Low-Latitude Seasonal Frost on Mars

Abstract

Surface water frosts on Mars, while thin, are an important component of the water cycle, and their timing and abundance provide valuable information about when and where condensation occurs. The lowest latitudes are the least favorable for such frosts and thus provide important constraints. Widespread early-morning water frost, unlikely to last much past dawn, has been observed on Arsia Mons near 8.7S [1] around LS=45–150. Indirect evidence for nighttime frost in Gale crater (4.5S) has also been reported near winter solstice [2], although they estimated thicknesses <1 micron which would be unlikely to last long past sunrise. Early-morning water frost was observed on the deck of the Opportunity rover at 2S, but not on the nearby regolith due to superior radiative cooling for metal surfaces [3]. Detections of afternoon frost, likely to indicate seasonal H2O frost persisting through the day, have thus far been at somewhat higher latitudes than the morning frost detections. Thermal and near-infrared spectral data have revealed afternoon water frost on steep pole-facing slopes at latitudes as low as 13S [4–7]. Vincendon et al. [6] conducted climate modeling that broadly matched their spectral observations but also predicted occurrences of thin frost (2–5 microns thick) at 3 PM on pole-facing slopes at even lower latitude. Those predicted frost deposits would occur around Noctis Labyrinthus and Arsia Mons around LS=80–100 at latitudes near 5S but were not observed in their data. In this abstract, we report observations of afternoon water frost in both Noctis Labyrinthus and Arsia Mons, located in and near shadows cast by steep local slopes. This corroborates the predictions of [6] and provides the lowest-latitude evidence of seasonal water frost persisting through the day on Mars observed to date.