Saturn experiences 'megastorms' every 20-30 years, finds study

A new study has shed light on the "megastorms" forming on Saturn. 
Mrigakshi Dixit
In the optical, Saturn's banded atmosphere appears to smoothly shift from color to color.
In the optical, Saturn's banded atmosphere appears to smoothly shift from color to color.

S. Dagnello (NRAO/AUI/NSF), I. de Pater et al (UC Berkeley) 

Jupiter's colossal Great Red Spot is the most famous storm in our solar system. Recent research, however, has cast light on the lesser-known "megastorms" forming on Jupiter’s neighboring gas giant: Saturn

Astronomers from the University of California, Berkeley, and the University of Michigan, Ann Arbor, discovered that Saturn creates megastroms every 20-30 years. Surprisingly, the influence of such storms is so great that the atmosphere retains chemical evidence for centuries. 

“Understanding the mechanisms of the largest storms in the solar system puts the theory of hurricanes [on Earth] into a broader cosmic context, challenging our current knowledge and pushing the boundaries of terrestrial meteorology,” said lead author Cheng Li, and an assistant professor at the University of Michigan, in an official release. 

Radio emissions from the planet

As per the official release, megastorms on the ringed planet are similar to hurricanes on Earth but far greater. 

However, scientists are unsure what atmospheric factors are causing such massive storms in Saturn. Its atmosphere is mainly made up of hydrogen and helium, with trace amounts of methane, water, and ammonia.

Astronomers examined radio emissions from the planet's surface using the Karl G. Jansky Very Large Array in New Mexico for this study.

​​The telescope's radio emission data revealed long-term disruptions of ammonia gas.

The researchers measured ammonia concentrations at various atmospheric altitudes because they move through the planet's atmosphere like water. 

“At radio wavelengths, we probe below the visible cloud layers on giant planets. Since chemical reactions and dynamics will alter the composition of a planet’s atmosphere, observations below these cloud layers are required to constrain the planet’s true atmospheric composition, a key parameter for planet formation models,” said Imke de Pater, a UC Berkeley professor.

“Radio observations help characterize dynamical, physical, and chemical processes, including heat transport, cloud formation and convection in the atmospheres of giant planets on both global and local scales,” added de Pater.

Saturn experiences 'megastorms' every 20-30 years, finds study
Huge storm dominates the rather featureless surface of Saturn in an image taken by the Cassini spacecraft on Feb. 25, 2011, about 12 weeks after the powerful storm was first detected in the planet's northern hemisphere.

Ammonia distribution anomaly

The researchers began by looking at ammonia concentrations in the aftermath of Saturn's most recent storm in 2010. Surprisingly, scientists discovered evidence that storms that occurred as early as 1876, and maybe even earlier, were still affecting Saturn's atmosphere.

The data evaluation found irregularities in the ammonia distribution of ammonia gas in Saturn's atmosphere. The authors hypothesized that the ammonia concentration anomaly might be connected to previous megastorms in the planet's northern hemisphere.

They discovered that the distribution of ammonia is depleted at mid-altitudes (below the topmost ammonia-ice cloud layer). Still, it is relatively abundant at lower altitudes — around 100 to 200 kilometers deeper in the atmosphere. 

The ammonia is most likely carried from the upper to the lower atmosphere by precipitation and re-evaporation. And such unusual ammonia concentrations at various levels may linger for hundreds of years.

The study of such massive storms on gas giants might help scientists understand how they arise on exoplanets in the future. On the other hand, it could also provide valuable insights into Saturn's winds and currents.

The results have been published in the journal Science Advances.

Study abstract:

Planetary-scale giant storms erupt on Saturn quasiperiodically. There have been at least six recorded occurrences of past eruptions, and the most recent one was in 2010, with its whole life span captured by the Cassini mission. In 2015, we used the Very Large Array to probe the deep response of Saturn’s troposphere to the giant storms. In addition to the remnant effect of the storm in 2010, we have found long-lasting signatures of all mid-latitude giant storms, a mixture of equatorial storms up to hundreds of years old, and potentially an unreported older storm at 70°N. We derive an ammonia anomaly map that shows an extended meridional migration of the storm’s aftermath and vertical transport of ammonia vapor by storm dynamics. Intriguingly, the last storm in 2010 splits into two distinct components that propagate in opposite meridional directions, leaving a gap at 43°N planetographic latitude.

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