HIGH-VELOCITY IMPACTS OF DUST PARTICLES IN LOW-TEMPERATURE WATER ICE

Laboratory experiments to measure the water vapour release caused by impacts of fast dust particles on a pure water ice surface were performed with an electrostatic dust accelerator. The dust particles have masses between 10(-14) and 8 x 10(-11) g and sizes in the micrometer range. The velocity range of the dust particles varies from 1 to 50 km s-1, depending on the size of the dust particles ; this corresponds to projectile energies of 10(12) eV. The target consists of an aluminum block, cooled by liquid nitrogen, which contains a pure water ice kernel of 1.2 cm diameter and 0.5 cm depth. The experimental set-up of the dust accelerator permits us to select iron dust particles of a specified velocity and mass range. About 800 impacts were detected, and the pressure difference from the sublimented ice in the target chamber was recorded before and after the impact. Typical pressure differences had values of 10(-6) mbar. The sensitivity of the pressure measurement was 10(-8) mbar and the volume of the target chamber 100 cm3. The duration of each pressure pulse was below 100 ms, and after this time the pressure in the target chamber reached its initial value. Crater masses and volumes were calculated from the obtained data and compared with crater parameters from Frisch (in Hypervelocity Impacts in Space, University of Kent, 1992) and Lange and Ahrens (Icarus 69, 506-518, 1987). Both works had projectile energies several orders of magnitude higher than those used in this work (Frisch 10(16) eV, Lange and Ahrens 10(21) eV). The power laws for crater volume vs impact energy were compared. This work extends the knowledge for micrometeroid crater formation on icy planets and presents data for quantitative analysis of micrometeroid erosion of planetary rings.