Sputtering Contributions to Mercury’s Exosphere During Average and Extreme Solar Wind Conditions

Solar wind ions that impact Mercury contribute to alteration of its surface and erosion via sputtering [1]. The exact contribution of sputtering to the weathering of Mercury’s surface is, however, still unclear. To provide updated estimates of sputtering as a source for Mercury’s exosphere, we use a combination of two simulation approaches: the Amitis hybrid code and the SDTrimSP-3D ion-surface interaction model [2,3]. In doing so, we first simulate the global interaction of Mercury’s magnetic field with the solar wind to derive energy-resolved precipitation maps (see Figures 1 and 2). These precipitation fluxes are then combined with sputter yields for a regolith-covered surface to calculate sputter flux maps and global sputter source rates. In this work, we specifically consider the sputtering of the refractory elements Ca and Mg, which have been observed by both NASA’s MESSENGER mission and ESA and JAXA’s BepiColombo [4-7]. Due to their strong bonds at the surface, only sputtering and micrometeoroid impact vaporization are candidate processes for releasing these atoms into the exosphere.

Figure 1: Due to Mercury’s magnetic field, ions impact the surface only at specific locations.

 

Our simulations consider a range of solar-wind upstream parameters, as the variability of Mercury’s space environment affects the flux of ions to the surface significantly. First, we consider average solar wind conditions and find that sputter rates of Ca and Mg are 1-2 orders of magnitude too low to explain MESSENGER observations. This is in line with micrometeoroid models that predict the seasonal dependence of their exospheric sources, and the dawn-centered emission well [8]. Our study suggests that the sputter yields in previous models have been overestimated, and it supports that micrometeoroid impact vaporization dominates refractory exospheres at most times.

         

Figure 2: Compared to average precipitation conditions (left, dynamic pressure of 9 nPa), the ion flux to the surface under strong CME impacts is vastly increased (right, 428 nPa).

 

However, under extreme space weather conditions during the impacts of coronal mass ejections (CMEs), we see a significant change of the exospheric regime. The solar wind pressure has been observed to increase by up to around a factor of 50 compared to regular conditions [9]. This has been reported to lead to compression and even vanishing of Mercury’s dayside magnetosphere [10]. As the planet’s dayside surface becomes exposed directly to the solar wind, our simulations show that surface precipitation and sputtering increase by as much as two orders of magnitude (see Figure 2). Under such conditions, sputtering exceeds the source rates for Ca and Mg exospheres from micrometeoroids. We thus infer that a temporary reconfiguration of Mercury’s exosphere occurs during strong CME impacts, especially when the solar wind dynamic pressure reaches more than 100 nPa. This is further underlined by Direct Simulation Monte Carlo (DSMC) simulations of the spatial distribution of atoms in Mercury’s exosphere [11]. Compared to a micrometeoroid-dominated dawn-centered exosphere under average SW conditions [12], we demonstrate the distribution of refractory atoms becomes centered on the dayside due to the vast sputtering increase. The upcoming BepiColombo mission will provide plenty of opportunities to observe such events to better understand how Mercury’s exosphere, surface, and space environment are connected.

 

References

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