Carbon in Lunar Polar Regions

Carbon-bearing material has been detected on the Moon, including ices in lunar permanently shadowed regions (PSR). The Lunar Crater Observation and Sensing Satellite impacted into a lunar PSR. This experiment released materials from the crater floor that were detected in the plume including CO, CO2, and some hydrocarbons. In addition, reflectance data from lunar PSR have been interpreted to indicate the presence of a more-volatile frost than water in the coldest PSR, potentially CO2. These highly-volatile materials exist in unknown abundances on the Moon because the spatial and volumetric extent are not well constrained with existing data.

This presentation investigates the delivery of carbon-bearing molecules through the lunar exosphere to lunar PSR. We model the migration and delivery of CO, CO2, and CH4 from a variety of sources. Potential sources of these volatile carbon compounds include exogenous delivery from comets, meteoroids, and the solar wind. A potential endogenous source is outgassing from the lunar interior, especially early in lunar history.

The model tracks particles as they follow ballistic trajectories in the lunar exosphere. Each time a particle encounters the surface, the model determines the residence time of the particle on the surface, whether it is re-released, and the trajectory of the particle at re-release. We explore the parameter space for this exosphere-surface interaction and determine how surface properties affect the delivery of material to PSR.

Lunar Atmosphere and Dust Environment Explorer (LADEE) Neutral Mass Spectrometer (NMS) observations indicate that methane exists in the lunar exosphere presently and likely stems from solar wind C and H implantation on the dayside of the Moon. CH4 hops through the exosphere on the dayside of the Moon and cold-traps on the lunar nightside even though the stability temperature is lower than the nightside temperature. However, the high instrumental background of NMS in the 44 and 28 u channels precluded detection of CO and CO2 in the equatorial exosphere. We investigate cases where these molecules cold trap to the lunar nightside and cases where they do not. The case without nightside cold-trapping dramatically increases the delivery rate to PSR because molecules are allowed to hop around in shadow without loss to photodissociation.