LAUSR

Abstract The morphology of the dark polar oval seen at the Venus cloud top in visible and ultraviolet wavelengths has been unclear because the portion of the oval on the nightside is invisible. We analyzed in detail the variability of the whole shape of the oval by connecting VMC/Venus Express visible images taken on different days after shifting the images in zonal direction based on the estimated zonal advection speed. The shape of the oval was found to be changing over time between elongated shape and near-circular shape. The dominant period of this variation changes with time in the range of 200–350 Earth days, and does not seem to coincide with the orbital period, the rotation period, and the length of the day. This suggests that the variation of the oval shape is driven by some internal nonlinear process. The mechanism of oval formation is discussed.