Saturn’s Super Storm

If you live in the US, you will remember the great February snowstorm of 2010 – which entered history as “Snowmageddon” – that covered the East Coast in thick snow and paralyzed cities and airports. It was one of the largest winter storms in recent history.

Yet, the same year in the outer solar system another storm developed that dwarfed Snowmageddon – in fact, it dwarfed all storms combined on our planet. This much larger, much colder, and arguable much more mysterious storm has developed in the atmosphere of Saturn.

This was not the first such storm on Saturn: roughly every Saturnian year (29.3 Earth years) a dramatic mega-storm develops. These storms have been observed three times by now, always occurring on the northern hemisphere on Saturn during its summer.

Although the mysterious storms have been seen before, what was different this time was that a spacecraft was present in the saturnian system. Cassini got a first row seat to observe the megastore develop, engulf the northern hemisphere and eventually dissolve, after several months.

Cassini‘s amazing images of the gigantic storm have been published before, but the nature of the storm remained unexplained. Now, in a a University of Wisconsin team led by Lawrence Sromovsky presents a detailed analysis of the storm. The group has worked on trying to figure out the composition of the material dredged up by the storm.

To understand this monster storm let me tell you a bit about Saturn itself. Saturn is a very cold world — at least its upper atmosphere which is visible to us. At 1 bar (the same pressure as at sea level on Earth) Saturn’s atmosphere is only 134 K. Saturn has as much mass as 95 Earths would have – and this massive, cold planet rotates fully around every 10.7 hours!

Like the Solar System’s other gas giant, Jupiter, Saturn is mostly made up of hydrogen and helium, the most common elements in the universe. Most of Saturn’s hydrogen is in its molecular form (), concentrated to the upper layers of the atmospheres (down to about 2 million bars!). Below these immense pressures hydrogen is thought to be compressed to its metallic form, in which electrons are stripped from individual hydrogen atoms and can wander freely among the protons, like they would in “regular” metals.

Based on observations of the previous storms decades ago it was suspected that the storms may dredge up gas that is of different composition than the molecular hydrogen that dominates Saturn’s upper atmosphere. However, lacking detailed observations the actual components could not be identified. This time was different: Cassini’s VIMS (Visible and Infrared Mapping Spectrometer) obtained spectra of the storm head and its vicinity. Sromovsky and colleagues compared the spectra of the gas from the storm’s head to the “ambient” spectra to figure out what components does the storm carry with it.

Comparison of the in- and out-of-storm spectra showed a prominent difference at 3 micron: Sromovsky and his team use sophisticated atmospheric models to try to figure out what causes the difference in the spectra. They conclude that this feature must be caused by small particles present in the storm, but not found otherwise in Saturn’s upper atmosphere. The detailed analysis of the spectra suggests that Sromovsky’s team has observed ice particles, made of a mixture of water and ammonia (which gives urine its smell). Water ice has never been seen in Saturn’s atmosphere previously and thought to exist in Saturn at depths of 200 km and below!

So, how large is Saturn’s Super Storm? It has emerged from a depth of at least 200 km and covered at least 7 degrees latitude when it was first seen in the atmosphere. And that 7 degrees at mid-latitude Saturn corresponds to about 1 Earth radius – making this a monster storm compared to Snowmageddon, which only covered part of the US and did not even smell that bad.