White storms in Saturn

The atmosphere of planet Saturn normally displays a rich meteorology. Moreover, once during its long year that spans 29.5 terrestrial years, Saturn develops a unique phenomenon in the Solar System, an extraordinary storm popularly known as Great White Spot, that grows until reaching a size of 10 000 km, almost the diameter of the Earth. The atmospheric perturbation induced by the storm expands under the influence of winds and, eventually, it encircles the whole planet with a turbulent cloudy band. Saturn, a huge gaseous planet without solid surface placed at 1500 million km from the Sun (ten times the Sun-Earth distance), loses then, for some months, its usual pale face of yellowish haze and clouds.

During the last 130 years of regular telescopic observations, only five Great White Spot events had been recorded: one for each Saturn year. These storms tended to arise during the northern hemisphere summer of the planet. Given that the last storm happened close to the equatorial region of Saturn in 1990, another one was not expected until around 2020. But surprisingly, almost nine years in advance, Japanese amateur astronomers announced at the beginning of December 2010 the apparition of a very bright spot in the mid-latitude zone of Saturn northern hemisphere, the first sign of the giant storm.

Early developement of the Storm in Saturn

In issue number 475 of the scientific journal Nature, an international research team leaded by Prof. Agustín Sánchez Lavega, from the Higher Technical School for Engineering of the University of the Basque Country (UPH-EHU), Spain, presented the results of their initial observations and the first interpretation of this phenomenon. In this study participated, too, researchers from the European University Miguel de Cervantes at Valladolid, the Foundation Observatory Esteve Duran (Catalonia), Calar Alto Observatory (Almería), Oxford University (United Kingdom) and Paris Observatory (France). Special attention deserves the fundamental work done by an international network of observers who, coordinated from UPV-EHU, kindly contributed taking images of the planet.

saturn_cafos

Saturn Storm observed with CAFOS camera in December 2010

The data obtained in December 2010 at Calar Alto Observatory came from the Zeiss 2.2 m reflector equipped with the instrument CAFOS, that yielded a set of images taken in red light. On these images, the brightness of the planetary disc was compared with the intensity of the rings (known before hand). This way, curves showing the reflectivity of the clouds at different latitudes and from East to West were obtained. Thus, models were fitted to the observed data, in order to describe the vertical structure of the Saturnian clouds. Also, during the last months Calar Alto is performing a follow-up of the storm evolution using the fast camera Astralux attached to the same telescope.

Image composites produced with Astralux camera attached to the 2.2 m telescope of Calar Alto. The combination of information from different filters shows the structure of the storm at different vertical levels in the atmosphere of Saturn.

This research complements another previously published in digital form in May 20th 2011 by the journal Science, in which the UPV-EHU team also took part. There, the perturbations induced by the storm on the temperature field and atmospheric chemical composition were described.

Prof. Sánchez Lavega, who has been studying this class of phenomena for many years, declared than “More than six months after the storm eruption, its original source is weaker but still active, what represents a huge surprise and a challenge to our understanding of these violent meteorological events”.

These storms seem to arise following a seasonal pattern. Seasons are very well defined in Saturn, because the rotation axis is highly inclined with respect to the orbit of the planet, as happens in the case of Earth. But it is still a mystery how changes in the extremely weak solar illumination that reaches Saturn, and that penetrates only a few kilometres into the upper layers of ammonia clouds, is capable of triggering such huge storms at depths of more than 250 km, in the underlying, hidden water clouds.

Furthermore, according to the observations of the process, the irruption of jet columns of hot gas that induces the formation of white clouds almost does not modify the normal wind flow that follows directions aligned with Saturn parallels. This is an important aspect, because there are two competing theories to explain where does the energy for these processes come from. The energy source could be the light from the Sun, in which case the winds would be only “superficial”. Or the motor could be in the internal heat of Saturn and, then, the winds would be “deep”. As published in the article in Nature, and in words of the researchers themselves, ”our models best fitting the storm and the subsequent perturbation at planetary scale require the winds to be extended in depth down to the water clouds, there where sunlight does not reach. In this case, this work would confirm what was already suggested in our previous works on Jupiter, the other gas giant, and on Saturn, pointing to winds moved by an internal source of heat.”

Beyond the curiosity for knowing the physical processes underlying the formation of these giant storms in Saturn, the study of these phenomena allows testing models used to study the behaviour of the Earth’s atmosphere, but under very different conditions impossible to emulate in laboratories. Saturn storms offer, thus, in some sense, a test bench for the physical mechanisms behind the formation of violent storms at the equatorial and tropical areas of the Earth, of for other closer phenomena like those known in Spain with the name of gota fría (“cold drop”).

Images

Early development of the storm in Saturn. (286 kB)

Storm in Saturn, Hubble Space Telescope image taken on March 12th 2011. (1.75 MB)

Storm image and numerical model. (128 kB)