On this page you’ll find links to some of the better coverage of Saturn’s springtime storm, along with a series of questions and answers inspired by my conversations with journalists.
Media Coverage:
Oxford Science Blog - What might a skydiver see on Saturn?
NASA Press Release - Cassini Spacecraft and Ground-Based Telescope See Violent Saturn Storm
ESO Press Release - Looking Deep into a Huge Storm on Saturn
Popular Mechanics - A Peek Inside Saturn's Enormous (and Unexpected) Swirling Storm
Space.com - Monster Storm Rearranges Saturn Before Our Eyes
USA Today - Saturn Storm Swirls Ringed World
Der Spiegel - Super-Saturn-Sturm lässt Forscher staunen
Wired - Planet-wide storm tears across Saturn's northern hemisphere
Daily Mail - Once-in-a-generation storm batters Saturn and offers scientists an unprecedented view
Cosmos - Massive Storm Erupts on Saturn
Sky and Telescope - Dissecting Saturn’s Big Storm
Overview:
Compared to Jupiter, Saturn is typically thought of as being a calmer, more serene place, with fewer high-contrast dynamical features. We know that the atmosphere evolves slowly with seasons, and that small-scale storms and vortices crop up occasionally. But every so often, approximately once per Saturnian year, we have a gigantic, violent and complex eruption of bright cloud material, which is sheared east-west by Saturn's winds to ultimately encircle the entire planet. One of these global storms erupted in December 2010.
This is the sixth eruption to be recorded on Saturn. Normally storms break out after northern summer solstice (after 2017 for this Saturn year), but this springtime eruption is rather early. That's lucky for us, as Cassini was around to see it!
Previous storms have been studied by the sunlight reflected from the clouds. But the combination of Cassini/CIRS and the VISIR instrument on the VLT 8-m observatory has allowed us to get the first ever observations of the thermal structure within a Saturnian storm. Thermal wavelengths are sensitive to the heat energy emitted by the planets, and we can use the images and spectra to measure the storm's temperature, winds (horizontally and vertically), composition and cloud structures.
Crucially, thermal data provides a vertical dimension to the visible images you see, which are sensitive to the cloud decks. Our results show that this tropospheric storm affects the atmosphere at much higher altitudes than we ever expected, creating enormous perturbations to temperatures high in the stratosphere (the stratospheric beacons described below).
Our data shows that the storm had a huge effect on the atmospheric structure, transporting energy and material over great distances horizontally and vertically, modifying the atmospheric winds (causing meandering jet streams and the formation of giant vortices) and disrupting Saturn's slow seasonal evolution. This storm could affect Saturn's northern mid-latitudes for some time to come.
Questions and Answers about Saturn’s Storm:
What’s the most important implications of these findings, and the most surprising?
The most fascinating thing for me is that a storm deep within Saturn's weather layer (the region of the atmosphere prone to convection, turbulence and cloud formation) can have such dramatic effects much higher up in the stratosphere (where we typically think the air is more stable), hundreds of kilometers above the cloud tops. This means that a deep storm can affect the emission we measure from the stratosphere, and generate the intense beacons that have been observed by both Cassini and the VLT. In essence, this study has shown that Saturn's planetary-scale storms can affect the normal seasonal evolution of the atmosphere over hundreds of kilometers vertically.
In addition, these results confirm that Saturn's giant storms mainly occur when the planet is near northern spring/summer, and that raises a lot of fascinating questions - what has changed to allow this intense storm to develop now? We think the answer lies in the change in environmental conditions (e.g., warmer temperatures in the spring) which allows these convective storms to penetrate to the altitudes where we can see them, both in visible light and the thermal infrared.
What makes this storm so special?
Eruptions of this type happen about once a year on Saturn, so while this might appear rare from our point of view (that’s once every 30 years), they’re clearly a seasonal phenomenon on Saturn. This storm is only the sixth to be recorded since 1876, and has occurred much earlier in the seasonal cycle than usual (northern spring, whereas they're mostly after summer solstice), and is the first to be characterised in the thermal infrared and the first to be observed by an orbiting spacecraft. That powerful combination is revealing insights into the meteorology of the storm, and showing that their are incredible effects at high altitude that are invisible in reflected sunlight.
What might be the cause of the stratospheric beacons? How did the disturbance create them? What might make them so warm compared to their surroundings?
The disturbance is a convective storm, rather like a system of thunder clouds, deep within Saturn's weather layer, and yet they have generated these beacons high up in the stratosphere. The images show that the stratosphere over the disturbance is cold (suggesting rising air), but flanked by warmer regions to the east and west. By continuity, what goes up must come down, so these beacons may be formed by air subsiding and warming up in the stratosphere. It's like the stratosphere is adjusting to the intense perturbations going on deeper down.
These beacons are invisible in reflected sunlight, and they only show up in thermal infrared images of the disturbance. They are the strongest stratospheric perturbations we've detected on Saturn (15-20 degrees celsius from minimum to maximum). We call them beacons because they completely outshine the emission from the rest of the planet at certain wavelengths. It's like looking at a lighthouse - as Saturn rotates once every ten hours, these incredibly powerful beacons swing into view. And all this is being produced by a tropospheric storm at much higher pressures (i.e., much deeper down in the weather layer of the atmosphere).
How wide are the beacons? How long do they last?
The beacons have persisted from at least January 2nd (the first thermal data we got after the storm eruption on December 5th) to the present day, where we're continuing to track their evolution with Cassini and VLT. In fact, the beacons have grown stronger, and continue to dominate the emission. They're at least 30'000 km wide, and there was one either side of the central disturbance. Both moved westward with time (being pushed around by winds in the stratosphere), at slightly different speeds, which means that eventually they came into very close contact with each other.
How did the storm develop?
Models suggest that these storms can be explained as powerful convective plumes - a small anomaly deep in the water cloud is able to expand and grow, rising upwards and punching through Saturn's usually serene veil. So what we see in the visible images is a manifestation of dynamics occurring deep down in the atmosphere in regions hidden from view. The rising convective columns inject fresh ices into the upper troposphere (the bright white material seen in visible images).
What might be the cause of the cold oval vortex? How did the disturbance create it? What might make it so cold compared to its surroundings?
Our results show a well-defined dark oval, circulating clockwise in the northern hemisphere (an anticyclone) that's been generated as a result of this storm. The vortex is a feature of the deeper atmosphere, in the weather layer itself, and we can see it as a dark feature in visible light, and a cold core in the thermal data. We believe it's cold for the same reasons as Jupiter's Great Red Spot - you have a central core of upwelling air, which expands when it reaches the top of the troposphere, and that expansion causes cooling. We didn't see its formation directly, but the turbulence of the storm causes temperature differences and pressure differences, and air tends to flow along the isobars of constant pressure. In most cases, this just causes a jet stream to meander like a wave, but in this instance the flows formed a spinning vortex, which might be analogous to the formation of spinning weather systems on Earth.
We didn't see anything like this vortex in previous storms, but then again, we didn't have the resolution or the infrared capabilities to do so! I don't think we expected something quite so complex or beautiful to emerge from this storm system, and given that it's embedded in this turbulent weather system we can't say how long it might last.
How might you expect the disturbance to influence Saturn's northern hemisphere for the next few years?
The cold vortex might be ephemeral in nature, but the disturbance in general has had a dramatic effect already, moving the atmosphere around and redistributing Saturn's thermal energy and composition. The ice clouds (e.g., ammonia ice and other solid materials) that were created in the troposphere may persist until they either rain out, sublimate or sink into the deeper atmosphere. The temperature changes and winds will ultimately settle back down to the usual quiescent conditions. And the beacons are so new that we're uncertain what their fate may be, but we'll be continuing to track them to see how they evolve over the coming months.
What specific directions do you think your research might or should go from here?
The key thing about these measurements is that we're studying the after-effects of a storm that initiated deep within the atmosphere, at levels we cannot probe with Cassini or the VLT. We see their dramatic effects on the uppermost clouds and the atmospheric temperatures, but then we have to model the underlying causes. Our understanding of the deep atmosphere would improve with multiple atmospheric probes into Saturn's cloud decks, or with microwave observations (just like the ones we'll get from the Juno mission to Jupiter). The trick will be to understand the processes deep within Saturn's water clouds, where these intense storms are thought to begin.
What are the wider implications of this research?
We've shown that a huge disturbance on a gas giant can lead to intense modulations of infrared emission. This could possibly occur on other giant planets in our solar system, and might even modulate the emitted light from extrasolar planets. Crucially, it shows that you need continuous monitoring, rather than just snapshots, to really understand these complicated atmospheres, both in our solar system and beyond.
Finally, I think it's exciting that we can take meteorological concepts developed for our own atmosphere, and apply them to a giant planet ten times further from the Sun. Such studies move Saturn out of the realm of astronomy, and into the messy and chaotic field of planetary meteorology.
How was this storm first discovered?
The storm was first detected by Cassini’s radio and plasma wave science instrument and was tracked by amateur astronomers since December 2010, as the disturbance expanded rapidly. The storm started out as a small white spot, and amateurs noted its rapid westward motion and expansion as it was sheared by the zonal winds. It was in response to the amateur observations that we launched our ground-based campaign with ESO to observe the thermal effects of the storm. Luckily we had an observing run scheduled for the spring of 2011, which ESO allowed us to pull forward to observe the storm as soon as possible. It was a stroke of luck that Cassini/CIRS observed the storm at the same time, so we had imaging from VLT and spectroscopy of Cassini to compare.
Is there a good analogy to this storm on Earth?
Not really! This storm is tens of thousands of kilometers in size, and possibly grew from a single storm complex (possibly with multiple convective plumes) that transported material upwards from the deeper troposphere. The reason it can grow to encircle the planet is that there are no boundaries to the winds (like continents that disrupt our atmospheric and oceanic currents). The material moving west from the storm core eventually encountered material moving east from the storm core, so it wrapped around the entire planet.
Can you learn anything about the Earth’s weather by studying Saturn’s storms?
It’s more realistic to turn this question around - we can apply our knowledge of storms on Earth to understand the emergence of this storm on Saturn. That tells us that similar physical processes generating Earth’s storms may be responsible for Saturn’s storms, and that our understanding of weather physics can extend to the more extreme environments of the giant planets. In a way, it’s an extreme test of our models, and by studying weather phenomena across the solar system we can place the Earth’s weather into a broader context.
