Mercury's Sodium Exosphere: Interpretion of MESSENGER Observations

Mercury's sodium atmosphere has been the subject of many studies since it's discovery in 1985, with its extended corona and the tail-like structure probably forming its most famous attribute [5, 8, 10, 12]. In addition, the Na atmosphere is temporally and spatially variable, concentrated on the dayside, often enhanced close to the north and south poles (with a moderate north-south asymmetry), and correlates with in situ magnetic field observations. [3, 4, 6, 7, 9, 11]. MESSENGER observations, carried out between 2011 and 2015, show little or no year-to-year variation and a dawn-dusk asymmetry in Mercury's sodium atmosphere [2]. In this presentation we analyse different sodium surface release mechanisms and determine their contribution to the Hermean Na exosphere by comparing our model results to published MESSENGER measurements.

1. MESSENGER observations

The MESSENGER measurement we use as a baseline for this study was presented by [1] in their Figure 7. According to their observations, the sodium column density ranges from >10¹² cm^–2 at the surface to ~10⁷ cm^–2 at ~3500 km altitude, exhibits a steep fall-off close to the surface, and starts to flatten out at ~700 km.

2. Monte-Carlo Model

The Monte-Carlo model used herein was first presented in 2003 [13]. In this model, a large number of particles (typically on the order of 10⁶) are traced individually ab initio. Source mechanisms relevant to Mercury's exosphere are (i) thermal desorption, (ii) photon-stimulated desorption, (iii) sputtering, and (iv) micro-meteorite impact vaporization. Figure 1 shows the energy distribution functions associated with these four release processes. For this specific analysis, the effect due to radiation pressure, and an improved energy distribution for photon-stimulated desorption was added [14]. Particles are removed from the calculation domain when they (i) are ionised, (ii) fall back and stick to the surface, or (iii) reach the Hill-sphere.

Figure 1: Normalized energy distribution functions for (i) thermal desorption, (ii) photon-stimulated desorption, (iii) sputtering, and (iv) micro-meteorite impact vaporization. The vertical line shows the escape energy.

3. Simulation Parameters & Geometry

Limb scan data from the MASCS/UVVS instrument on MESSENGER presented in [1] Figure 7 was taken on 16 June 2012 at a local time of ~12:00. On that date, Mercury was at a true anomaly angle of TAA = 99°, which means that the radiation pressure was almost at its maximum. The scan itself extends from just above the surface up to an altitude of several thousand km.

4. Results and Discussion

Our analysis shows that the measurement presented by [1] can be quantitatively reproduced by particles originating from two different source mechanisms. Close to the surface, thermal desorbed Na particles with a surface temperature of 594 K dominate the exosphere. At higher altitudes, Na particles released by micro-meteorite vaporization dominate. Both the shape and the absolute value of the modelled tangent column density agree well with the measurements.

5. References