Experimental studies of ice grain ejection by massive gas flow from ice and implications to Comets, Triton and Mars

摘要

This is an experimental study of ice grain ejection when trapped gases are released from water ice. When ice is formed by adherence of water molecules at low temperatures, it forms an amorphous structure with many pores, where gas molecules can reside. When further ice layers are formed, the gases are trapped in the ice. Upon its warming-up, the ice structure changes, releasing fractions of the trapped gas. If they do not encounter obstacles, they are released quiescently by dynamic percolation. In a non-dense ice a huge flux of ice grains emanates from the ice, propelled by gas jets and covering its entire surface. When the overlying ice is denser, due to back-migration of water vapor during its sublimation, gas trying to escape from below cannot penetrate the dense ice and breaks it, producing non-circular craters and a chaotic terrain, as observed experimentally and in close encounters with Comets Wild 2, Tempel 1 and Hartley 2. These experimental findings explain several observations of Solar System bodies: ice grain ejection from Comets Temple 1 and Hartley 2. Also explained are the dark jets observed on Triton, where their ejection speed suggests a deep source. On Mars, dark streaks are observed in the southern pole in spring, most likely by plumes carrying dark dust, carried by winds and falling on the surface. As found by us experimentally, only frozen CO_2 covered by water ice or mixed with it will work to form jets, whereas pure frozen CO_2 will sublimate quiescently.