Retrieving the Properties of Martian Aerosols at Jezero Crater over Two Martian Years using SuperCam Passive Sky Observations

Introduction

The SuperCam instrument onboard the Mars2020 Perseverance rover is a suite of remote sensing instruments that is operating on the Martian surface since February 2021 [1, 2, 3]. It notably includes a Visible-InfraRed (VISIR) spectrometer covering the 385–465 nm, 536–853 nm, and 1.3–2.6 µm spectral ranges [4], which regularly performs observations of the Martian atmosphere using the passive sky geometry to retrieve information on the composition of the atmosphere [5, 6]. At these wavelengths, scattering by aerosols is strongly sensitive to the particle size. The ability of the passive sky technique to retrieve the atmospheric dust content has been demonstrated in the VIS spectral range with MSL/ChemCam [7], and SuperCam is now able to probe for the first time the Martian atmosphere from the ground for both the VIS and near-IR domains, which provides further information on the aerosol properties.

Dust and water ice aerosols play an important role in the current Martian climate: they affect the thermal structure of the atmosphere as they absorb and scatter the incoming sunlight, and play an important role in the global water cycle of the planet [8]. Thus, monitoring the properties of these aerosols is of importance to better understand and model the current Martian climate. On Perseverance, the optical depth of the aerosols above the rover is monitored on a seasonal and local time basis by the MEDA and MastCam-Z (ZCAM) instruments [9-11].

Data & Methods

Regular Passive Sky

By measuring the spectra of the sky luminosity at two different elevation angles, and by comparing the measurement with the results of a multiple scattering radiative transfer model, we are able to retrieve the aerosol properties for both the dust and water ice over more than two Martian years. Here we use the DIScrete Ordinate Radiative Transfer (DISORT) code in version 4 [12] through the pyRT_DISORT Python module [13] to retrieve the respective optical depth of dust and water ice from the VISIR passive sky measurements of SuperCam performed since the beginning of the mission in 2021, and constrain their particle size. We assume asymmetric hexahydra dust particles and droxtals shapes for the water ice crystals, and we use vertical atmospheric profile from the Mars Climate Database version 6.1 [14, 15].

These retrievals complement the ones performed by the rover’s other instruments, notably ZCAM. While it is highly challenging with their measurements to distinguish between dust and water ice contributions in the total optical depth, their results can be directly compared with those from SuperCam, as the wavelength ranges of the two instruments overlap in the visible.

Thus, we assume the total column-integrated optical depth of the aerosols to be the same as the one derived by ZCAM at λ=880 nm, then we perform the retrievals on the relative amount of dust and ice, and on their particle size. In order to address the issue of local minima of the χ² when performing the retrievals, we run the solver on a range of values for the particle sizes, then compare the χ² values of all the runs to select the best fit.

Aerosols-specific Passive Sky

In addition to the regular passive sky performed at two altitudes, an other observation sequence has been implemented and run a few times on Mars with a series of measurements at the same altitude but for different azimuths. This allows to scan the lobe of the phase function of the scattering of the aerosols.

Comparison with MastCam-Z

On sols 1523 and 1563, two coordinated observations between SuperCam and MastCam-Z have been performed to compared the radiances measured by both instruments in the same regions of the sky, using the ZCAM filters whose wavelengths overlap with the SuperCam spectra range. These observations have been designed to investigate a potential straylight effect in the SuperCam passive sky data, but also provide an opportunity to compare and cross-calibrate the atmospheric retrievals between the two instruments.

Conclusion & Perspectives

For more than two Martian years now, the SuperCam instrument has been performing passive sky observations of the Martian atmosphere in the VISIR. Despite being challenging to perform, the aerosols retrievals from SuperCam provide information on the nature and particle size of the Martian aerosols in the lower layers of the atmosphere, and complement the measurements from the other instruments of the rover.

References

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