The Great Red Spot of Jupiter, will it disappear by the year 2100?
The “Great Red Spot” of Jupiter is getting smaller and smaller.
Once three times the Earth's diameter in size, or about 40 thousand kilometres,
the most famous feature of the giant of our solar system seems to be shrinking more and more,
also changing its shape from elliptical to almost circular. The phenomenon has been
going on for decades, but in recent years it seems to have accelerated abruptly.
When we think of Jupiter the first thing that comes to mind is its gigantic dimensions. With
a diameter of just under 143,000 kilometres its mass is 2.5 times those of all the
other planets in the Solar System, so much so that it would take 11 planets like the Earth
lined up to cover its diameter, and it would take 317 to equal its mass. However, the planet is
not as rocky as the Earth: it is a giant ball of gas formed mostly by hydrogen and helium.
Its size suggests that it was the first planet to form in the Solar system when it incorporated
the leftovers of the gases that had formed the Sun. Jupiter also turns very fast on itself:
a day there lasts only 10 hours. And this is the main cause of its extraordinary multicoloured
livery with parallel bands, the origin of which, however, is not completely clear. The simplest
interpretation is that in the "zones" (the lighter colour bands) ascending motions take place,
while in the " belts" (darker bands) descending motions take place. The clear confinement of
the bands is controlled by very strong "jet streams" that blow in opposite directions.
A not secondary issue is that of the different colouring of the atmospheric structures.
The only hypothesis that seems reasonable at the moment is that the different shades
derive from substances present in traces, probably organic molecules and phosphorous compounds, which
interact with light depending on the physical and chemical conditions of the environment. The
determining parameters would be the temperature and radiation of solar ultraviolet radiation.
But surely the characteristic that most gives an idea of the size of the planet
is the enormous and bubbling red hurricane that has been swirling in the atmosphere of the giant
of the Solar system for centuries: the Great Red Spot, the structure that has fascinated
terrestrial astronomers for generations. The storm is located in the southern hemisphere
of the planet, at a latitude of -20° and contained between two atmospheric bands, themselves bounded
by jet streams pushing in opposite directions. The atmospheric circulation inside it is
counterclockwise, which classifies the great vortex as an immense high-pressure system,
the largest in the Solar system. An anticyclone in which winds rotate at over 500 km per hour,
more than twice the speed of the strongest hurricanes on Earth,
and which, according to recent observations by the Juno probe,
towers 8 km above the surrounding cloud layer and has roots that reach at least 350 km deep.
Despite many attempts to formulate a simple model of the Great Red Spot based on knowledge
of vortex dynamics and turbulence, a fully satisfactory explanation of the mechanism capable
of keeping an atmospheric structure of this size alive for so many years has not yet been obtained.
Among the main hypotheses is the lack of friction with a solid surface underneath
and the reduced loss of energy by the outer layers. On Earth, in fact, hurricanes gain
strength on the ocean and begin to slow down and attenuate when they reach solid surfaces,
but on Jupiter, with no coast to discharge onto, the storm could rage for an extremely long time.
The losses induced by viscosity could then be compensated by "cannibalism" towards smaller
vortexes. In fact, the Great Red Spot applies the law of the strongest and incorporates without much
compliment the small anticyclones that are passing in its vicinity. And in doing so,
he probably acquires their rotational energy. In short, the formation of vortices seems to be
inevitable in a rapidly rotating atmosphere, where the vortices themselves can combine over time
to produce larger vortices. The Great Red Spot is an extreme example of this mechanism:
it is nothing more than a gigantic vortex that has lasted for centuries.
To this series of extraordinary characteristics, the Spot has for some time now been adding that of
a worrying mutation in shape and size... A story which, however, needs a summary of the history of
the telescopic observations that have taken place over the last few centuries to be well understood.
Several astronomy historians still attribute the discovery of the Red Spot to Robert Hooke
(1635-1703), while others claim that the one observed by the English scientist in May
1664 was not the Spot we know today (too far away towards the equator of Jupiter); and that
the primogeniture is probably due to Giovanni Domenico Cassini (1625-1712), who observed it
repeatedly from central Italy during the summer of 1665. The Italian astronomer also calculated
its longitude and the period of revolution, describing it as almost circular in shape
and one-tenth the apparent diameter of Jupiter. In his reports he speaks of "an island", but
without mentioning its colour. The Spot remains visible until 1674, when it disappears again. It
reappears weak and poorly defined in 1677 and then disappears again and only becomes visible again
in April 1680. After the umpteenth disappearance, Cassini sees it again at the end of March 1685.
Evidently some other astronomer of the time noticed the prevalent reddish hue, and the news
went around the cultural circles and reached the ears of the Roman painter Donato Creti, who at the
beginning of the 18th century painted a series of paintings with astronomers with their instruments
inserted in romantic landscapes. One of the paintings, preserved in the Vatican Museums,
clearly represents Jupiter with its satellites and a reddish elliptical spot.
For a long time it was believed that all these observations referred to the
Great Red Spot that we know today. However, this is a hypothesis that we cannot prove
since for a good part of the eighteenth century there will be a decline in "physical" astronomy.
The efforts of astronomers, in fact, are now directed towards position astronomy,
and the large, uncomfortable telescopes with which the great discoveries about Jupiter and
Saturn were made are soon put aside. And the study of planetary surfaces ceases almost entirely. A
period almost a century long during which no one has any way of confirming the presence on the
planet of a spot similar to that seen by Cassini. The stagnation in planetary research, produced
also by the events linked to the French Revolution and the Napoleonic wars,
seems to have come to an end around 1830. The planets were once again observed, and
on 5 September 1831, the German astronomer Samuel Heinrich Schwabe (1789-1875), the discoverer of
the Solar Cycle, reported the appearance of a large loop in the Southern Temperate Band,
at the same latitude where astronomers used to see the Spot decades earlier. This is not, as many
sources erroneously report, "direct" observation of the spot, but only the outline, within which a
uniform and faded elliptical region appear. Was it the genesis of a new spot, unrelated to the
one first seen by Cassini in 1665? Or simply the "rebirth" of that of the Italian astronomer?
Between the autumn and winter of 1869-1870, the Englishman Joseph Gledhill (1837-1906)
saw the cavity clearly and with a markedly elliptical shape, yet still empty; but two years
later, in 1872, he observed that something more substantial was beginning to emerge in the area:
a very elongated spot that was easily followed even with small telescopes. In that year,
and the following year, Lord Rosse (1800-1867) points out for the first time the red colour of
the structure in formation, perceived thanks to its gigantic reflector of 1.83 metres.
In 1877, Henry C. Russell (1836-1907)
often observed it from Sydney and ironically called it "the pink salmon",
so much so that its shape appears elongated. Sightings are becoming more and more numerous,
but most observers do not believe they are in the presence of a stable detail,
let alone a re-edition of the Cassini Spot. For each one of them, the spot observed was
one of the more or less passenger details of which the surface of the great planet is rich.
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This is a story rather difficult to understand for those who, like us,
live in an age where all scientific information is carefully archived... but the memory of the
spot observed in Cassini's time had faded so much that the new formation was considered
completely new, to the point that it became common opinion that the discovery of the Great Red Spot
was credited to the American astronomer Carr Waller Pritchett (1823-1910), who was among
the first to publish articles on the subject. Paradoxically, denying or ignoring any affinity
with the spot observed by Cassini, the scientific myth of the "Great Red Spot" as it is known in our
times will be born in this very period. Only in 1899, with the article "Early
History of the Great Red Spot on Jupiter" the English astronomer William Frederick Denning
re-established the historical "truth", rehabilitating Cassini's observations
and proposing a temporal continuity for all the subsequent "reappearances" of the spot.
In our day we are not certain that this is the truth, but what we can guarantee is that since
the rigorous measurements of its size began, the Red Spot seems to have become smaller and smaller,
changing also repeatedly of shape and colour. Between 1878 and 1879, its length in longitude
reaches as much as 43°, which correspond to 50,000 kilometres; while in the period 1879-1882
it is already reduced to 34°. At the beginning of the twentieth century,
the visibility of the Spot diminished, and its colour became an increasingly faded pink.
In 1908 it is difficult to perceive, but in 1923 it becomes perfectly visible again:
it is very elongated and occupies a quarter of the equatorial diameter of Jupiter. It appears
vaguely hexagonal in shape and adheres, as in Cassini's time, to the Southern Temperate Band.
In 1928, the length is around 30°, but in 1946 it is only 21°, one of
the minimum values ever recorded until then. In 1956 the Spot is "blood red", while in 1960
it appears dark orange. The following year it is very dark, the darkest for a century.
Its average length is 25°, but in the Sixties and Seventies it shrinks again, approaching 21°.
The colour is bright red again in 1975, and in 1979 the Voyagers who fly over it
estimate it to be 20° long, equal to a linear dimension of 23,500 km.
Its colour varies in the Eighties from orange to very dark red,
and in 1989 the dimensions do not differ from 20°. Also in 1990-1991, the Spot remained small and not
very bright, and at the end of the decade, in 1998, it reached a new negative record:
even 16th, equal to 19,000 km! But that's not all! The descent
continued even today, reaching an absolute minimum of 16,000 km in 2017...
From Voyager measurements to today’s, it’s been about 40 years, and in this period the diameter
has fallen by 7500 km... almost 200 km a year... At this rate, what we used to consider one of
the most spectacular and enduring peculiarities of the solar system risks disappearing by 2100!
In short, we have gone from assuming that the gigantic anticyclonic vortex that has been
perpetuating itself for centuries on Jupiter could go through phases of instability,
to even fear that the hurricane will be completely extinguished,
like any other characteristic of the turbulent Jovian atmosphere!
Obviously ours is just a provocation, because in reality we do not know if this
trend will continue for a long time or if it will even change its sign.
But perhaps, the understanding of the mechanism that originates and feeds this enormous vortex
may come from witnessing its disappearance and then its possible resurrection...
Who knows, we will probably know something more about it within the next decade.