Icy cyanide cloud detected over Saturn’s moon Titan

Nature : Over the past two years, a cloud has been forming over the south pole of Titan, the largest of Saturn’s moons. The discovery was not unexpected, as the southern hemisphere has been shifting to winter, and a cloud had appeared over the north pole a decade earlier when the northern hemisphere was experiencing winter. However, data from NASA’s Cassini spacecraft indicate that the cloud is much higher, at 300 km, than expected. It is also composed of micrometer-sized particles of hydrogen cyanide ice, indicating that the air temperature is at least 100 K colder than predicted. Because the cloud only appeared two years ago, and there was no indication of cyanide ice at that time, the atmospheric temperature must be dropping relatively rapidly. The surprising findings indicate that Titan may be more dynamic than researchers thought and that global-circulation models of Titan’s atmosphere may need to be revised.