Jupiter may be the stormiest place in the Solar System, but Saturn is no slouch either. A new study has found that the ringed giant also has persistent mega-storms that can last a century and leave deep atmospheric scars that last much longer.

An analysis of radio waves emitted from Saturn by a team of astronomers led by Cheng Li of the University of Michigan has revealed long-lasting signatures of giant storms, including equatorial storms that occurred hundreds of years ago. This is a fascinating insight into Saturn's dynamics and could help us understand the cause of the strange mega-storms that occur every few decades.

"Understanding the mechanisms of the largest storms in the Solar System challenges our current knowledge and pushes the boundaries of terrestrial meteorology by putting hurricane theory into a broader cosmic context," says Li.

Aside from its incredible ring system, Saturn appears rather soft and beige when seen in purely optical wavelengths. But in radio light, a series of vividly contrasted atmospheric bands can be seen surrounding the entire planet. Because ammonia blocks radio light, this striking perspective offers astronomers a way to map the ammonia in Saturn's atmosphere.

Li and colleagues made detailed radio observations of Saturn using the National Radio Astronomy Observatory's Very Large Array and used them to study the distribution of ammonia in the atmosphere.

Saturn's atmosphere is composed mainly of hydrogen and helium, with only traces of water, methane and ammonia; however, ammonia dominates the upper layer of the cloud. Radio observations allow scientists to see what lies beneath this top layer. Researchers have found something interesting here.

"At radio wavelengths, we probe beneath the visible cloud layers of giant planets. Since chemical reactions and dynamics will change the composition of a planet's atmosphere, observations beneath these cloud layers are essential to constrain the true atmospheric composition of the planet, an important parameter for planet formation models," says astronomer Imke de Pater of the University of California Berkeley.

"Radio observations help characterise dynamical, physical and chemical processes such as heat transport, cloud formation and convection in the atmospheres of giant planets on both global and local scales."

The brighter bands seen in the radio images indicate a much lower concentration of ammonia in these bands. As expected, the team found ammonia in Saturn's uppermost cloud layer, but they also found anomalous ammonia concentrations 100 to 200 kilometres (62 to 124 miles) deep in the atmosphere. The mid-altitude region between the two is relatively ammonia-free.

The team's analysis shows that megastorms, which occur on Saturn every 28 to 30 years (i.e. once every Saturnian year), precipitate ammonia deep into the atmosphere, where it evaporates and is transported back to the cloud tops. Deep ammonia concentrations are therefore records of ancient megastorms that can persist for hundreds of years, long after the storm itself has dissipated.

The researchers were able to trace the anomalies they detected in all six megastorms recorded on Saturn since 1876. They also found an anomaly that they believe is a precursor to an older megastorm.

Since both are gas giants, we tend to associate events on Saturn with those on Jupiter. The team's findings also show that the two are more different than we think. In Jupiter, temperature anomalies are associated with alternating dark and light cloud bands. On Saturn, it's all about storms. This is a clue that could help us piece together the different ways in which gas giant exoplanets can evolve.

By the way, the next mega-storm is predicted to happen in 10 or 20 years. It will be fascinating to see what new things it can teach us.

 

Source: https://www.sciencealert.com/