Io, a Jovian moon

.......... So far 32 moons have been discovered, orbiting Jupiter. The four largest ones are called galilean satellites as they were first observed on 07 january 1610, by Galileo Galilei, using a small reflecting telescope. The four large saellites orbit Jupiter in its equatorial plan, from east to west as seen from Earth.
The orbits of Io, Europa , Ganymede and Callisto are locked together in a pattern. Io does 4 orbits around Jupiter while Europa does 2 and Ganymede 1. This is due to the Laplace Resonance phenomenon.
Io is the third largest of Jupiter's satellites, slightly larger than the Earth's moon.
The four large moons, Ganymède,Europe, Callisto and Io were named after the Zeus lovers.
Io is the innermost of the galilean moons and is somewhat similar in bulk composition to the terrestrial planets. It is about the same size as our moon , and the only body in the solar system , except earth, to posses such volcanic activity.
The Voyagers observed ten volcanoes erupting on Io, and found over 200 other dormant volcanoes. Interestingly, according to the Galileo survey, they are supposed to be more than one hundred active volcanoes.
The energy for all this activity derives from tidal interactions between Jupiter, Io but also Ganymède and Europa. The tidal bulging on Io's surface is estimated as great as 100 meters.Scientists think that Jupiter is powering the volcanoes by squeezing Io everytime it goes around Jupiter. This tidal pumping heats the inner part of the satellite and most of the rocks become molten. Some of the hottest spots on Io may reach temperatures as high as 2000 K but the average surface temperature is about 140 K. We must not forget that Io is far from the sun (more than 5 a.u.)
Io is peculiar enough a moon to draw the attention of scientists from all over the world..

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Io, a priestess of Hera

.......... Io was one of Hera's young priestess and the daughter of the water God, Inachus and his wife Melia.
One day she dreamt she had to meet Zeus on the banks of the Lerne lake. After asking her father permission,she went there.When Zeus saw her, he fell in love, as usual and desired her. In order to fool Hera, he covered the world with clouds and turned himself in cloud. Hera grew suspicious and came down from Mont Olympus and began dispersing the clouds.
To decieve the jealous Hera, Zeus turned the lovely maiden Io into a white heifer. Seeing right through all this, Hera claimed the heifer as a gift. Zeus couldn't turn such a reasonable request down , so he gave the cow to his wife.Hera gave the cow to Argos to watch over. Argus was a fine watchman, as he was a monster with one hundred eyes, fifty slept whilst the other fifty watched Io.
Zeus asked his son Hermes to help him. Disguised as a shepherd, Hermes played music and told stories to gain Argus confidence then put him to sleep, killed him with his sword and freed Io.Hera , infuriated at being decieved again, sent a gladfly to torment Io.This pushed Io near madness and she wandered the world. She met Prometheus who gave her hope and predicted that she would be changed back into human form and would bear a child.Many geographical features were named after her, as the Ionian Sea. Io was identified with the Egyptian goddess Isis.

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Io's Passport
An artist view of Galileo, Io and Jupiter

Mean distance from Jupiter 421600Km (5.905 jovian radius)
Equatorial diameter 3630 Km
Rotational period 42 h 27min
Orbital period 42 h 27 min
Mean density 3.57
Mass 8.94E22 kg (0.015 Earth mass)
Surface gravity 0.183g(Earth)
Escape velocity 2.56 km/s
Orbital inclination 0.040°
Orbital velocity 17.34 km /s
Visual geometric albedo 61%
Thermic emission 10E14 watts, a little more than 2W.m-²

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The exploration of Io
.......... The discovers :
Galileo Galilei (1564-1642) was a physicist who devised the astronomical telescope and who had the same point of view as Copernicus about the Heliocentrism theory. In France, Galilée is said to have discovered Io but a German, Marius Simon claimed to have made the same discovery. The truth is that each dicovered the moons independantly. Marius Simon named the Galilean moons after Jupiter's lovers.
.......... Spacecrafts :
Voyager I
The two first spacecrafts Voyager I and II were launched in 1977 from Cap Canaveral (Florida). They discovered the existence of volcanoes on Io, in 1979, and send a lot of pictures.
Galileo :Launched in october 89, to study Jupiter and its moons: atmosphere
com ponents, pressure and temprerature, dust detection, magnetic field detection and of course a lot of views.


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Io, a huge "pizza"

.......... Stanton Peale, Patrick Cassen and Ray Reynolds from California University, were the only scientists to suppose Io to be very hot and have volcanoes. Their conjoncture was correct. In 1979, when the Voyager spacecrafts flew past Jupiter, they noticed at a strange bulge on one side of Io. After eliminating a computer malfunction, they realised Voyager had seen the first volcanic eruption on the "outside world". Now we know about more than one hundred active volcanoes.

.......... Every year ,these volcanoes spout out billions tons of lava, explaining the surface of Io's everchanging appearance.
The ground near the volcanoes is flat. Lava flows, hundreds of kilometers long, composed mostly of various compounds of molten sulfur and molten silicate rocks. These compounds take in a wide range of colors which are responsible for Io's variegated appearance. Hence the nickname of "deep frozen pizza" due to its yellowish and red deposits.
Recent images show very large volcanic eruptions, spouting lava and plumes as high as 300 km,
with an ejection celerity over 1000 m/s.
Instead of the expected crater impacts, Voyagers found hundreds of volcanic caldeiras (crater formed by an explosion or the collapse of a volcanic vent). We now have striking photos taken by Galileo and HST.
This two caracters are read Iô and they mean...Sulphur!!

About the volcanic activity of Io, we founded an odd coincidence. Reading chinese texts you may found these two caracters wich mean...sulphur and which are read Iô

.......... The best examples of Io's volcanic activity are its main volcanoes.
First, volcano Pele: the highest of Io's volcano with a very hot spot, probably due to a lava lake. Chimical data from the HST have been studied by researchers at Washington University in St Louis who conclude that Io has a differentiated mantle similar to that on Earth: various rock types are produced from a common magma. They obtained the same results in comparing the magma temperature inferred from HST data (magma and plume pressure of Pele) and the temperature deduced from infrared measurements taken by Galileo spacecraft orbiting Jupiter, a challenge for expensive planetary missions. Pele is one of the most active moon's volcano, with lava flows 10 km long and 50 m high.
Loki is the more powerful volcano in the solar system spouting more than all of the Earth's volcanoes put together It shows a huge caldera (larger than Maryland) ,often flooded with lava A large dark area, in the center of the caldera might be a lake of sulfur with a "raft" of solid sulphur.
Haemus Mount : one of the few mountains on Io , near the south pole. Its base measures about 200 by 100 km. Some mountains reach 16 km high

Focus on an Io's volcano

It is odd to find so much energy in a moon the same size as our own. Its activity can't be explained by natural radioactivity (it is 250 times higher). Only tidal interactions between Jupiter , Io, Ganymede and Europe whose orbits are locked into resonance, can produce all this activity.
Io is squeezed, and the resulting friction creates heat and melts rocks, producing geysers ans lava flows.
Io always faces the same side toward the planet, but it wobbles under the combined attractions of Ganymède and Europe creating a 100 m tide.

.......... Recently,researchers evaluated the oxydation state of the magma, and suggested a lack of iron metal from the mantle.Intense oxydation could have occurred during the first billion years, maybe with water as the oxidizing agent.
Water has disappeared from the Io's surface.
It is supposed that Io has a two-layer structure with a metallic core, probably made of iron and iron compound, but about 900 km in radius (half of the moon's radius).
Galileo also detected a magnetic field around Io (the first moon shown to have one) and , what was completely unexpected, a hole in Jupiter's magnetic field.
Io is probably the source of high velocity dust streams stretching millions of km around Jupiter.

.......... The study of Io could help us understand more about the earth's past ,according to Torrence Johnson, Galileo Project Scientist.

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1- Using Kepler's third law, find the orbital period of Amalthee, Jupiter's moon discovered in 1892. Data : Io (Orbital period 42h27 mn, mean distance from Jupiter :421 600 km
Almathe (mean distance from Jupiter : 181 000 km)

2- An object is spouted out by Io's volcano Pele with an vertical ejection velocity of 500 m/s.
It is supposed to obey the law of gravity, whithout other interaction and it is launched from ground level.
How long it will take to rise and fall back to the ground. ?
What is the covered distance during the travel ? g(io)= 0,183 g(earth)

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.......... Websites for Io (Thanks to the webmasters):

.......... Books

Astronomie-Astrophysique by Benoit Villeneuve and Marc Seguin6 MASSON
Etoiles et constellations by A.Benhamouda (SNED)
Ciel et espace n° 366 and 369

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This little document about Io was made by :
AUBERT Emmanuel