Jupiter and its Galilean Moons

By: Logan Brink
Astronomy 101
April 19, 2011
The solar system is an intriguing place. There are objects in our solar system that have unfathomable beauty; a few of these beautiful objects being Jupiter and its four Galilean moons. Jupiter is one of the most interesting bodies in the solar system, so it makes sense that its four largest moons are equally fascinating. The Galilean moons are some of the most curious bodies in our solar system. From dead worlds to water worlds to fire worlds, these four moons may hold a lot of answers to some of the mysteries of the solar system. At 5.2 A.Us (Astronomical units – 93,000,000 miles) from the Sun, Jupiter is the fifth planet from the Sun and the first of the gas giants (Bennett et. al A-14-15). Being so far away from the Sun, it takes Jupiter about 11.9 Earth years to orbit the Sun (Kerrod 148). Jupiter is the largest planet in the solar system; in fact, it is more massive than all the other planets combined! At its equator, the gas giant measures 88,850 miles in diameter (Kerrod 148). Jupiter is so huge that it is eleven times bigger across than Earth. Jupiter’s size, however, does not seem to have any effect on how fast the planet spins on its axis. Earth spins on its axis in about 24 hours, one Earth day. Jupiter has eleven times the diameter as Earth and spins on its axis in approximately 9 hours and 55 minutes (Kerrod 148). This means that the massive planet is spinning inordinately fast, this, as one can imagine, causes some pretty intense weather with 300-500 mile per hour winds.
Jupiter’s most impressive feature is probably its massive hurricane type storm people know as the Great Red Spot. The Great Red Spot (GRS) measures 25,000 miles across (Kerrod 148). It is red because of the phosphorous in the atmosphere (Kerrod 148). Astronomers have been observing the Great Red Spot for 300-500 years (Kerrod 148). The GRS lies in the South Equatorial Belt and, for the most part, stays in the same spot (Kerrod 148). Hovering about five miles above the surrounding cloud tops, the GRS is spinning counterclockwise, making its way around the center in approximately six days (Kerrod 148). Because there are no oceans, mountains, or land on Jupiter, there is nothing to slow the raging storm down, and it sustains enough energy to keep going (astronomyonline).
Along with the GRS, Jupiter also has interesting white ovals that surround the Great Red Spot. The white ovals are much less persistent than the GRS and come and go as they please. They only last a couple of months before they are replaced by another white oval (Kerrod 148). According to David Eicher of NASA, a semi-new discovery has been made on Jupiter – the Red Spot Jr. This is the second largest storm on Jupiter and it is as big as about 70% of Earth’s diameter (Eicher para. 5).
Along with the massive storms on Jupiter, the planet holds another beauty, its ring system. Though not nearly as impressive as the rings of Saturn, Jupiter has a very faint, thin ring system surrounding the planet. Jupiter’s rings – like all ring systems – lie almost directly on the planet’s equatorial plane (Bennett et. al 342). The particles that make up the rings around are much less numerous than the particles that make up Saturn’s rings (Bennett et. al 342). The particles are also much less bright and often smaller that Saturn’s ring particles (Bennett et. al 342).
Jupiter’s atmosphere is what makes it truly beautiful to look at. The massive planet’s atmosphere is about 1000 km thick (astronomyonline). Jupiter has three layers of clouds because of the different gases at different altitudes in Jupiter’s atmosphere (Bennett et. al 325). The top layer is composed of ammonia that has condensed into a cloud, the second layer is condensed ammonium hydrosulfide, and the bottom layer is condensed water (Bennett et. al 325). The clouds form the fascinating patterns people love to observe. All over Jupiter, there are regions known as belts and zones. Belts are areas where the gas is falling instead of rising (astronomyonline). They appear darker and have a lower pressure (astronomyonline). Zones are just the opposite of belts. Zones are areas of rising gas and they appear lighter than the belts; they are areas of high pressure (astronomyonline).
The beautiful colors people observe on Jupiter are due to its composition. Jupiter contains compounds such as ammonia (NH3), hydrogen sulfide (H2S), and methane (CH4) (windows2universe). The atmosphere also consists of hydrogen, helium, sulfur, and phosphorous (windows2universe). The gases in Jupiter’s atmosphere are highly flammable here on Earth. Jupiter also has plenty of lightning to ignite the gases. Jupiter, however, does not have oxygen in its atmosphere, and oxygen is required for fire to exist (Bennett et. al 325). Like Earth, Jupiter has a thermosphere, a stratosphere, and a troposphere (Bennett et. al 325). Earth’s basic structure is composed of three layers known as the crust, mantle, and core. Because Jupiter has no solid surface, its layers are not as distinct as Earth’s. Jupiter has a thick “crust” made of liquid hydrogen (astronomyonline). Beneath the “crust”, there is a “mantle” made of liquid metallic hydrogen (astronomyonline). At the center, Jupiter has a small rocky core (astronomyonline).
Jupiter has an extremely strong magnetic field; it is about 20,000 times stronger than Earth’s (Bennett et. al 329). Jupiter’s strong magnetic field gives the gas giant a massive magnetosphere. The magnetosphere surrounding Jupiter is so large that it can deflect solar winds up to 3 million kilometers away from the planet (Bennett et. al 329). If Jupiter’s magnetosphere were visible, it would be as large as the full Moon in the night sky (windows2universe). Not only can the magnetosphere surround all of the moons of Jupiter, the Sun could even fit inside of Jupiter’s magnetosphere (windows2universe). Jupiter’s magnetosphere catches charged particles that create the planets auroras – the same process that causes the auroras on Earth (Bennett et. al 329). Jupiter, unlike Earth, receives its charged particles not from solar winds but from its moon Io (Bennett et. al 329).
Jupiter is also curious due to its large amount of internal heat. Like anything that has internal heat, the heat is gradually lost with time. This is what is happening to Jupiter, but because Jupiter is so massive it loses heat very slowly. Jupiter actually releases about twice as much thermal energy as it receives from the sun (Bennett et. al 324). Radioactive decay is one of the causes of Jupiter’s intense internal heat, but it is not enough for the planet to stay that hot; scientists now believe that because Jupiter is so massive, it is still contracting causing internal heating to continue (Bennett et. al 324).
Many people have called Jupiter a “failed star”. Jupiter does have a similar composition compared to a star, but even though Jupiter is massive, it is not near massive enough to be a star. Jupiter’s gravity is extremely strong, but not strong enough to begin the nuclear fusion needed for all stars (Bennett et. al 319). In order for Jupiter to become a star, it would have to grow 80 times its current size in order to reach the internal temperature and pressure needed to begin the fusion process (Bennett et. al 319).
Not only is Jupiter unique because of its size and composition, it also has more than 60 moons (Kerrod 148). It is believed by astronomers that Jupiter has had several generations of moons in the past. The disc material in the solar system interacted with the growing moons and caused them to begin to spiral in towards Jupiter and eventually be consumed completely (Chown para. 6). Because of all of the rocky material and dust around Jupiter, as soon as one set of moons began to disappear, new moons began to form immediately. The Galilean moons are the last moons standing from the previous five generations of moons that once circled Jupiter.
Jupiter’s four Galilean moons are some of the most interesting bodies in the solar system. They were all discovered by Galileo Galilee in 1610. Each of the four moons has a curious surface and history. Three of the four moons have ice surfaces while the other is covered in active volcanoes. Needless to say, the four Galilean moons of Jupiter have been fascinating scientists and astronomers since their discovery in 1610.
The Galilean moon Callisto is the third largest moon in the solar system (second largest of the Galilean moons) with a diameter of 4,800 kilometers – that is almost as big as Mercury (NASA.gov). Callisto is the outermost of the Galilean satellites at 1,883,800 kilometers away from Jupiter (astronomyonline). With a four billion year old surface, Callisto has the oldest landscape in the entire solar system (NASA.gov). Due to its age and lack of geological activity, Callisto also has the most heavily cratered surface in the whole solar system as well (NASA.gov). Like two of the other moons, Callisto is covered in ice and it also has a dark mineral of what appears to be dirt scattered on the surface – the dark mineral is unknown (astronomyonline). Compared to the other four moons, Callisto may seem kind of dull. It is the darkest of the moons and does not have the same beautiful features as the other moons have (astronomyonline). Even though Callisto is the darkest of the Galilean moons, its reflection is still two times brighter than our Moon (NASA.gov).
For years, scientists and astronomers believed that Callisto was a dead world - meaning that it had no geological activity, no magnetosphere, or no atmosphere (astronomyonline). Surprisingly, Callisto has a thin atmosphere mostly consisting of carbon dioxide (CO2), the carbon dioxide coming from the poles where it is cold enough to have CO2 ice (astronomyonline). Callisto is also the least dense of the four moons with a density of 1.86 grams per cubic centimeter (NASA.gov). Scientists believe that Callisto has a small, non-metallic core – this is why scientists are baffled at the fact that Callisto has a small magnetic field (astronomyonline). Astronomers believe that a subsurface salty ocean may exist beneath the crust of Callisto (Bennett et. al 336). Researchers now believe that the cause of this magnetic field is due to a liquid ocean just beneath Callisto’s crust (astronomyonline).
Not only is Jupiter the largest planet in the solar system, it also has the largest moon in the solar system (NASA.gov). Ganymede is three quarters the size of Mars and larger than Mercury and Pluto (NASA.org). Technically, if Ganymede were to orbit the Sun instead of Jupiter, it would classify as a planet. For many years, Ganymede was thought to be the second largest satellite in the solar system next to Saturn’s moon Titan; however, Ganymede has a diameter of 5,268 kilometers while Titan has a diameter of 5,150 kilometers (windows2universe). Ganymede’s orbit is 1,070,000 km away from Jupiter, just inside of Callisto’s orbit (astronomyonline). Interestingly enough, Ganymede’s atmosphere is composed mostly of oxygen and ozone (astronomyonline). A strong magnetic field has also been discovered, and it is even possible that Ganymede has its own magnetosphere (astronomyonline).
The surface of Ganymede provides an interesting history. There are impact craters, volcanic deposits, and tectonic structures (Lunar and Planetary Institute). Ganymede is made up of three main layers: a layer of icy rock for the crust, a layer of rock beneath the ice, and a core of metallic iron (Fe) at the center (NASA.gov). The metallic core could be an explanation for Ganymede’s magnetic field (astronomyonline). Like Callisto, Ganymede also has the possibility of having a subsurface salty ocean that could also be contributing to Ganymede’s magnetic field. Similar to Earth’s Moon, Ganymede consists of many light and dark areas. The surface of Ganymede is covered in patches of a mysterious dark material along with the icy rock (astronomyonline). Specifically, about 40% of Ganymede’s surface is a heavily cratered, dark landscape that is believed to be the original crust of the moon (NASA.gov). The other 60% of the surface is a grooved landscape with fewer craters and was most likely formed from the release of water from beneath the surface (NASA.gov). Similar to Ganymede and Callisto, Europa is also an icy world. In fact, Europa is a frozen water world. Europa has a diameter of 3,138 kilometers and is the roundest body in the solar system (astronomyonline). Europa’s surface is completely covered in cracks in the ice. These cracks are a result of Jupiter and the other Galilean moons pulling on Europa, a process known as tidal flexing (astronomyonline). It is believed that when the cracks appear, liquid water flows to the surface through the cracks and quickly freezes; this – along with plate tectonics similar to those on Earth – would explain why the surface is so round. On Europa’s surface, there is only a handful of crater impacts that have been discovered. Like Ganymede, Europa also has a warm, metallic core (astronomyonline). A magnetic field on Europa is the result of the metallic core and alleged liquid oceans beneath the ice (astronomyonline).
Scientists and astronomers have a lot of hype about Europa. Life as we know it would not exist without liquid water. Scientists believe that because Europa’s core is warm due to tidal heating from Jupiter, there is a possibility for life on Europa ocean floors (Bennett et. al 711). What is most interesting about life on Europa is that the life would not have to be microscopic (Bennett et. al 711). If there were life on Europa, it could be larger beings swimming through the vast subsurface oceans. Because light does not penetrate the thick ice surface, photosynthesis would not be possible so there would be no plant life, but there could still be life growing on Europa’s ocean floors just as there is life growing on Earth’s ocean floors (Bennett et. al 711).
The last of the Galilean moons, and probably the most intriguing, is Io. With a diameter of 3,630 kilometers, Io is the third largest of the Galilean moons (The Planetary Society). Unlike Jupiter’s other Galilean moons, Io’s orbit is slightly elliptical instead of being almost perfectly circular (Bennett et. al 333). This elliptical orbit is caused by its orbital resonance with Ganymede and Europa (Bennett et. al 333). The three moons share a relationship that is for every one orbit that Ganymede makes, Europa makes two full orbits and Io makes four full orbits (astronomyonline). This means that periodically, the moons align. Because the moons are constantly pulling on each other in the same direction, the orbits become slightly elliptical (Bennett et. al 333).
Because Io has a somewhat elliptical orbit, the moon undergoes a great deal of tidal flexing. Io’s elliptical orbit also causes its orbital speed and its distance from Jupiter to change (Bennett et. al 332). When Io is closer to Jupiter, Io has large tidal bulges and when Io is farther away from Jupiter, the tidal bulges are smaller (Bennett et. al 333). These tidal bulges can be up to 100 meters (NASA.gov). Io is constantly being flexed in different directions. This causes a great deal of friction in Io; it also causes Io to heat to extreme temperatures (NASA.gov).
Looking at Io, one would probably think that it looks very similar to a pizza. There are no impact craters on Io’s surface though (Bennett et. al 331). Io is covered in active volcanoes. It is the most volcanically active body in the entire solar system (NASA.gov). Because of all of the volcanic activity, Io’s surface is fairly new. There are frequent enough eruptions that Io’s surface is constantly changing (Bennett et. al 332). When volcanoes on Io erupt, they release sulfur dioxide gas (SO2) and a little bit of sodium (Bennett et. al 332). A lot of the gas supplies Jupiter’s magnetosphere and the Io torus (Bennett et. al 332). Some of the gas also gives Io a very thin atmosphere, but the majority of the gas condenses and falls back to the surface; the sulfur is what gives Io is distinct red and orange appearance (Bennett et. al 332). Some volcanoes on Io can spew material up to 300 kilometers in the atmosphere when they erupt (NASA.gov). Lava temperatures on Io are also very extreme, some reaching between 1,700-2,000 kelvins or more (NASA.gov).
The Io torus, as described by the Hayden Planetarium, is a “doughnut-shaped ring of gas” that encircles Jupiter (Hayden Planetarium). Because Io has so many active volcanoes that are erupting almost constantly, the gas erupting from the volcanoes is continuously being shot into space. The gas is thereby stripped of its electrons, or ionized, and caught in Jupiter’s magnetic field, fueling the planet’s auroras (Hayden Planetarium). If people could see the torus, it would look like a belt that follows Io’s orbit around Jupiter (Bennett et. al 329).
Astronomers rely heavily on their space crafts to relay information about the planet back to them. There are four categories the space crafts can fall into: a flyby spacecraft, an orbiter spacecraft, a lander or probe spacecraft, or a sample return spacecraft (Bennett et. al 216). A flyby craft does just that; it flies by the planet only one time and records as much data as it can in the time it has near the planet. An orbiter spacecraft does exactly how it sounds as well. The spacecraft orbits the planet under study allowing scientists and astronomers to collect long-term data. Sample return crafts fly to a planet, land on the planet, and return to Earth with samples collected from the planet. Finally, a combination spacecraft is a craft that carries another craft with it. An example of a combination spacecraft would be an orbiter and a probe travelling together. A number of these different types of crafts have been used to study Jupiter and its Galilean moons. Throughout history, there have been at least eight crafts that have provided researchers with information about Jupiter; the first series being Pioneer 10 and Pioneer 11. Pioneer 10 was launched in March of 1972 and was the first craft to ever pass through the asteroid belt (astronomyonline). The Pioneer 10 did a very successful flyby of Jupiter, getting 200,000 kilometers from Jupiter’s cloud tops (The Planetary Society). Astronomers were amazed at the high levels of radiation the craft received when passing by the massive planet (The Planetary Society). On the Pioneer 10 craft, there are simple drawings on the front that show Earth’s location in the galaxy (Bennett et. al 719). Like Pioneer 10, Pioneer 11 also has similar drawings on it (Bennett et. al 719). The Pioneer 11 craft was launched shortly after Pioneer 10 in April of 1973 (NASA.gov). This craft also did a successful flyby of Jupiter, coming within 34,000 kilometers of the planet’s cloud tops (The Planetary Society). Pioneer 11 photographed Jupiter and a few of its moons and it studied Jupiter’s magnetic field and atmosphere (The Planetary Society). These two crafts were the first to ever explore the outer solar system (The Planetary Society).
The next series of crafts is the Voyager series. Voyager 2 was actually launched 16 days before Voyager 1, but Voyager 1 reached Jupiter about four months before Voyager 2 (The Planetary Society). During their flybys, both of the crafts took about 18,000 pictures each (The Planetary Society). Using the crafts, astronomers and scientists discovered three new moons, Jupiter’s rings, and active volcanism on Io (The Planetary Society). The Voyager crafts actually recorded videos of volcanoes on Io erupting (Bennett et. al 331). Both Voyager crafts have a phonograph record which contains images, greetings, and music from Earth in case the crafts eventually encounter another intelligent being (Bennett et. al 719).
Perhaps the most fascinating probe sent to Jupiter was the combination spacecraft named Galileo. It was launched in October of 1989 (astronomyonline). Galileo was the first craft to ever send a probe into Jupiter (The Planetary Society). The probe began its decent on December 7, 1995 and lasted about an hour inside of Jupiter reaching a depth of about 300 kilometers; the probe was not able to sample the interior, however (Bennett et. al 323). The probe relayed information back concerning the temperatures on Jupiter as well as wind speeds and pressure (The Planetary Society). Along with being the first spacecraft to send a probe into a Jovian atmosphere, Galileo was also the first probe to stay in Jupiter’s magnetosphere long enough to study the planet’s magnetic field and determine its global structure (The Planetary Society). A day after the probe was sent into the depths of Jupiter’s atmosphere, the Galileo’s orbiter entered Jupiter’s orbit (The Planetary Society). Galileo studied the rings of Jupiter and found them to be the result of dust from interplanetary meteoroids crashing into the inner moons (The Planetary Society). On September 22, 2003, the Galileo spacecraft was sent into the depths of Jupiter, burning itself up (The Planetary Society).
The next spacecraft to send back information about Jupiter was named Ulysses. Ulysses mission was not to study Jupiter, but to study the Sun. In order to reach the Sun on the proper trajectory, the craft had to first fly out to Jupiter so the Jovian gravity could redirect the craft (The Planetary Society). When it was by Jupiter, however, Ulysses did capture information regarding Jupiter’s magnetic field and radiation (The Planetary Society).
Along with Ulysses, this next craft was also not meant specifically for Jupiter. The Cassini-Huygens craft was on its way to Saturn when it did its Jupiter flyby. This spacecraft uses Jupiter’s gravity to keep its orbit around the Sun (astronomyonline). The Cassini-Huygens spacecraft captured spectacular images of Jupiter and its moons during its flyby in December of 2000 (The Planetary Society).
On its way to Pluto and the Kuiper Belt, the New Horizons spacecraft studied Jupiter for five months before continuing on to the far reaches of the solar system (The Planetary Society). New Horizons needed to use Jupiter’s gravity to slingshot itself on the right path to Pluto (planetary.org). The spacecraft sent back astonishing images of Jupiter and its Galilean moons, primarily Io (Eicher). The images of Io showed its volcanoes in action along with bright, hot lava pooling on the surface (Eicher). The most amazing image was the picture of Io’s volcano called Tvashtar spewing lava 320 kilometers off of Io’s surface (Eicher). Along with capturing fascinating images of Io’s volcanoes, New Horizons was also the first spacecraft to document Jupiter’s Little Red Spot (The Planetary Society).
Preparing for its launch to Jupiter in August of 2011 is the spacecraft called Juno (The Planetary Society). Juno will orbit Jupiter from pole to pole, for the first time (NASA.gov). During its orbit, Juno will study the interior of Jupiter and see if there is a small, rock-ice core (The Planetary Society). It will also study the atmospheric composition along with weather patterns and the magnetic field (The Planetary Society). Prometheus One is another spacecraft destined for Jupiter, launching in 2015 (The Planetary Society). It will orbit Jupiter’s icy moons studying the water that exists on the moons (NASA.gov).
Jupiter and its Galilean moons have dazzled astronomers for hundreds of years. The brilliant colors that make up Jupiter’s atmosphere are absolutely mesmerizing and its moons are full of mysteries. Worlds composed of fire and ice orbiting the largest planet in the solar system catch the eyes of people all over the globe. So curious that space programs have sent numerous space crafts and probes to study the planet. Since their discovery in 1610 by Galileo Galilee, the four moons Callisto, Ganymede, Europa, and Io have fascinated astronomers, scientists, and onlookers. Jupiter and its Galilean moons have answered many questions posed by researchers. But with discovery, there are always new mysteries that arise about the fascinating bodies that lie within the solar system.

Works Cited
Bennett, Jeffery, et al. The Cosmic Perspective. 6th ed. San Francisco: Pearson Addison-Wesley, 2010. Print.

Chown, Marcus. “Jupiter ate its own Moons.” New Scientist 201.2698 (2009): 11. Academic Search Premier. Web. 18 Apr. 2011.

Cohen, Ellen. “The Io Torus of Jupiter.” Hayden Planetarium. American Museum of Natural History, 2011. Web. 20 Apr. 2011. <http://www.haydenplanetarium.org/‌index.php>.

Eicher, David J. “New Horizons Dazzles Scientists.” Astronomy 35.8 (2007): 6. Academic Search Premier. Web. 18 Apr. 2011.

“Ganymede.” Ganymede. Lunar and Planetary Institute, n.d. Web. 18 Apr. 2011. <http://www.lpi.usra.edu/‌resources/‌outerp/‌gany.html>.

“Jupiter - Missions to Jupiter.” AstronomyOnline. N.p., 2011. Web. 19 Apr. 2011. <http://www.astronomyonline.org>.

Kerrod, Robin. The Stargazer’s Guide to the Universe. Hong Kong: Barron’s Educational Series Inc. , 52005. Print.

The Planetary Society. The Planetary Society, Spring 2011. Web. 18 Apr. 2011. <http://planetary.org/‌explore/‌topics/‌jupiter/‌missions.html>.

Russell, Randy. “A Look at Jupiter’s Magnetosphere.” Windows to the Universe. National Earth Science Teachers Association, 3 June 2003. Web. 29 Apr. 2011. <http://www.windows2universe.org>.

“Solar System Exploration.” National Aeronautics and Space Administration. NASA, 4 Mar. 2011. Web. 18 Apr. 2011. <http://solarsystem.nasa.gov/‌planets/‌profile.cfm?Object=Jup_Ganymede>.

“Space Topics: Jupiter.” The Planetary Society. N.p., n.d. Web. 18 Apr. 2011. <http://www.planetary.org/‌explore/‌topics/‌jupiter/‌io.html>.