Mariner 9 ultraviolet spectrometer experiment: Afternoon terminator observations of Mars

It is possible to model the ultraviolet terminator observations of Mariner 9 either with a homogeneous atmosphere or with a scattering layer in addition to a homogeneous atmosphere. The characteristic shape of the twilight intensity variation is dependent on the latitude of observation. Observations north of 55N require a homogeneous model. Twilight measurements south of 55N indicate that a scattering layer may be a general Martian occurrence, at least in the evening. The day-to-day surveillance of the Martian terminator during the first 216 Mariner 9 revolutions indicates haze formation in the evening, and its continued presence until early morning is a daily occurrence in mid-latitudes up to 50N.