High-altitude Martian plume is likely an ordinary twilight cloud

Sánchez-Lavega et al. (2015) analyzed ground-based and space-based images of Mars that they interpreted as showing two high-altitude plumes. Their derived plume altitudes of 200 to 250 km or higher defied atmospheric processes known at the time. No supporting observations have been published in the decade since, despite ample observations by the fleet of spacecraft currently orbiting the planet. Instead, more recent observations provide strong constraints against the interpretations proposed at the time. We therefore reanalyzed the images used in that work and reviewed the methods employed by Sánchez-Lavega et al. (2015). We have identified several issues in the original data processing that may have affected the altitude determination, including surface-feature registration, black-level selection, color-image alignment, inconsistent map gridding, unknown image preprocessing, and the propagation of systematic uncertainties. Our independent analysis indicates that the features in these images are consistent with post-terminator clouds with altitudes between 50 and 100 km. Such clouds are relatively common at Mars and require no new atmospheric physics or chemistry to understand.