Equinoctial atmospheric activity over Titan dune fiels revealed by Cassini/VIMS

Abstract : Introduction: Titan, the largest satellite of Saturn, is the only satellite in the solar system with a dense atmosphere. The close and continuous observations of Titan by the Cassini spacecraft, in orbit around Saturn since July 2004, bring us evidences that Titan tropo-sphere and low stratosphere experience an exotic, but complete meteorological cycle similar to the Earth hy-drological cycle, with hydrocarbons evaporation, con-densation in clouds, and rainfall. Cassini monitoring campaigns also demonstrate that Titan's cloud cover-age and climate vary with latitude. Titan's tropics, with globally weak meteorological activity and widespread dune fields, seem to be slightly more arid than the poles, where extensive and numerous liquid reservoirs and sustained cloud activity were discovered. Only a few tropospheric clouds have been observed at Titan's tropics during the southern summer [1-4]. As equinox was approaching (in August 2009), they oc-curred more frequently and appeared to grow in strength and size [5-7]. VIMS observations: We present here the observa-tion of intense brightening at Titan's tropics, very close to the equinox. These detections were conducted with the Visual and Infrared Mapping Spectrometer [8] (VIMS) onboard Cassini. Figure 1 presents the VIMS color composite images of the three individual events detected so far, observed during the Titan's flybys T56 (22 May 2009), T65 (13 January 2010) and T70 (21 June 2010). T56, T65 and T70 observations show an intense and transient brightening of large regions very close to the equator, right over the extensive dune fields of Senkyo, Belet and Sangria-La. They all appear spectrally and morphologically different from all tran-sient surface features or atmospheric phenomena pre-viously reported. Indeed, these events share in particu-lar a strong brightening at wavelengths greater than 2 µm (especially at 5 µm), making them spectrally dis-tinct from the small tropical clouds observed before the equinox [1,3,7] and the large storms observed near the equator in September and October 2010 [6]. Discussion: In this paper, we will discuss the pos-sibility that these singular events may have occurred very close to the surface, having a strong link with the underlying dune fields. Radiative transfer calculations indeed show that these singular brightenings are due to the transient appearance of an additional atmospheric layer, confined at very low altitudes and loaded with few but large particles. Gathering all the observational and modeling constraints, we conclude that the most probable explanation for these events is the local and transient occurrence of huge dust storms, directly orig-inating from the underlying dune fields. We will also discuss the possible implications of the equinoctial occurrence of such events for Titan's tropical wind regimes and for the present-day activity of equatorial dunes.
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S. Rodriguez, S. Le Mouélic, J.W. Barnes, M. Hirtzig, Pascal Rannou, et al.. Equinoctial atmospheric activity over Titan dune fiels revealed by Cassini/VIMS. Third International Planetary Dunes Workshop, Lunar and Planetary Institute, Jun 2012, Flagstaff, United States. 2 p. ⟨hal-01077692⟩

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