Fluidization and multiphase transport of particulate cometary material as an explanation of the smooth terrains and repetitive outbursts on 9P/Tempel 1

摘要

The Deep Impact mission discovered repetitive outbursts on Comet 9P/Tempel 1 and the presence of several smooth terrains on its surface. We present new measurements of the extent of the smooth terrains, the slopes along their centerlines, and the areas of their likely source regions. Our analysis of these features indicates that they are <700 orbits old and probably the result of an ongoing process. The implications of the recently found locations of the source regions of the repetitive outbursts are also analyzed. We propose that the origins of these phenomena are in the different regimes of fluidization and gas transport in a weakly bound particulate mixture of ice and dust above an assumed amorphous/crystalline H_2O phase change boundary where CO and/or CO_2 gas is released. The depth of this boundary is estimated to lie between 30 and 100 m below the surface. The smooth terrains are visualized as occurring about once every ～70 orbits at random locations of the nucleus where a spurt in CO production occurs over a limited region of the phase change boundary. The weak (tensile strength ～ 10~2 Pa) crystalline and dust overburden is locally ruptured and fluidized by the CO gas pressure and is then extruded onto the surface at speeds of ～0.003-0.03 m/s, well below the escape velocity of 1.3 m/s. Once on the surface a base pressure of only 2.5 Pa is required to ensure fluidization of the extruded material and it can remain fluidized for typically ～20 h against diffusive loss of CO. As the material accelerates down the local topography it deflates due to diffusive gas loss. The flow becomes increasingly viscous until it is no longer fluidized at which point it quickly halts forming a terminal scarp. The mean speed of the laminar flow is estimated at 0.3 m/s for an emplacement time of ～3 h. Topographic features on the flow >0.3 m in size should become fully relaxed during the emplacement time explaining the smooth texture seen in the images. In contrast, the repetitive outbursts require a gas-laden reservoir to have formed in the vicinity of the phase change boundary well below their preferred location. We visualize the outbursts to be the result of either spouting or bubble transport to the surface where the release of gas is diurnally modulated by either thermal stresses or H_2O sublimation back pressure. The source region for the i2 smooth terrain is found to coincide with an H_2O-ice rich area and we propose that the process of elutriation, i.e., the separation of different classes of particulates depending on their drag properties, occurs in the fluidized material as it flows up to and through the surface. In this way the material becomes enhanced in H_2O crystals relative to siliceous and carbonaceous particulates.