Jupiter’s Great Red Spot (GRS) is a prominent feature in our Solar System, known since the 1600s as a colossal anti-cyclonic storm larger than Earth, with winds exceeding 400 km/h (250 mph). Its formation and longevity have been debated, raising questions about whether it has been consistently observed over time.

The GRS was possibly first noted in 1632 by a German Abbott using a telescope. Later observations in the 1660s by Giovanni Cassini and others documented its presence, which was then dubbed the Permanent Spot. However, it disappeared from records for 118 years until astronomer S. Schwabe observed a similar structure in the same location, suggesting its reformation.

New research published in Geophysical Research Letters combines historical observations with computer simulations to understand the origin of this enigmatic storm. Lead author Agustín Sánchez-Lavega from the University of the Basque Country in Bilbao, Spain, explains that the GRS’s rotational slowdown since around 2010 raises intriguing planetary mechanics questions.

Recent measurements and simulations indicate that the current GRS is unlikely to be the same observed by Cassini, suggesting a disappearance between the mid-18th and 19th centuries. This reformation implies that the GRS has existed for at least 190 years.

The GRS’s size has diminished from 39,000 km in 1879 to 14,000 km today, becoming more rounded over time. Space telescopes and missions like NASA’s Voyager, Galileo, and Juno have provided unprecedented insights into its structure. Juno’s close-up images and measurements revealed a shallow and thin storm with a vertical length of about 500 km.

Supercomputer simulations suggest that the GRS formed from a long cell resulting from Jupiter’s South Tropical Disturbance (STrD), capturing and compacting winds into an anti-cyclonic vortex over time. This process began in the mid-1800s, indicating that the current GRS is approximately 150 years old.

The study concludes that while alternative theories involving superstorms or vortex mergers were considered, the STrD hypothesis best aligns with observed data and historical accounts. Future research aims to further refine our understanding of Jupiter’s atmospheric dynamics and the evolution of its iconic Great Red Spot.