Physical Properties of the Hermean Surface (A Review)

Abstract

Two new missions to Mercury are planned in the next few years (according to the NASA “Messenger” project in 2004 and the ESA BepiColomboproject in 2009). Many aspects of the study of Mercury concerning the origin of the planet, its interior structure, the formation and composition of the regolith, the surface cratering processes, the magnetosphere, the very tenuous atmosphere (exosphere) of Mercury, the orbital and rotational dynamics, and the thermal history of the planet's surface and interior are intensely developing at present. The presence of rocks on Mercury's surface, such as anorthosites (consisting mainly of calcium plagioclase) and feldspars, was reliably established in the course of such investigations. There are obvious signatures of old lava outflows and the heterogeneous composition of the crust depleted in FeO (less than 3%) and enriched with feldspar, with the possible presence of low-iron pyroxenes and alkali basalts. The sole spectral feature in the near infrared, observed at some longitudes, is a possible pyroxene absorption band at 0.95 μm, which can be used to investigate the abundance and distribution of FeO in the regolith. Mercury represents a geologically intriguing planetary object. Its exosphere contains Na and K, the origin of which is undoubtedly related to the nature of Mercury's surface. The physical properties of Mercury's regolith, its structure, the grain sizes, the refractive index, and even the characteristic sizes of block material, lend themselves, in principle, to investigation by remote sensing methods. It is possible that deposits of buried water ice and/or elemental sulfur are present in the polar regions of the planet. The results of the study of the structure, physical properties, and composition of Mercury's regolith can be used to single out fundamental features in the origin of Mercury's surface. Thermal infrared spectra are also indicative of the presence of feldspars, pyroxenes, and igneous nepheline-bearing alkali syenites. The wavelengths of the thermal emissivity maxima indicate intermediate or slightly mafic rocks with a pronounced heterogeneous composition. The iron absorption bands give evidence for the presence of FeO in the Hermean crust and mantle. To some extent, the physical properties of the crustal layers may be associated with the odd magnetic field of the planet. The resulting Hermean magnetic field may be produced, at least partly, by randomly oriented paleomagnetic fields of individual large magnetized blocks of the planet's crust.