Hua Feng heard the teacher say that Europa may have been caused by the accretion of Jupiter's sub-nebula, the disk of gas and dust that surrounds Jupiter after its formation, and the accretion process of Calymede lasted about 10,000 years, with a clear distinction between the fainter Nicholson zone and the brighter Hapagia groove.

Much shorter than Callisto's 100,000 years. By the time Galileo's moons began to form, Jupiter's subnebulae already contained relatively little gas, which led to a longer accretion time for Callisto. Conversely, because Ganymede formed immediately after Jupiter, the sub-nebulae were still dense, so their accretion took less time.

The relatively short formation time allows less escape from the heat generated during accretion that leads to the melting of the ice and the differentiation of the internal structure of Europa: that is, the rock and ice are separated from each other, and the rock sinks into the center of the star to form the core. In this respect, Europa differs from Callisto, which, due to its long formation time, allows the accretion heat to escape and thus fail to melt the ice and differentiate the internal structure in the early stages. This hypothesis reveals why two moons, which are so close in mass and composition, look so different.

After its formation, the core of Ganymede retains most of the heat formed during accretion and differentiation, slowly releasing a small amount of heat into the icy mantle layer, just like a thermal battery operates. The mantle then conducts heat to the surface of the star by convection. Soon the radioactive elements contained in the rock began to decay, and the heat generated further heated the core, which intensified the differentiation of its internal structure, resulting in the formation of an iron-ferrous sulfide core and a silicate mantle. At this point, the internal structure of Ganymede is completely differentiated.

In contrast, the radioactive heat energy produced by Callisto, which is not internally differentiated, can only cause convection in its icy interior, which effectively cools the stars and prevents large-scale ice melting and rapid differentiation of the internal structure, and at most it can only cause partial differentiation of ice and rock. Today, Callisto's cooling process is still very slow. The heat released from the inner core and silicate mantle allows the subterranean ocean on Europa to exist, while the slowly cooling, flowing iron-ferrous sulfide core continues to drive thermal convection within the star and sustain the magnetosphere. It is likely that Calmede has a higher external heat flux than Callisto.

Ganymede's orbit is 1,070,400 kilometers from Jupiter, the third closest of Galileo's moons, and its orbital period is 7 days and 3 hours. Like most known moons of Jupiter, Europa is locked to Jupiter and is always facing Jupiter on the same side, in a state of Laplace resonance between Europa, Europa, and Ganymede.

Its orbital eccentricity is very small, and its orbital inclination is also very small, close to Jupiter's equator, and the eccentricity and inclination of the orbit are also affected by the gravitational perturbations of the Sun and Jupiter as a function of periods over hundreds of years. The variation ranges from 0.0009-0.0022 and 0.05-0.32°, respectively, and the change of the orbit makes the inclination of the axis vary between 0-0.33°.

Calliste maintains an orbital resonance relationship with Europa and Europa: i.e., when Europa is at periaparch and Europa is at the far arch, there is an upward conjunction between Ganymede and Europa, and when Europa is at periaparch, there is also an upward conjunction between it and Europa.

Io, Europa, and Europa's upper alignment positions will move at the same rate, so there is a possibility of a three-star conjunction between the three. This complex orbital resonance is known as Laplace resonance. Today's Laplace resonance does not raise Europa's orbital eccentricity to a higher value.

The eccentricity value of 0.0013 may have been left over from earlier times – an increase in the orbital eccentricity was possible at that time. But Ganymede's orbital eccentricity is still confusing: if it does not increase at this stage, it must indicate that its eccentricity value is gradually depleting due to tidal dissipation within it.

This means that the last loss of eccentricity values occurred hundreds of millions of years ago. Since the eccentricity of Calliste's orbit today is relatively low—0.0015 on average—the tidal heat of Ganymede should be correspondingly weak. However, in the past, Europa may have experienced one or more Laplace-like resonances, resulting in high orbital eccentricities of 0.01-0.02.

This may have caused significant tidal heat effects within Europa, and this multi-stage internal heating ultimately led to the trench topography of the current surface of Europa, and it is not known exactly how the Laplace resonance between Europa, Europa, and Ganymede formed. There are two hypotheses: one is that this state existed at the beginning of the formation of the solar system, and the other is that it developed after the formation of the solar system.

One possible formation process is as follows: first, due to Jupiter's tidal effect, which causes Io's orbit to move outward until a certain point resonates with Europa's orbit by 2:1, and then its orbit continues to move outward, while transferring part of the rotational moment to Europa, which also causes the latter's orbit to move outward, and this process continues until Europa reaches a point where it forms a 2:1 orbital resonance with Europa. In the end, the position movement rate of the two pairs of UPPER junction between the three is consistent to form a Laplace resonance.

The first ones were Pioneer 10 and Pioneer 11, both of which sent back less information about Europa. Later, Voyager 1 and Voyager 2 flew by Ganymede in 1979. They accurately determined its size, and it turned out to be larger than Titan, which was once thought to be larger. In addition, the two ships discovered the trench terrain on Ganymede.

In 1995, Galileo entered orbit around Jupiter. Between 1996 and 2000, it flew by Ganymede six times at close range. These 6 flybys were named G1, G2, G7, G8, G28, G29. In its closest flyby, G2, Galileo was only 264 kilometers above the surface of Ganymede. During the 1996 G1 flyby, it discovered Europa's magnetic field. Later, Callisto's subterranean ocean was discovered and unveiled to the public in 2001.

Galileo sent back a large number of spectral images and discovered several non-icy compounds on the surface of Europa. The probe that went to probe Ganymede up close was New Horizons, which flew past Ganymede on its way to Pluto in 2007 and took a topographic map and composition of Ganymede during its acceleration.

In February 2009, NASA and the European Space Agency confirmed that the program would be implemented in priority over the Titan-Saturn program. The Europa-Jupiter program includes the Jupiter-Europa orbiter hosted by NASA and the Jupiter-Europa orbiter, and possibly the Jupiter Magnetic Field Probe hosted by the Japan Aerospace Exploration Agency.

The canceled orbital exploration program around Europa is the Jupiter Ice Moon Orbiter. It was planned to use a nuclear fission reactor as its power source, which would allow it to carry out detailed surveys of Europa. However, due to budget cuts, the program was scrapped in 2005. There was also a canceled program called "The Magnificent Ganymede."

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On March 12, 2015, the National Aeronautics and Space Administration (NASA) announced that NASA's Hubble Space Telescope recently observed the aurora phenomenon generated by Ganymede's magnetic field and measured the existence of a saltwater ocean with a certain salinity under the ice of Ganymede.

It is estimated that the depth of this subterranean ocean is about 100,000 meters, which is more than 10 times the deepest ocean on Earth. It exists 150 kilometers below the surface of the earth, which is mainly composed of ice.

Jupiter's largest moon---- Europa, is also the only moon it has with a strong magnetic field. Using thousands of images obtained by the Hubble Space Telescope, scientists have found that the very spectacular aurora seen in Jupiter's polar regions was formed under the gravitational influence of Ganymede's magnetosphere.

As Ganymede and the very active Europa orbit Jupiter, they interact with the planet's plasma, producing bright spots in Jupiter's polar regions called "aurora footprints."

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By analyzing images taken by the Hubble Space Telescope, the researchers measured the exact size of Ganymede's footprints, which they believe are too large to be projections of the moon on the planet, and that their diameter closely matches the diameter of Ganymede's protective magnetic field. Scientists also measured the size and shape of Io's Aurora footprints, which are caused by charged particles from active volcanoes on Io

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"Each of these aurora structures tells us an ongoing story – massive energy transfer is taking place on distant Jupiter," T) said.

By analyzing the exact location of these aurora lights, as well as the changes in the shape and brightness of Europa and Europa as they orbit Jupiter, we have created the most detailed simulation to date that mimics the electromagnetic interaction between Jupiter and these moons. Grenton detailed the findings at the European Congress of Planetary Science in Germany,

In addition to combining Ganymede's auroral footprint with its magnetic field, Glendont and his research team accidentally discovered periodic changes in the brightness of the moon's aurora, which occurred at three different moments. The researchers believe that each change reflects an interaction between Jupiter's plasma and Ganymede's magnetic field, but they still don't know what caused the interaction.

"By mapping the surface of Europade, we can more accurately answer scientific questions about the formation and evolution of this truly unique moon," said Wes Patterson of Johns Hopkins University's Applied Physics Laboratory, the research leader. ”

Published by the U.S. Geological Survey, this map technically illustrates the various geological features of Europa's surface and is the first complete map of a cold outer planet moon. Patterson, Collins and colleagues created the map using images captured by NASA Travelers and the Galileo space probe.

Since its discovery in January 1610, it has been the focus of repeated observations. Scientists made their first observations of Europa with the Earth telescope, and then with a close-up probe and a spacecraft orbiting Jupiter. These studies uncovered a complex, icy world.

Its surface is characterized by a striking contrast between the two main terrain types. The two types of terrain are the dark, cold, crater-ridden area and the brighter, younger (but still very old) area, the latter characterized by a large number of trenches and ridges.

At 3,280 miles (5,262 kilometers) in diameter, Ganymede is larger than both the planet Mercury and the dwarf planet Pluto. It is also the only known moon in the solar system that has its own magnetosphere. This map details the geological features of Ganymede's formation and evolution over most of the history of the solar system. These geological features record evidence of the internal evolution of Ganymede, the dynamics of Ganymede and the interaction between other Galilean moons, and the evolution of small objects impacting the surface of Callisto.

The new map is an important tool for researchers to compare the geological features of other icy moons, because any type of feature found on other icy moons is similar to that of a place on Callisto. The surface of Ganymede is more than half of all land area of the Earth.

The satellite provides scientists with a wide variety of observation sites. "Europa exhibits both ancient and recently formed geological features," Collins said.