Dopamine acts through Cryptochrome to promote acute arousal in Drosophila.
tor, in contrast to Earth’s tropical rainforest climate? Earth’s ITCZ can migrate only within the tropics because the large latitudinal temperature contrast at mid-latitudes gives rise to mid-latitude weather systems, which prevent the ITCZ from shifting further poleward. Because the tropical rain belt is often wider than its seasonal excursion, the equator can barely escape rainfall in any season. Conversely, Titan’s ITCZ is predicted to migrate nearly from pole to pole, so that it rarely stays near the equator ( 6). This dramatic global migration of the ITCZ is a consequence of Titan’s slow planetary rotation (15 days, 22 hours), which requires the ITCZ to extend far beyond Titan’s tropic (26.7°) and proceed to the summer pole to ensure conservation of angular momentum and energy ( 7). This pattern reverses twice in the course of an orbital cycle of Saturn (29.5 years). During recent years, the ITCZ of Titan was presumably located near the south pole and is now on the way to the north pole. Although Titan’s precipitation pattern is generally explained by global atmospheric circulation, it is also thought to be highly sensitive to several uncertain factors such as surface moisture ( 6), tropospheric ethane abundance ( 8), or chemical composition of polar lakes ( 9). On regional scales, orography (terrain) may also affect the rain distribution substantially ( 10). The lack of knowledge of many of these parameters, and the difference in physicochemical behavior between water and methane, thus makes rain prediction a challenging task. An individual rainstorm on Titan could cause surface precipitation of a few hundred millimeters within hours ( 11, 12), but the question is how often such rainstorms occur at individual places. Titan’s precipitation climatology is clearly different from that of Earth, and exotic climate zones unknown in Köppen’s classifi cation ( 1) may exist. Observations such as those by Turtle et al. reveal the geographical distribution, seasonal timing, interval between rainstorms, and relative intensity of rainstorms, although smaller-scale or weaker rainfall might escape detection. It will also be helpful to search for changes in cloud top altitude ( 12), rivers, or polar lakes, as they may be diagnostic of surface precipitation.