Science & Discovery
A Complex Storm and its Complex History
Scientists have amassed a multi-century database of Jupiter’s Great Red Spot.
By Wendy Orlando
NASA, ESA, A. Simon (Goddard Space Flight Center) and M.H. Wong (University of California, Berkeley)
Scientists in Spain have collected modern instrumental observations and dug through manual records extending back to the 1600s to collect a data trove about Jupiter. This comprehensive collection even includes centuries-old hand sketches of the Giant Planet. The observations form a more complete picture of Jupiter and one of its most obvious features.
Using those data, the researchers have found that Jupiter’s Great Red Spot (GRS, a warm-hued super storm in the planet’s thick atmosphere) has changed size, shape, and color since the late 1800s. These changes suggest that the atmospheric feature seen today in the 21st century may not be the same storm historic astronomers observed in the 17th century. But if humankind has been enamored with this phenomenon for almost 400 years, why has it taken so long to understand if it has been the same storm all along?
Many hurdles have slowed the accumulation of knowledge, from funding biases to humans’ Earth-tinted glasses to GRS’s frequent invisibility. Fortunately the dedication of astronomers, plus technology’s relentless march, have gradually — though not steadily — revealed the spot’s secrets. Our learning about the GRS has been as complicated as the GRS itself.
Jupiter's swirling atmosphere is on display in this image captured by the Juno spacecraft.
[Image data: NASA/JPL-Caltech/SwRI/ MSSSImage processing by Kevin M. Gill, © CC BY]
Italian astronomer Giovanni Cassini observed Jupiter and its "permanent spot" many times between 1665 and 1713. Shown here are drawings from that time period.
[G. Cassini]
1665 – 1713: The first telescopes
The first documentation of the long-standing storm likely originates in 1665. Giovanni Cassini used a 34-foot-long telescope to observe the feature, and the Italian astronomer was the first to announce what he called a “permanent spot” south of Jupiter’s equator. (Multiple other observers — Leander Bandtius, Robert Hooke, and Eustachio Divini — each claimed to have seen it earlier, however.) Cassini and his nephew, Jacques Philippe Maraldi, observed the permanent spot many times between 1665 and 1713.
Cassini had already documented Jupiter’s bands of darker and lighter colors parallel to its equator, and Francesco Fontana argued that those bands prove the planet rotates. Cassini noticed that spots near Jupiter’s equator rotate faster than those near the poles, suggesting two rotation rates. This idea was rejected by other scientists, however, as Jupiter was assumed to have a fixed surface like Earth. Indeed, the very notion that a planet could have a spot, a blemish, went against another assumption — one theorized by Aristotle — that all the planets were heavenly bodies, perfect in form. Cassini’s announcement of the spot, a giant defect on the supposedly flawless jovian surface, was gutsy.
1713 – 1878: A silent century
After so many sightings, suddenly in 1714 no one wrote about observing Jupiter’s permanent spot. Not one observer in any country; not for 118 years.
Finally, in 1831, a drawing by German amateur astronomer Samuel Heinrich Schwabe included a large spot in Jupiter’s southern hemisphere, and two other works from other observers in 1870 showed “Mayer’s Ellipse” similarly placed. But there was little else until 1878.
One explanation of this silence is that the spot every so often changed color — to that of the nearby clouds. As Amy Simon, Senior Scientist at Nasa’s Goddard Space Flight Center, describes, “sometimes you could barely see it because there was almost no contrast with the surroundings.” It would melt into the background for months or years at a time, only to reappear again later.
Also, astronomers in the 18th century (and crucially their funders) shifted attention from the planets to the stars. Subsequently, in the 19th century, “professional astronomy was about the position of things in the sky…not physical observations,” says Thomas Hockey, Professor of Astronomy at the University of Northern Iowa. Amateur astronomers were the ones left looking for spots — and not getting much attention.
French artist and astronomer Étienne Léopold Trouvelot created this pastel artwork of Jupiter following his November 1, 1880, observation of the planet.
[Rare Book Division, The New York Public Library. (1881 - 1882). The planet Jupiter.]
Scientists combined a 2017 view of Jupiter's Great Red Spot from the Juno spacecraft's camera with a computer model of winds to create this animation.
[NASA/JPL-Caltech/SwRI/MSSS/Gerald Eichstadt/Justin Cowart]
1878 – 1973: A brilliant spot
In 1878, a spot on Jupiter roared back into astronomical conversations. American Carr Pritchett noted a large bright atmospheric disturbance south of Jupiter’s equator. He described it as a perfect oval, rose-tinted. Many others quickly corroborated with their own observations (or assertions that they had seen it first), including Australian George Hirst, who reported seeing a bright red “fish” in 1876. Ironically, the term “Great Red Spot” was coined in 1880, not by a scientific journal but by a popular London magazine. This vivid appearance of the GRS lasted until 1882, sometimes viewable with only binoculars.
In spite of the astronomic hubbub, further GRS investigation “started up slowly,” says Hockey. Although amateurs were inspired to look for this and other spots, he points out that at that time the “study of the planets was just a backwater.” However, in the late 19th and early 20th century, tools and equipment improved and thus observatories flourished — often on mountaintops to limit atmospheric distortion. Atop one of those, Mt. Wilson in California, scientists began taking pictures of Jupiter. Photography changed everything. With photos, one can quantify what previously had been subjective descriptions or drawings. Astronomers began keeping thorough records of many observations, including the longitudinal position of the GRS to understand why it drifts westwardly across Jupiter’s cloud tops. (What determines the drift rate is still unknown.)
1973 – present: The computer age
In the late 20th century, scientists could explore Jupiter up-close. In 1973 and 1974, the Pioneer 10 and 11 space probes flew past Jupiter, and more missions followed. Scientists now have extensive data of the GRS from 1979 to the present, including the GRS’s longitudinal length and latitudinal width. Because most observations look down from above, scientists are only now beginning to piece together the spot’s third dimension, height. But researchers have confirmed that the GRS is a rotating storm that towers over the surrounding clouds.
Computer modeling has also accelerated astronomers’ understanding of the storm, but it brings new challenges. For example, knowledge of Earth’s atmosphere should — in theory — serve as a good starting point to simulate Jupiter’s atmosphere and hence the GRS, but it’s not that simple. Instead, says Simon, scientists find themselves “data-starved on a much bigger planet with a lot more atmosphere,” and there is no firm lower surface like Earth’s land to slow the atmosphere down.
There is also the danger of home-planet bias. Jupiter’s storms are not driven by energy from the Sun as Earth’s storms are, but rather by Jupiter’s internal energy, because Jupiter is five times farther away from the Sun. Thus, asserts Hockey, Jupiter is an “example of a whole different kind of planet.”
In summary, as Simon asserts, it is difficult “to replicate a storm — a vortex — on Jupiter.”
Then there is the spot’s redness. Simon says that the big question about color in the last 20 years has been “What is making that color?” Scientists have identified a haze sitting above the GRS’s clouds that, she explains, could be red due to its chemical makeup.
It could be phosphene, or a sulfur compound, adds Hockey. Alternatively, the color could come from the different conditions very high up, or even some other factors. He acknowledges, “We still don’t know what it is.”
The Great Red Spot is prominent in this early photograph of Jupiter. Astrophotographer Andrew Ainslie Common captured this view September 3, 1879, through his 36-inch telescope.
[from A popular history of astronomy during the nineteenth century by Agnes Clerk, 1887]
Today, the Great Red Spot takes on a rusty-brown-orange hue, as seen in this processed image from the Juno mission.
[NASA/JPL-Caltech/SwRI/MSSS/Gerald Eichstadt/Sean Doran © CC NC SA]
Two spots or one?
Jupiter’s GRS has changed not just hue and brightness over time, but also size and shape, raising the age-old query: Was Cassini’s permanent spot the early GRS? The answer isn’t clear. Many scientists state that the smaller spot observed 350 years ago could not have grown to the size of the late 19th century GRS within the mostly undocumented years of 1713–1878. “We have definitely not” seen any spot on any outer planet with that kind of growth rate, says Simon.
However, some researchers, including Hockey, assert that we cannot know for certain if the permanent spot is or is not the GRS, because “the absence of observations … really doesn’t tell you anything.”
What is not a mystery is why the GRS has been a solar system darling for so long. Although its color today is no longer a rosy tint but rather a muted orange-brown, its original redness is what caught people’s attention. It is, as Hockey remarks, what will cause the GRS to “never go out of observation.” ✰
(Published October 10, 2024)
WENDY ORLANDO is a scientist-turned-science writer in Boulder, CO. She used to research fluid flows in the atmosphere, ocean, and human heart. Now she writes about such topics, and many more, for the general public.
See more of Wendy's writings
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