Five Mars Years of Cloud Observations at Gale Crater: Opacities, Variability, and Ice Crystal Habits

Abstract

We update the record of cloud opacity observations conducted by the Mars Science Laboratory (MSL) Curiosity rover to cover the first five Mars Years (MYs) of the mission (L s = 160° of MY 31 to L s = 160° of MY 36). Over the three MY period that we add to the previously analyzed two MY record, we achieve good diurnal coverage between 07:00 and 17:00 with nearly 1200 new observations. We derive a new scattering phase function for the clouds of the Aphelion Cloud Belt (ACB) using results from the Zenith and Suprahorizon movie data sets. Our phase function is generally smooth and featureless, which is consistent with the overall lack of atmospheric optical phenomena on Mars aside from a single instance of an observed halo. Applying our new phase function to the data, we find that there is very minimal variability in the ACB's opacity, either diurnally, intraseasonally, or interannually, noting that our observations are only sensitive to ice clouds and cannot detect any ice hazes that may be present over Gale. This contrasts with previous results, which observed a 57% difference in the opacity of morning and afternoon clouds in MY 33. The MY 33 results now appear to be an outlier, not replicated at any point during the MSL mission. We conclude that the higher morning opacities in MY 33 were a consequence of an incomplete understanding of the nature of the scattering phase function close to the Sun.

Three Years of ACB Phase Function Observations from the Mars Science Laboratory: Interannual and Diurnal Variability and Constraints on Ice Crystal Habit

“…While we initially used the same method as Cooper et al (2019), using MCS optical depths for Δτ WIC , we also computed the phase function replacing these optical depths with those derived from MSL cloud observations (Hayes et al 2023). This has the advantage that the MSL values are calculated for specific cloud observations, whereas the MCS tau values are average values.…”

“…That being said, neither approach is without weakness. The MSL-derived tau values are calculated using an assumed phase function, the weaknesses of which are discussed in Hayes et al (2023). However, given the advantages in seeing either the same clouds, or at least temporally nearby clouds, we have made the choice to continue using the phase-function values calculated with MSL-derived tau values.…”

“…There are a number of cloud observations performed by the rover both during the ACB season and throughout the rest of the year. Observations of the ACB at Gale Crater have shown low interannual variability in cloud opacity (Kloos et al 2018;Hayes et al 2023) and altitude (Campbell et al 2020) but potential diurnal variability in both.…”

“…We attempt to use our larger data set to better constrain a dominant ice crystal geometry and to determine if there are any interannual or diurnal variations in ice crystal habit at Gale Crater. Recent evidence from zenith and suprahorizon cloud movies (Hayes et al 2023) suggests that Martian clouds over Gale Crater resemble roughened particles more so than regular geometric crystals. Because of these findings, we will examine the PFSS record of the ACB at Gale Crater using formulations that consider dust in addition to water ice through a different method of comparison than that used by Cooper et al (2019).…”