M. Turbet, H. Tran, O. Pirali, F. Forget, C. Boulet, and J.-M. Hartmann. Far infrared measurements of absorptions by CH4 + CO2 and H2 + CO2 mixtures and implications for greenhouse warming on early Mars. Icarus, 321:189-199, 2019. [ bib | DOI | arXiv | ADS link ]
We present an experimental study of the absorption, between 40 and 640 cm-1, by CO2, CH4 and H2 gases as well as by H2 + CO2 and CH4 + CO2 mixtures at room temperature. A Fourier transform spectrometer associated to a multi-pass cell, whose optics were adjusted to obtain a 152 m path length, were used to record transmission spectra at total pressures up to about 0.98 bar. These measurements provide information concerning the collision-induced absorption (CIA) bands as well as about the wing of the CO2 15 μm band. Our results for the CIAs of pure gases are, within uncertainties, in agreement with previous determinations, validating our experimental and data analysis procedures. We then consider the CIAs by H2 + CO2 and CH4 + CO2 and the low frequency wing of the pure CO2 15 μm band, for which there are, to our knowledge, no previous measurements. We confirm experimentally the theoretical prediction of Wordsworth et al. (2017) that the H2 + CO2 and CH4 + CO2 CIAs are significantly stronger in the 50-550 cm-1 region than those of H2 + N2 and CH4 + N2, respectively. However, we find that the shape and the strength of these recorded CIAs differ from the aforementioned predictions. For the pure CO2 line-wings, we show that both the χ-factor deduced from measurements near 4 μm and a line-mixing model very well describe the observed strongly sub-Lorentzian behavior in the 500-600 cm-1 region. These experimental results open renewed perspectives for studies of the past climate of Mars and extrasolar analogues.
T. Encrenaz, T. K. Greathouse, E. Marcq, H. Sagawa, T. Widemann, B. Bézard, T. Fouchet, F. Lefèvre, S. Lebonnois, S. K. Atreya, Y. J. Lee, R. Giles, and S. Watanabe. HDO and SO2 thermal mapping on Venus. IV. Statistical analysis of the SO2 plumes. Astronomy Astrophysics, 623:A70, 2019. [ bib | DOI | ADS link ]
Since January 2012 we have been monitoring the behavior of sulfur dioxide and water on Venus, using the Texas Echelon Cross-Echelle Spectrograph (TEXES) imaging spectrometer at the NASA InfraRed Telescope Facility (IRTF, Mauna Kea Observatory). We present here the observations obtained between January 2016 and September 2018. As in the case of our previous runs, data were recorded around 1345 cm-1 (7.4 μm). The molecules SO2, CO2, and HDO (used as a proxy for H2O) were observed, and the cloudtop of Venus was probed at an altitude of about 64 km. The volume mixing ratio of SO2 was estimated using the SO2/CO2 line depth ratios of weak transitions; the H2O volume mixing ratio was derived from the HDO/CO2 line depth ratio, assuming a D/H ratio of 200 times the Vienna Standard Mean Ocean Water (VSMOW). As reported in our previous analyses, the SO2 mixing ratio shows strong variations with time and also over the disk, showing evidence of the formation of SO2 plumes with a lifetime of a few hours; in contrast, the H2O abundance is remarkably uniform over the disk and shows moderate variations as a function of time. We performed a statistical analysis of the behavior of the SO2 plumes, using all TEXES data between 2012 and 2018. They appear mostly located around the equator. Their distribution as a function of local time seems to show a depletion around noon; we do not have enough data to confirm this feature definitely. The distribution of SO2 plumes as a function of longitude shows no clear feature, apart from a possible depletion around 100E-150E and around 300E-360E. There seems to be a tendency for the H2O volume mixing ratio to decrease after 2016, and for the SO2 mixing ratio to increase after 2014. However, we see no clear anti-correlation between the SO2 and H2O abundances at the cloudtop, neither on the individual maps nor over the long term. Finally, there is a good agreement between the TEXES results and those obtained in the UV range (SPICAV/Venus Express and UVI/Akatsuki) at a slightly higher altitude. This agreement shows that SO2 observations obtained in the thermal infrared can be used to extend the local time coverage of the SO2 measurements obtained in the UV range.
Y. Nishikawa, P. Lognonné, T. Kawamura, A. Spiga, E. Stutzmann, M. Schimmel, T. Bertrand, F. Forget, and K. Kurita. Mars' Background Free Oscillations. Space Science Reviews, 215:13, 2019. [ bib | DOI | ADS link ]
Observations and inversion of the eigenfrequencies of free oscillations constitute powerful tools to investigate the internal structure of a planet. On Mars, such free oscillations can be excited by atmospheric pressure and wind stresses from the Martian atmosphere, analogous to what occurs on Earth. Over long periods and on a global scale, this phenomenon may continuously excite Mars' background free oscillations (MBFs), which constitute the so-called Martian hum. However, the source exciting MBFs is related both to the global-scale atmospheric circulation on Mars and to the variations in pressure and wind at the planetary boundary layer, for which no data are available.
To overcome this drawback, we focus herein on a global-scale source and use results of simulations based on General Circular Models (GCMs). GCMs can predict and reproduce long-term, global-scale Martian pressure and wind variations and suggest that, contrary to what happens on Earth, daily correlations in the Martian hum might be generated by the solar-driven GCM. After recalling the excitation terms, we calculate MBFs by using GCM computations and estimate the contribution to the hum made by the global atmospheric circulation. Although we work at the lower limit of MBF signals, the results indicate that the signal is likely to be periodic, which would allow us to use more efficient stacking theories than can be applied to Earth's hum. We conclude by discussing the perspectives for the InSight SEIS instrument to detect the Martian hum. The amplitude of the MBF signal is on the order of nanogals and is therefore hidden by instrumental and thermal noise, which implies that, provided the predicted daily coherence in hum excitation is present, the InSight SEIS seismometer should be capable of
F. Ferri, Ö. Karatekin, S. R. Lewis, F. Forget, A. Aboudan, G. Colombatti, C. Bettanini, S. Debei, B. Van Hove, V. Dehant, A.-M. Harri, M. Leese, T. Mäkinen, E. Millour, I. Muller-Wodarg, G. G. Ori, A. Pacifici, S. Paris, M. Patel, M. Schoenenberger, J. Herath, T. Siili, A. Spiga, T. Tokano, M. Towner, P. Withers, S. Asmar, and D. Plettemeier. ExoMars Atmospheric Mars Entry and Landing Investigations and Analysis (AMELIA). Space Science Reviews, 215:8, 2019. [ bib | DOI | ADS link ]
The entry, descent and landing of Schiaparelli, the ExoMars Entry, descent and landing Demonstrator Module (EDM), offered a rare (once-per-mission) opportunity for in situ investigations of the martian environment over a wide altitude range. The aim of the ExoMars AMELIA experiment was to exploit the Entry, Descent and Landing System (EDLS) engineering measurements for scientific investigations of Mars' atmosphere and surface. Here we present the simulations, modelling and the planned investigations prior to the Entry, Descent and Landing (EDL) event that took place on 19th October 2016. Despite the unfortunate conclusion of the Schiaparelli mission, flight data recorded during the entry and the descent until the loss of signal, have been recovered. These flight data, although limited and affected by transmission interruptions and malfunctions, are essential for investigating the anomaly and validating the EDL operation, but can also contribute towards the partial achievement of AMELIA science objectives.
O. L. White, J. M. Moore, A. D. Howard, W. B. McKinnon, J. T. Keane, K. N. Singer, T. Bertrand, S. J. Robbins, P. M. Schenk, B. Schmitt, B. J. Buratti, S. A. Stern, K. Ennico, C. B. Olkin, H. A. Weaver, L. A. Young, G. New Horizons Geology, and Imaging Theme Team. Washboard and fluted terrains on Pluto as evidence for ancient glaciation. Nature Astronomy, 3:62-68, 2019. [ bib | DOI | ADS link ]
Distinctive landscapes termed `washboard' and `fluted' terrains1,2, which border the N2 ice plains of Sputnik Planitia along its northwest margin, are among the most enigmatic landforms yet seen on Pluto. These terrains consist of parallel to sub-parallel ridges that display a remarkably consistent east-northeast-west-southwest orientationa configuration that does not readily point to a simple analogous terrestrial or planetary process or landform. Here, we report on mapping and analysis of their morphometry and distribution as a means to determine their origin. Based on their occurrence in generally low-elevation, low-relief settings adjacent to Sputnik Planitia that coincide with a major tectonic system, and through comparison with fields of sublimation pits seen in southern Sputnik Planitia, we conclude that washboard and fluted terrains represent crustal debris that were buoyant in pitted glacial N2 ice that formerly covered this area, and which were deposited after the N2 ice receded via sublimation. Crater surface age estimates indicate that this N2 ice glaciation formed and disappeared early in Pluto's history, soon after formation of the Sputnik Planitia basin. These terrains constitute an entirely new category of glacial landform.
I. Ordonez-Etxeberria, R. Hueso, A. Sánchez-Lavega, E. Millour, and F. Forget. Meteorological pressure at Gale crater from a comparison of REMS/MSL data and MCD modelling: Effect of dust storms. Icarus, 317:591-609, 2019. [ bib | DOI | ADS link ]
We examine the record of atmospheric pressure in Gale crater measured in-situ by the Rover Environmental Monitoring Station (REMS) instrument (Gómez-Elvira et al., 2012) on the Mars Science Laboratory (MSL) rover over two Martian years. We compare the data with pressure predictions from the Mars Climate Database (MCD) (Forget et al., 1999; Millour et al., 2015) version 5.2, which is a climatological database derived from numerical simulations of the Martian atmosphere produced by a General Circulation Model run over several Martian years. Seasonal and daily trends in pressure data from REMS are well reproduced by the standard climatology of the MCD using its high resolution mode. This high-resolution mode extrapolates pressure values from the nominal model into the altitude of each location using a high-resolution topography model and a fine tuning of the vertical scale height that was chosen to mimic effects of slope winds not directly accounted for in the General Circulation Model on which the MCD is based. Differences between the synthetic MCD pressure data and the REMS measurements are produced by meteorological features that are identified on particular groups of sols and quantified in intensity. We show that regional dust storms outside Gale crater and dust abundance at the crater are important factors in the behaviour of the pressure exciting larger amplitudes on the daily pressure variations and causing most of the largest REMS-MCD differences. We compare the pressure signals with published data of the dust optical depth obtained by the REMS ultraviolet photodiodes and the Mastcam instrument on MSL, and with orbital images of the planet acquired by the MARCI instrument on the Mars Reconnaissance Orbiter (MRO). We show that in some cases regional dust storms induce a characteristic signature in the surface pressure measured by REMS several sols before the dust arrives to Gale crater. We explore the capability of daily pressure measurements to serve as a fast detector of the development of dust storms in the context of the MSL, Insight and Mars 2020 missions.
W. Pluriel, E. Marcq, and M. Turbet. Modeling the albedo of Earth-like magma ocean planets with H2O-CO2 atmospheres. Icarus, 317:583-590, 2019. [ bib | DOI | arXiv | ADS link ]
During accretion, the young rocky planets are so hot that they become endowed with a magma ocean. From that moment, the mantle convective thermal flux control the cooling of the planet and an atmosphere is created by outgassing. This atmosphere will then play a key role during this cooling phase. Studying this cooling phase in details is a necessary step to explain the great diversity of the observed telluric planets and especially to assess the presence of surface liquid water. We used here a radiative-convective 1D atmospheric model (H2O, CO2) to study the impact of the Bond albedo on the evolution of magma ocean planets. We derived from this model the thermal emission spectrum and the spectral reflectance of these planets, from which we calculated their Bond albedos. Compared to Marcq et al. (2017), the model now includes a new module to compute the Rayleigh scattering, and state of the art CO2 and H2O gaseous opacities data in the visible and infrared spectral ranges. We show that the Bond albedo of these planets depends on their surface temperature and results from a competition between Rayleigh scattering from the gases and Mie scattering from the clouds. The colder the surface temperature is, the thicker the clouds are, and the higher the Bond albedo is. We also evidence that the relative abundances of CO2 and H2O in the atmosphere strongly impact the Bond albedo. The Bond albedo is higher for atmospheres dominated by the CO2, better Rayleigh scatterer than H2O. Finally, we provide the community with an empirical formula for the Bond albedo that could be useful for future studies of magma ocean planets.