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Exospheric Redistribution of Elements in Mercury’s Soil

Presentation #106.06 in the session Mercury (Oral Presentation)

Published onOct 23, 2023
Exospheric Redistribution of Elements in Mercury’s Soil

This work uses Monte Carlo modelling to test whether the cold-pole enhancements in Mercury’s sodium exosphere could be attributed to an adsorbed surface reservoir that is created naturally via preferential deposition. MESSENGER UVVS has measured enhancements in the exospheric Na column density above 90 and 270 E longitude, locations that are termed the cold-poles because they alternately face the Sun at each orbital aphelion. Using the laboratory measurements of the temperature-dependent bouncing and sticking coefficients, along with a thermal model of Mercury’s surface, simulations show that the exosphere distributes a surface reservoir of sodium atoms towards regions with colder annual mean surface temperatures. An initial isotropic distribution with known thermally dependent gas-surface interactions naturally produces a reservoir that is concentrated at high latitudes and cold-pole longitudes. Although the Na surface concentration is unconstrained longitudinally, MESSENGER measurements sensitive to ~10 cm depths have shown that sodium is enhanced at high latitudes within Mercury’s soil and potassium concentrations in the northern hemisphere are longitudinally bi-model. These characteristics in the global alkali elemental abundances at depth may reflect the chemical composition of the northern plains. Alternatively, their consistency with exospheric deposition may indicate that the planet’s exosphere can affect its surface mineralogy on geological timescales. BepiColombo’s coverage of Mercury’s southern hemisphere soon will be able to distinguish between these two scenarios.

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