The most striking visual feature of the largest planet in the Solar System is contracting and no one knows why.
It is difficult to get an idea of its enormous proportions. The Great Red Spot is the largest and longest-lasting storm in the Solar System, with a diameter larger than our planet. Its clouds rise eight kilometers above Jupiter’s other clouds. Hurricane-force winds hit the edge of the storm at more than 400 kilometers per hour. And this is not a passing phenomenon: while storms on Earth usually last a few days, this storm has been active for at least 350 years.
The Great Red Spot is, without a doubt, one of the most recognizable features of the Solar System. No one knows for sure how or when it came about. But the truth is that it has been contracting for several decades. According to the latest observations, the Great Red Spot has shrunk to almost a third of its size at the beginning of the 20th century. If this trend continues in the coming years, it is possible that by 2040 this iconic storm will be nothing more than a beautiful memory.
spotted planet
In 1664, the English scientist Robert Hooke was the first to claim that he had observed a spot on Jupiter. But that spot was near the north pole of the planet, so it cannot be what we know today as the Great Red Spot (GMR), located in the southern hemisphere. More convincing is the description of “a permanent spot,” made a year later by the Italian astronomer Giovanni Cassini. The Cassini spot was in a region close to today’s spot, which is why Cassini is often considered the first astronomer to observe GMR.
Several decades passed before another Italian astronomer, Giacomo Maraldi, saw the spot again, in 1713. Two years earlier, the painter Donato Creti had painted a series of oil paintings of celestial bodies that included a painting of Jupiter with a huge red spot. It was the first time that color was attributed to it.
For more than a century there was not a single reference to the GMR. It is strange that none of the great astronomers of the second half of the 18th century, for example William Herschel, overlooked the existence of the GMR. That is why some experts raise the possibility that the GMR is not as old as is thought. Another option is that a change in color or a decrease in size at that time would help it go unnoticed.
The modern history of the GMR began in 1831, when German astronomer Samuel Heinrich Schwabe depicted it in detailed drawings. In 1878, the American astronomer Carr W. Pritchett baptized this formation with its current name, and since then this fascinating formation has been observed regularly and with increasing precision. This distance portrait has been refined with visits to Jupiter by space probes Voyager 1 and 2 (1979), the probe Galileo (1995-2003) and, more recently, the probe Juno which has been in orbit around that planet since 2016.
Extended view of Jupiter’s surface (Hubble Space Telescope, June 27, 2019). The bands are created by differences in the thickness and height of the ammonia ice clouds: the lighter ones rise higher and have thicker clouds than the dark bands.
Jupiter’s atmosphere
Jupiter’s atmosphere is basically made up of hydrogen and helium. Other substances, such as methane, ammonia and hydrogen sulfide, are also present, although in quantities less than 1%. When observing Jupiter, the first thing that catches your attention is a series of bands parallel to the equator. There are two types of these bands: zones (light in color) and belts (dark in color). Zones and belts alternate from north to south of the planet in a pattern characteristic of the Jovian giant. These bands are formed by different layers of clouds that cover the entire planet. The zones are believed to be lighter than the belts due to the presence of high ammonia clouds. The ocher tone of the belts is due to an intermediate layer of clouds composed of ammonium hydrogen sulfide. Below these two layers would be a third layer of water clouds in the deepest part of the atmosphere.
For decades, experts have debated the structure of these bands. Are they a surface feature of the atmosphere reaching a few hundred kilometers deep, as traditional models predict? Or is this the tip of the iceberg of much deeper structures, according to the most recent theories? Thanks to the probe Juno, today we know that the roots of these bands penetrate up to 3,000 kilometers deep, much deeper than previously thought. If these bands were on our planet, they would reach halfway to the center of the Earth.
Jupiter’s bands are driven by strong winds directed alternately to the east and west, with speeds reaching hundreds of kilometers per hour. These currents are dotted with thousands of cyclones and anticyclones of multiple sizes. The largest and longest-lived of all is the GMR.
Core
It is believed to have a small solid core, made up of solid iron and other heavy metals. There the pressure is so high that the electrons have separated from the hydrogen atoms and move freely. This is what is known as metallic hydrogen, which would favor the enormous magnetic field that this planet has: 20,000 times more powerful than that of the Earth.
Day/year
It takes only 9.93 hours to make one revolution on itself, but its movement around the Sun is slower than that of our planet, since it takes almost 400 days to complete an orbit around our star.
Characteristics
It emits more energy than it receives from the Sun. The source of this energy comes from the gravitational contraction of the planet, which generates heat outwards. If Jupiter had been about 80 times more massive, it would have been able to initiate the nuclear reactions of the stars and today we would have two suns in our sky.
A storm like no other
Cyclones on Earth can be more than 1,000 kilometers in diameter, with winds reaching speeds of 300 kilometers per hour. They are capable of devastating entire cities and wreaking havoc in their wake in a matter of hours. This power, however, is insignificant compared to that of the GMR.
From a meteorological point of view, the GMR is an anticyclonic storm, that is, the pressure in the center is greater than in the periphery. It has a slightly oval shape and rotates counterclockwise, with a period of six Earth days. As occurs in hurricanes on our planet, its center remains relatively calm, with fairly mild winds. In the periphery, however, the situation is very different: the average wind speed is 400 kilometers per hour, with peaks of 600 kilometers per hour. Regarding its size, the latest measurements made by the Juno space probe indicate that the GMR currently measures just over 15,000 kilometers in diameter, enough to contain the Earth. The GMR remains fixed in latitude (north-south position), but moves in longitude with respect to the clouds, dragged by the planet’s rotation. How can a storm of this size last for so many years without weakening? The Earth has a solid surface, so any storm loses energy due to friction with the ground. That is why cyclones and hurricanes, which originate at sea, usually weaken when they make landfall and disappear within a few days. On the other hand, Jupiter does not have a solid surface. Its gaseous atmosphere becomes denser with depth, but a storm can continue to grow because there is no friction with the ground.
More information
Saturn has the so-called Great White Spot, a periodic storm that appears in the northern hemisphere or in the tropical zone. It is characterized by its white color, a core the size of the Earth and a tail several thousand kilometers long. First seen in 1876, six such storms have been observed in the last 140 years, most recently in 2010. Although the cause is unknown, the storms typically occur when Saturn’s northern hemisphere is facing the Sun. It is believed that their appearance is related to the cooling of the Saturnian atmosphere during winter, although that does not explain why they are so rare.
Similar in appearance to the GMR, the so-called Great Dark Spot of Neptune was detected in the southern hemisphere of that planet in 1989 by the Voyager 2 probe. The spot was almost the same size as the Earth and winds were measured around it with speeds exceeding 2,400 kilometers per hour, the fastest ever recorded in the Solar System. When the Hubble Space Telescope wanted to photograph it in 1994, the Great Dark Spot had completely disappeared, leaving scientists wondering about its true nature. Was it a hole in Neptune’s immense methane atmosphere, similar to Earth’s ozone hole? Or a storm that dissipated?
Since then, four other similar dark spots have been observed in Neptune’s atmosphere. These spots can appear in any region of the planet and last about two years, until they disappear due to the powerful winds originating in the high areas of the atmosphere.
The mutant stain
Although it is common for storms to change size and shape, what has happened to the GMR in recent decades is surprising experts. According to annual records, dating back to 1878, the GMR has only declined since then. In images from the end of the 19th century it had a width of more than 40,000 km. When the Voyager 1 and 2 space probes visited Jupiter in 1979, it had shrunk to 23,000 km and today it barely reaches 15,000. Since 2012 the rate of contraction has accelerated. The GMR shrinks at a rate of 1,000 kilometers a year without the cause being known.
As the storm has been contracting, scientists expected the powerful internal winds to become even more intense, like an ice skater spinning faster when he curls his arms. Instead, it appears that the storm is stretching in the vertical direction. This is reminiscent of clay on a potter’s wheel. As the wheel turns, a craftsman can transform a compact mass of clay into a tall, thin vase by squeezing with his hands. The smaller the base, the taller the pot will grow. In the case of GMR, the height change is small relative to the area covered by the storm, but is still significant.
Nor has the color of the GMR remained uniform over these decades of observations. In fact, as of 2014 it is turning intensely orange. Experts aren’t sure why this is happening, and are considering two alternatives. One is related to the presence of…