Presentation #502.05 in the session “Solar Wind & Exospheres”.
The importance of sputtering in promoting atoms into the exospheres of planets exposed directly to solar wind ion irradiation has been widely debated. The surface binding energy (SBE) for each element sputtered from a given compound determines both the corresponding elemental sputter yield and also the velocity distribution of the ejected atoms. The SBE has often been taken to be the sublimation energy of the element ejected from a monatomic substrate. We have shown that the SBE is often much larger than this sublimation energy. Specifically, we have simulated the Na exosphere of Mercury with values of SBE ranging from 0.27 to 7.9 eV, incorporating values from published works and our newly calculated results for Na-bearing albite that were obtained using Molecular Dynamics simulations with the ReaxFF forcefield. We note that the peak in the velocity distribution of the ejected atoms increases as the SBE does, but the sputter yield decreases with increasing SBE. Incorporating our results into a model exosphere, we find that the higher the SBE, the more extended the exosphere, but the lower the density. The lowest SBE of 0.27 eV produces a high column abundance near the planet, but a smaller scale height. The highest SBE of 7.9 eV results in more than an order of magnitude lower column abundance, but a larger scale height with trajectories that are almost vertically escaping from the planet. Solar wind ions precipitate to the surface of Mercury through the northern and southern cusp regions of the planet’s magnetosphere. The northern cusp has been found to be approximately confined to a width of 10° around 65° latitude. For our modeling, we have used a similar cusp for the southern hemisphere. Here, we will show the results of these simulations for the exosphere of Mercury and discuss their implications for the importance of sputtering as a mechanism for producing the Na exosphere of Mercury.