Titan is one of the most intriguing moons in the solar system: Not only is it the largest, it’s the only one with a sizable atmosphere full of aerosols and complex organic molecules—not so dissimilar to the primeval soup from which life emerged on Earth.
The chemical reactions that produce this gaseous cocktail begin in the atmosphere’s upper reaches, where the Sun’s ultraviolet rays break molecules apart and strip them of electrons, helping to create the layer of plasma that makes up the ionosphere. A new study finds that Saturn’s magnetic field is also important, in two different ways.
Edberg et al. used data from NASA’s Cassini spacecraft, which has been orbiting Saturn since 2004—monitoring the planet and its magnetic field and performing flybys of its moons. The mission includes 110 passes of Titan in which the craft used a stubby probe to collect electrons and measure the density of the plasma as it flies through.
The team found that high above Titan’s surface (at altitudes of 1600 kilometers or greater), the plasma density depended greatly on whether Titan was on the nightside or dayside of Saturn: On the nightside, the plasma was roughly 2.5 times thicker. This may seem counterintuitive since the Sun’s rays replenish plasma, but this is not an issue: Even on the nightside of Saturn, Titan is almost always in sunlight because its orbit is sufficiently tilted and distant.
Instead, the team suggests that Saturn’s magnetic field is to blame—it’s stronger on the dayside, forcing the high-altitude density to lower in order to maintain a pressure balance. On the nightside, the reverse is happening. The difference in strength is due to the solar wind: The surge of protons streaming from the Sun bludgeons the dayside of Saturn’s magnetic field, compressing it and making it stronger. As Titan barrels through this region, its ionosphere naturally compresses to balance the greater magnetic pressure. On the nightside, when the magnetic pressure drops, it expands, explaining the higher density readings at high altitudes.
Interestingly, Cassini found a totally different effect at work when it dipped lower to Titan’s surface: The plasma density was affected not by night or day but by how close Titan was to Saturn’s equator. That’s because within Saturn’s magnetosphere, electrons are densest in a disk around its equator—and they can ionize molecules in Titan’s atmosphere when they collide. When Titan was plowing through this disk, its own levels of plasma jumped by 40% compared to when it was outside of it, at the apexes of its tilted orbit.
The results help paint a clearer picture of the chemistry in Titan’s atmosphere and may even have implications for understanding alien life. Titan is a habitable world: It has oceans of water beneath its icy surface. If Titan has biology, it possibly sprang forth from the chemical reactions that begin in the ionosphere. (Journal of Geophysical Research: Space Physics, doi:10.1002/2015JA021373, 2015)
—Mark Zastrow, Freelance Writer
Citation: Zastrow, M. (2016), How Saturn alters the ionosphere of Titan, Eos, 97, doi:10.1029/2016EO044209. Published on 28 January 2016.