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The Role of Solar Wind Sputtering for Mercury’s Sodium Exosphere

Presentation #207.01 in the session Surface Processes on Mercury: ’Tis but a Scratch.

Published onOct 20, 2022
The Role of Solar Wind Sputtering for Mercury’s Sodium Exosphere

A long-standing controversy concerns the role of sputtering in the creation and variability of Mercury’s exosphere. The surface binding energy (SBE) of various minerals, which determines both the yield and energy distribution of sputtered ejecta, has recently been calculated using a molecular dynamics code. To better constrain the role of solar-wind sputtering in the creation of Mercury’s sodium exosphere, we have simulated Mercury’s sodium exosphere with a Monte Carlo code. We include the source processes photon-stimulated desorption (PSD), impact vaporization (IV) and solar wind sputtering with SBEs of 0.27, 2.6, 4.4, and 7.9 eV, respectively. The lowest value SBE, 0.27 eV, is commonly used in exosphere models. The highest value, 7.9 eV, is calculated using molecular dynamics for the Na-bearing feldspar endmember, albite. We find that the processes responsible for generating Mercury’s Na exosphere are separable by measuring line-of-sight column densities tangent to the planet at various altitudes and positions around the planet. The low-altitude exosphere is dominated by PSD, whereas the altitudes above ~1000 km are dominated by IV. We conclude that high-altitude data (> 1000 km above the surface) are consistent with a small portion of the Na being sputtered from a high-SBE material such as feldspar, which has been predicted to be abundant on the Mercury’s surface, and that the SBE for that surface is generally consistent with values on the order of 7.9 eV. A high SBE is favored given that there is little high-altitude variability seen in MESSENGER observations of the sodium exosphere. This suggests that for the high-altitude component of Mercury’s Na exosphere, solar wind sputtering is a small contribution on top of a primarily dominant IV component.

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