Evidence of 210 Po on Martian dust at Meridiani Planum

Since the Surveyor and Apollo missions and up to the recent Lunar Prospector mission, 222 Rn and 210 Po have been key isotopes for understanding gas release events and their spatial and temporal variations on the Moon. Comparatively, these isotopes have drawn much less attention on Mars, if any, despite the wealth of information it could bring on the uppermost meters of the regolith, the exchange of volatiles at the surface, and the atmospheric aerosol cycle. Here we present a statistical analysis of the high‐energy end of alpha spectra obtained by the alpha particle X‐ray spectrometer onboard Mars Exploration Rover Opportunity and report evidence of 210 Po, a decay product of 222 Rn, attached to atmospheric dust. The 210 Po surface activity on rocks and soils at the landing site is lower than 3.1 × 10 −4 Bq cm −2 , but analysis of spectra obtained on the dust capture magnet reveals a 210 Po activity of (4.6 ± 2.4) × 10 −3 Bq cm −2 (±2σ). This difference is due to the very low dust cover index at the landing site. Owing to frequent dust devils, regional and global dust storms that mobilize substantial amounts of dust and homogenize the dust surface layer, we infer that the global average 222 Rn exhalation rate is significantly greater on Mars than on the Moon. This comparison supports the hypothesis that on Mars, radon emanation could be comparatively enhanced by the presence of water in the surficial soil. Analysis of atmospheric spectra yields a radon activity upper limit of 16 ± 5 Bq m −3 during nighttime at the landing site.