1,2,4- 1Space Telescope Science Institute, Baltimore, United States of America (mburger@stsci.edu)
- 2Goddard Space Flight Center, Greenbelt, United States of America (rosemary.killen@nasa.gov)
- 3Johns Hopkins University Applied Physics Laboratory, Laurel, United States of America (Ron.Vervack@jhuapl.edu)
- 4Boston University, Boston, United States of America (schmidtc@bu.edu)
Mercury’s escaping atmosphere is known to form an extended, comet-like sodium tail that extends as far as 1500 Mercury radii (RM) from Mercury (Baumgardner et al. 2008, Schmidt et al. 2010). The tail forms due to radiation pressure, a consequence of the resonant scattering of sunlight, the primary sodium emission mechanism at Mercury. Resonant scattering is the process by which photons at wavelengths corresponding to resonant transitions in an atom are absorbed and quickly reemitted. Because incident photons originate from the direction of the Sun and are reemitted isotropically, there is a change in the total momentum of the photons. This momentum is imparted tot he atoms, resulting in a net force directed radially away from the Sun.
The magnitude of radiation acceleration is proportional to the photon scattering coefficient, the so-called g-value, the product of the incident photon flux at the resonant wavelength and the scattering probability per atom (Chamberlain 1961). The Na g-value varies by over an order of magnitude during a Hermean year due Mercury’s changing distance from the Sun and variations in the solar flux at the Doppler-shifted resonance wavelength caused by deep Fraunhofer absorption lines in the solar spectrum. Figure 1 shows the radiation acceleration as functions of velocity (Panel a) and true anomaly angle (TAA, Panel b) for Na, Ca, and Mg at Mercury.
We will explore seasonal variations in the extended Na tail out to 1500 RM for proposed exospheric Na source processes from Mercury such as photon stimulated desorption, micrometeoroid impact vaporization, and ion sputtering. Because of the difficulty of observing the tail from ground-based telescopes, observations have only been made at a small range of true anomaly angles. Our models will result in predictions of the extent of the tail at other true anomalies and suggest methods by which it can be observed.
Figure 1: (a) Radiation acceleration, ar, for Na (red), Mg (magenta), and Ca (blue) as a function of an atom’s radial velocity relative to the Sun at a solar distance of 0.39 AU. The dotted lines show ±10 km s-1, Mercury’s minimum and maximum radial velocity relative to the Sun. (b) Radiation acceleration of the same species as a function of Mercury’s TAA for an atom at rest relative to Mercury. The scale of the left shows the magnitude of ar near Mercury. The scale on the right shows for both (a) and (b) the ratio of the radiation acceleration to the surface gravitational acceleration. From Burger et al., submitted (2025).
References
Baumgardner, J., Wilson, J., Mendillo, M. 2008. Imaging the sources and full extent of the sodium tail of the planet Mercury. Geophysical Research Letters 35. doi:10.1029/2007GL032337
Burger, M. H., et al., 2025. Effects of the Changing g-Value on Mercury’s Exospheric Structure, Planetary Science Journal, submitted.
Chamberlain, J. W. 1961. Physics of the aurora and airglow. International Geophysics Series, New York: Academic Press, 1961.
Schmidt, C. A., Wilson, J. K., Baumgardner, J., Mendillo, M. 2010. Orbital effects on Mercury’s escaping sodium exosphere. Icarus 207, 9–16. doi:10.1016/j.icarus.2009.10.017
How to cite: Burger, M., Killen, R., Vervack, R., and Schmidt, C.: Models of Seasonal Variations in Mercury’s Extended Sodium Tail Due to Variations in the g-Value, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–12 Sep 2025, EPSC-DPS2025-685, https://doi.org/10.5194/epsc-dps2025-685, 2025.
