2015 .

S. Lebonnois, V. Eymet, C. Lee, and J. Vatant d'Ollone. Analysis of the radiative budget of the Venusian atmosphere based on infrared Net Exchange Rate formalism. Journal of Geophysical Research (Planets), 120:1186-1200, 2015. [ bib | DOI | PDF version | ADS link ]

A detailed one-dimensional analysis of the energy balance in Venus atmosphere is proposed in this work, based on the Net Exchange Rate formalism that allows the identification in each altitude region of the dominant energy exchanges controlling the temperature. Well-known parameters that control the temperature profile are the solar flux deposition and the cloud particle distribution. Balance between solar heating and infrared energy exchanges is analyzed for each region: upper atmosphere (from cloud top to 100 km), upper cloud, middle cloud, cloud base, and deep atmosphere (cloud base to surface). The energy accumulated below the clouds is transferred to the cloud base through infrared windows, mostly at 3-4 μm and 5-7 μm. The continuum opacity in these spectral regions is not well known for the hot temperatures and large pressures of Venus's deep atmosphere but strongly affects the temperature profile from cloud base to surface. From cloud base, upward transport of energy goes through convection and short-range radiative exchanges up to the middle cloud where the atmosphere is thin enough in the 20-30 μm window to cool directly to space. Total opacity in this spectral window between the 15 μm CO2 band and the CO2 collision-induced absorption has a strong impact on the temperature in the cloud convective layer. Improving our knowledge of the gas opacities in these different windows through new laboratory measurements or ab initio computations, as well as improving the constraints on cloud opacities would help to separate gas and cloud contributions and secure a better understanding of Venus's atmosphere energy balance.

B. Charnay, E. Barth, S. Rafkin, C. Narteau, S. Lebonnois, S. Rodriguez, S. Courrech Du Pont, and A. Lucas. Methane storms as a driver of Titan's dune orientation. Nature Geoscience, 8:362-366, 2015. [ bib | DOI | arXiv | PDF version | ADS link ]

The equatorial regions of Saturn's moon Titan are covered by linear dunes that propagate eastwards. Global climate models (GCMs), however, predict westward mean surface winds at low latitudes on Titan, similar to the trade winds on Earth. This apparent contradiction has been attributed to Saturn's gravitational tides, large-scale topography and wind statistics, but none of these hypotheses fully explains the global eastward propagation of dunes in Titan's equatorial band. However, above altitudes of about 5 km, Titan's atmosphere is in eastward super-rotation, suggesting that this momentum may be delivered to the surface. Here we assess the influence of equatorial tropical methane storms-which develop at high altitudes during the equinox-on Titan's dune orientation, using mesoscale simulations of convective methane clouds with a GCM wind profile that includes super-rotation. We find that these storms produce fast eastward gust fronts above the surface that exceed the normal westward surface winds. These episodic gusts generated by tropical storms are expected to dominate aeolian transport, leading to eastward propagation of dunes. We therefore suggest a coupling between super-rotation, tropical methane storms and dune formation on Titan. This framework, applied to GCM predictions and analogies to some terrestrial dune fields, explains the linear shape, eastward propagation and poleward divergence of Titan's dunes, and implies an equatorial origin of dune sand.

S. Vinatier, B. B├ęzard, S. Lebonnois, N. A. Teanby, R. K. Achterberg, N. Gorius, A. Mamoutkine, E. Guandique, A. Jolly, D. E. Jennings, and F. M. Flasar. Seasonal variations in Titan's middle atmosphere during the northern spring derived from Cassini/CIRS observations. Icarus, 250:95-115, 2015. [ bib | DOI | PDF version | ADS link ]

We analyzed spectra acquired at the limb of Titan in the 2006-2013 period by the Cassini/Composite Infrared Spectrometer (CIRS) in order to monitor the seasonal evolution of the thermal, gas composition and aerosol spatial distributions. We are primarily interested here in the seasonal changes after the northern spring equinox and interpret our results in term of global circulation seasonal changes. Data cover the 600-1500 cm-1 spectral range at a resolution of 0.5 or 15.5 cm-1 and probe the 150-500 km vertical range with a vertical resolution of about 30 km. Retrievals of the limb spectra acquired at 15.5 cm-1 resolution allowed us to derive eight global maps of temperature, aerosols and C2H2, C2H6 and HCN molecular mixing ratios between July 2009 and May 2013. In order to have a better understanding of the global changes taking place after the northern spring equinox, we analyzed 0.5 cm-1 resolution limb spectra to infer the mixing ratio profiles of 10 molecules for some latitudes. These profiles are compared with CIRS observations performed during the northern winter. Our observations are compatible with the coexistence of two circulation cells upwelling at mid-latitudes and downwelling at both poles from at last January 2010 to at least June 2010. One year later, in June 2011, there are indications that the global circulation had reversed compared to the winter situation, with a single pole-to-pole cell upwelling at the north pole and downwelling at the south pole. Our observations show that in December 2011, this new pole-to-pole cell has settled with a downward velocity of 4.4 mm/s at 450 km above the south pole. Therefore, in about two years after the equinox, the global circulation observed during the northern winter has totally reversed, which is in agreement with the predictions of general circulation models. We observe a sudden unexpected temperature decrease above the south pole in February 2012, which is probably related to the strong enhancement of molecular gas in this region, acting as radiative coolers. In July and November 2012, we observe a detached haze layer located around 320-330 km, which is comparable to the altitude of the detached haze layer observed by the Cassini Imaging Science Subsystem (ISS) in the UV.