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Measurement of Heating in Mercury’s Alkali Exotail

Presentation #116.10 in the session Mercury (Poster + Lightning Talk)

Published onOct 23, 2023
Measurement of Heating in Mercury’s Alkali Exotail

With R~150,000 resolving power and fast tip-tilt image stabilization, the Extreme Precision Spectrometer at the 4.3m Lowell Discovery Telescope is ideally suited for measuring line broadening in planetary gases (Jurgenson et al. 2016; Petersburg et al. 2020; Brewer et al. 2020). Here we present measurements sampling between 0 and 5 planetary radii along the cometlike tail of Mercury’s sodium and potassium exosphere. Data were obtained surrounding maximum radiation pressure and at a 90° phase angle, where Mercury’s tail is oriented perpendicular to the line of sight. Spectra show steadily broadening linewidths in both sodium and potassium with distance from the planet. Effective temperature estimates are obtained by convolving a model of the Doppler-broadened hyperfine structure with the instrumental line spread function. The result is a dramatic heating in sodium gas from ~1,200 K to nearly 10,000 K as the gas is sampled from 0 to 5 Mercury radii downtail. Potassium effective temperatures over this range show an even greater increase from ~700 K to 11,000 K. Small-scale effective temperature variations are also seen on Mercury’s dayside with sodium at high latitudes being 100-200 K hotter than gas at the sub-solar point.

We theorize that this heating results from the recoil of photon momenta as light is re-emitted from the excited atoms. Photon re-emission is nearly isotropic in alkali D line transitions, and while direction vectors cancel on average, recoil events could still heat the gas by imparting a momentum of ℎ/ onto each atom at a variable resonance scattering rate. Due to radiation pressure, resonance scattering rates increase down-tail since the incident sunlight available to excite the gases Doppler shifts from the Fraunhofer absorption wells into the solar continuum. As atoms move down-tail with Mercury in the outbound portion of its orbit, there is positive feedback in the scattering rate and each photon scatter imparts its momentum, steadily heating the population in a random-walk. The observed difference between sodium and potassium heating supports this theory: heating would scale with radiation pressure, which is nearly 50% higher in potassium. Observations have not yet been made at the inbound leg of Mercury’s orbit where negative feedback occurs, however, and numerical modelling is needed to test this hypothesis.

This heating mechanism is previously unconsidered in dynamical models of escaping planetary atmospheres. The finding has broad implications to collisionless gas dynamics and is relevant to studies of the lunar exosphere, cometary comae and exoplanetary transit spectra.

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