The Mars Science Laboratory record of optical depth measurements via solar imaging
The Mars Science Laboratory Curiosity rover has monitored the Martian environment in Gale crater since landing in 2012. This study reports the record of optical depth derived from visible and near-infrared images of the Sun. Aerosol optical depth, which is mostly due to dust but also includes ice, dominates the record, with gas optical depth too small to measure. The optical depth record includes the effects of regional dust storms and one planet-encircling dust event, showing the expected peaks during southern spring and summer and relatively lower and more stable optical depth in fall and winter. The measurements show that there is a seasonally varying diurnal change in dust load, with the optical depth peaking in the morning during southern spring and summer, correlated with thermotidal pressure changes. However, there was no systematic diurnal change during autumn and winter, except after one regional storm. There were indications that the dust was relatively enhanced at high altitudes during high-optical-depth periods and that high-altitude ice was significant during winter. The observations did not provide much information about particle size or composition, but they were consistent with a smaller particle size after aphelion (in southern winter). No scattering halos were seen in associated sky images, even when there was visual evidence of ice hazes or clouds, which suggests small or amorphous ice particles. Unexpectedly, the measurement campaign revealed that the cameras collected saltating sand in their sunshades 1.97 m above the surface. As a result, the measurement strategy had to be adjusted to avoid high-elevation imaging to avoid sand covering the optics.