CLOUDS IN THE ANCIENT LUNAR ATMOSPHERE.

Today's moon is vastly different from what it was 3 billion years ago. At that time, it was home to a collisional atmosphere formed through massive amounts of volcanism, releasing enough subsurface gas to sustain surface pressures of up to 1 kPa. Observations of our solar system have taught us that all dense atmospheres are host to clouds and aerosols, and we expect the Moon's to be no different. Knowing when, where, and under what conditions cloud particles form is important for understanding the evolution of the lunar atmosphere, how it reacted to temperature gradients, and how it cycled volatiles. To study this, I investigated the immersion freezing properties of three aerosols: JSC- 1A lunar simulant, which is similar in composition and properties to lunar sample #14163 returned by the Apollo 14 Mission; Exolith LMS- 1D Lunar Mare simulant, which simulates volcanic deposits on the Moon's surface; and Arizona Test Dust (ATD), a standard for Earth atmospheric studies. A Peltier thermoelectric cooler and droplet freezing array were used to isolate Millipore water droplets with aerosols cooled to temperatures as low as - 18 ° C. Aerosol type, aerosol concentration, and droplet size was varied across these experiments. Both lunar simulants were found to nucleate ice less frequently than ATD, with JSC- 1A nucleating more droplets than LMS- 1D, which exhibited freezing properties similar to pure water. The results indicate that water ice nucleation is possible but highly variable on lunar regolith aerosols, and cloud formation on volcanic dust may have been possible in the ancient lunar atmosphere. [ABSTRACT FROM AUTHOR]