Jupiter has a number of moons. Four of these were known since Galileo days – Io, Europa, Ganymede and Callisto. They stand out form the rest of the moons due to their size (varies from the size of the Moon to the size of Mercury) and their proximity to the planet. There are few other satellites that rotate even closer to Jupiter: three very small ones and Amalthea, which has an irregular shape (its size is approximately 130 x 80 km). These small satellites together with the Galilean satellites form a so-called accurate system, which has a distinctive coplanarity (location of satellite orbits in the equatorial plane of the planet) and almost circular shape of the orbits.
If you compare them to the position of Earth’s Moon, Io is 10% further, and Callisto – 4.9 times further from the Jupiter than Moon is from Earth. But because of Jupiter’s huge mass it only takes them 1.8 and 16.7 days to orbit the planet.
Since their discovery, those four moons remain one of the most popular objects of astronomical observations. But if only astronomers knew then what wonders are hidden in these celestial bodies!
Relatively short history of space exploration is full of accidents and unexpected discoveries. Gradually experts have developed a specific slang, which is only known to people related to space exploration. No wonder space experts came up with their own version of Murphy’s law: “Everything that can go wrong – will go wrong. Anything that can not go wrong – will go wrong too.” But, fortunately, this law can also work the opposite way. The discovery outlined below is a prime example of such amazing luck.
In order to accurately determine spacecraft’s position on the orbit of a distant planet, specialists do not rely on navigation information only. They also use pictures of visible satellites, which are transmitted by the spacecraft. Relative positions of celestial bodies are then entered into a computer, which determines the coordinates of the spacecraft. One legend says that when Voyager 1 was approaching Jupiter, a computer reported an error in the image of Io. The reason for the error was unclear, but in the end scientists found that Io’s image transmitted by the spacecraft did not match the expected image of a round celestial body. Something was protruding on the side of Io. That “something” turned out to be a huge gas cloud, which rose about 250-300 km above the active volcano.
An emission of oxygen, sodium and sulfur was discovered along the orbit of Io several years ago. The question is – How does such cloud remain in space? At first, scientists thought that images of Io explained this phenomenon: 7-8 active volcanoes on its surface constantly emit gaseous fountains that rise hundreds of kilometers above its surface. If we assume that only part of the eruption products is dissipated into space, the origin of the gas cloud along Io’s orbit can be explained.
But the fact is that Io is rather massive celestial body – its mass is 20% greater than the Moon. The average density of Io is 3.53 g/cm3. Its diameter is 3620 kilometers (Moon is 3476 kilometers). Calculations show that the free fall acceleration at its surface is also large enough – 1.81 m/s2. Heavy sulfur dioxide and sulfur vapor ejected from the volcanic caldera quickly condense in low temperatures, and fall on Ios` surface in the form of frost and snow. This process is faster than the destruction of gas molecules by ultraviolet radiation from the Sun. At the same time, the free-fall acceleration is not enough to retain an atmosphere, as it is on Mars, although Io has some traces of the atmosphere. In order for gas clouds to be several hundred kilometers high, it requires gas outflow of around about 1 km / sec from the volcano. Io’s atmosphere has very small density – 10 to 100 million times smaller than Earth. This is another factor contributing to the high gas outflow.
But let’s leave a question of how do sulfur and sodium get into space for now, and look at Io’s magnificent volcanos.
Energy for Io’s volcanoes
Io is not large enough for the radioactive decay of elements in its core to cause strong heating of the crust as it does on Earth. Energy for heating is drawn from an entirely different source: from tidal effects of the second Galilean satellite – Europe, Jupiter itself, and, to a lesser extent, the third satellite – Ganymede.
These tidal effects force Io’s lithosphere to bend, and it heats up – just like a wire does when you bend it. Enormous energy is released because of the tidal effects – 60-80 million MW. This energy is most likely distributed unevenly – the majority of it is probably released in the surface layers of the moon.
Volcanoes and hot spots on Io
Around 2% of Io’s surface is covered by active hot spots. There are more than 10 of these known to scientists. Temperatures in these spots are believed to be between 310 and 600 K, with spot sizes varying from 75 to 250 km. “Voyager 1″ found 8 active giant eruptions. When 4 months later “Voyager-2” reached Io, it found that 7 of them still continued to erupt. The only one that stopped erupting was one of the largest volcanoes called Pele (in honor of the god of volcanoes).
It is interesting to note that the center of the eruption is for some reason dark, while products of eruptions are orange. Scientists believe that these products accumulate in a molten state in deep reservoirs under Io’s surface.
Io’s volcanoes can be divided into two main categories. Volcanoes of the first group have temperatures of around 350 – 400 K and release gaseous products at a rate of about 500 m/sec. Their gas clouds are usually around 100 km high, and precipitations are white in colour. The majority of Io’s volcanos are of the first type. Volcanos in the second group have very high caldera temperatures; have emission rate of about 1 km/s and heights of their gas clouds can reach around 300 km. Abovementioned Pele, and later found Surt and Aten are examples of volcanos in the second group.
One of the volcanic objects, known as Patera Ra, has a very unusual form and deserves special mentioning. The serpentine lava flows that start at Patera Ra extend over distances of up to 200 km. Their colours vary from shades of brown to light orange and snow-white tones. The nature of these volcanic flows remains unclear, as is nature of even more mysterious objects – lava lakes.
The strongest signal recorded by Voyager’s thermal radiometers, came from a strange object, which was later called Loki. On high-resolution images it appears as a slightly truncated dark circular formation. This formation is around 250 km in diameter and has a bright yellow object of an angular shape in its center. It is believed that the yellow object is a 100-kilometer “iceberg” of solid sulfur, which floats in the center of a lake of molten sulfur! More of these small floating fragments can be seen around the main one.
Topography of Io
Few words about Io’s relief. The moon is mostly flat. There are several large mountainy areas and the mountains in the center of the Pele area. There are also high mountains at the South Pole, covering an area of about 150×80 km. An interesting object was detected on one of the photos – 2.5 km high conical shape mountain with a base diameter of about 85 km.
Interaction with Jupiter
Io’s orbit is located in in the center of Jupiter’s radiation belt – a part of magnetosphere, where the streams of charged particles are especially dense.
Electric and magnetic phenomena are very intense in Jupiter’s magnetosphere. Io is a part of one of the nature’s wonders – a powerful natural electrical generator. Perhaps “powerful” is not the right word though. A current of around 5 million Amperes flies between Jupiter and Io. This is more than 20 times more than the total energy produced by all Earth power plants. The mechanism by which this fantastic power is produced, is probably associated with a very peculiar structure of the so-called current shells in the Jupiter’s plasmasphere. Some scientists argue that strong electrical currents on Io’s surface can focus on small areas. And potentially volcanic eruptions could be related to these electric activities. So, to summarize, Io “works” as a part of a giant natural particle accelerator.
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