By: Heramb Sawant, Tracy Highway, Stephen Patrick, Meghan Bryshun, Curtis Vinish

The discovery and study of Jupiter and its moons, Ganymede, Castillo, Io, and Europa have unraveled the mysteries that surround our universe. The heliocentric model which was one of the most revolutionary discoveries in astronomy was made by Galileo through observation and data collection of Jupiter moons. The moons of Jupiter consist of some of the most volcanic activity in the solar system along with promising potential of life. These discoveries have not only increased our knowledge about the universe but also pushed us to innovate and invent technologies to aid these discoveries. This website will explore Jupiter and the wonders of its moons along with the tools that were used to aid these discoveries. It will also contain some of the most up to date discoveries about Jupiter and its moons.


Jupiter's Moons

The following section will outline each of Jupiter's largest moons and some interesting facts about them. The section following will go into more detail about how these discoveries were made and the tools used to aid them.

Ganymede (The Alpha Moon)

With a diameter of 5,286 km and a mass of 1.4819×1023kg Ganymede is not only the largest of Jupiter's moons but also the largest satellite in our Solar System. Ganymede is larger than the planet Mercury and is ⅔ the size of Mars.In 1996, the Hubble Space Telescope discovered oxygen in Ganymede's atmosphere which brought a lot of questions with it. The mission led to the inquiry of potential salty liquid oceans and possible life. Since then there have been a countless number of discoveries made about this moon, a significant one being Ganymede’s composition. This large moon is made up of three main layers: a metallic Iron Core, a rocky mantle, and a combination of rock and ice that makes up the crust. The surface of the moon also contains a subsurface ocean which suggests that the moon had salty oceans beneath its layer of ice.


Figure 1: Courtesy of NASA/JPL-Caltech.

Europa is slightly smaller than Ganymede and Earth's moon1 with a diameter of 3100km. Based on the lack of craters, Europa's surface may not be as old as the other moons of Jupiter.1 The Hubble telescope was able to collect an abundance of information about this moon,1 including the detection of an atomic oxygen emission. This could possibly be due to a very thin atmosphere (roughly 10^-11 the pressure of earth at sea level).2

Europa was known to be covered by an icy crust more than 150 km deep and a rocky interior.1 NASA’s Galileo Mission explored the Jupiter system from 1995 to 2003. This mission shed light on its icy crust and what's beneath it. It discovered an entire ocean under Europa's thick crust which sparked questions about life on Europa. This water may have retained its liquid form due to tidal heating from the core. More recent studies have even suggested that the lines visible on Europa's surface may be a result of salt rising from the ocean. This existence of this ocean suggests that there may be life on Europa, since we know the characteristics of an ocean can provide a habitable environment. One difficult question to answer is the potential energy source for microbial life on Europa.1 The ice is too thick to permit photosynthesis to occur underneath, therefore other pathways have been suggested.1 One alternative theory suggests that a radiation-driven ecosystem could exist on Europa.1 Many missions have been planned to further investigate potential life on Europa.


Unlike the relatively smooth surface of Europa, Callisto is the most heavily cratered object in our Solar System. Due to this cratering, Callisto is estimated to be the oldest landscape in our Solar System. With a diameter of 4800 km, Callisto is the third largest moon in our Solar System and almost the size of mercury. Callisto is the outermost Galilean satellite and has minimal geological activity.3

The interior of Callisto has similarities to Ganymede, but with a smaller rock core covered in a 200km thick ice mantle.4 Historically significant is the fact that Callisto was discovered January 7th, 1610 by Galileo Galilei, which was the first time a moon was discovered orbiting a planet other than earth.3 As discovered by the Galileo spacecraft, Callisto is likely to contain a salty subsurface ocean.4 Unfortunately, evidence collected also suggests this subsurface ocean is an inhospitable environment.5




Io, Jupiter’s third largest moon, displays multiple similarities to Earth’s moon. In addition to its radius being only slightly larger at 1131.7 miles, its topography and density (3529 kg m-3) also suggest a similar interior composed of silicates, although Io’s interior likely also contains heavier metals.7 Io has a slight elliptical shape, and is covered with sulfur, sulfur compounds, and silicates.

Io’s orbital period is shortening, which is causing the moon to also move out of resonance. With this movement, some predict that it will lose eccentricity altogether which will result in Io losing all its volcanic activity as a result. It should be noted that this discovery of volcanic activity in the Solar System was made by Voyager 1 and 2 when they captured an actual eruption.8

Space Missions

Many of the observations researchers have made about Jupiter’s moons have come from the spacecraft missions designed by NASA to specifically explore the planet, or gather information on a fly-by. These observations include Doppler measurements, photography, and even launching probes into Jupiter's extreme atmosphere to take readings.8 Overall these spacecrafts are responsible for gathering information that would otherwise be inaccessible by other means, and for giving researchers the data that allow conclusions to be made about its satellites.

The many missions designed to explore Jupiter and its four main satellites began in March 1972, when Pioneer 10 was launched from earth. The spacecraft’s mission was to make observations on Jupiter, which included exploring its physical characteristics, and its satellites, specifically Io.8 Some key work done by Pioneer 10 was an S-band (a portion in the microwave band of the Electromagnetic spectrum) radiation occultation measurement. The important finding that resulted from this measurement was that Io contains an ionosphere, and therefore a neutral atmosphere. Up to this point evidence had been inconclusive as to whether or not Io had an atmosphere. Pioneer 10 gave a value of the pressure at Io’s surface to be around 10-8 to 10-10 bar.9 In comparison, the pressure at the surface of the earth is roughly 1 bar. The discovery of an atmosphere on Io is significant because it is one measure scientists use to determine the present or past composition of a satellite, specifically whether or not it contains a suitable environment for water.

Pioneer 11 was launched a year after Pioneer 10. During its mission, Pioneer 11 was able to take photographs of Jupiter as well as further examine some of its atmospheric properties.8 In addition to Doppler tracking data collected from Pioneer 10, Pioneer 11 collected more data that lead to a better understanding of Jupiter’s gravity field, and more accurate values for the masses of its moons.10

After the Pioneer missions, data was beginning to be collected about Jupiter’s satellites, but many questions remained. Apart from the masses of the satellites, and some atmospheric data of Io, the composition of Jupiter’s moons were still a definite mystery. Voyager arrived at the planet Jupiter in 1979, and something notable about this mission was its vast improvement in telescopic resolution in comparison to the Pioneer missions.11 The apparent improvement in resolution was comparable to the improvement in resolution from naked-eye observations of Earth’s moon to resolution with the best ground-based telescopes.11 This major improvement in technology allowed Voyager 1 to make much more detailed observations of the Jupiter system. The first photos of the Jupiter's satellites were taken by Voyager 1 in 1979.12 Voyager photographed all four major Jovian moons and the results were indeed quite novel. Initial observations included “bizarre color patterns on Io, global-scale linear patterns that traverse Europa, complex interwoven and twisted albedo patterns that lace Ganymede, and enormous concentric systems of multiple bright rings on the crater-riddled surface of Callisto”.11

One of the most interesting findings of the mission was the high amount of volcanic activity on Io.12 This was the first time active volcanoes were observed that were not located on earth.12 This volcanic phenomenon was later explained by the fact that a combination of Jupiter’s gravitational force and gravitational interaction with other satellites causes extreme friction in the interior of Io.13 The extreme friction leads to high heat, and ultimately volcanic activity.13

Voyager was launched sixteen days before Voyager 1, but embarked on a trajectory that took it longer to reach Jupiter.14 The images captured by Voyager 2 were used to both confirm and support the data collected by Voyager 1.14 On Io, changes were observed in regards to volcanic activity, eruptive clouds, and plume structure. The lack of evidence of meteorite impact was used to identify Europa. It also contained a series of bright and dark linear features that were not understood at the time.14 Ganymede was observed to have at least one portion of heavy craters, while Callisto seemed to be entirely covered in craters.14 This data is important because one method that is used to determine age of the satellites is by comparing the amount of craters. Typically the more cratered a planet or satellite is, the greater its age.

Galileo began its mission after launching in 1989 by first heading to Venus, then returning to earth to slingshot towards Jupiter. This technique is called gravity assist, and is used to make the spacecraft’s journey quicker and easier.15 Again what was seen with the Galileo mission was an increase in resolution. This allowed for novel observations, and better clarification on existing data.

Examining the surface of Europa, the Galileo spacecraft could better resolve the crater morphology observed by the Voyager mission.16 Multiple types of craters such as the “classic” type seen on earth’s moon, and other non-rimmed types were observed.16 Based on these images and crater morphologies, scientists began to develop hypotheses about the composition of Europa. Researchers believed the craters could be explained if the surface of Europa was either solid ice, or a thin layer of ice overlying a deep ocean.16

The Galileo mission also provided evidence that Callisto has a subsurface ocean. The evidence provided for a subsurface ocean is that Callisto’s magnetic field fluctuates in response to Jupiter’s magnetic field.17 An underground ocean of salt water seemed the most plausible explanation, as salt water conducts electrical current, which could interact with Jupiter's magnetic field to produce the fluctuations.18 Although the presence of water makes it possible for there to be forms of life on Callisto, it’s likely that the ocean's depths are packed with dense balls of ice and rock, which would block the circulation of heat and make it an inhospitable environment.18

One study used the information gathered by the Galileo probe to compare the differences in landform degradation between Ganymede, Europa, and Callisto.19 According to the data, Callisto displays the most degraded surface with large mass movements. In comparison, Ganymede shows signs of consistent erosion, and Europa shows very small land mass movements.19 Movement patterns can provide important information about the surface of a satellites, such as sediment particle size.19

Perhaps most interestingly, Galileo gathered more information regarding the volcanic activity on Io. Explosive volcanic activity was discovered at four previously unrecognized locations.20 Also, the 100 kilometer plumes produced by the volcanic activity were better characterized based on data from a five year observation period.20 Although there is consistent volcanic activity on Io, this activity appears to be localized. Therefore at least 83% of Io’s surface was unchanged throughout the course of the Galileo observation.

After the Galileo mission, the Ulysses required Jupiter for a gravity assist in 1992. In the time of the fly-by further observations of Jupiter’s magnetic field were conducted for comparison to the data from the previous four missions.21 Essentially, they discovered that the magnetosphere was much larger than initially detected with Pioneer 10.21 Therefore, they concluded that Jupiter's magnetosphere is able to change size under different conditions.21

For the first time since the Galileo mission Io was again observed, this time by the New Horizons spacecraft in 2007.22 Again, the main focus was Io’s tidal driven volcanic activity, and examining the huge plumes that speckle the surface. Surprisingly Io’s surface only changed in 19 locations since the last observation by Galileo in 2001, this is much lower (25%) than the amount of changes observed by the Galileo spacecraft in a five year period.23

The New Horizons spacecraft also made observations of Europa and Ganymede at visible and infrared wavelengths.24 Using new technology known as Linear Etalon Imaging Spectral Array (LEISA), New Horizons was able to map bands of H20 emission, but was not able to determine what the non-ice material is that distorts the imaging bands.24

Despite the evidence that Callisto, Ganymede, and Europa may contain subsurface oceans of some kind, many questions remained. If there were subsurface oceans, what was the non-water component being observed? There was some speculation that Europa’s ocean may contain hydrated magnesium and/or sodium sulfate salts, which may constrain the chemistry of the potential ocean and therefore make it inhabitable.24 Other considerations were whether a significant energy source could be present in these oceans if they are covered by thick layers of ice. Since the ice is so thick, it would be nearly impossible for photosynthesis to take place. An alternative pathway was proposed that radiation could provide an energy source for microbial life in a subsurface ocean.25

Overall, the space missions delving into the Jupiter system have provided us with a lot of raw data about Jupiter's moons. All of this data has allowed scientists to make conclusions about the physical nature of each moon. Observations of Io provided evidence of the first volcanic activity in the universe, beyond earth. Through surface observation and scanning technologies, it was determined likely that Callisto, Ganymede, and especially Europa contain subsurface oceans of some kind. Advancement of technology is the common theme throughout the Jupiter space exploration missions. As technological advances are made, more accurate and diverse data can be collected.

Future Research & The Possibility of Life

The search for life beyond Earth is ongoing, and through all the years of research we have yet to discover a source that hosts life as we know it in present day. We know that for life to exist it requires three components: liquid water, an energy source (typically a star, orbiting within the habitable zone), and organic material. Jupiter is approximately 779 million kilometers from the Sun, well outside of what we call the habitable zone, so why are we investing so much time and effort into examining its’ moons and their possibility for even microbial life sources if there is no energy source? The discovery of their composition and the existence of subsurface oceans tells us that their cores exude enough heat to preserve the water in its liquid form. Although we have thus far been unable to assess the water sources we know exist on Ganymede, Callisto, and Europa, scientists have ruled out the possibility of Callisto being a hospitable environment as a result of its ocean depths being packed with ice and rock, blocking heat circulation.

An exciting observation occurring multiple times throughout 2014, was the expelling of water vapour plumes from Europa’s southern hemisphere.28 While not proven, the evidence that Europa is spraying ocean water into space makes the possibility of studying its prospect for life much simpler. Instruments may fly through the plumes and assess trace chemicals and particles that it finds within the water. Previous space missions and research on Europa has already led us to believe that it holds the strongest possibility for life, and this new discovery may reveal whether there is metabolic activity in its subsurface ocean. Even more exciting is the launch of the Europa Clipper, due in 2022, which may mean we are only years away from discovering life within our Solar System.28

In addition to the launch of the Clipper, the JUICE (Jupiter’s Icy Moons Exploration) mission is set to launch toward Jupiter in 2022 and intends to look for ocean layers or water reservoirs, look at the atmosphere, figure out the interior, and map the surface.3 More information can be found in the following video

While scientists dedicate their research careers to finding life within our Solar System, they are also expanding their search to stars within the Milky Way Galaxy to discover other Solar Systems. Just recently in February of 2017, TRAPPIST-1 was found to host seven Earth-sized planets orbiting it.29 All seven planets have an orbital period shorter than two weeks, and while this may indicate that these planets are much too close to the energy source in order to be habitable, this dwarf star does not generate anywhere near the energy of our star, and therefore indicates the potential for life. With TRAPPIST-1 having a mass that is 8% of the Sun’s, and a roughly similar size to Jupiter, every planet is within the habitable zone and suggests the possibility of surface liquid.29 With TRAPPIST-1 only 39 light years distance from Earth, and the launch of the Hubble’s scientific successor, the James Webb Space Telescope, which is set to launch in late 2018, we soon may know much more about this Solar System.

The many scientific discoveries over the last 150 years tell us there is the possibility for life in our Solar System and our galaxy, but the Milky Way is just one of 100 billion galaxies. Typically, this would lead one to believe that the chances of life beyond Earth is exponentially large. However, this may not be the case as research indicates that only one in ten galaxies can support life, a result of something called gamma ray bursts.30 These stellar explosions would regularly wipe out all possibilities of life. Although there is a chance that microbial life may survive a gamma ray burst, more complex life forms would not. Additionally, in spiral galaxies such as our own, supernova explosions that occur in the galactic centers create a very dangerous, inhospitable environment, whereas the more diffuse spiral arms of the galaxy are more likely to host life.31 Earth itself is 27 000 light years from our galactic center, on the edge of what would be considered a hospitable environment. So despite the universe being unimaginably large, the existence of life such as our own may be much more limited than previously expected. However, an idea that is currently being explored is that life may form out of components beyond the three that we are know of. For example, rather than liquid water as is found on Earth, the liquid methane-ethane lakes of Titan, Saturn’s moon, could possibly host life. As technology continuously advances we are consistently becoming closer to the discovery of life beyond Earth, one of the most exciting prospects of astronomical research.


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