Using Engineering Cameras on Mars Landers and Rovers to Retrieve Atmo- Spheric Dust Loading

Introduction: Dust in the Martian atmosphere influences energy deposition, dynamics, and the viability of solar powered exploration vehicles. The Viking, Pathfinder, Spirit, Opportunity, Phoenix, and Curiosity landers and rovers each included the ability to image the Sun with a science camera that included a neutral density filter. Direct images of the Sun provide the ability to measure extinction by dust and ice in the atmosphere. These observations have been used to characterize dust storms, provide ground truth sites for orbiter-based global measurements of dust loading, and to help monitor solar panel performance [1]. In the costconstrained environment of Mars exploration, future missions may omit such cameras, as the solar-powered InSight mission has. We will present a simulation-based assessment of imaging strategies and their error budgets, as well as a validation based on archival engineering camera data. Procedure and Preliminary Validation : We seek to provide a robust capability of determining atmospheric opacity from sky images taken with cameras that have not been designed for solar imaging, such as lander and rover engineering cameras. One such method of deriving optical depth from non-solar sky images involves taking the ratio of sky radiance at two different elevation angles and generating a brightness profile. Near 90◦ away from a low Sun, the scattering angle is nearly orthogonal to elevation angle. In this case, the scattering angle controls the phase function (P) and the elevation angle controls the airmass (η). For the low optical depth limit, radiance (I) is proportional to optical depth (τ ) × airmass (η), but for high airmass and/or optical depth, multiple extinction becomes important [1]. To reduce errors from absolute calibration in the high airmass and/or optical depth case, we take the ratio of two measurements at the same scattering angle, but different elevation angles. The ratio method is a simplification: one can gain accuracy using an elevation ( ) profile of dln(I)/d from a sky image, taken at constant scattering angle. This is analogous to multiple sun images at different elevation angles to calibrate solar optical depth. The following equation can be used to generate a brightness profile and thus derive optical depth: