For nearly 400 years, the swirling red vortex known as Jupiter’s Great Red Spot has captured the attention and imagination of astronomers. But surprising new research suggests this planetary icon is actually not the same one described by Italian astronomer Giovanni Domenico Cassini. A detailed analysis of historical observations dating back to the 1600s reveals that what we now know today as Jupiter’s Great Red Spot may have actually formed in the 19th century and is far smaller than its predecessor.
Back in 1665, Cassini noted a large dark oval feature near the location of today’s Great Red Spot. This blemish, dubbed the “Permanent Spot,” was observed intermittently until 1713. Many assumed this was an early sighting of the Great Red Spot, implying the massive storm had been churning for centuries.
However, in a new paper published in Geophysical Research Letters, a team of astronomers scrutinized measurements of both the Permanent Spot and the Great Red Spot over time. To their surprise, the two spots don’t match up. In the late 1800s, the Great Red Spot was two to three times bigger than the Permanent Spot had been. There are also no recorded observations of either spot for over 100 years between 1713 and 1830. This gap strongly suggests the Permanent Spot had disappeared, and the Great Red Spot is a more recent storm that first flared up in 1831.
(a) In this 1711 painting by Donato Creti, a red spot is shown prominently on Jupiter—likely influenced by communications with astronomers Cassini or Manfredi. Two late-1800s drawings (b,c, Trouvelot and Elger, respectively) show elongated spots on the planet. Researchers used these illustrations and others to track Jupiter’s red spots through time. They determined today’s Great Red Spot is a different one than that which Cassini first observed.
What’s more, the Great Red Spot has been steadily shrinking since it was first definitively observed. The researchers found that its length has been decreasing by about 210 kilometers (roughly 130 miles) per year. At this rate, if the Great Red Spot and Permanent Spot were the same storm, the Permanent Spot would have had to grow dramatically in size between 1713 and 1830 to reach the dimensions of the Great Red Spot in the 1800s. There are no observations supporting this, and such growth would be uncharacteristic of Jupiter’s vortices.
“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,” adds research leader Agustín Sánchez-Lavega, a planetary scientist at the University of the Basque Country, in a media release. “The ‘Permanent Spot’ probably disappeared sometime between the mid-18th and 19th centuries, in which case we can now say that the longevity of the Red Spot exceeds 190 years.”
Methodology: How Did Scientists Tell These Storms Apart?
To reach these conclusions, the researchers carefully combed through centuries of historical records, notes, and drawings from past astronomers. They compiled as many measurements as possible of the size and position of both the Permanent Spot and the Great Red Spot.
For the earliest records of the Permanent Spot between 1665 and 1713, the researchers analyzed drawings from Giovanni Cassini and others. Precise measurements were difficult due to the limitations of early telescopes, but they were able to approximate the spot’s dimensions. They then compared these to more detailed records of the Great Red Spot starting in the 1800s, including drawings, photographs starting in 1879, and modern digital images.
For each observation, the researchers carefully measured the east-west length and north-south width of the spots. When photographs weren’t available, they analyzed astronomers’ drawings, accounting for potential inaccuracies. They also tracked the longitude of the spots over time to calculate their drift rate and velocity. By compiling data points across 350+ years, they could chart long-term trends in the spots’ size and motion.
Source: https://studyfinds.org/history-jupiter-great-red-spot/