-spectral data analysis, LBLMS has been extended to the visible and near infrared with the inclusion of surface bi-directional reflectance properties. A number of scenes are evaluated: two clear scenes, one with nadir geometry and one cross-track encompassing sun glint, and three cloudy scenes, all with nadir geometry. <br><br> CPL data is used to estimate the particulate optical depth at 532 nm for the clear and cloudy scenes and cloud upper and lower boundaries. Cloud optical depth is retrieved from S-HIS infrared window radiances, and it agrees with CPL values, to within natural variability. MAS data are simulated convolving high resolution radiances. The paper discusses the results of the comparisons for the clear and cloudy cases. LBLMS clear simulations agree with MAS data to within 20% in the shortwave (SW) and near infrared (NIR) spectrum and within 2 K in the infrared (IR) range. It is shown that cloudy sky simulations using cloud parameters retrieved from IR radiances systematically underestimate the measured radiance in the SW and NIR by nearly 50%, although the IR retrieved optical thickness agree with same measured by CPL. <br><br> MODIS radiances measured from Terra are also compared to LBLMS simulations in cloudy conditions, using retrieved cloud optical depth and effective radius from MODIS, to understand the origin for the observed discrepancies. It is shown that the simulations agree, to within natural variability, with measurements in selected MODIS SW bands. <br><br> The impact of the assumed particles size distribution and vertical profile of ice content on results is evaluated. Sensitivity is much smaller than differences between measured and simulated radiances in the SW and NIR. <br><br> The paper dwells on a possible explanation of these contradictory results, involving the phase function of ice particles in the shortwave

Titan's atmosphere as observed by Cassini/VIMS solar occultations: CH4_4, CO and evidence for C2_2H6_6 absorption

We present an analysis of the VIMS solar occultations dataset, which allows us to extract vertically resolved information on the characteristics of Titan's atmosphere between 100-700 km with a characteristic vertical resolution of 10 km. After a series of data treatment procedures, 4 occultations out of 10 are retained. This sample covers different seasons and latitudes of Titan. The transmittances show clearly the evolution of the haze and detect the detached layer at 310 km in Sept. 2011 at mid-northern latitudes. Through the inversion of the transmission spectra with a line-by-line radiative transfer code we retrieve the vertical distribution of CH$_4$ and CO mixing ratio. The two methane bands at 1.4 and 1.7 {\mu}m are always in good agreement and yield an average stratospheric abundance of $1.28\pm0.08$%. This is significantly less than the value of 1.48% obtained by the GCMS/Huygens instrument. The analysis of the residual spectra after the inversion shows that there are additional absorptions which affect a great part of the VIMS wavelength range. We attribute many of these additional bands to gaseous ethane, whose near-infrared spectrum is not well modeled yet. Ethane contributes significantly to the strong absorption between 3.2-3.5 {\mu}m that was previously attributed only to C-H stretching bands from aerosols. Ethane bands may affect the surface windows too, especially at 2.7 {\mu}m. Other residual bands are generated by stretching modes of C-H, C-C and C-N bonds. In addition to the C-H stretch from aliphatic hydrocarbons at 3.4 {\mu}m, we detect a strong and narrow absorption at 3.28 {\mu}m which we tentatively attribute to the presence of PAHs in the stratosphere. C-C and C-N stretching bands are possibly present between 4.3-4.5 {\mu}m. Finally, we obtain the CO mixing ratio between 70-170 km. The average result of $46\pm16$ ppm is in good agreement with previous studies.Comment: 51 pages, 28 figure

Solar Carbon Monoxide, Thermal Profiling, and the Abundances of C, O, and their Isotopes

A solar photospheric "thermal profiling" analysis is presented, exploiting the infrared rovibrational bands of carbon monoxide (CO) as observed with the McMath-Pierce Fourier transform spectrometer (FTS) at Kitt Peak, and from above the Earth's atmosphere by the Shuttle-borne ATMOS experiment. Visible continuum intensities and center-limb behavior constrained the temperature profile of the deep photosphere, while CO center-limb behavior defined the thermal structure at higher altitudes. The oxygen abundance was self consistently determined from weak CO absorptions. Our analysis was meant to complement recent studies based on 3-D convection models which, among other things, have revised the historical solar oxygen (and carbon) abundance downward by a factor of nearly two; although in fact our conclusions do not support such a revision. Based on various considerations, an oxygen abundance of 700+/-100 ppm (parts per million relative to hydrogen) is recommended; the large uncertainty reflects the model sensitivity of CO. New solar isotopic ratios also are reported for 13C, 17O, and 18O.Comment: 90 pages, 19 figures (some with parts "a", "b", etc.); to be published in the Astrophysical Journal Supplement

Resolving the Surfaces of Extrasolar Planets With Secondary Eclipse Light Curves

We present a method that employs the secondary eclipse light curves of transiting extrasolar planets to probe the spatial variation of their thermal emission. This technique permits an observer to resolve the surface of the planet without the need to spatially resolve its central star. We evaluate the feasibility of this technique for the HD 209458 system [..]. We consider two representations of the planetary thermal emission; a simple model parameterized by a sinusoidal dependence on longitude and latitude, as well as the results of a three-dimensional dynamical simulation of the planetary atmosphere previously published by Cooper & Showman. We find that observations of the secondary eclipse light curve are most sensitive to a longitudinal offset in the geometric and photometric centroids of the hemisphere of the planet visible near opposition. To quantify this signal, we define a new parameter, the ``uniform time offset,'' which measures the time lag between the observed secondary eclipse and that predicted by a planet with a uniform surface flux distribution. We compare the predicted amplitude of this parameter for HD 209458 with the precision with which it could be measured with IRAC. We find that IRAC observations at 3.6um a single secondary eclipse should permit sufficient precision to confirm or reject the Cooper & Showman model of the surface flux distribution for this planet. We quantify the signal-to-noise ratio for this offset in the remaining IRAC bands (4.5um, 5.8um, and 8.0um), and find that a modest improvement in photometric precision (as might be realized through observations of several eclipse events) should permit a similarly robust detection.Comment: AASTeX 5.2, 24 pages, 5 figures, accepted for publication in ApJ; v2: clarifications, updated to version accepted by ApJ; v3: try to reduce spacin

CLOUDS search for variability in brown dwarf atmospheres

Context: L-type ultra-cool dwarfs and brown dwarfs have cloudy atmospheres that could host weather-like phenomena. The detection of photometric or spectral variability would provide insight into unresolved atmospheric heterogeneities, such as holes in a global cloud deck. Aims: It has been proposed that growth of heterogeneities in the global cloud deck may account for the L- to T-type transition as brown dwarf photospheres evolve from cloudy to clear conditions. Such a mechanism is compatible with variability. We searched for variability in the spectra of five L6 to T6 brown dwarfs in order to test this hypothesis. Methods: We obtained spectroscopic time series using VLT/ISAAC, over 0.99-1.13um, and IRTF/SpeX for two of our targets, in J, H and K bands. We search for statistically variable lines and correlation between those. Results: High spectral-frequency variations are seen in some objects, but these detections are marginal and need to be confirmed. We find no evidence for large amplitude variations in spectral morphology and we place firm upper limits of 2 to 3% on broad-band variability, on the time scale of a few hours. The T2 transition brown dwarf SDSS J1254-0122 shows numerous variable features, but a secure variability diagnosis would require further observations. Conclusions: Assuming that any variability arises from the rotation of patterns of large-scale clear and cloudy regions across the surface, we find that the typical physical scale of cloud cover disruption should be smaller than 5-8% of the disk area for four of our targets. The possible variations seen in SDSS J1254-0122 are not strong enough to allow us to confirm the cloud breaking hypothesis.Comment: 17 pages, 14 figures, accepted by A&

Water in HD 209458b's atmosphere from 3.6 - 8 microns IRAC photometric observations in primary transit

The hot Jupiter HD 209458b was observed during primary transit at 3.6, 4.5, 5.8 and 8.0 microns using the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. We detail here the procedures we adopted to correct for the systematic trends present in the IRAC data. The light curves were fitted including limb darkening effects and fitted using Markov Chain Monte Carlo and prayer-bead Monte Carlo techniques, finding almost identical results. The final depth measurements obtained by a combined Markov Chain Monte Carlo fit are at 3.6 microns, 1.469 +- 0.013 % and 1.448 +- 0.013 %; at 4.5 microns, 1.478 +- 0.017 % ; at 5.8 microns, 1.549 +- 0.015 % and at 8.0 microns 1.535 +- 0.011 %. Our results clearly indicate the presence of water in the planetary atmosphere. Our broad band photometric measurements with IRAC prevent us from determining the additional presence of other other molecules such as CO, CO2 and methane for which spectroscopy is needed. While water vapour with a mixing ratio of 10^-4-10^-3 combined with thermal profiles retrieved from the day-side may provide a very good fit to our observations, this data set alone is unable to resolve completely the degeneracy between water abundance and atmospheric thermal profile.Comment: 14 pages, 6 tables, 10 figures, Accepted for publication in MNRA

Cloud Atlas: Rotational Spectral Modulations and potential Sulfide Clouds in the Planetary-mass, Late T-type Companion Ross 458C

Measurements of photometric variability at different wavelengths provide insights into the vertical cloud structure of brown dwarfs and planetary-mass objects. In seven Hubble Space Telescope consecutive orbits, spanning $\sim$10 h of observing time}, we obtained time-resolved spectroscopy of the planetary-mass T8-dwarf Ross 458C using the near-infrared Wide Field Camera 3. We found spectrophotometric variability with a peak-to-peak signal of 2.62$\pm$0.02 % (in the 1.10-1.60~$\mu$m white light curve). Using three different methods, we estimated a rotational period of 6.75$\pm$1.58~h for the white light curve, and similar periods for narrow $J$- and $H$- band light curves. Sine wave fits to the narrow $J$- and $H$-band light curves suggest a tentative phase shift between the light curves with wavelength when we allow different periods between both light curves. If confirmed, this phase shift may be similar to the phase shift detected earlier for the T6.5 spectral type 2MASS J22282889-310262. We find that, in contrast with 2M2228, the variability of Ross~458C shows evidence for a {color trend} within the narrow $J$-band, but gray variations in the narrow $H$-band. The spectral time-resolved variability of Ross 458C might be potentially due to heterogeneous sulfide clouds in the atmosphere of the object. Our discovery extends the study of spectral modulations of condensate clouds to the coolest T dwarfs, planetary-mass companions.Comment: Accepted in ApJ

Venus: interaction surface/atmosphere in the framework of EnVision mission

Tese de Mestrado, Física (Astrofísica e Cosmologia), 2022, Universidade de Lisboa, Faculdade de CiênciasVénus é o planeta do Sistema Solar mais parecido com a Terra dimensionalmente, apresentando valores de massa e densidade muito parecidos, mas as semelhanças terminam aí. Vénus apresenta uma atmosfera extremamente densa estando a sua superfície completamente oculta por uma espessa camada de nuvens de ácido sulfúrico. As condições à superfície são hostis à vida como a conhecemos apresentando uma temperatura de cerca de 700 K e uma pressão 92 vezes mais elevada que à superfície da Terra. Vénus foi visitado por várias missões espaciais ao longo dos anos tendo sido as mais recentes as missões Venus Express da ESA e a missão japonesa Akatsuki cuja contribuição para o conhecimento da atmosfera de Vénus continua a ser relevante. A principal ferramenta utilizada neste trabalho foi o Planetary Spectrum Generator (PSG) Villanueva et al. 2018 [43]. Trata-se de uma ferramenta que simula modelos de transferência radiativa para sintetizar espectros planetários. Considera vários parâmetros para reproduzir condições reais relacionadas com geometria do objecto, atmosfera e a sua composição, e ainda parâmetros do instrumento de observação, seja ele já existente, ou com características novas. O PSG baseia a sua simulação de linhas espectrais em repositórios espectroscópicos como o HITRAN (Gordon et al. 2022 [15]) sendo capaz de simular uma elevada quantidade de linhas espectrais. No estudo de Irwin et al.2008, [21] usaram-se dados da VIRTIS-M, instrumento a bordo da Venus Express, com o objetivo de estudar a variabilidade de monóxido de carbono (CO) da banda (1-0) em torno dos 4.7 µm na mesosfera, logo acima da camada de nuvens, em diferentes latitudes do planeta. No entanto, devido a uma inversão de temperatura nas latitudes polares, no espectro de observações feitas a estas latitudes, as linhas espectrais de absorção do CO estavam escondidas pela inversão. O aumento de radiância no contínuo do espectro, tornou impossível constranger a abundância de CO para estas latitudes. Sugerem que um estudo com uma maior resolução seria necessário para obter um valor de abundância fiável para as latitudes polares. Numa primeira fase deste trabalho, para levar a cabo o estudo da abundância de CO a diferentes latitudes, recorremos ao PSG, e a dados do instrumento VIRTIS-H, cuja resolução é maior que a VIRTISM. Considerando sets de dados equivalentes aos usados em Irwin et al. 2008 [21] mas para a VIRTIS-H, usamos dois espectros, um para o perfil nominal a latitudes médias, e outro com a inversão de temperatura a altas latitudes. A simulação do espectro para o perfil nominal pelo PSG resultou num contínuo muito elevado e as linhas de absorção do CO em torno dos 4.7 µm eram demasiado profundas quando comparadas com as obtidas com os dados da VIRTIS-H. Estas características mostraram que a temperatura da zona da atmosfera era mais elevada do que a retirada pelos dados, uma vez que a radiância do contínuo era mais elevada que o esperado. A maior profundidade das linhas de CO mostrava que o PSG estava a considerar uma abundância de CO maior do que aquela apresentada pelos dados. Consequentemente a simulação foi ajustada de forma a determinar a que temperatura se encontravam os dados. Considerando que a camada de nuvens poderia ser aproximada a um corpo negro, calculou-se a radiância para dois comprimentos de onda distintos no espetro. Partiu-se de uma temperatura de 235 K, para avaliar se a diferença entre os valores de radiância era igual ao apresentado no espetro. Não apresentando esta o melhor ajuste do contínuo, foram feitas várias tentativas em torno do valor base de 235 K para encontrar a temperatura que melhor se ajustava aos dados. Uma vez encontrada a melhor aproximação para a temperatura, procurouse a abundância de CO que melhor se ajustava com as linhas de absorção dos dados. Para o perfil nominal obtivemos uma abundância de CO na mesosfera de 30 ppm. Para a simulação do espetro da inversão, reproduziu-se a inversão de temperatura apresentada pelos dados. Como ponto de partida, usou-se o modelo construído para o perfil nominal. Uma vez construído o novo perfil da temperatura para a inversão a partir do fit dos espectros, procuramos os valores referentes para abundância de CO para altas latitudes, resultando numa abundância de cerca de 50 ppm. No entanto, verificámos que a simulação ainda apresentava uma baixa sensibilidade para o CO devido à inversão de temperatura. Na segunda parte deste trabalho, e tirando partido das potencialidades do PSG, exploramos a possibilidade de detecção de possíveis plumas vulcânicas que possam vir a ser detectadas com o instrumento VenSpec-H a bordo da missão EnVision. Várias novas missões estão a ser preparadas para estudar Vénus sob uma nova luz e tecnologia. Uma dessas missões é a missão espacial EnVision da ESA. Sendo que as preparações para uma missão espacial começam muito antes do lançamento desta, apesar da missão só estar prevista ser lançada daqui a uma década, os preparativos e simulações já estão a ser levadas a cabo. Um dos principais objetivos da missão EnVision é entender se Vénus ainda é ativo geologicamente, e, sendo, quais as características dessa atividade. Várias pistas de atividade vulcânica em Vénus foram aparecendo ao longo dos anos, seja a evolução de espécies químicas na atmosfera do planeta (H2O e SO2), assinaturas de pontos quentes a baixas altitudes, ou características da superfície que aparentam origem vulcânica (coronas). No entanto, nenhuma destas pistas confirmou a deteção direta de atividade vulcânica ativa. Havendo atividade geológica nos dias de hoje, a missão EnVision estará preparada para detetar estas erupções. Começou por se construir um modelo no PSG com as características esperadas das observações com a VenSpec-H com uma resolução R=20000 em torno de duas bandas com comprimentos de onda entre 2340- 2420 nm e 2450-2480 nm. Neste espectro determinou-se quais as espécies químicas responsáveis pelas linhas espectrais de absorção, e o comportamento de absorção destas nas bandas em questão. Considerouse o CO e devidos isótopos, H2O, HDO e SO2, para preparar a simulação de possíveis plumas vulcânicas. Uma vez identificadas as linhas de absorção no espectro, procurou-se identificar as camadas da atmosferas mais sensíveis em termos de radiância quando alterado o volume mixing ratio (VMR) em 10% para cada espécie química. Estas seriam as camadas nas quais um aumento de VMR, devido a uma pluma vulcânica, seriam mais facilmente detectados no espectro. Identificadas as camadas mais sensíveis, procedemos à simulação de plumas de H2O e SO2, aumentando o VMR para as camadas mais sensíveis em 30% para o caso da H2O e em 1% para o caso do SO2. Para o caso da H2O houve uma possível detecção que resultou em linhas espectrais de absorção mais profundas do que as previstas pelo modelo de referência. No caso de SO2 não houve qualquer deteção, nem quando se aumentou o VMR em 10%. Numa tentativa de melhorar as nossas simulações, recorremos a dados da VIRTIS-H para comparar com os modelos construídos no PSG. Estes dados foram retirados de Marcq et al. 2008 [28] para o lado noturno. Quando comparados dados e simulação, os valores de radiância revelaram-se bastante distintos. Para tentar melhorar o ajuste testaram-se diferentes valores de temperatura para uma nova camada base da atmosfera. Não apresentando resultados relevantes, procedeu-se à alteração da escala dos aerossóis vulcânicos no modelo do PSG, que melhorou o nosso modelo. Procuramos então, identificar as camadas da atmosferas mais sensíveis a uma mudança nos aerossóis vulcânicos, e voltaram-se a simular as plumas de H2O e SO2 considerando um valor dos aerossóis vulcânicos igual a metade do original. Verificou-se que para a banda considerada, esta mudança não causou qualquer alteração na simulação das plumas de H2O e SO2, mantendo-se os resultados.We have used infrared spectra of the dark side of Venus, recorded by the VIRTIS-H spectrometer [9] aboard Venus Express [37], to analyze the CO (1-0) band around 4.7 µm. The resolving power of VIRTIS-H (about 1200) is sufficient to separate the individual lines of CO. We have selected two sets of spectra, the first one at mid-latitude (43°S) and the other in the polar collar (69-83°S). The CO individual lines appear in absorption in the first case, and in emission in the second case, as a consequence of a temperature inversion occurring at high latitude at the level of the upper cloud top. Synthetic models have been calculated using the Planetary Spectrum Generator (PSG) [43]. This work illustrates the capabilities of high-resolution infrared spectroscopy for monitoring minor atmospheric species in the mesosphere of Venus, in the perspective of the EnVision mission [13]. We have used PSG to simulate observations of VenSpec-H [20], a spectrometer aboard the future EnVision mission. We created reference models on PSG for bands 2#a and 2#b (2340-2420 and 2450-2480 nm) for the day side of Venus, and studied the chemical origin of their absorption lines, in order to prepare simulations of tentative volcanic plumes of H2O, and SO2. The results of these plume simulations are presented here. This work prepares for high-resolution observations of possible active volcanic plumes on Venus