The Moons of the Solar System

More Questions Than Answers

Europa Moon of Jupiter
Europa – Moon of Jupiter. Credit NASA

Think of our solar system, and for most of us the first thing to come to mind would be the eight planets orbiting our Sun. But perhaps even more interesting are the moons which orbit the planets. Our Moon seems to be a lifeless body, with hardly any atmosphere and no dynamic activity. But the more data we collect, and the more we learn about its formation, the more fascinating it becomes. The same is true of the moons of the outer gas giants, some of which are comparable in size to the Moon. Little was known about these bodies until we began to send spacecraft armed with a host of sensors in order to analyse them in much greater detail. Each has its own uniquely distinctive characteristics in terms of overall composition, and their widely diverse range of surface features give tantalizing clues to what is going on beneath. The image below shows the size of the Earth’s Moon in relation to other moons and planets in the Solar System.

Moons of the Solar System
Moons of the Solar System / Credit NASA

Galileo and the Beginning of Modern Astronomy

In 1610 Italian astronomer Galileo Galilei discovered the four largest moons of Jupiter; Io, Europa, Ganymede and Callisto. He used a homemade telescope to observe the motion of these bodies, which he first took to be stars. But after a few weeks he realized that they never left the vicinity of Jupiter, and they changed position in relation to each other and the planet. Galileo therefore concluded that they must be planetary bodies in orbit around Jupiter. This discovery, along with his measurements of the phases of Venus, proved that not everything in the Universe revolves around the Earth, and led to conflict with the Catholic Church by refuting the geocentric view of the Solar System. But importantly for science, his work also marked the beginning of modern astronomy.

All that’s required to see the Galilean moons of Jupiter is either a good pair of binoculars or a small telescope. Credit: Jan Sandberg,

The Inner Solar System

Phobos (left) and Deimos (right), Moons of Mars. Credit NASA

Both Mercury and Venus have no moons at all. One theory for this is that if they ever possessed a moon in the past, it would have eventually been stolen by either the gravitational pull of the Sun or the gravitational pull of the host planet. Then there’s Mars which has two moons, Phobos and Deimos. These are very small bodies, with Phobos having a diameter of 22.2 km and Deimos 12.4 km, so they haven’t got enough mass to enable them to form into the roughly spherical shapes that larger bodies are able to do. The orbital radius of Phobos is just 9,377 km, with an orbital period of 0.32 days. It is being pulled 1.8 m closer to Mars every century, and so will eventually either crash into the planet or break up and form a ring of material around it. Deimos, on the other hand, has an orbital radius of 23,460 km, and is gradually moving in the opposite direction, away from Mars, just like our own Moon is moving away from the Earth. So one day both Deimos and the Moon will cease to be influenced by the pull of their host planets and be set free into space.

The Outer Solar System

Io, moon of Jupiter shown with plume. Credit: NASAIt seems reasonable to assume that the further from the Sun the colder the temperature of the bodies that exist there. At Jupiter, which lies five times further from the Sun than the Earth, the average surface temperature of its moons is -170°C. At Saturn the average surface temperature is -200°C, at Uranus -210°C, and at Neptune -235°C. Before it was possible to closely inspect the moons of the outer planets in any great detail, scientists assumed that, due to these low temperatures, they would be cold, lifeless, inactive bodies. But since Voyager 1, launched in 1977, the Galileo mission, launched in 1989, and Cassini-Huygens in 1997, it has become clear that their theories were very far from the truth. Io for example, the innermost of the four Galilean moons of Jupiter, is the most volcanically active body in the solar system. Whilst having an average surface temperature of -130°C, its volcanoes can reach 1,650°C. It is covered in sulfur due to the hundreds of active volcanoes on its surface, and has lava lakes, floodplains of liquid rock and plumes of sulfur reaching as high as 300km. The above image (credit: NASA) of Io showing an active plume from a volcano. Io’s colorful appearance is due to various materials produced by its volcanism, including silicates, sulfur and sulfur dioxide.


The reason for Io’s volcanic activity is due to the orbital resonance of three of the four Galilean moons, meaning that their orbital periods are multiples of each other. Io revolves around Jupiter four times in the same period it takes Europa to revolve twice and Ganymede to revolve once. This regular alignment results in a gravitational pull which has caused their orbital paths around Jupiter to become elliptical. This in turn creates immense tidal forces, causing the physical rock on Io’s surface to rise up and down a hundred meters during the course of each Io day, or about every 42 hours. A huge amount of heat is therefore generated, which is enough to melt a large proportion of Io’s interior and bring about the conditions we have observed on its surface from pictures relayed by Voyager 1 in the late seventies and the Galileo mission in the late 1990’s and early 2000’s. Resonance is common in the solar system, and accounts for the geysers and the jets on Enceladus for example, and the liquid water ocean beneath the surface of Europa.


Image taken by NASA’s Cassini probe of jets of water ice being emitted from the surface of Enceladus. Credit: NASA/JPL/SSI
Image taken by NASA’s Cassini probe of jets of water ice being emitted from the surface of Enceladus. Credit: NASA/JPL/SSI

Volcanism does not only occur on rocky bodies like Io. In March 2006 the Cassini probe observed icy jets being emitted from the south pole of Enceladus, a moon of Saturn. The volcanoes erupting these jets were however not spewing out molten rock. When Cassini flew through the plume of one of these emissions, it detected predominantly salty water-ice, with small amounts of carbon dioxide, ammonia, methane and other hydrocarbons. The contaminants lower the melting temperature of the ice on the crust of Enceladus, allowing the generation of cryomagma, which can be erupted in plumes reaching hundreds of kilometers above its surface. Other icy moons exhibiting cryovolcanism include Ariel and Miranda orbiting Uranus, and Triton, the largest moon of Neptune.

Is There Life in the Solar System?

Mars has traditionally been the place to look for alien life in the solar system, but the icy moons of the outer planets are exciting for scientists to study because their surfaces are comprised of large amounts of water ice. Although solid at the surface, it has been proven that vast amounts of liquid water can exist underneath. The Galileo mission to Jupiter gave evidence that Europa has an ocean of water beneath its icy surface totaling more than all the oceans, rivers and lakes existing on the Earth. And where there’s water there’s the potential for life, at least life as we know it. Tidal distortion has created cracks on the surface of Europa, enabling liquid water to escape to the surface. These could be the places where life is most likely to occur, as sunlight could create the conditions for photosynthesis to take place. Other places with the potential for life are Ganymede, Enceladus and Titan, Saturn’s biggest moon.

Plate tectonics on Europa. Image Credit: NASA
Plate tectonics on Europa. Image Credit: NASA

But life can also be created without the need for sunlight. At the bottom of our oceans on Earth, hydro-thermal vents exist that harbor microbes which, in a process called chemosynthesis, convert chemicals from the vent into usable energy. So why can’t this same process take place elsewhere in the solar system, or elsewhere in the Universe for that matter?

Future Exploration

NASA’s New Horizons spacecraft is one of the latest ongoing missions to explore the solar system. It is on its way to a rendezvous with the newly termed dwarf planet Pluto and its moon Charon, both members of the vast region beyond Neptune known as the Kuiper Belt. Launched in 2006, and moving at almost one million miles per day, it will reach Pluto in summer 2015, armed with much more state of the art sensors than earlier missions. For instance, it will include LORRI, one of the highest resolution telescopes ever sent into space. Scientists believe that the Kuiper Belt contains a totally different class of world than the rest of the Solar System, so to better understand it, we need to understand worlds like Pluto and Charon.
In 2022 the European Space Agency is planning to launch the Jupiter Icy Moons Explorer (JUICE), which will study Ganymede, Europa and Callisto, and their potential for containing life. NASA too is hoping to launch its own mission to Europa in 2022, called the Europa Clipper. It will fly down to within 25km of its surface, and may even include a lander. So the future of space exploration is alive and well, with plenty of exciting encounters to look forward to.

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