Evidence for Impact Vaporization As a Source of Calcium in Mercury's Exosphere

Abstract:

Mercury is surrounded by a surface-bounded exosphere with six known components: H, He, Na, K, Ca, and Mg. Of these components, Ca is of extreme temperature and has been observed in Mercury's exosphere for the past decade, after its discovery by Bida et al. (2000) from observations with the Keck telescope on Mauna Kea. Observations of the Ca exosphere by MESSENGER show a source concentrated on the dawn side. The time variation in that Ca signal repeats every Mercury year (Burger et al., 2014). Figure 1 shows how the Ca source rate varies with true anomaly angle, ν, which is the planet's orbital longitude measured from the direction of periapse. We suspect that these variations are due to impact vaporization resulting from infalling interplanetary dust. Our models of this scenario show that much of the seasonal variation in Ca is due to Mercury's substantial radial motion through the interplanetary dust cloud that results from Mercury's large orbital eccentricity (e=0.2). The seasonal Ca variation is enhanced further by Mercury's large orbital inclination (7° relative to the ecliptic), which causes additional periodic variations in the dust infall rate as Mercury's vertical motion carries it repeatedly across the dust-disk's midplane. Note also the strong Ca enhancement near ν~20° (Fig 1) which requires an additional source of localized dust. The location of that peak is close to (but not precisely coincident with) Mercury's orbital longitude as it crosses the orbital plane of comet 2P/Encke.

Bida, T.A. et al., 2000. Nature 404, 154-161.

Burger, M.H. et al., 2014, Icarus 238, 51-58.

Figure 1. Vaporization rate of Ca at Mercury from interplanetary dust (green line) versus true anomaly angle, plus an assumed cometary source centered at 25° with a Gaussian half width of 15°. The modeled source rates from Burger et al. (2014) are plotted in black. The sum of the disk and cometary sources is plotted in red.