37th DPS Meeting, 4-9 September 2005
Session 61 Planetary Rings
Poster, Thursday, September 8, 2005, 6:00-7:15pm, Music Lecture Room 5

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[61.12] Multilayer model for thermal infrared brightness of Saturn's rings revisited

R. Morishima, H. Salo (Univ. of Oulu), K. Ohtuski (LASP)

We are modeling the thermal infrared brightness of Saturn's rings based on a multilayer approximation. In the model, the vertical heterogeneity of spin frequency of particles and the heat transport due to particles' vertical motions are both taken into account. Provided that a ring has an extended size distribution, we assume that smaller particles have higher orbital inclinations and faster spin frequencies. If we choose a proper optical depth fraction of fast spinning particles and observe the lit side of the ring, we mainly see fast spinning cooler small particles for small viewing elevations, whereas slowly spinning warmer large particles start to be better visible near the maximum elevation ~26\circ. Further, the vertical heat transport reduces/increases the ring lit/unlit side brightness, which effect is particularly important for small solar elevation angle. Due to these two effects the thermal brightness of Saturn's rings seen from Earth (viewing angle ~ solar elevation angle) is somewhat enhanced with elevation angle. However, it is still insufficient to explain the observed elevation angle dependence for A and B rings (tile effect). Vertical heterogeneity of particle albedo can further enhance the elevation angle dependence, but it is more likely that large particles in these dense rings are rather packed and the effect of finite size of particles for mutual shadowing, which is neglected in a multilayer approximation, is significant.

This work is supported by the Academy of Finland and the Oulu University special research-unit grant.

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