Saturn's mega-storms challenge planetary formation models

Scientists are reassessing theories of how gas giants form after observing that fallout from Saturn's cyclical mega storms can last for centuries. 

Observations dating as far back as 1876 have shown that visible storms lurk beneath Saturn's normally calm outer atmosphere. Some of the storms have been so large they left visible surface disturbances when viewed from a telescope. 

Evidence of all but the most recent storm is not possible to see, but the rest are still detectable from radio emissions, which were the topic of study for an international team led by University of Michigan professor Cheng Li. Those images, the team concluded, point to the storms continuing to affect Saturn's inner atmosphere.

The images, data for which was captured way back in 2015 using the National Radio Astronomy Observatory's Very Large Array, depict radio bands present in Saturn's atmosphere. Lighter bands that appeared across Saturn's northern hemisphere were indicative of higher temperatures, which Li's team believes are warmer because Saturnian storms caused ammonia vapor in the planet's atmosphere to condense into "ammonia-rich mushballs" that rained down into the depths of the gas giant. 

Radio band images of Saturn - lighter bands indicate hotter areas where giant storms passed - Click to enlarge

Because of cold, "nearly inviscid" conditions in Saturn's deeper layers it would take years for convection currents to push the ammonia back into the higher atmosphere. "This is exactly what we have seen using longer wavelength radio observations," the boffins noted in their paper. 

Batten down the hatches - better yet, seal 'em shut

When ammonia rains and snows down from the upper limits of Saturn's atmosphere following one of the mega storms that form every 20 to 30 years on the planet, it stays down for a long time. 

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The team said they found three distinct patches of ammonia anomalies in Saturn's atmosphere, and all six storms that have been tracked on Saturn since 1876 fell inside the latitude of the three patches. The team even found evidence of a seventh storm that likely occurred before 1876. 

Li describes the storms as akin to giant hurricanes, but ones that form in different ways to their terrestrial cousins. "Based on the theory of hurricane genesis on Earth, a warm ocean surface is needed," Li told The Register but of course, there's no solid surface on Saturn, "How a huge hurricane can be generated and maintained without the energy supply from the warm surface beneath it is a mystery," Li added.

Even more planetary mysteries

Aside from raising questions of storm formation, the researchers also pointed out that their findings point to Saturn's atmospheric dynamics being "substantially different from Jupiter's."

Take Jupiter's bands, for example - the researchers said that those bands are related to temperature differences, and recent research pointing to Jupiter's magnetic field as a cause for their formation. Saturn's bands, however, are dominated and caused by the giant storms that form on its surface. 

Because they were using radio imaging to detect the bands, Li said his team was also able to determine that ammonia concentration is generally less on Saturn than on Jupiter. While he said that data is also still preliminary, it suggests a shortcoming in our current theory of planetary formation. 

"We know that Saturn should have more water than Jupiter based on planetary formation models," Li told us - but ammonia is another thing altogether. "If Saturn has less ammonia than Jupiter, then the theory of planetary formation should be revised." ®