ИСТИНА

The surfaces of airless bodies like Mercury undergo processes of gardening, i.e. meteoritic bombardment, and regolith alteration, due to the direct and continuous exposition of the regolith to the solar and cosmic radiation. The long exposure to the meteoritic and to the energetic ion bombardments causes deep changes in the surface features. Eventually, to have a real knowledge of the body, it would be necessary to study the layers below the exposed surface, down to at least tens of meters. The most conventional method to investigate the underlying surface layers and composition is to use a drill in a probe/lander to the surface. On the contrary, producing an artificial fresh impact crater with a massive projectile hitting the surface would allow to access the subsurface material. Part of it volatilizes and can be measured remotely from an orbiting spacecraft. Part of it is ejected and re-deposited around the cavity, so that it could be analyzed and studied by remote sensing instruments. The present mission idea consists in launching a massive and high velocity projectile, able to volatilize an adequate surface volume and generate a crater of up to a hundred meters. The projectile should be built of a special alloy that would be easily distinguishable from the one of the planet surface. The science objectives of this mission will be to study i) the lower (and pristine) layers of the surface, ii) the impact by itself and its measurable consequences on the surface and iii) the variability induced in the exosphere due to the vaporization process. Two possible mission scenarios are under investigation: 1. Using the electric propulsion or a ballistic trajectory, the cruise could last less than 6 months, but the maximum impact velocity will be about 16 km/s, thus producing a smaller crater and allowing only shallow impacts in the polar regions. In this case, the Technological Readiness Level (TRL) is much higher and the development phase will be shorter, thus the BepiColombo MPO spacecraft could be still in orbit around Mercury; hence, its payload could observe the impact effects. 2. Using the solar sail propulsion, the cruise will last 1.5-3 years, the impact velocities could arrive up to more than 100 km/s in the case of a retrograde impact, thus producing a big and deep crater at any desired position onto the surface. The TRL is still low; hence, a relatively long development time will be required. The mother spacecraft will include a dedicated payload (mass spectrometer, UV-Vis imager, IR spectrometer, high res camera). The choice of the best suited target sites and the determination of the minimal requirements of crater size and depth is under analysis.