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要旨

This work presents a comparative study of microwave properties and 1-μm emissivity for five Venus crater-associated radar-dark parabolas along with neighboring plains and some other geologic units. Craters of interest are Adivar, Bassi, Bathsheba, Sitwell, and du Chatelet with nearby crater Caccini having non-parabolic radar-dark halo. Microwave properties under study include emissivity, Fresnel reflectivity, decameter-scale surface roughness, and radar cross-section from the Magellan mission. 1-μm emissivity was derived using Venus Monitoring Camera (VMC) measurements on board of Venus Express (VEx). Results of our analysis show that 1-μm emissivity which characterizes the uppermost hundreds microns-thick layer of parabola mantles usually relates to microwave properties as following: the lower 1-μm emissivity, the lower Fresnel reflectivity, the higher microwave emissivity. For loose mantles having the same composition, lower 1-μm emissivity indicates smaller particles. Differences in bulk properties of parabola units showing the same 1-μm emissivity appear to reflect differences in a packing style of mantles consisting of particles having similar sizes. Properties of radar bright inner parabola parts, observed within the three of five studied parabolas possibly indicate more turbulent (comparing to radar-dark parts) deposition environment, thinner parabola mantles and/or partial coverage of the underlying surface. The non-parabolic halo of the crater Caccini exhibits characteristics close to dark parabolas suggesting that in a process of shrinking of a parabolic-like deposit into non-parabolic one parabola mantle properties remain mainly unchanged. Observed differences in microwave emissivity and Fresnel reflectivity between parabolas and adjacent plains may indicate highly weathered loose parabola material (with oxidation of their iron into hematite) that is more easily accessible for atmospheric gases. Studied properties of tessera terrains confirmed suggestions of earlier works for a possible non-basaltic composition of tesserae material. The distinctive (from plains) composition of tesserae also could indicates an effective down-slope movement of these material uncovering new portions of pristine tessera material. Otherwise considered in this work horizontal transportation of the very thin layer of surface material would equalize the observed properties.