Simulations of Halos Produced by Carbon Dioxide Ice Crystals in the Martian Atmosphere

It has been known for decades that carbon dioxide (CO2) ice clouds exist in the Martian atmosphere. According to remote sensing observations and previous modeling studies, the Martian CO2 ice crystals may be sufficiently large to generate halos. However, observations of CO2 ice crystal halos have not been reported so far. This study simulates the scattering and polarized phase functions at a wavelength of 0.48 mu m based on state-of-the-art light-scattering computational capabilities. The specific CO2 ice crystal habits considered in the simulations include cubes, octahedrons, cubo-octahedrons, and truncated octahedrons of various sizes. The halos produced by CO2 ice crystals peak at approximately 29 degrees and 42 degrees. Moreover, large CO2 ice crystals may cause strong scattering peaks at 155 degrees and 180 degrees. An ensemble of water (H2O) ice crystals and CO2 ice crystals with appropriate mixing fractions might be responsible for a halo occurrence recently observed on Mars. Plain Language Summary On Earth, optical phenomena, particularly halos, glories, and rainbows, caused by ice crystals and water droplets, are often observed in the sky. In the Martian atmosphere consisting of approximately 95% carbon dioxide (CO2) by volume, CO2 ice crystals can exist and result in optical phenomena similar to those observed on Earth. In the present study, the optical properties of CO2 ice crystals are computed to explain halos and other optical features caused by these particles. Because the habits (shapes) of CO2 ice crystals are different from those of water droplets and ice crystals, the optical phenomena produced by the former have different positions in the sky compared to the counterparts caused by water droplets and ice crystals. Furthermore, a halo observed recently on Mars might be caused by a mixture of water ice crystals and CO2 ice crystals.

Automated summary

The study shows that carbon dioxide ice crystals in the Martian atmosphere can create halos, and a recent halo observed on Mars may be caused by a mixture of water ice crystals and CO2 ice crystals.