Twilight Limb Observations of the Martian North Polar Hood by MAVEN IUVS

In northern winter, a broad distribution of ice aerosols appears in the north polar atmosphere of Mars, commonly referred to as the North Polar Hood (NPH). The NPH is thought to be formed as a result of condensation from lowered temperatures associated with both seasonal and diurnal variations in solar heating. The spatial extent and density of the NPH is highly variable, with a maximum latitudinal extent spanning 30-80°N, and a maximum density at 10-30 km altitude.The NPH has been extensively observed by both ground-based telescopes and instruments in orbit around Mars. However, the majority of these observations are nadir-pointing. This observation geometry has two significant limitations. Firstly, they poorly probe the vertical structure of the NPH. Secondly, column densities are determined by monitoring the intensity of various spectral features associated with the ice composing the NPH, against a strong background with similar features from the frost that has condensed on the surface in the winter season, resulting in low sensitivities. Limb observations removes both limitations, allowing us to study the vertical distribution of the aerosols that make up the NPH at high sensitivities.We present new limb observations of the NPH by IUVS (Imaging Ultraviolet Spectrograph) on the MAVEN (Mars Atmospheric and Volatile Evolution) spacecraft. These observations represent the first ultraviolet limb observations of the NPH, opening a new window for understanding the structure and composition of the NPH. The observations are also of the twilight limb, with sunlight being scattered from the dayside into the nightside over large solar zenith angles. This illumination geometry allows us to avoid the high dayside intensities that would drown out the signal from the thinner sections of the NPH. We determine the latitudinal extent of the NPH to be 30-60°N. We also find that an exponential altitude distribution of aerosols is able to reproduce the observed intensities, with a scale height similar to the atmospheric scale height. Finally, we observe an almost mutual exclusion of the NPH and nitric oxide nightglow emissions, an effect of the global circulation that drive both phenomena.