The modifications of Mercury surface induced by solar wind ions

Abstract

Airless bodies of the Solar System, such as Mercury, are continuously exposed to the bombardments of photons, meteorites of different size, cosmic rays and solar particles (ions from flares and solar wind, a rarefied plasma made of electrons, protons, heavier ions and magnetic fields streaming radially from the Sun). Such projectiles, hitting the surfaces, can induce on them sputtering, implantation, photo or thermal desorption, vaporization, erosion. These processes alter the physical and optical properties of the surfaces, contribute to the mass loss of the bodies and constitute the so called space weathering. Mercury is the innermost Solar System planet, with the highest orbital eccentricity and some peculiar orbital characteristics which give it a great thermal excursion (~700 K) between Sun-exposed and the dark sides, and make this planet a very interesting subject for the astrophysical community because many questions remain open as e.g. information on its principal chemical constituents. Some ground-based observations of Mercury have revealed a heterogeneous mineralogical surface composition and the presence of sodium, potassium and calcium, nevertheless, the greatest contribution to the knowledge of the planet has been obtained thanks to space missions. The future joint European-Japanese space mission, BepiColombo, will add complementary information and new laboratory experiments are also requested for testing its instruments, prepare and validate the results. The aim of this thesis work is to characterize some Mercury analog materials in preparation to ELENA (Emitted Low-Energy Neutral Atoms) experiments aboard BepiColombo. The candidate materials, here chosen on the basis of existing data in literature, are old and fresh erupted etnean basalts, together with other silicate rocks (sodalite, nepheline, jadeite) containing sodium and potassium. The variations in the spectral reflectance of these candidate Mercury materials, before and after irradiation with energetic ions (200 keV H+ and Ar+) aimed at simulate cosmic ion bombardment, have been studied, with the intent to be compared with observational Mercury spectra. Some possible analogies have been tested operating an opportune scaling of the laboratory ion energy and fluences, in order to estimate the correspondent exposure time scale at Mercury. Our Mercury analog materials seem to be the best candidates so far found, even if a wider study on different materials is needed.