Jupiter | .. . . o * O O . . . . . . . .
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Europa (pronounced "yoo roh puh") is the sixth moon from Jupiter. It weighs in at 4.80*10^22 kg and a diameter of 3138 km which puts it slightly smaller than the Moon (7.35*10^22 kg and 3476 km) and the fourth largest of Jupiter's moons (smallest of the Galilean Moons).
While the moons are called the Galilean Moons, they were discovered independently by Galileo and Simon Marius in the year 1610. Simon Marius was a German astronomer who lived from 1573 to 1624 and gave them the names that we have today (though the suggestion came from Johannes Kepler in 1613) - Galileo wanted to name them the "Medicean planets" after his patron, Cosimo II de Medici and used numbers to distinguish them. Both systems were in use until the mid 1800s when the names we are familiar with were officially adopted (it was realized that numbers would be a headache as new moons were found). While Marius claims to have been observing them in November 1609 he did not publish his findings right away and Galileo was first to the presses. Furthermore, Galileo's work was more extensive and accurate than that of Marius.
Galileo's discovery was made on January 7, 1610 when he saw three 'stars' in a line with Jupiter. The following evening the 'stars' had moved - though not in the way that he had expected them to. Over the course of a week a fourth 'star' appeared. None of these 'stars' had left the area around Jupiter and moved with respect to each other and Jupiter. This supported the Copernican system for the solar system.
Europa is composed of materials that are very similar to that of the terrestrial planets - rock made of mostly silica. Recent data from the Galileo probe also seems to indicate that there are several layers beneath the surface and the possibility of a small metallic core.
The surface of Europa is unique within the solar system being very, very smooth and with a very high albedo (0.63) (surface reflectivity). There are no mountains, and very few craters implies a rather young and active surface. The closest resemblance to Europa's surface is that of great sheets of ice on the earth.
It is believed that under this surface, there is a giant ocean of water that is kept liquid by the heat produced by the tidal squeezing from Jupiter (the same force that keeps Io fairly liquid). This belief is
reinforced by a weak magnetic field that varies as Europa passes through Jupiter's magnetic field - indicating a conducting layer under the surface, possibly a deep layer of salt water. The presence of water leads many to wonder if there is the possibility of life under that layer of ice, much as there is life at the deep sea vents on Earth
(known as black smokers).
Through the Hubble Space Telescope, it has been found that Europa also has a very thin atmosphere (10^-11 bar) of oxygen. However, this is believed to be created by the bombardment of the surface with light and cosmic rays that split the ice into hydrogen and oxygen. The hydrogen then escapes leaving only the oxygen.
Europa is locked into an orbital resonance with Io and Ganymede at with the ratios of 1:2:4 - Europa orbits about Jupiter half as fast as Io and twice as fast as Ganymede.
Europa is the target of some interesting missions in the future that would explore the ocean under the ice.
This probe would drill through the ice of Europa to look under it. It is uncertain how thick this ice is - estimates range from 1km to 20km thick. Chances are, this wouldn't actually drill, but rather melt through (its much easier to make something hot than it is to drill - and less chance of a failure). The probe would then send messages back to the surface and/or orbiter which would then be sent back to Earth. See cryobot for more on this type of project.
- Ice Clipper
- Originally proposed to launch in December of 2001 and reach Europa in 2009 this probe would shoot a hollow copper ball about the size of a softball into Europa at 10km/sec. This impact would send up some ejecta which would be collected by the probe at 50km above Europa in aerogel and then head back to Earth (around 2014).