UC Santa Cruz

Water ice clouds on Mars are an important component of the hydrologic cycle as well as the overall climate system of the planet. The goal of this research is to better understand water ice cloud formation and behavior on Mars. We use modeling and laboratory experiments to explore different processes related to water ice cloud formation and evolution.

The first goal of this work is to examine how well the Martian water cycle is simulated by the NASA Ames Mars General Circulation Model. The simulation predicts atmospheric water vapor amounts approximately half of those observed, globally. We identify water ice clouds as being a major contributor to this discrepancy. The model closely reproduces the convective aphelion cloud belt at the equator, but deviates substantially from observations over the North Polar Cap region. Modifying the nucleation scheme within the cloud microphysical model brings model results closer to observations and affects the surface radiative balance, which affects the annual cycle of sublimation and deposition of water ice at the residual North Polar Cap. The most realistic global water vapor and cloud patterns come from limiting the nucleation rate of particles at the poles. Our simulations show that the North Polar Cap region exhibits atmospheric dynamics where stratiform clouds form. We hypothesize that the modified nucleation scheme compensates for biases in the radiative properties of the stratiform clouds expected over the North Polar Cap. More broadly, this study illustrates the strong sensitivity of the Martian global water cycle to clouds over the North Polar Cap region.

The second goal of this work is to assess the ability of various salts to serve as water ice cloud condensation nuclei under Martian conditions. We use a vacuum chamber to simulate the cold, lower pressure atmospheric conditions on Mars and find the critical saturation ratios at which the substrates nucleate water ice. We find no significant difference between sodium chloride nucleation and that of the control of a bare silicon wafer. In contrast, sodium perchlorate nucleates at significantly lower saturation ratios than the control, suggesting that some atmospheric salts could serve as effective cloud condensation nuclei on Mars. All substrates examined demonstrate an exponential temperature-dependence for the critical saturation ratio, indicating that at colder temperatures, nucleation requires increasingly higher saturation ratios. Our results suggest that airborne sodium perchlorate may enable water ice cloud formation at partial pressures lower than would otherwise be required in its absence. For example, at 155 K, sodium perchlorate could nucleate water ice at a partial pressure 40 percent lower than other cloud substrates.

The third goal of this work is understand the impacts of temperature-dependent optical parameters on the radiative effects of water ice clouds on Mars. The optical properties of water ice vary with temperature, however, past Mars climate modeling have used optical properties based on water ice refractive indices relevant to Earth's atmospheric temperatures. In this chapter we use water ice refractive indices at temperatures relevant to the Martian atmosphere with Mie scattering code to provide input into the NASA Ames Mars General Circulation Model. We compare the instantaneous effects of using these optical parameters, versus values that have been traditionally used, on daytime and nighttime radiative fluxes. We find that the updated optical properties amplify existing radiative flux trends over most of the planet, which would lead to increased energy fluxes at the North Polar cap during the Northern hemisphere summer as well as increased atmospheric warming at equatorial clouds. Atmospheric warming is increased at night with more relevant optical parameters and could result in temperatures differences of several Kelvin a day.