Dust column opacity and size variability at Gale crater during the 2018/a storm

The Mars Science Laboratory's Mastcam has used solar filters to image the Sun and monitor 440-nm and 880-nm atmospheric opacity though the mission. Through sol 2075 (solar longitude 189.5, 8 June 2018), the 880-nm opacity ranged from lows near 0.3 from aphelion to solar longitude 140 to highs around 1-1.5 around solar longitude 230 and 330. Opacity increased from 0.8 on sol 2075 to approximately 8.5 on sol 2085 (solar longitude 195.5, 18 June 2018). The opacity then declined to about 4.8 after the next 27 sols on sol 2113 (solar longitude 212.2), during which time non-solar-filtered Mastcam and Navcam images were used to supplement standard monitoring. During that first phase of the decline, a decay of about 1-1.5%/sol was seen, with opacity spikes of 0.5-1 in several mornings. Such diurnal variation has been rare, but may be related to >10% pressure drop during the sol [Harri et al. 2014].

The angstrom exponent derived from the ratio of 440-nm to 880-nm opacity has normally been -0.05 to -0.10, corresponding to >1-micron radius dust particles. The precise size constraint depends on both the width of the size distribution and the particle shape, and these constraints will be discussed. The angstrom exponent has varied slightly (+0.10 or less) during aphelion season, and remained near or slightly above -0.10 during the storm onset and initial decay. Non-solar-filtered color images have illustrated the slight bluing of the Sun during the storm.

Sky images were taken before the storm, during the rising opacity, and during the initial decay. The image sequences were designed to allow for monitoring of dust properties including particle size. The near-Sun sky ranged from blue, with a rapid fall-off of brightness away from the Sun due to diffraction by dust, to brown with no azimuthal structure due to high-opacity. Analysis is ongoing, and results will be presented.

[Harri, A.-M., et al., 2014. Pressure observations by the Curiosity rover - Initial results. J.Geophys.Res. 119, 82-92. DOI: 10.1002/2013JE004423.]