Callisto

Image Credit:

NASA/JPL/DLR

The farthest of the Galilean moons, and the second largest of all Jovian moons, Callisto has the smallest density of the four major moons of Jupiter of 1.81 g/cm³. From the density, we know that Callisto is mostly water and ice. It has a radius of 2410 km, giving it a total mass of 0.018 Earth masses. Orbiting at 1.883 million km from Jupiter, it takes 16.7 Earth days or 40 Jupiter days. Like Ganymede, Io, and Europa, it is tidally locked to Jupiter, so it rotates on its axis once ever 40 Jupiter days, as well. It does not share a resonance with Io, Europa, and Ganymede, which, if you recall, is 1:2:4. Callisto's resonance would be 9.4 if you would want to know that.

Since it is the farthest Galilean moon, it does not go as intense tidal friction as do the others. Because of this, it is not as active as the other three, and is not as well differentiated even though it is larger than both Io and Europa. It may have a liquid ocean below the ice crust, but it is less likely to harbor life than Europa.

The Four Galilean Moons of Jupiter. Clockwise from top left:

Io, Europa, Callisto, and Ganymede. Note how Callisto is the only one that has a uniform interior.

Image Credit:

NASA/JPL

 

Since it is not as strongly affected by tidal forces, Callisto's surface is slightly older than the surfaces of Io, Europa, and Ganymede. It is dark, dusty, and pockmarked with multiple craters. The dark and dusty material is from millennia of asteroid and comets impacts, disintegrating and covering the surface.

Callisto, however, is the best bet for human presence in the Jupiter system. Since it is farther from Jupiter than the rest, the magnetic field of Jupiter is less intense at the Callisto than the other Galilean moons. Also, the radioactivity from the moon itself is not as strong since the moon is geologically inactive. The thin atmosphere contains both carbon dioxide and molecular oxygen which can be used by the colonists or scientists on the surface, and with the liquid ocean below the crust, water is in abundance, as well. Though it is farther from Jupiter, it is still well within Jupiter's gravity well, so any asteroids or comets coming nearby will most likely be capture by Jupiter and crash into Jupiter (see Comet Shoemaker-Levy 9).

Callisto, like the other Galilean moons, formed at roughly the same time as Jupiter, but they were far enough away to be prevented from being accreted by the newly formed planet. They, obviously, are still under the gravitational influence of Jupiter itself.

Ganymede

Surface of Ganymede

Image Credit:

USGS Astrogeology Science Center/Wheaton/ASU/NASA/JPL-Caltech

 



Ganymede is the third closest Galilean Moon, and the largest of all Jupiter's moons. In fact, it is actually larger than Mercury and our Moon, and only three quarters the size of Mars. It has a density of only 1.9 g/cm³, about two times the density of water, implying that Ganymede is composed mostly of water and ice, though it does contain some rocky material and a small iron core. The iron core is inferred from Ganymede's size. It is large enough that in the past, radioactivity in the interior made the iron and rock molten, allowing for the heavier iron to sink to the center of the moon. Its ice crust is about 500 km thick, but it does have a 5-km thick liquid water layer about 170 km below the surface. Despite it being larger than Europa, its ice crust is too thick to allow life to exist, so we believe.

 

 Ganymede orbits at approximately 1.07 million km from Jupiter, giving it an orbital period of 7.15 days (twice that of Europa and four times that of Io). Its radius is 41.3% that of Earth, but its mass is only 0.025% of Earth, as Earth is mostly iron, nickel, and rocky material while Ganymede, as mentioned above, is mostly ice and water. Much like the Moon as it orbits the Earth, Ganymede is tidally locked to Jupiter. For that matter, so are Io, Europa, and Callisto. This means that for every orbit the Galilean moons make around Jupiter, each one rotates on its axis once. So much like the same face of the Moon is facing Earth, the same faces of Io, Europa, Ganymede, and Callisto are pointed to Jupiter.

 

Tidally locked moon orbiting a planet

Small line pointed to planet at all times

 

 

Ganymede has a surface that is not uniform. About a third of the surface is old, dark, and heavily cratered. We know that this makes the surface old as the early Solar System was heavily bombarded by asteroids and comets, though there are some impacts today, but not on the scale seen when the Sun was just being born. A way to look at this is to compare the surface of the Earth with that of the Moon. The Earth has a very active surface, from earthquakes, volcanoes, flowing water, and the weather, where the Moon is not active at all. A larger percentage of the surface of the Moon is cratered compared to the Earth. We know that we are not being impacted as much now as the Earth was in the beginning, as we do not see large meteors every day. However, Ganymede also has a large percentage of its surface that is much younger. Much like Io and Europa are affected by tidal forces from Jupiter and the other Galilean moons, Ganymede is as well. The tidal forces can cause fractures in the surface crust, allowing liquid water to come up to the surface and flow. This flowing water creates grooves on the surface, which flow through the craters. If the grooves formed before the craters, then the grooves would be broken, but we see the grooves as continuous.

"Old" surface vs. "New" surface

The left side of the image is the older surface of Ganymede, dark and cratered. The right side is younger, with less craters and grooves running along the surface

Image Credit:

NASA/JPL/DLR

 

Europa

Galileo Images of Europa

Image Credit:

NASA/JPL/DLR

 

Europa is the fourth largest satellite of Jupiter, the smallest of the fours Galilean moons, and the second closest to Jupiter of the Galilean moons. It orbits 671,100 km from Jupiter and takes only 3.55 Earth days to go around Jupiter (twice that of Io). It has a density of 3.013 g/cm³, making it about 3 times that of water. It does have a lower density than Io, which will be explained in the next paragraph. It's radius is about 1561 km, making it smaller than the Moon, which is about 1738 km in radius.

Europa has a surface covered with ice and is believed to have subsurface liquid water under the ice. The water is kept liquid by two processes, the insulation from the ice and from tidal forces heating the interior, much like tidal forces keep the interior of Io molten. If life exists anywhere else in our Solar System, this may be the most likely place to look for life. However, any mission we send to Europa to find life would have to be very careful to be sterilized as any microbes hitching a ride from Earth would contaminate any detection techniques used on Europa to search for life.

Just recently, Europa was found possibly to experience plate tectonics, much like Earth. Slabs of ice may slide under each other, creating "Europa-quakes", much like earthquakes experienced on Earth as continental plate slide under and above each other. More information about plate tectonics can be found on space.com's article about Europan plate tectonics.

Artist's Concept of Plate Tectonics on Europa

Credit: Noah Kroese, I.NK

Close-up view of possible plate spreading on the surface of Europa

Credit: NASA/JPL

 

Io

Mosaic of the Voyager Missions

Image Credit: 

NASA/JPL

 

Io is the third largest Galilean moon and the closest to Jupiter. It has vey little water and ice, making it the dryest body in the Solar System. It is mostly rocky material which is reflected in its density, 3.55 g/cm³, comparable to the density of the Moon. It is has a larger diameter than the Moon, with an average radius of 1816 km (the Moon has a radius of 1737 km). It only takes 1.77 days to orbit Jupiter, at a semi-major axis distance of 421,700 km. Io has a very hot surface even though it is at the same distance from the Sun as Jupiter. Why?

The quick and simple answer is that tidal forces from Jupiter's immense gravity pull and squeeze Io, keeping the interior hot. As Io orbits Jupiter, those differential forces from Jupiter, as well as the other Galilean moons, keep the interior hot and molten. As we know from Earth, molten interiors lead to volcanoes on the surface and Io has plenty of those. At least 150 active volcanoes have been observed on Io, erupting continuously.

Jupiter also has a much stronger magnetic field than Earth, so large, that Io itself orbits within its confines. The magnetic field lines actually capture ionized particles from the solar wind also ionizes atoms in the thin atmosphere of Io. This ionizing radiates the surface and helps keep it hot even at its extreme distance from the Sun.

The combination of the volancism and the magnetic field of Jupiter gives Io one of the youngest surfaces in the Solar System. The surface is estimated to be only a million years old and making it a place that would not be fun to visit. Io does have an atmosphere, making it one of the few satellites in the Solar System to have an atmosphere. The atmosphere is composed of mainly sulfur compounds, so would not be a pleasant place to smell, even if you could stand the heat and didn't have to breathe.

A comparable place to Io, and Io is in fact the inspiration for this place, is Mustafar in the Star Wars universe. At the end of Episode III: Revenge of the Sith, Obi-Wan and Anakin fight on the surface of Mustafar which is covered in lava lakes and has a lot of volcanic activity. Mustafar is compressed and pulled in the same way as Io, but with two gas giant planets in neighboring orbits providing the gravity, rather than a gas giant and fellow satellites.

Eruption on the surface of Io

Image Credit:

NASA/JPL/University of Arizona

Plume on the limb of Io

Image Credit:

NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Galilean Moons

Callisto, Ganymede, Europa, and Io

From Astronomy Online

 

 

The Galilean moons are the four largest moons orbiting Jupiter (though they are not the closest). They were first discovered by Galileo Galilei (and independently, by Simon Marius) in 1609 and 1610 after Galileo turned his telescope towards Jupiter. He noticed these bright dots near Jupiter and as he followed Jupiter in the sky over the course of weeks, he observed those dots staying with Jupiter as it moved in the sky. He correctly concluded that these dots, which he first called stars, were gravitationally bound to Jupiter. He was the first to discover satellites orbiting another body other than the Earth. As we learned in the post about Galileo, there could be objects that did not orbit the Earth. This put another nail into the coffin, successfully debunking the theory that everything orbited the Earth, the Geocentric model of the Solar System.

 

When Galileo discovered these four moons, he wanted to name them for his sponsor, Cosimo de Medici, calling them Cosmian stars, or the Medici family, the Medician stars. He labeled them Jupiter I, Jupiter II, Jupiter III, and Jupiter IV based on increasing distance from Jupiter. This convention held up until the mid 20th century until moons closer to Jupiter were discovered. The four names we use today (Io, Europa, Ganymede, and Callisto), were actually proposed by Johannes Kepler and adopted by Simon Marius who had also observed the moons. Galileo did not like the names and kept his convention, never using the names we use today.

 

All four moons are bigger than any dwarf planet, including Pluto. In fact, Ganymede has a larger diameter, though a smaller mass, than Mercury. The three inner Galilean moons also experience a 4:2:1 resonance. For every four orbits around Jupiter completed by Io, Europa completes two orbits and Ganymede completes one. Callisto has an orbital period of 9.4 times that of Io, so it doesn't really fit the pattern.

 

All the moons are actually visible with amateur telescopes and binoculars, as long as none are obscured by Jupiter's face. The next four posts will go into more detail about these fascinating moons.

The Moons of Jupiter

"Jupiter family" by NASA/JPL - PIA01481This image or video was catalogued by Jet Propulsion Laboratory of the United States National Aeronautics and Space Administration (NASA) under Photo ID: PIA01481. Licensed under Public domain via Wikimedia Commons

 

Jupiter has 67 known natural satellites, with 51 of them having diameters of 20 km or less. Many of these moons are probably captured asteroids that got caught in Jupiter's massive gravitational field. Most of these moons have only been discovered since 1975 with improvements in telescopes and the Pioneer and Voyager missions.

Jupiter's moons are generally divided into groups based on proximity to Jupiter, composition of the moon, and other orbital characteristics.

These groups are:

• The Inner Group: as the name implies, these are the inner most moons of Jupiter. There are four of these moons are they are all less than 200 km in diameter with semi-major axes less than 200,000 km from Jupiter. They have generally low eccentricities and have inclinations close to 0°, i.e. their orbits are almost directly above the equator of Jupiter.
• The Galilean moons: These are probably the most well-known of all of Jupiter's moons. These are the four moons discovered by Galileo in 1609 or 1610 after Galileo pointed his telescope towards Jupiter. He noticed these small objects moving along with Jupiter as Jupiter went around the Sun. These four moons will be discussed in more detail later on.
• Themisto is a single moon that does not belong to any group. It is farther away from Jupiter than the Galilean moons but closer to Jupiter than the next group, the Himalia group. Themisto is located approximately halfway between Callisto and the innermost Himalia moon, Leda. It has a semi-major axis of 7.39 million km, an eccentricity of 0.2006, and an inclination to Jupiter's rotation of 47.48°.
• The Himalia group: This group is named after the largest member, Himalia. There are five known satellites in this group with orbits ranging from 11.15 million km to 11.75 million km. They orbit at an inclination of 26.6° to 28.3° and have eccentricities of 0.11 to 0.25. These moons have compositions similar to C-type asteroids, which lead astronomers to believe that this group is made up of a captured asteroid that was ripped apart by tidal forces from Jupiter's gravity. Any moons that are found in this group will have a name ending with "-a".
• Carpo is another group made up of a single moon. It has an orbit at the inner edge of the next group, the Ananke group, but its orbital parameters are different. It has an inclination of 55°, a semi-major axis of 17.15 million km, and an eccentricity of 0.4316.
• The Ananke group: This group is named after the largest satellite in the group, Ananke. Theses asteroids range in eccentricities from 0.02 to 0.28 (much less than Carpo), semi-major axes from 19.3 million km to 22.7 million km, and inclinations from 145.7° to 154.8°. From the inclination, and if you remember from the post about a day on Venus, the inclinations of these satellites tell you that these satellites orbit retrograde. Looking down on Jupiter from the north, Jupiter rotates counter-clockwise (or for you Europeans, anti-clockwise), but these satellites orbit clockwise around Jupiter. Any satellites found in this group will have a name ending in "-e".
  
• The Carme group: Named after the largest satellite, Carme. They have semi-major axes of 22.9 million km to 24.1 million km and eccentricities between 0.23 to 0.27. The inclinations range from 164.9° to 165.5°, which means these also orbit retrograde around Jupiter. It is believed that these moons formed after a D-type asteroid was captured by Jupiter and broke up. Like the Ananke group, any future Carme group moons will end in an "e". Two exceptions to these moons are Kalyke is much redder than the other asteroids, and Taygete has a much higher eccentricity (e=0.3678).
• The Pasiphae group: These satellites share similar orbital distances but at a slightly lower inclination (144.5° to 158.3°). The largest moon in this group is Pasiphae.

Jupiter

Jupiter: Note the Great Red Spot to the lower right and the shadow of Europa on the lower left

Image is from the Cassini Spacecraft

 

Jupiter is by far the largest planet in the solar system in terms of mass and size. However, it is still less then 99% of the Sun's mass. It has a semi-major axis distance of 5.2 AU which gives it an orbital period of about 11 years. It rotates once on its axis every 10 hours, making it one of the fastest rotating bodies in the Solar System.

Previously, we had talked about Galileo and how he had discovered the first moons around a planet other than our Earth. This discovery helped prove the geocentric theory of the solar system was incorrect (Objects can orbit something other than the Earth). Galileo originally wanted to call them the Medician moons after his sponsors, the Medicis, but it was eventually agreed to name them the Galilean moons and to name them after four of the lovers of Zeus; Io, Europa, Ganymede, and Callisto.

Saturn is no the only planet with rings. In fact, all four Jovian planets have a ring system, though not quite as magnificent as Saturn's rings. Jupiter's rings are not as extensive as the rings around Saturn and were not discovered until 1979 by Voyager 1.

Jupiter is also home to the largest storm in the Solar System. The Great Red Spot is at least 183 years old and may be older. It may have been first observed in 1665, but it isn't clear if it was observed again until 1831. It is between 24,000 to 40,000 km east-west and 12,000 to 14,000 km north-south, making it capable of holding 2 to 3 Earths.

Jupiter also shares its orbit with a two groups of asteroids known as the Greek and Trojan asteroids. These asteroids were discussed previously here. These asteroids, much like the Amor asteroids, will never impact Jupiter. However, Jupiter's gravity is strong enough to capture both asteroids and comets. Many of Jupiter's moons are possibly captured asteroids, and back in 1994, Comet Shoemaker-Levy 9 entered Jupiter's gravitational field, broke up, and collided with Jupiter.

Jupiter has a very thick atmosphere, as the majority of its volume are the many layers of gas. There is a rocky core, approximately Earth-sized, but is only a small fraction of the entire diameter of Jupiter. The combination of its large volume and the majority of the composition of the planet give Jupiter the largest mass of all the planets, but also a low density, just a little bit above that of water.