Titan’s hazy secrets

Saturn's moon Titan is the second largest in the solar system - and the only one with a dense atmosphere. The atmosphere, nitrogen and methane, resembles that of the early Earth.

NASA’s Cassini spacecraft peered through the atmosphere, imaged the haze layers - and ESA’s Huygens probe landed on the surface.

A detector led by the University College London (the Electron Spectrometer, part of the CAPS instrument) on the orbiter detects an unexpected component in Titan's high atmosphere - extremely heavy hydrocarbon-based negative ions. Their mass is at least 10 000 times that of a hydrogen atom, detected at 953 km above the surface; about the distance from London to Milan. The image shows Titan’s haze and the heavy ions.

These form part of the haze in the atmosphere, and may fall towards Titan’s surface as organic gunk. They may become Carl Sagan’s tholins; a brown residue appearing in the Miller-Urey experiment, where a spark excites a mixture of gases resembling that of Earth’s early atmosphere.

The right hand side of the image shows the negative ion signature on the T16 encounter where CAPS-ELS sees the ions. The vertical stripes show the ions seen as the instrument is scanned through Cassini's direction of travel. Increasing numbers of ions are shown by redder colours as they are rammed into the sensor. Energy, and the mass of ions, increases vertically.