11:30 〜 11:45
[PPS04-10] Using Atomic Modelling to Better Explain the Surface-Exosphere Connection on Airless Bodies
★Invited Papers
キーワード：Molecular Dynamics, Sputtering, Surface Exosphere Connection
For nearly 40 years, planetary science studies of the exospheres of Mercury, the Moon, and other airless bodies have been hindered because of uncertainties in our understanding of the surface processes influencing exosphere formation. The surfaces of these airless bodies can be subjected to several different emission processes including solar wind induced sputtering, photon stimulated desorption, and micrometeorite impact vaporization. However, the relative contributions of these various processes to the body’s exosphere remains contested for many observed elemental species. To obtain a true understanding of the surface-exosphere connection of airless bodies we must first improve our understanding of the interplay of these different processes and how they are affected by the specific characteristics of the surface. However, many of these key emission processes are occurring on the atomic scale, meaning global exosphere models often require atomically derived parameters as inputs. These inputs are difficult to obtain experimentally, and are therefore typically derived via fitting, often overlooking important complexities that can significantly affect predicted results. Further complicating this picture is the fact that some proportion of the ejected atoms leave the surface at energies lower than the escape energy of the body and thus return to the surface. A portion of these atoms can then be reaccommodated on the surface at an energy and composition unique from the mineral bulk. However, current global exosphere models are unable to consider the effects of adsorbed atoms nor the contribution of emission from the newly formed adsorbed layers.
Here, we will discuss how molecular dynamics (MD) modelling on the atomic scale can be a critical tool to provide physically realizable and surface-specific input parameters for global exosphere models. We will discuss a series of our previous studies that first focused on the surface binding energy (SBE) of surface atoms, a key parameter affecting the yield and energy distribution of different emission processes. We derive the first mineral specific SBE values for minerals relevant to planetary science and demonstrate that MD-derived values are highly discrepant from commonly assumed values in exosphere modelling. These results are then used as inputs into exosphere models, showing the effect on the predicting sputtering behavior and exosphere composition.
Next, we will show more recent results on how adsorbed volatiles are significantly changing the atomic structure of the exposed surface and thus the SBE. Our models consider different Na coverage cases (from 0 to 100% coverage 1 ML) onto a range of important minerals. The first case represents when individual Na atoms are adsorbed onto an initially pure surface. The second case represents when Na atoms have formed an initial monolayer (ML) and are instead adsorbed onto Na atoms, which will be referred to as 100% coverage.
We show that for low coverages the Na is tightly bound to the surface, forming strong bonds with exposed oxygen largely in line with what is found for Na bound in the mineral. However, when coverage increases to a ML there is a distinct drop in the SBE. We attribute these differences to the unique bond types formed in each case. In the 0% case, it is expected that the adsorbed Na atoms form ionic bonds with the free oxygen atoms on the mineral surface which have a high bond strength. At 100 % coverage, there are only Na-Na bonds available to be formed, which are instead metallic and have a comparatively lower bond strength. Next, we will show how diffusion and intermediate coverage scenarios may make the SBE a dynamic value that preferentially samples only specific surface sites. We will then conclude the talk by discussing other areas these approaches could be applied.
Here, we will discuss how molecular dynamics (MD) modelling on the atomic scale can be a critical tool to provide physically realizable and surface-specific input parameters for global exosphere models. We will discuss a series of our previous studies that first focused on the surface binding energy (SBE) of surface atoms, a key parameter affecting the yield and energy distribution of different emission processes. We derive the first mineral specific SBE values for minerals relevant to planetary science and demonstrate that MD-derived values are highly discrepant from commonly assumed values in exosphere modelling. These results are then used as inputs into exosphere models, showing the effect on the predicting sputtering behavior and exosphere composition.
Next, we will show more recent results on how adsorbed volatiles are significantly changing the atomic structure of the exposed surface and thus the SBE. Our models consider different Na coverage cases (from 0 to 100% coverage 1 ML) onto a range of important minerals. The first case represents when individual Na atoms are adsorbed onto an initially pure surface. The second case represents when Na atoms have formed an initial monolayer (ML) and are instead adsorbed onto Na atoms, which will be referred to as 100% coverage.
We show that for low coverages the Na is tightly bound to the surface, forming strong bonds with exposed oxygen largely in line with what is found for Na bound in the mineral. However, when coverage increases to a ML there is a distinct drop in the SBE. We attribute these differences to the unique bond types formed in each case. In the 0% case, it is expected that the adsorbed Na atoms form ionic bonds with the free oxygen atoms on the mineral surface which have a high bond strength. At 100 % coverage, there are only Na-Na bonds available to be formed, which are instead metallic and have a comparatively lower bond strength. Next, we will show how diffusion and intermediate coverage scenarios may make the SBE a dynamic value that preferentially samples only specific surface sites. We will then conclude the talk by discussing other areas these approaches could be applied.