Abstract
The degradation in electrical output of solar arrays on Mars landers and rovers is reviewed. A loss of 0.2% per Sol is typical, although observed rates of decrease in ‘dust factor’ vary between 0.05% and 2% per Sol. 0.2%/Sol has been observed throughout the first 800 Sols of the ongoing InSight mission, as well as the shorter Mars Pathfinder and Phoenix missions. This rate was also evident for much of the Spirit and Opportunity missions, but the degradation there was episodically reversed by cleaning events due to dust devils and gusts. The enduring success of those rover missions may have given an impression of the long-term viability of solar power on the Martian surface that is not globally-applicable: the occurrence of cleaning events with an operationally-useful frequency seems contingent upon local meteorological circumstances. The conditions for significant cleaning events have apparently not been realized at the InSight landing site, where, notably, dust devils have not been detected in imaging. Optical obscuration by dust deposition and removal has also been observed by ultraviolet sensors on Curiosity, with a similar (but slightly higher) degradation rate. The observations are compared with global circulation model (GCM) results: these predict a geographically somewhat uniform dust deposition rate, while there is some indication that the locations where cleaning events were more frequent may be associated with weaker background winds and a deeper planetary boundary layer. The conventional Dust Devil Activity metric in GCMs does not effectively predict the different dust histories.
Highlights
The occurrence of global dust storms visible even from Earth attests to the vigor of the Martian atmospheric dust cycle, e.g. (Kahre et al, 2017)
Dust attenuation of solar array output was first observed on the Mars Pathfinder mission, but the mission was short in duration and so the power loss was not a major mission constraint
That the InSight ‘lull’ between Sols 200 and 400 is associated with a higher dust devil activity index predicted by the MarsWRF model, and the number of moderate (>0.5 Pa) and large (>2 Pa) pressure drops associated with convective vortices (Spiga et al, 2021) that were detected by InSight's instrumentation were roughly a factor of 2 higher than in the Sol 0–200 period
Summary
The occurrence of global dust storms visible even from Earth attests to the vigor of the Martian atmospheric dust cycle, e.g. (Kahre et al, 2017). In addition to being a key factor in the Martian climate, influencing atmospheric and surface temperatures and the surface/atmosphere exchange of water and carbon dioxide, dust is of vital importance in spacecraft operations. This can be true even of radioisotope-powered vehicles, since science operations may require adequate visibility, and thermal control can be affected by dust deposition. The environmental challenge to the InSight mission, where the arrays have become steadily more and more dust-covered (Fig. 1) and array output has fallen after one Mars year to about a quarter of that at landing (e.g. Lorenz et al, 2021) shows that the MER experience of dust removal (Fig. 2) is not universally-applicable, but may be contingent upon the local meteorological conditions and/or the specifics of the solar panel (e.g. surface properties, motion-induced vibration). The present paper critically reviews the solar array performance on landed Mars missions to document the record of dust deposition and removal
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