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]
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[...] the moon also has Laplace resonance, which occurs because of the forces from the satellites Io and Europa. Every time Ganymede revolves around Jupiter once, Europa, the satellite just inside Ganymede, goes [...]