Mars lost most of its atmosphere long ago to the solar wind due to a lack of a meaningful electromagnetic field. Saturn's Titan has an atmosphere denser than Earth's, yet Titan is a smaller body than Mars. Why hasn't the solar wind carried Titan's atmosphere away?
Roughly speaking, at the distance of Saturn, the solar electromagnetic power per unit area and solar wind flux are sufficiently low that elements and compounds that are volatile on the terrestrial planets tend to accumulate in all three phases. Titan's surface temperature is also quite low, about 90 K. Therefore, the mass fractions of substances that can become atmospheric constituents are much larger on Titan than on Earth.
In fact, current interpretations suggest that only about 70% of Titan's mass is silicates, with the rest consisting primarily of various H2O ices and NH3-H2O (ammonia hydrates). NH3, which may be the original source of Titan's atmospheric N2, may constitute as much as 8% of the NH3-H2O mass.
Much of the original atmosphere appears to have been lost over geologic time. But since Titan began with a proportionally greater volatile budget than Earth or Mars, atmospheric pressure on its surface remains nearly 1.5 times that of Earth's. It is possible that most of the atmospheric loss was within 50 Ma of accretion, from a highly energetic escape of light atoms carrying away a large portion of the atmosphere (hydrodynamic blow off event). Such an event could be driven by heating and photolysis effects of the early Sun's higher output of X-ray and ultraviolet (XUV) photons.
We don't really know why only Titan has a thick atmosphere, while the structurally similar Ganymede and Callisto don't. Temperatures may have been too high (well above ~40K) in the Jovian subnebula due to the greater gravitational potential energy release, mass, and proximity to the Sun, greatly reducing the NH3-hydrate inventory accreted by Callisto and Ganymede. The resulting N2 atmospheres may have been too thin to survive the atmospheric erosion effects that Titan has withstood.
Alternatively, cometary impacts may release more energy on Callisto and Ganymede than they do at Titan due to the higher gravitational field of Jupiter. The higher energies could erode the atmospheres of Callisto and Ganymede, while the cometary material would build Titan's atmosphere. Nevertheless, D/H ratios suggest that cometary input is unlikely to be the major contributor to Titan's atmosphere.
As with Mars, Titan's internal magnetic field is negligible, and perhaps even nonexistent. Furthermore, relative speed between Saturn's magnetic field and Titan actually accelerate reactions within Titan's atmosphere, instead of guarding the atmosphere from the solar wind.
If you would like to know the underlying research and more technical details, I have posted them as a co-edited entry online.
This page was last updated on July 18, 2015.