A spherical Monte-Carlo model of aerosols: Validation and first applications to Mars and Titan

TL;DR

This paper introduces a spherical Monte-Carlo radiative transfer model for planetary aerosols, validates it with observations from Mars and Titan, and demonstrates its ability to analyze aerosol vertical distribution and surface reflectance.

Contribution

The paper presents a novel fast Monte-Carlo model in spherical geometry for planetary aerosols, with applications to Mars and Titan data analysis.

Findings

Mars dust scale height varies seasonally between 6 and 12 km.

Titan aerosols have a scale height of approximately 80 km.

Aerosol optical depth decreases with wavelength following a power-law.

Abstract

The atmospheres of Mars and Titan are loaded with aerosols that impact remote sensing observations of their surface. Here we present the algorithm and the first applications of a radiative transfer model in spherical geometry designed for planetary data analysis. We first describe a fast Monte-Carlo code that takes advantage of symmetries and geometric redundancies. We then apply this model to observations of the surface of Mars and Titan at the terminator as acquired by OMEGA/Mars Express and VIMS/Cassini. These observations are used to probe the vertical distribution of aerosols down to the surface. On Mars, we find the scale height of dust particles to vary between 6 km and 12 km depending on season. Temporal variations in the vertical size distribution of aerosols are also highlighted. On Titan, an aerosols scale height of 80 \pm 10 km is inferred, and the total optical depth is found to decrease with wavelength as a power-law with an exponent of -2.0 \pm 0.4 from a value of 2.3 \pm 0.5 at 1.08 {\mu}m. Once the aerosols properties have been constrained, the model is used to retrieve surface reflectance properties at high solar zenith angles and just after sunset.