Establishing the age and origin of Jupiter's Great Red Spot

Research staff at the University of the Basque Country (UPV/EHU), the Universitat Politècnica de Catalunya—BarcelonaTech (UPC) and the Barcelona Supercomputing Center (CNS-BSC) have analyzed historical observations since the 17th century and developed numerical models to explain the longevity and nature of Jupiter's Great Red Spot.

They have published the findings of their observations and numerical models in the journal Geophysical Research Letters.

As a popular icon among objects in the solar system, Jupiter's Great Red Spot (GRS) is probably the best-known atmospheric structure. Its large size (right now, its diameter is that of the Earth) and the contrast of its reddish color against the planet's pale clouds make it an object that can be easily seen even with small telescopes.

Jupiter's Great Red Spot is a huge anticyclonic vortex with winds traveling at 450 km/h around its periphery. It is the largest and most long-lived vortex among all those existing in the atmospheres of the planets in the solar system, but its age is a matter for debate and the mechanism that led to its formation remains obscure.

Speculation about the origin of the GRS dates back to the first telescopic observations made by the astronomer Giovanni Domenico Cassini, who in 1665 discovered a dark oval at the same latitude as the GRS and named it the "Permanent Spot" (PS), since it was observed by him and other astronomers until 1713.

Astronomers subsequently lost track of it for 118 years and it was not until 1831 and later years that S. Schwabe again observed a clear structure, roughly oval in shape and at the same latitude as the GRS; that can be regarded as the first observation of the current GRS, perhaps of a nascent GRS.

Since then, the GRS has been observed regularly by means of telescopes and by the various space missions that have visited the planet right up to the present day.

In the study, the authors first analyzed the evolution of its size over time, its structure and the movements of both meteorological formations, the former PS and the GRS; to do so, they used historical sources dating back to the mid-17th century, shortly after the invention of the telescope.

"From the measurements of sizes and movements we deduced that it is highly unlikely that the current GRS was the PS observed by G. D. Cassini. The PS probably disappeared sometime between the mid-18th and 19th centuries, in which case, we can say that the longevity of the Red Spot now exceeds 190 years at least," explained Agustín Sánchez-Lavega, professor of physics at the UPV/EHU and who led this research.

The Red Spot, which in 1879 was 39,000 km in size at its longest axis, has been shrinking to about the current 14,000 km and simultaneously becoming more rounded.

What is more, since the 1970s, several space missions have studied this meteorological phenomenon closely.

Recently, "various instruments on board the Juno mission in orbit around Jupiter have shown that the GRS is shallow and thin when compared to its horizontal dimension, as vertically it is about 500 km long," explained Sánchez-Lavega.

To find out how this immense vortex could have formed, the UPV/EHU and UPC teams carried out numerical simulations on Spanish supercomputers, such as the BSC's MareNostrum IV, part of the Spanish Supercomputing Network (RES), using two types of complementary models of the behavior of thin vortices in Jupiter's atmosphere.