The hydrocarbon lakes and the seas on Saturn's large moon, Titan, may sometimes erupt with patches of bubbles which are caused when nitrogen gets dissolved in extremely cold liquid methane, Nasa scientists have found.
The discovery sheds a light on one of the most intriguing observation made by the Nasa Cassini spacecraft over the course of its Saturn mission – the so-called "magic islands".
The Cassini mission is a joint project between Nasa, ESA (the European Space Agency) and the Italian Space Agency. It is the first in-depth, up-close study of Saturn and its system of rings and moons, which started in 1997 when the orbiter and an ESA probe were launched into space.
Seven years later, they reached the giant planet, and the Cassini spacecraft became the first to orbit the planet. The mission has allowed scientists to uncover a lot about the sixth planet from the Sun and its moons, and it will come to an end this year.
Among the most important discoveries were the identification of an ocean beneath the icy crust of Saturn's moon, Enceladus, and liquid methane seas on another moon, Titan.
It is these seas full of methane which are at the heart of the new Nasa experiments. Scientists at Nasa's Jet Propulsion Laboratory in Pasadena, California, modelled the frigid surface conditions on Titan, and showed that significant amounts of nitrogen can be dissolved in the extremely cold liquid methane of Titan's lakes and seas.
However, the scientists find that slight changes in temperature, air pressure or composition can result in nitrogen's inability to stay in solution. As it separates out of solution, nitrogen creates bubbles in the liquid methane – a phenomenon known as exsolution.
Lead study author Michael Malaska explained: "Our experiments showed that when methane-rich liquids mix with ethane-rich ones – for example, from a heavy rain, or when runoff from a methane river mixes into an ethane-rich lake – the nitrogen is less able to stay in solution".
Exsolution can also happen when methane seas warm slightly during the changing seasons on Titan.
Investigating 'magic islands'
The patches of bubbles that result may explain the phenomenon of "magic islands" which Cassini has reported back on. During several flybys, Cassini's radar has revealed small areas on the seas that appeared and disappeared. In one case, one of these areas has subsequently reappeared.
There could be a number of explanations behind these "magic islands", but one triggered a lot of interest – the idea of fields of bubbles in Titan's seas. With their experiments, the scientists give credit to this hypothesis and detail a potential mechanism that could be forming such bubbles.
"Thanks to this work on nitrogen's solubility, we're now confident that bubbles could indeed form in the seas, and in fact may be more abundant than we'd expected," said Jason Hofgartner, co-author of the study.
Although fascinating to study, the bubble patches on Titan could potentially cause problems in the future. They could impinge on the work of future robotic probes sent to float on Titan's seas. Scientists especially worry that the heat emanating from the probes may cause bubbles to form around its structures making it difficult to steer or to keep it stable.
The spacecraft is now about halfway through its penultimate mission phase, which involves 20 orbits diving past the outer edge of the main ring system. This will go on until late April. During the 22 finale orbits, Cassini will repeatedly plunge through the gap between the rings and Saturn and could yield other interesting discoveries about the planet and its moons.