Jupiter’s moon Europa is one of the most interesting bodies of the Solar System. It was discovered by Galileo Galilei and Simon Marius (Simon Marius did it a bit later, and it was him who has offered the names to moons discovered by Galilei) in 1610. According to Greek mythology, Europa was a Phoenician princess stolen by Zeus who had transformed her into a white bull.

Tidal forces inside Europa are much smaller compared to another Jupiter’s moon – Io. First readings from spacecrafts orbiting Jupiter did not show any signs of eruptions on Io. But further examinations found a sultan over the moon’s limb, which included water, ammonia and other products. Nevertheless, gas eruptions that are so typical for Io seem to be very rare on Europa.

Photos of Europa show that the entire surface of the moon is covered by ice. It looks very unusual. A thrilling idea of artificial origin of channels on Mars was proposed around 100 years ago. However, these lines turned out to be an optical illusion caused by hardly distinguishable details on the distant planet. But here on surface of orange-brown Europa scientists found a really dense network of crossed lines. Pictures of Europa’s surface look very similar to the pictures of the Arctic Ocean made from the orbit. In the beginning scientists were very careful about arising analogy. But spectral measurements did not leave a place for doubts — Europa’s surface is covered by ice.

The size and average density of Europa allowed scientists to calculate the amount of ice in the total mass of the moon. With a diameter of 3138 km and average density of 3,04 g/sm3, Europa should have much more water than Io or the Moon. Because of this, first calculations predicted that Europa’s ice cover should be about 100 km thick. Further research, however, has led to smaller figures.

According to the latest research Europa’s ocean could be tens of kilometers deep, and its ice cover is probably just a few kilometers thick. This cover is fragile and sometimes breaks under tidal pressure. These breaks give liquid water access to moon’s surface, which has no atmosphere.

It is likely that a global network of lines visible on Europa’s surface is cracks in the thick ice cover, caused by tectonic processes. These breaks are not accompanied by any movements of the ice cover, and cracks are quickly filled with orange liquid. Breaks can be tens of kilometers to 100s of kilometers wide and more than 3000 km long. Water instantly begins to boil and simultaneously freezes. Evaporated water returns to the moon’s surface in the form of snow and frost. The boiling carries away a lot of heat, and it only takes few minutes for water to form a half-meter layer of ice.

Europa is a very flat moon. Its highest ‘mountains’ are less than 50 meters high.  All of this can be explained in two ways: either Europa is a very young moon or there is a ‘mechanism’, which smoothens its surface. Few facts speak in favor of the second option – relatively high temperature (ocean of liquid water) and ability of ice to move.

Interest to the ocean lying under the ice of Europa had been stimulated by the assumption that life could exist in it. Even if this life is in elementary form. If Europa’s ocean is 50—60 km deep, its volume should be close to the terrestrial oceans. Free fall acceleration on the surface of the moon is 1.32 m/s2. This means that the pressure at the bottom of Europa’s ocean is the same, as it is at the 4-kilometer depth on Earth. It is well known that life on Earth has started in oceans. But there is one fundamental difference – Europa’s ocean does not have a constant energy source. Sunlight is such source on Earth. Life and photosynthesis are inseparable. However, there is one exception – sulfur compounds formed at rather high temperatures of underwater volcano eruptions are used by some microorganisms in chemosynthesis (chemical synthesis under the influence of heat). There are other, equally speculative ideas, such as light absorption by microorganisms while new cracks are still free of ice.

Existence of an ice cover on Europa had been proven and there are no doubts about it.  As for the ocean and the assumptions associated with it – very little is known about what is hidden under the ice cover and most hypotheses are only speculative at this stage.

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[images: nasa.org]

• Diameter of Jupiter: 85,788 miles. Jupiter is the largest planet of the solar system – more than 12 Earths could line up across it
• Temperature Range: -163° C to -121° C
• Distance from Sun: about 466 million miles
• Atmosphere of Jupiter: Mostly hydrogen and helium
• Rotation of its axis: 9 hours, 55 minutes in Earth time (the length of one rotation).  Jupiter spins faster than any other planet of the solar system.
• Rotation around the Sun: 12 Earth years
• Magnetic Field: Yes
• Number of Moons: at least 63
• Ganymede, Jupiter’s moon is the largest moon in our solar system. It is even larger than Mercury and Pluto. Ganymede is the only moon that has its own magnetic field.
• Four of Jupiter’s moons are known since Galileo days – Io, Europa, Ganymede and Callisto. They are all in synchronous rotation with Jupiter – always faces Jupiter with one side.
• On Jupiter you will weigh more than you weigh on the Earth, because of the fact that Jupiter is larger planet and has stronger gravity field than Earth.  If you weigh 70 pounds on Earth, on Jupiter you would weigh 185 pounds.

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Jupiter, the largest planet of our Solar system is the fifth planet from the Sun. It is over 11 times the size of Earth and is so large that it can fit 1,400 Earths. It is assumed that if Jupiter was larger, it might have been a sun. After the Sun, the Moon and Venus, Jupiter is the fourth brightest object in the sky. It has been known since prehistoric times as a bright “wandering star”.  Jupiter is a very stormy planet and most of the storms seem to never end. Different cloud formations and storms in the atmosphere make Jupiter a very colorful planet.

Jupiter has 1400 times the volume of Earth however the mass is only 318 times more. Hence, the mean density of Jupiter is approximately one-fourth that of Earth, signifies that the Planet Jupiter must consist of gases rather than the metals and rocks of which the Earth and other inner planets are composed.

Jupiter’s Atmosphere

Jupiter has the largest planetary atmosphere in the Solar System – expanding over 5000 km in altitude. Major part of atmosphere is molecular hydrogen (H2). Samples tested by the Galileo probe indicated 89.8 percent molecular hydrogen, 10.2 percent helium, minor amounts of methane, ammonia, hydrogen deuteride, ethane, water, ammonia ice aerosols, water ice aerosols, ammonia hydrosulfide aerosols. The core of the planet must have essentially the same composition taking into account the observed low density. It seems that, this huge world must be made mostly of the two lightest and most abundant elements in the universe, a mixture resembling that of the Sun’s and other stars’.

Jupiter’s clouds have bright colors, which are probably the result of subtle chemical reactions of the trace elements in Jupiter’s atmosphere. It possibly involves sulfur whose compounds take on a wide diversity of colors, but the details are unidentified. Jupiter’s atmosphere also features large storm structures; one of them is famous Great Red Spot (GRS).  The GRS is an oval about 12,000 by 25,000 km, which is very big and is enough to hold two Earths. Other smaller but comparable spots have been discovered for decades. Infrared observations and the direction of its rotation specify that the GRS is a high-pressure region and its cloud tops are considerably higher and colder than the neighboring regions. However it is not known yet, how such formation can remain active for so long.

Internal structure of the Jupiter

Most likely Jupiter has a core of rocky material amounting to somewhat like 10 to 15 Earth-masses. Over the core lies the bulk of planet’s mass in the form of liquid metallic hydrogen. Liquid metallic hydrogen on Jupiter has ionized protons and electrons just like the interior of the Sun, however at a much lower temperature. Due to high temperatures and pressure inside of Jupiter, hydrogen is kept in a liquid form, not a gas. Hydrogen is an electrical conductor and the cause of Jupiter’s magnetic field. This layer most likely also includes some helium and traces of various “ices”. The outermost layer consists mainly of ordinary molecular hydrogen and helium that is liquid in the interior and gaseous outwards. The atmosphere we can observe is only the very top of this layer. Water, carbon dioxide, methane and other molecules are also there but in tiny amounts.

Magnetic Field

It is assumed that the Jovian magnetic field is generated deep within the layers of the planet. At the Jupiter’s surface, this field is 14 times stronger than Earth’s. Its polarity is the opposite of Earth’s so that a terrestrial compass at Jupiter would point south. This Magnetic field is accountable for the huge belts of trapped charged particles which circle the planet out to a distance of 10 million km. It is approximately ten times as strong as the Earth’s Van Allen radiation belts and holds mysterious high-energy helium ions from an unknown source.

Jupiter spins faster than any other planet

Jupiter rotates on its axis every 9.9 hours, compared with 24 hours for Earth. This rapid spin causes a bulge at its equator and flattening at the poles, making the planet roughly about 7 percent wider at the equator than at the poles. Jupiter makes a complete orbit around the sun in 11.9 Earth years. It completes the orbit around the sun at a mean distance 5.2 times greater than one astronomical unit.

Jupiter’s Rings

NASA’s Voyager 1 spacecraft discovered Jupiter’s three rings. The first ring, which is the main ring is flattened. It is roughly about 20 miles thick and more than 4,000 miles wide.

The second ring, called the halo, is roughly 12,000 miles thick. The main ring and halo, both are composed of small, dark particles.

The third ring, called the gossamer ring is perhaps made up of dust particles less than 10 microns in diameter. It extends to an outer edge of about 80,000 miles from the center of the planet and inward to about 18,600 miles.

Jupiter’s Moons

Jupiter has at least 63 moons. The four largest moons of Jupiter, which were discovered by Galileo himself, are known as Io, Europa, Ganymede, and Callisto. These moons are more commonly known as the Galilean satellites.

Io is the most volcanically active body in our solar system. The sulfur emitted by these volcanoes emit gives Io a blotted yellow-orange appearance. As this moon orbits Jupiter, the planet’s immense gravity results in ‘tides’ in Io’s solid surface which rise 300 feet (100 meters) high, creating enough heat for volcanic activity.

The frozen crust of Europa is constituted mostly of water ice, and it might have a liquid ocean holding twice the water Earth does. Icy oceans might also be present under the crusts of Callisto and Ganymede.

Callisto has the lowest reflectivity, of the four Galilean moons. This implies that its surface might have dark, colorless rocks.

Potential prospects for life on Jupiter

After Mars, the Jovian system is probably the place in the solar system to sport life. Although the planet itself cannot have life and most of its moons cannot either, the four moons discovered by Galileo present some hope. Especially, the icy moon Europa is the leading candidate for life. Many NASA scientists expect with strong evidence that an ocean is concealed under the moon’s solid and icy surface. NASA is currently working to find a way to send a probe to Europa that would finally dig deep beneath its surface. And if life does not exist on Europa, the larger Jovian satellites Callisto and Ganymede have an environment potentially suitable for upcoming human colonization.

Jupiter was primary visited by Pioneer 10 in 1973 and then by Pioneer 11, Voyager 1, Voyager 2 and Ulysses. The spacecraft Galileo orbited Jupiter for eight years and is still regularly observed by the Hubble Space Telescope.

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