Titan is unveiled by Cassini–Huygens, showing an active hydrological cycle involving methane forming rivers, lakes and seas and exotic clouds of methane, ethane and hydrogen cyanide. Vast fields of sand dunes form in an equatorial desert (shaped by the Hadley circulation on this slowly rotating world). The dunes and seas shed light on present and past climate on Titan. Structure of the Martian polar cap is revealed by radar, amid efforts to decode its record of Mars climate variations. Sulfur dioxide varies on Venus – a signature of Volcanism?