Solid Carbon Dioxide is Stable in Lunar Polar Craters

Map of possible locations where dry ice may be found, showing the Moon south of 80°S. Colored areas show locations where the dry ice sublimation rate is less than 2 inches per billion years (100 kg/m2/Gyr). Black contours encircle locations where the water ice sublimation rate is less than 4 inches per billion years (100 kg/m2/Gyr). Enlarged maps of Cabeus and Amundsen Craters are shown at right, with logarithmic sublimation rates plotted. Credit: Schorghofer et al., 2021.

The Moon has deep polar craters with permanently dark floors shielded from direct sunlight. These craters can be cold enough to maintain water ice, as demonstrated by NASA’s Lunar CRater Observation and Sensing Satellite (LCROSS) impact experiment in Cabeus Crater. They may even be cold enough to maintain dry ice (solid carbon dioxide). When water ice and dry ice are delivered to the Moon, for instance by comets, the low ambient temperatures in the lunar polar craters can result in these materials being trapped as solids.

To determine where dry ice might be located in the lunar polar craters, Norbert Schorghofer of the Planetary Science Institute and colleagues determined temperatures in these craters using 11 years of observations by the Diviner instrument aboard NASA’s Lunar Reconnaissance Orbiter. Diviner measures the amount of visible and infrared light coming from the Moon. Using these measurements, surface temperatures can be computed with a precision of ±7°F (4°C). From these temperatures, the team calculated the sublimation rate of dry ice, which is the rate at which solid carbon dioxide becomes gaseous and would escape from the Moon. Schorghofer and colleagues estimate that the amount of dry ice deposited on the Moon is between about 0.02 inches (roughly the thickness of a fingernail) and 2 inches every billion years. For the sublimation rate to be less than this delivery rate, the temperature must be below -370°F (-223°C, 50 K). Based on this result, between 41 and 147 square miles (106 and 381 square kilometers) of the area near the lunar south pole are cold enough to trap dry ice, about 40% of which are within Amundsen Crater. These results reveal a potentially important resource for future lunar colonies since dry ice is useful in fuels and for sustaining life. READ MORE