Processes Producing the Extremely Hot Ca and Mg Exospheres at Mercury

Abstract:

The Mercury Atmospheric and Surface Composition Spectrometer on the MESSENGER spacecraft observed calcium emisison in Mercury's exosphere on a near-daily basis for >16 Mercury years. The calcium was persistently concentrated in the dawn hemisphere and was of extreme temperature (>50,000K). Any mechanism producing the Mercurian Ca exosphere must explain the facts that the Ca is extremely hot, that it is seen almost exclusively on the dawnside of the planet, and that its content varies seasonally, not sporadically. In this paper we consider the energization processes. We start with the assumption that calcium is ejected into the exosphere by impact vaporization, initially in the form of simple molecules, and that they are subsequently dissociated by some process. We consider here simple diatomic molecules or their clusters, focusing on calcium oxides while acknowledging that Ca sulfides may also be the precursor molecules. The pathways we discuss are (1) impact vaporization producing Ca and Ca-oxide clusters, (2) electron-impact dissociation of CaO molecules, (3) spontaneous dissociation of Ca-bearing molecules following impact vaporization, and (4) photodissociation. The most likely origin of extremely hot Ca seen in Mercury's exosphere is photodissociation or dissociative photoionization of a precursor CaO molecule or clusters, or possibly some other diatomic molecule such as CaS, produced by impact vaporization. Photodissociation of the CaO molecule results in a hot Ca atom and an even hotter oxygen atom. This explains both the observation of hot refractories in Mercury's exosphere and the absence of observable oxygen (Vervack et al., 2015), which would rapidly escape from the exosphere. It has been shown previously that CaO clusters and even dimers are produced by evaporation of diopside, and that the dawnside origin of the Ca is the result of preferential impact vaporization in the ram direction as Mercury moves through the interplanetary medium. In addition to Ca, there has been some indication that Mg in Mercury's exosphere is perhaps as hot as 20,000 K and may be concentrated in the dawn hemisphere (Sarantos et al., 2011). Some of the arguments put forth for Ca may also apply to Mg.