Cassini Imaging Science Subsystem observations of Titan’s south polar cloud

Abstract

In May of 2012 images of Titan obtained by the Cassini Imaging Science Subsystem (ISS) showed a newly-formed cloud patch near the southern pole. The cloud has unusual morphology and texture suggesting that it is formed by condensation at an altitude much higher than expected for any of the known organics in Titan’s atmosphere. We measured the altitude to be 300±10km from images when the feature was on the limb. Limb images suggest that the initial stages of the formation began in late 2011. It was just visible in images obtained in 2014 but is not expected to be visible in the future due to enveloping darkness as the season progresses. The feature has a slightly different color than the surrounding haze. Its optical thickness is near 2 at 889nm wavelength and the particle imaginary refractive index must be less than 5×10−4 at that wavelength. Wind vectors derived from a time series show that it is rotating about a center offset by 4.5° from Titan’s solid-body spin axis, consistent with that found from the temperature field by Achterberg et al. (Achterberg, R.K., Conrath, B.J., Gierasch, P.J., Flasar, F.M., Nixon, C.A. [2008a]. Icarus 197, 549–555) and subsequent measurements. The feature rotates at an angular velocity near the rate expected for transport of angular momentum from the low latitudes to the pole. The clumpy texture of the feature resembles that of terrestrial cloud fields undergoing open cell convection, an unusual configuration initiated by downwelling.