Impact gardening is a mixture of excavation by impacts and burial under continuous proximal ejecta. An existing analytical model describes the rate at which impacts excavate material on the Moon (Gault et al., 1974; Costello et al., 2018, ; Costello et al., 2020, ). We expand the model to include a treatment of burial under proximal ejecta. Using the models for excavation and burial, we explore the effects of impacts in the evolution of the lunar surface over the last few billion years. We find that excavation of material by gardening outpaces burial in all reasonable ejecta coverage test scenarios. Thus, gardening does not act as a shield for ice in permanent shadow. However, gardening fails to eradicate the surface expression of compositional contrasts, such as those associated with pyroclastic deposits and compositional rays, which are not vulnerable to removal by thermal or ionization processes. Explorers seeking ice at the lunar poles should not expect regions of permanent shadow to have pure ice within the top 1-10 m because that ice will have been disrupted by gardening.

The Moon is continually bombarded by interplanetary meteoroids. While many of the meteoroid sources are near the ecliptic plane, a significant population of high-inclination meteoroids exists at 1 au that bombards the lunar polar regions. Building on previous measurements of the response of the lunar impact ejecta cloud to known meteoroid sources, we develop an ejecta model for the entire lunar surface by incorporating the high-inclination sources. We find that the polar regions of the Moon experience similar quantities of impact ejecta production as the equator. Due to the enhanced impactor fluxes near the equator and at high latitudes on the dawn side, lunar regolith is preferentially distributed to the mid to high-latitude regions over long timescales, providing a pathway to mix regolith from different regions. We find impact ejecta yields at the Moon to be significantly lower than the Galilean moons, suggesting meteoroids deliver more energy to the local regolith and can be an important driver in the evolution of volatiles near the lunar surface. Additionally, we find that a polar orbiting spacecraft equipped with a dust analyzer can measure appreciable quantities of lunar ejecta near the poles to constrain the water content in the polar regions. Plain Language Summary The Moon is continually impacted by small particles shed primary from comets, which impact the Moon from a variety of organized directions. These impacts kick up a significant amount of the lunar soil above the lunar surface and sustain a permanently present cloud of ejecta around the Moon. Previous work categorized the ejecta cloud in the Moon's equatorial plane. Here we extend that work to understand the ejecta environment in the high-latitude polar regions of the Moon. We find that there are significant quantities of impact ejecta generated in the polar regions. Over long periods of time, lunar material is preferentially distributed to the mid to high-latitude regions, providing a pathway to mix equatorial and polar regolith. Additionally, we find that a polar orbiting spacecraft equipped with a dust analyzer can measure appreciable quantities of lunar ejecta near the poles to constrain the water content in the polar regions.

[1] Both steady and episodic sources have been proposed as sources of hydrogen observed by Lunar Prospector in association with the regions of permanent shadow at the poles of the Moon. Either source could supply significant quantities of water to the poles. However, space weathering processes affect the retention of water in the cold traps. We investigate those effects by simulating the evolution of a column of regolith in the region of permanent shadow over time. We determine the hydrogen concentration as a function of depth using a Monte Carlo model of discrete impacts and of delivery from the solar wind. We treat the delivery, sublimation, sputtering, and very small scale impacts as continual processes. Comparing the amount of water delivered to the poles to the amount remaining after space weathering, we find a retention efficiency of 5.6%. The retention efficiency of the polar cold traps is adequate for preserving volatile deposits over long periods of time. The average hydrogen concentration in the regolith column is 4100 ppm in the top meter after 1 Gyr. This is a saturation level in the regolith. Increasing the amount of time deepens the enriched layer but does not lead to increased concentrations. In 1 Gyr, about 1.6 m of the regolith is gardened. Therefore the top meter, which is probed by the neutron spectroscopy technique, has reached steady state in the simulations. The 4100 ppm saturation level is about half of the amount of hydrogen inferred from the Lunar Prospector neutron data.

[1] The common wisdom that water ice may exist in lunar polar cold traps has become a significant factor in the selection of space research objectives. The purpose of this paper is to address two topics that are missing from the discourse on lunar water: the effect that the pristine cleanliness of the regolith has on water transport on the moon, and the limits on water exposure implied by the extremely high adsorption potentials of the surfaces of soil grains. Water transport is characterized by chemisorption on soil grains and the mixing of wet'' grains into the regolith by meteoritic gardening. Ballistic lateral flow, which is generally thought to be an efficient conduit for moving water to the poles, is actually a secondary phenomenon that is facilitated by solar wind and micrometeor erosion but not by thermal desorption, as is the case for the dominant lunar exospheric gases, He and Ar-40. Simulation results show that even under the most optimistic conditions, less than 7% of the water accumulated in the regolith resides in the polar cold traps, where water concentrations cannot be greater than 350 ppm. More important, when realistic transport parameters are used in the simulator, the polar water concentration is reduced by almost 2 orders of magnitude. In a word, the concept of water ice at the lunar poles is insupportable.