Atmospheric Dust monitoring derived from Orbital Near-Infrared Imaging Spectroscopy and Implications for RSL Formation

Introduction: Dust is a high albedo material, which is omnipresent on Mars: both on the surface and suspended in the atmosphere. These micrometer-sized particles modify heat balance while being highly mobile: it is a major contributor to current Martian atmosphere and surface dynamic. Information on dust spatial and time distribution was already provided from numerous orbital and surface studies (e.g., [1,2,3]). However, some characteristics of the dust cycle remain uncertain, which is notably related to the large range of spatial scales of the dust movements: from Recurring Slope Lineae (RSL) to global dust storms, and with important interannual variability ([4,5]). Further global monitoring of dust movements with high spatial sampling and over several years may help improving our Martian dust knowledge. Here we present a new method used to derived near-infrared atmospheric dust optical depth using OMEGA observations acquired between 2004 and 2010.

 

Data and method: We use the near-infrared (NIR) imaging spectrometer OMEGA [6] onboard Mars Express, which acquired about 8300 observations of the Martian surface over that time range, with a spatial sampling typically close to 1 km.

Firstly, we quantify the prominent 2 µm atmospheric CO₂ absorption band which is sensitive to airborne dust that reduces the atmospheric optical paths of the photons. We have developed a model to predict the CO₂ optical depth under clear atmosphere conditions (notably depends on solar incidence angle, albedo and pressure). This allows to compare observed CO₂ optical depth to these predictions and to derive the difference "Δτ_CO2" (more details in [7]).

Then, we transform this Δτ_CO2 into dust optical depth by a calibration with the visible (VIS) dust optical depth (from 880 nm images) measured by the Mars Exploration Rovers (MER) [2]. Concomitant MER/OMEGA observations are used to characterize the relation between Δτ_CO2 and the ground optical depth (notably depends on incidence angle and albedo). Then, we extrapolated this relation to the whole OMEGA dataset to compute a VIS-NIR dust optical depth "τ_dust".

Results and discussion: We produce latitude/solar longitude diagrams, such as in Figure 1 for the Martian year 28. We observed well-known dust cycle characteristics: low dust optical depth values (<0.5) in the "clear atmosphere period" (solar longitude L_s=0-180°) and medium to high values (>1) during the dusty season (L_s>180°) such as the 2007 global dust storm during L_s=260-310°. We also observed medium-high values at northern and southern latitudes during L_s=220-250° that can correspond to dust storms identified in Hellas and Chryse [8]. Other main investigations were made with thermal-infrared (TIR) data [3,9]. We can notice an overall good agreement between our VIS-NIR study and TIR studies. But there are still differences that we studied by computing the dust optical depth ratio between our VIS-NIR values derived at 2 µm (and converted to 880 nm using MER) and the thermal ones at 9 µm. This ratio distribution reaches a maximum at 2.4, which is in agreement with the value (2.6) considered in [3] and also with the values found locally with MER [2]. This distribution is very large: 2.0 equivalent standard deviation, which may be related to the mean dust particle size variations according to [2] and to potential observational biases of each method (e.g., uncertainties in the vertical distribution of dust).

Figure 1: VIS-NIR dust optical depth (derived from the OMEGA dataset: see text for details) as a function of solar longitude (L_s) and latitude for Martian year 28.

We also produce global seasonal maps of dust optical depth with a spatial coverage typically greater than 50% for most 60° wide intervals in solar longitude. We provide two seasonal maps of Martian year 29 in Figure 2 (L_s=180-240° and 300-360°). Firstly, we can notice some localized high values, such as in 355°E and 55°S on the L_s=180-240° map, that can be associated to a dust storm. Some main dust storm travel routes [8,10] can be identified, such as the Acidalia-Chryse route which is associated to medium-high dust optical depth values in these two maps. Activity is also observed at RSL sites in this area during both periods [11] represented in Figure 2: rapid disappearance events over L_s=180-240° suggesting dust deposition/horizontal dust transport [12], and RSL formation/lengthening over L_s=300-360° which suggest dust removal. This indicates that local surface movements may be related to large-scale atmospheric dust movements occurring at RSL location.

Figure 2: Two seasonal atmospheric VIS-NIR dust optical depth maps of Mars during Martian year 29. Top: L_s=180-240°; bottom: L_s=300-360°.

 

Conclusion: We have developed a new method to derived a VIS-NIR dust optical depth using near-infrared data (OMEGA). The method relies on CO₂ absorption band at 2 µm and on a calibration procedure that we implemented, based on MER “ground truth” measurements. We constructed latitude/solar longitude diagrams (Figure 1) and global seasonal maps (Figure 2). Overall, our results are in a good agreement with previous thermal-infrared studies, but with some differences noticed by a particular distribution of the dust optical depth ratio between VIS-NIR and thermal-infrared, with a maximum occurrence value of 2.4. Then, preliminary results indicate that large scale atmospheric dust events may be related to local RSL activity occurring in the Chryse-Acidalia area. We will present further comparisons between atmospheric dust and RSL activities at the conference.

 

Acknowledgments: The OMEGA/MEx data are freely available on the ESA PSA at https://archives.esac.esa.int/psa/#!Table%20View/OMEGA=instrument.

 

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