IMPACT MELT RHEOLOGY

Impact melt is a common product of cratering with amounts proportional to impact energy. At simple craters, melt occurs as rim coatings, floor ponds, and flank flows. At complex craters, it occurs on the floor, perched on rim blocks and downslope beyond the rim. Analyses of impact melt provide insight into composition, distribution, dynamics of ejection and deposition, and physical properties. As melt flows are silicate liquids, they behave similar to lava flows, thus lava flow rheology can be used to constrain impact flow rheology. Similar to lava flows, impact melts cool by radiation. Estimates of radiative cooling of flows suggest time-scales for solidification of hours-days. Flows with an anorthositic composition cool significantly more slowly than basaltic composition. Entrainment of "cold" debris will also influence both the viscosity and the cooling history. Flows at Giordano Bruno, Byrgius, Das G, Gibbs, Korolev X, and other craters were examined. At Bruno and an unnamed 9 km diameter crater (13.5°N, 234°E) flows are similar: they begin as channelized flows with levees and evolve into tabular flows. Varying morphology allows for use of different parameters that are, in part, independent of each other. Flows at the south rim of Giordano Bruno are narrow (~50 m), ~350 m long, on a slope of 4.2°. Yield strength is 1-2 x 10^2 Pa. At the unnamed 9 km crater, a flow on the NE rim begins as a broad flow (~150 m) and divides into two independent flows with levees on a steep slope (14.4°). The flows are 60-70 m wide, levees 15-20 m wide, central channel is 2 m deep and the flow is 8-10 m thick - suggesting yield strengths of 1 x 10^4 Pa. Yield strengths estimated for Bruno and the 9 km crater differ from earlier studies by as much as two orders of magnitude. Estimates based on LRO data are lower than earlier studies. However, is it not clear if the differences are real or an artifact of the lower resolution data used in earlier studies. Qualitatively, the data are better behaved on steep slopes and may result from minor slope variations represent being a smaller fractional change than on a shallow slope, thus reducing "noise." Similarly, flows on steeper slopes (other aspects held constant) have greater momentum and are less influenced by minor surface changes. It is interesting to note that flows at Bruno, the largest crater, have the lowest yield strengths.