At dawn on July 4, the engine of NASA’s Juno space probe left its long slumber and ignited for 35 minutes: its objective was to place itself in polar orbit around the planet Jupiter. It was the most complicated and dangerous maneuver this ship faced after almost five years of travel through the Solar System. And it will not be the last: on October 19 it was placed just over 4,000 kilometers from the outer layer of clouds of the gas giant. This has reduced its orbital period to fourteen days. Never before has a spacecraft gotten so close to the fifth planet, which is quite troublesome.
And it is that, in this way, the device will find itself fully submerged in one of the most dangerous places in our galactic neighborhood, the magnetosphere of Jupiter , which extends up to seven million kilometers in the direction of the star king and almost reaches the orbit of Saturn in the opposite direction. The study of this structure, which is the second largest in our system after the heliosphere – a huge bubble-like region through which the solar wind spreads – is precisely one of Juno’s goals.
The kingdom of eternal auroras
Apparently, the origin of this deformed donut-shaped magnetic envelope lies a few thousand kilometers under the vaporous cover of the planet, at a point where the pressure is so incredible that hydrogen is in a state in which it is capable to conduct electricity; it is known as metallic hydrogen. Planetologists believe that this degenerate phase of hydrogen, in combination with the rapid rotation of the gas giant – the days on Jupiter last just under ten hours – generates a powerful magnetic field around it, thousands of times stronger than that of the earth.
Now, the risk for Juno is not that field, but what moves inside it: electrons, protons and ions that travel around the planet at speeds close to the speed of light. It is this ring of charged particles accelerated to relativistic speeds that can destroy a spacecraft. That’s why NASA engineers have gone to great lengths to protect the delicate science instruments that Juno carries. Among all of them, the most sensitive component is the on-board computer, which is enclosed within a 172-kilo titanium safe capable of reducing the radiation that may reach it from outside by a factor of 800.
The probe has a system specially designed to study the magnetosphere, whose name seems taken from the Star Wars saga. It is JEDI, an acronym for Jupiter Energetic Particle Detector Instrument. This consists of three small detectors located at an angle of 120º to each other, thus covering the entire sky that surrounds the spacecraft. Thanks to them, scientists will obtain very valuable information about the high-energy particles that circulate near the planet and create the polar auroras of this gas giant, one of the most impressive phenomena in the Solar System. The JADE instrument (acronym for Jovian Auroral Distributions Experiment), which will study the low-energy particles that contribute to this process, will add more information.
The aforementioned Jovian polar auroras are a hundred times more energetic than ours . They were first captured by the Voyager 1 probe in 1979, as a thin, faint ring of visible light superimposed over the unlit area of the planet’s poles. Later, astronomers found that they were best seen in the ultraviolet and infrared range. Unlike what happens on Earth, where these lights are seen sporadically, the auroras on Jupiter are always there. It is an amazing sight that can even be captured with the Hubble Space Telescope.
A series of blue-white ovals about 1,000 km wide surround the magnetic poles, always in the same place; the only thing that changes is its luminosity, which depends on how intense the solar wind is. If we look at them, we will discover even more glittering spots. They are the traces of the magnetic field lines, which connect the ionosphere of the gas giant with those of three of its moons: Io –whose point is the brightest–, Europa and Ganymede.
As an addition to these fireworks caused by subatomic particles, we can also find arcs of light within the ovals themselves, created by the fluctuations of the aforementioned solar wind. But that Jupiter has permanent auroras is not due to the jet of charged particles coming from the Sun, but to those that come from a much closer object, its satellite Io.
This moon is undoubtedly one of the most interesting bodies in our system. On its surface of yellow and reddish tones, in which sodium and sulfur abound, several hundred active volcanoes rise. In fact, its peculiar coloration is due to the fact that the element of hell is found everywhere, in its two physical states: the monoclinic, which is red, and the rhombic, which is yellow. Even the scattered white spots that can also be seen are due to sulfur dioxide ice. The eruptions of its volcanoes, such as Prometheus, are more violent than those of Vesuvius, Etna or Krakatoa . The images taken by the probes that have approached the satellite have revealed that the materials are launched up to 500 km high; the ions expelled during the eruptions are what fuel the Jovian auroras.
67 known moons
It is estimated that the crust of Io has a thickness of about 18 km, but due to its proximity to Jupiter – it is the Galilean satellite closest to it – and as a consequence of the gravitational pull of the gas giant it undergoes a deformation of about 200 m with compared to its average shape. One of the effects that this planet has on its moons is that it tends to turn them into a kind of cigarette that is pointed towards it, a shape that they adopt to a greater or lesser extent depending on the elasticity of the materials with which they are made. Furthermore, bodies whose period of rotation differs greatly from their period of revolution around the planet are severely punished and undergo continuous shape changes, similar to what happens with ocean tides on Earth.
The consequence of all this is that the moon heats up – in the case of Io, this process is two hundred times greater than that caused by the radioactivity that is generated inside it – and loses rotational energy. This tidal effect gives Io its volcanic power. When this satellite reaches the end of its evolutionary period, it will end up showing Jupiter always the same face, as it happens with ours. But, despite its quirks, Io is just one of Jupiter’s 67 known moons. This strange procession is dominated by four large bodies, the so-called Galilean satellites, so named because they were discovered in 1610 by Galileo Galilei: the aforementioned Io, Ganymede and Europa, and Callisto.
Europe awakens a lively interest among astrobiologists. With a surface temperature of around -170 ºC, it appears completely frozen, but, apparently, under its frozen surface, between 10 km and 100 km thick, an immense ocean of salt water stretches. Different studies show that the conditions necessary for life to emerge could exist in it. NASA plans to send a probe in the 2020s whose main mission would be to confirm it and determine, where appropriate, the salinity and depth of that subterranean ocean.
Jupiter’s largest moon, Ganymede, is also the largest in our solar system. Its size exceeds even that of Mercury. It resembles ours in its landscapes, with dark regions saturated with craters and other lighter ones in which geological folds similar to those of the Earth are appreciated, but whose origin is unknown.
Finally, we have Callisto, 4,821 km in diameter – slightly smaller than the aforementioned Mercury – whose surface of ice and rock is covered with craters that cross it like scars. Among its most striking features is Valhalla, an immense basin whose core is surrounded by a series of practically concentric rings.
The long cohort of objects that is added to these four has its origin in asteroids – or in pieces of them – that were trapped in the gravitational well of the gas giant. Among them, the most interesting group is the one made up of its small and rocky interior satellites –Metis, Adrastea, Amalthea and Tebe–, of which, at the moment, very little is known. Curiously, the Solar System and the one that make up the moons of Jupiter show the same configuration; the innermost bodies are also the ones with the highest density. But to know its secrets we will still have to wait. The Juno probe, named after Jupiter’s wife, has no plans to study them. The colossus will focus your attention.
Jupiter has always been in the sights of astronomers. In the mythical age of astrology, when the planets were believed to be the gods themselves, he stood as the father of all of them. For example, to the Babylonians it was Marduk. In part, this is quite obvious, since Jupiter is easily distinguished. It is that bright gray-white star high above the horizon in the night sky. It is visible for nine to ten months of the year and its light does not flicker; in fact, it is the star that shines the least of all.
Unfortunately, observing Jupiter through an amateur telescope is not very spectacular. It looks very small, like a soccer ball located 1 km away. Thus, its most striking feature would be the four faint little points – the Galilean moons – that we find near it and that have starred in some memorable episodes in the history of science. The most important was his discovery. With it, Galileo showed that not everything revolved around our planet, until then considered the center of the universe: there were at least four bodies for which Jupiter was. Those same satellites were used in 1675 by the Danish Ole Rømer to make a first estimate of the speed of light.
A whirlwind that has existed for centuries
In any case, the most prominent thing that can be glimpsed on the planet’s surface are the bands, light and dark, that alternately cross it and, of course, the Great Red Spot. This tremendous anticyclone, in which two worlds like the Earth could fit, is anchored south of the equator and brings physicists of the atmosphere headfirst, as they still do not understand what keeps it there or why it presents red tones .
Such a whirlwind has been around for as long as we know of it, three centuries ago, and it might seem like it has been there forever. When Voyagers approached Jupiter in 1979, we discovered that it is accompanied by other, much smaller, short-lived eddies that rise and fall in no definite pattern. The motion and stability of the Great Red Spot is a direct consequence of the laws that govern fluid dynamics. Now, the equations that describe it are quite complicated and in many cases completely unpleasant.
As we are unable to solve them elegantly, with pencil and paper, we must do it numerically, with the help of a computer. In this way, we know that the laws that allow us to describe the calm behavior of the mega stain are the same that also apply to those nasty vortices. And all thanks to a brilliant Russian named Lev Landáu, who in the first half of the 20th century proposed a second theory of turbulent flow.
In essence, it maintains that the apparent irregularities – such as the aforementioned vortices – are small parts of a global order so complex that we cannot perceive it in its entirety. Today it is said that the person responsible for this behavior is a mathematical device that responds to the colorful name of chaos. But we are not talking about chance or disorder. In fact, a chaotic system exhibits a definite structure if we observe it long enough. Examples of this phenomenon, which applies to all natural processes, are the usual dripping from a poorly closed tap, atrial fibrillations, epilepsy, the weather, ecology or in other matters as exotic as the somersault movement of Hyperion, a moon of Saturn.
What there is no doubt is that everything that happens in Jupiter’s atmosphere is influenced by how fast it rotates on itself. A point on its equator travels 12.5 km in one second, while on Earth it advances about 460 meters. Due to this enormous speed of rotation, we can barely observe the movements of the clouds leaving the equator in the direction of the poles.
The color of the cloud masses is due, in turn, to chemical processes dominated by hydrogen. In the atmosphere we find hydrogenated compounds, such as methane, ammonia, hydrogen sulfide – with its characteristic smell of rotten eggs – and also water. The sun’s ultraviolet radiation produces different reactions, such as ammonia and hydrogen sulfide, which combine to form ammonium polysulfides. These are yellow or orange, but when the temperature drops, white tones appear. And if the polysulfide chain is long enough it tends to acquire reddish hues.
Amazing phenomena are observed in Jupiter’s atmosphere, from storms accompanied by lightning that last an average of four days and anticyclones that last four years to mysterious bright cloud-free regions through which the heat of the interior escapes. And, if we do without its nucleus, which is denser, Jupiter is similar to a well-mixed fluid, with a composition similar to that of the Sun and possibly that of the primitive solar nebula. Hence the interest of scientists to understand not only its dynamics, but what it is made of.
The two main constituents appear to be hydrogen and helium. The most curious thing is that, in addition to the compounds already mentioned, such as ammonia, water and methane, it also contains hydrocarbons. His discovery gave rise to a hypothesis about the possible abiogenic organic synthesis – the appearance of life from organic compounds – aided by ultraviolet radiation. The cosmologist Carl Sagan and the chemist Bishun Khare carried out a series of experiments in the 70s with which, in this way, they even obtained amino acids. Could some kind of microorganism have developed in the gas giant?
It is highly doubtful, although some researchers, such as Sagan himself, have speculated on that possibility. The only certainty is that these organic molecules function as a great palette that colors the atmosphere, although the main painters are amorphous red phosphorus and hydrogen and ammonium polysulfides.
In June 2016, a team of astronomers from the University of Berkeley presented the most detailed radio map to date of Jupiter’s atmosphere, showing the massive movement of ammonia. With it we can see up to 100 km below the cloud cover; the first results show how the gas mixtures rich in ammonia rise and form the clouds of the upper layers: some, of ammonium hydrosulfide, at -73 ºC; others, of ammonia ice, at -110 ºC. Air poor in this substance ends up sinking into the planet just as dry air descends from the upper layers of the Earth’s atmosphere. That’s up. But what happens in the core? At the moment, it is a complete mystery, although Juno is expected to shed some light on this matter as well. One hypothesis suggests that it could consist of a mixture of ice and rocks at very high pressures, which has led some scientists to suppose that there, under the weight of an atmosphere in which almost ten thousand Earths would fit, there is an immense diamond .
