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Aerosols are capable of having a huge influence on reflected, emitted and transmitted planetary spectra. The Near InfraRed Spectrograph (NIRSpec) using the PRISM mode on board of the James Webb Space Telescope (JWST) is providing valuable data of transit spectra over a wide spectral range that is able to cover the whole contribution of aerosols, potentially disentangling them from other constituents and thus allowing to constrain their properties. We aim to investigate whether NIRSpec/PRISM JWST transmission spectroscopy observations, in addition to being useful to detect and determine the abundance of gases more accurately than any previous instruments, are also capable of studying the physical properties of the aerosols in exoplanetary atmospheres. We perform nested sampling Bayesian retrievals with MultiNest library. We use the Planetary Spectrum Generator (PSG) and the Modelled Optical Properties of enSeMbles of Aerosol Particles (MOPSMAP) database as tools for the forward simulations and NIRSpec/PRISM JWST previously published observations of WASP-39b as input data. Retrievals indicate that models including an aerosol extinction weakly increasing or sharply decreasing with wavelength are decisively better than those with a flat transmission and that this increased degree of complexity is supported by the kind of data that JWST/NIRSpec can provide. Given other physical constraints from previous works, the scenario of weakly increasing particle extinction is favoured. We also find that this also has an effect on the retrieved gas abundances. JWST observations have the potential to study some physical characteristics of exoplanetary clouds, in particular their overall dependence of transmissivity with wavelength. In fact, it is important to implement more detailed aerosol models as their extinction may affect significantly retrieved molecular abundances., Comment: Accepted for publication in A&A, 12 pages, 5 figures

Dust is the main driver of Mars' atmospheric variability. The determination of Martian dust aerosol properties is of high relevance for radiative modelling and calculating its weather forcing. In particular, the light scattering behaviour at intermediate and large scattering angles can provide valuable information regarding the airborne dust particle shape. The angular distribution of sky brightness observed by the Mars Science Laboratory engineering cameras (Navcam and Hazcam) is used here to characterise the atmospheric dust single scattering phase function and to constrain the shape of the particles. An iterative radiative transfer based retrieval method was implemented in order to determine the aerosol modelling parameters which best reproduce the observed sky radiance as a function of the scattering angle in the solar almucantar plane. The aerosol models considered in this study for retrieving dust radiative properties were an analytical three term Double Henyey-Greenstein phase function, T-matrix calculations for cylindrical particles with different diameter-to-length aspect ratios and experimental phase functions from laboratory measurements of several Martian dust analogue samples. Results of this study returned mean DHG phase function parameter values in line with Wolff et al. (2009). Although differences were observed during the low opacity aphelion season (lower forward scattering values, presence of a peak in the backward region) compared to the rest of the year, no clear evidences of seasonal behaviour or interannual variability were derived. The obtained average D/L aspect ratios for T-matrix calculated cylindrical particles were 0.70{\pm}0.20 and 1.90{\pm}0.20, and the best fitting Martian dust analogue corresponded to the basalt sample.

In this paper we show that Sun-viewing images obtained by the Mars Science Laboratory (MSL) Navigation Cameras (Navcam) can be used for retrieving the dust optical depth and constrain the aerosol physical properties at Gale Crater by evaluating the sky brightness as a function of the scattering angle. We have used 65 Sun-pointing images covering a period of almost three Martian years, from MSL mission sol 21 to sol 1646 (MY 31 to 33). Radiometric calibration and geometric reduction were performed on MSL Navcam raw image data records to provide the observed sky radiance as a function of the scattering angle for the near-Sun region (scattering angle from 4{\deg} to 30{\deg}). These curves were fitted with a multiple scattering radiative transfer model for a plane-parallel Martian atmosphere model using the discrete ordinates method. Modelled sky brightness curves were generated as a function of two parameters: the aerosol particle size distribution effective radius and the dust column optical depth at the surface. A retrieval scheme was implemented for deriving the parameters that generated the best fitting curve under a least-square error criterion. The obtained results present a good agreement with previous work, showing the seasonal dependence of both dust column optical depth and the effectiveparticle radius.