For almost two centuries, astronomers have been arguing over whether the Great Red Spot we see on Jupiter today is the same feature as a similar mark first observed by Giovanni Cassini in 1665.
The Great Red Spot is an Earth-sized storm raging on Jupiter, and is the gas giant planet's most recognisable feature.
But is the storm we see today the same one that was recorded by the great Italian astronomer centuries ago?
A new set of simulations may have settled the debate once and for all, suggesting our spot and Cassini’s are two separate things.
History of observing the Great Red Spot
After Cassini first noticed what he referred to as the Permanent Spot, he and others observed the feature continuously until 1713, when it vanished from the record for 118 years.
Then in 1831, astronomers again started to see a clear, oval structure on the planet, at the same latitude as the Permanent Spot.
This Great Red Spot has been continuously monitored for over 190 years and is now known to be a huge vortex in Jupiter’s upper atmosphere.
However, it remained unclear whether the feature is the same as Cassini’s Permanent Spot or a completely new weather system.
The same storm?
To find the answer, a team used observations of the planet from the last few decades – including those taken by the Juno probe currently in orbit – to create simulations of how the Great Red Spot formed.
These investigated three different potential formation methods.
Two of these methods – one where the Great Red Spot is the product of a giant superstorm; another where it is several smaller vortices combined – didn’t produce the kind of storm we see today.
The third method, however, took a look at instabilities caused by the fast-flowing jet streams that flow parallel to the band where the Great Red Spot resides, but in the opposite direction.
In this case, the simulations did result in a system similar to the Great Red Spot.
The study then compared this simulation to historical records, tracking how the Great Red Spot has changed over time.
In 1879, the Great Red Spot was around 39,000km (24,200 miles) at its widest, but now measures just 14,000km (8,700 miles) across.
Comparing these changes with the simulations suggests the Great Red Spot first formed in the 19th century, long after the Permanent Spot seen by Cassini disappeared.
"From the measurements of sizes and movements, we deduced that it is highly unlikely that the current Great Red Spot was the ‘Permanent Spot’ observed by Cassini," says Agustín Sánchez-Lavega from the University of the Basque Country in Bilbao, Spain, who led the study.
Observing the Great Red Spot
Words: Chris Lintott
The Great Red Spot is the second most recognisable planetary feature behind Saturn’s rings.
I remember being gobsmacked when viewing the planet through Patrick Moore’s 12.5-inch reflector, where the Great Red Spot seemed to have faded very nearly completely.
More recently, the spot seemed to develop red ‘flakes’, which turned out to be gas being pulled into the eye of the living, breathing storm.
When it does fade away completely – which might happen in the next decade or so – it’ll be a sad day for back garden observers everywhere.
But this new research has a silver lining, suggesting a new spot will be along soon, just as our spot replaced Cassini’s.
This article appeared in the September 2024 issue of BBC Sky at Night Magazine
