EUROPA (SATELLITE): CHARACTERISTICS, COMPOSITION, ORBIT, MOVEMENT - ARTE - 2024

Europa is a natural satellite or moon of Jupiter, discovered in 1610 by the Italian astronomer Galileo Galilei (1564-1642). It is part of the so-called Galilean moons, along with Ganymede, Io and Callisto. Its name comes from a character in Greek mythology: Europa was the mother of King Minos of Crete, one of the many lovers of the king of the gods.

The German astronomer Simon Marius, a contemporary of Galileo, suggested the name in a work of his, which also credited the discovery of the Jovian satellites before Galileo announced it.

Figure 1. Natural color image of Europa taken by the Galileo mission, the lines are probably fractures in the crust with exposed rocks. Source: Wikimedia Commons. NASA / JPL / DLR / Public domain

Another designation used for this satellite and currently in disuse is the one that Galileo originally proposed, with Roman numerals. Thus, Europa is also Jupiter II, since it is the second Galilean moon close to the planet (Io is the closest but there are four other smaller moons).

Eventually the astronomers fell for the suggestion of Marius, who may have discovered the satellites independently of Galileo.

The discovery of the Galilean moons orbiting Jupiter was a milestone for science. It strengthened the heliocentric theory of Copernicus and made humanity realize that the Earth was not the center of the universe.

However, the Galilean moons remained for a long time as small points of light, seen with the telescope orbiting Jupiter.

That was until the unmanned missions Pioneer, Voyager, Galileo, and New Horizons brought a flood of information about Europa and the remaining satellites of the giant planets.

General characteristics

Possible habitability

Europa, slightly smaller than the Moon, has an ocean of water under the surface and is protected from the solar wind by the Jovian magnetic field, which gives it certain prospects for habitability.

Figure 2. Comparative size of Europa, bottom left, with the Earth and the Moon. Source: Wikimedia Commons. Apollo 17 Picture of the Whole Earth: NASATelescopic Image of the Full Moon: Gregory H. Revera Image of Europa: NASA / JPL / Public domain

Add to that the fact that Europe is possibly tectonic. And apart from Earth, until now no other celestial object with complex geology was known.

Atmosphere

It also has an atmosphere, tenuous but with oxygen, and its density, although not as high as the earth's, suggests that there is a good amount of rock in its composition.

Surface

The frozen surface is very smooth, barely crossed by the lines shown in figure 1.

These lines possibly reflect stresses in the 100-150 km thick icy crust that covers Europa, exposing the underlying rock, under which there is liquid water.

There is enough heat in the interior of Europa to maintain this ocean, due to tidal warming.

It is common to think of tides as phenomena typical of oceanic masses, however the gravitational attraction not only displaces the water, but also the rock. And these processes bring about friction that dissipates the energy of orbital motion into heat.

No magnetic field

Through measurements of the magnetic field made by unmanned missions, it is known that Europa lacks its own magnetic field. But they also detected the existence of an iron core and a layer of water rich in mineral content under the crust.

These measurements indicate that the compass of a traveler arriving in Europe would experience a wild swing, especially when the approach to Jupiter is maximum. And it is that the intense Jovian magnetic field interacts with the conductive material of the subsoil, causing these fluctuations.

The albedo of Europe

It is known that Europa has an icy and slightly uneven surface, not only because of the information obtained through images, but also because of the measurements made to its albedo.

The albedo of any object - astronomical or of another nature - is the fraction of light that it reflects. That is why its value ranges from 0 to 1.

If the albedo is 0 it means that the object absorbs all the light without reflecting anything, on the contrary, if it is 1 it reflects it completely.

Mirrors are objects with a large albedo and that of Europa is 0.69. This means that it reflects approximately 69% of the light that reaches its surface, an indication that the ice that covers it is clean and recent.

Therefore, the surface of Europa is relatively young, estimated to be around 10 million years old. Surfaces with old ice tend to be rather very dark and have less albedo.

Another fact in its favor is that Europa's surface has hardly any impact craters, suggesting enough geological activity to erase evidence of impacts.

One of these few craters appears at the bottom of figure 1. It is the light spot in the shape of a mole with a dark center, called Pwyll Crater, in honor of the Celtic deity of the underworld.

Summary of the main physical characteristics of Europe

Translation movement

Europa moves around Jupiter with a period of just over 3 and a half days, following a fairly circular orbit.

A peculiarity in the translational motion of Europa is that it is in synchronous rotation with Jupiter. Therefore it always shows the same face to the planet, just as the Moon does to the Earth. This phenomenon is also known as tidal coupling.

Figure 3. Europa always shows the same face to Jupiter thanks to the synchronous rotation. Source: NASA.

Tidal coupling is characterized by the fact that it takes the object the same time to orbit around the most massive body - Jupiter in this case - as it does one complete revolution on its own axis.

The explanation is that celestial bodies are not point masses, but objects with appreciable dimensions. For this reason, the force of gravity that Jupiter exerts on its satellites is not homogeneous, being more intense on the nearest side, and less intense on the far side.

This creates a periodic distortion in Europa, which is also affected by the force of gravity regularly exerted by the other nearby Galilean moons: Ganymede and Io.

The result is an amplification of gravitational forces in a phenomenon known as orbital resonance, as the other moons gravitationally pull on Europa at precise time intervals.

Laplace resonance

And of course Europe does the same with the other moons, creating a kind of harmony between all of them.

The mutual gravitational effects of the Galilean moons are called the Laplace resonance, after its discoverer, the French mathematician and astronomer Pierre Simon de Laplace in 1805.

There are several kinds of resonance in physics. This is a rare resonance in which the periods of revolution of the three moons are in a 1: 2: 4 ratio. Any force exerted on any of the members of this system is transmitted to the others, via gravitational interaction.

Figure 4. Animation of the orbital resonance between the Galilean satellites. Source: Wikimedia Commons. User: Matma Rex / Public domain.

Therefore the tidal forces make all of Europe is subjected to lugs and compressions that originate the heating described above. And it also causes Europa to have an ocean of liquid water within it.

Rotatory motion

Europa has a rotational motion around its own axis, which, as we have said, has the same duration as the orbital period, thanks to the tidal coupling it has with Jupiter.

Composition

The same elements are present in Europe as on Earth. In the atmosphere there is oxygen, iron and silicates are in the core, while water, the most striking substance, occupies the layer below the crust.

The water under Europa is rich in mineral salts, such as sodium chloride or common salt. The presence of magnesium sulfate and sulfuric acid may partly explain the reddish lines that cross the surface of the satellite.

It is also believed that in Europe there are tholins, organic compounds that are formed thanks to ultraviolet radiation.

Tholins are prevalent on icy worlds like Europa and Saturn's moon Titan. Carbon, nitrogen and water are required for them to form.

Internal structure

The internal structure of Europa is similar to that of Earth, since it has a core, a mantle and a crust. Its density, along with that of Io, is higher than in the case of the other two Galilean moons, indicating a higher silicate content.

Figure 5. Internal structure of the four Galilean moons, according to theoretical models. Source: Kutner, M. Astronomy: a physical perspective.

The core of Europa is not made of molten metal (as opposed to Io), which suggests that the water under the crust has a high mineral content, since Europa's magnetism comes from the interaction between a good conductor such as water with salts and the intense magnetic field of Jupiter.

Radioactive elements abound in the rocky mantle, which emit energy when decaying and constitute another source of internal heat for Europa, apart from tidal heating.

The outermost layer of water, partly frozen and partly liquid, is estimated to be 100 km thick in some areas, although others claim it is only about 200 m.

In any case, experts agree that the amount of liquid water in Europa can be twice as much as there is on Earth.

It is also believed that there are lakes in the crevices of the ice crust, as suggested in figure 6, which could also harbor life.

The icy surface receives the continuous interaction with charged particles sent from the Jovian radiation belts. Jupiter's strong magnetism accelerates electrical charges and energizes them. Thus the particles reach the surface ice and fragment the water molecules.

Enough energy is released in the process, enough to form the glowing gas clouds around Europa that the Cassini probe observed as it headed toward Saturn.

Figure 6. Internal structure of Europe according to the models created with the available information. Source: Wikimedia Commons.

geology

Unmanned missions have provided a wealth of information about Europa, not only in the multitude of high-resolution images they sent of the surface, but also because of Europa's gravitational effects on spacecraft.

The images reveal a very light yellow surface, devoid of noticeable landforms, such as towering mountains or notable craters, unlike other Galilean satellites.

But what is most striking is the network of sinuous lines that continuously intersect and that we see clearly in figure 1.

Scientists believe that these lines originate from deep fissures in the ice. Viewed more closely, the lines have a dark edge with a lighter central stripe believed to be the product of large geysers.

Figure 7. The geysers of Europa, seen by Hubble. Source: NASA.

These towering columns of steam (plumes) several kilometers high are made up of warmer water that rises from the interior through the fractures, as reported by observations from the Hubble Space Telescope.

Some analyzes reveal the traces left by water with a high mineral content and subsequently evaporated.

It is possible that under the crust of Europa there are subduction processes, as they occur on Earth, in which tectonic plates converge at the edges, moving relative to each other in so-called subduction zones.

But unlike Earth, plates are made of ice that move over liquid ocean, rather than magma, as it does on Earth.

Possible habitability of Europe

Many experts are convinced that the oceans of Europe can contain microbial life, as they are rich in oxygen. In addition, Europe has an atmosphere, although thin, but with the presence of oxygen, an element necessary to sustain life.

Another option to support life are the lakes encapsulated in the ice crust of Europa. At the moment they are assumptions and much more evidence is missing to confirm them.

Some evidence continues to be added to strengthen this hypothesis, for example the presence of clay minerals in the crust, which on Earth are associated with organic matter.

And another important substance that, according to new findings, is found on the surface of Europa is sodium chloride or common salt. Scientists have found that table salt, under the prevailing conditions in Europe, acquires the pale yellow color, which is seen on the surface of the satellite.

If this salt comes from the oceans of Europe, it means that they very possibly bear similarity to the terrestrial ones, and with it the possibility of harboring life.

These findings do not necessarily imply that there is life in Europe, but, if confirmed, the satellite has sufficient conditions for its development.

There is already a NASA mission called Europa Clipper, which is currently under development and could be launched in the next few years.

Among its objectives are the study of the surface of Europa, the geology of the satellite and its chemical composition, as well as the confirmation of the existence of the ocean under the crust. We will have to wait a little longer to find out.

References

1. BBC. Why is Jupiter's icy moon Europa the best candidate to find extraterrestrial life in the Solar System? Recovered from: bbc.com.
2. Eales, S. 2009. Planets and Planetary Systems. Wiley-Blackwell.
3. Kutner, M. 2003. Astronomy: a physical perspective. Cambridge University Press.
4. Pasachoff, J. 2007. The Cosmos: Astronomy in the new Millennium. Third Edition. Thomson-Brooks / Cole.
5. Seeds, M. 2011.The Solar System. Seventh Edition. Cengage Learning.
6. Wikipedia. Europe (moon). Recovered from: en.wikipedia.org.
7. Wikipedia. Europa Clipper. Recovered from: es.wikipedia.org.