The Moons of Jupiter and Saturn

The above image of the Galilean Satellites (Jupiter's moons) illustrates the diversity and complexity found in just a few of the bodies in our solar system. Given the size and composition of some these moons, we might even call them worlds. These particular moons (Io, Europa, Ganymede, and Callisto, from left to right) are all approximately the same size as our moon, and much more interesting. While the Earth is alive and blue, and the is moon dead and grey, these Galilean Satellites are seemingly somewhere in between. With such an abundance of worlds in the solar system, it is no wonder that there is still plenty to be discovered elsewhere; some of the most promising moons are described in this section.


Titan, Saturn's largest moon, is perhaps visually the most Earth-like body in our solar system. Unlike Mars, Titan holds a dense atmosphere, has an organic-rich chemistry, and even has mountains, rivers, rain, and large surface lakes. The difference between Titan and Earth is that Titan's rivers, lakes, rain, and atmosphere are all based on extremely cold methane. This raises an interesting question of whether methane, or any other liquid, is capable of substituting for water as a substrate for life. In the future, the Cassini mission will search for seasonal changes in Titan's atmosphere, and could one day confirm the presence of an underground liquid ocean.


Galileo Galilei first discovered Europa and the other prominent Jovian moons in 1610; it was the first time a moon was found orbiting a planet other than our own. This discovery was an important factor in revolutionizing our understanding of the Solar System's dynamics, and discovering that our Solar System revolves around the Sun rather than the Earth. Perhaps one day these moons will be a part of a new revolution, where not only are there other planetary moons, but also other worlds that harbour life.

Like Enceladus (discussed below), Europa is covered by a global ice sheet, and it is thought that underneath this sheet lies a deep ocean of liquid salt water. Europa is too far from the sun to form liquid water from solar energy alone; however, it's eccentric (or stretched) orbit means that it is closer to Jupiter at certain times and farther at others. This change in the gravity experienced by Europa causes it to stretch due to gravitational tidal forces, which in turn warms the subsurface ocean and likely causes many of the cracks that are seen on the icy surface.

The warming is also the source of speculation that the ocean may be habitable for simple microbial life. However, many scientists believe that although the ocean would have enough chemical reductants, it would not have enough oxidants for complex life to emerge (life is presumed to require a supply of both to survive). Earth’s ocean gets its oxidants mainly from interactions with the atmosphere. This process is simply not possible on Europa given the thick ice sheltering the ocean from above. However, models have been proposed where charged particles from the sun create oxidants at the ice surface which are able to migrate to the subsurface water below. It seems very possible that the right ingredients for life may exist on Europa, but perhaps not enough for complex life to form.


Enceladus is another interesting example among Saturn's moons. This moon, given it's age, reflectance, and distance from the sun, should have frozen long ago. However, there are surface features on Enceladus such as fissures, plains, regions with craters, and smooth craterless regions which all suggest major resurfacing events in its recent history. Even though Enceladus should be completely frozen, it is believed that gravitational tidal forces may be heating the interior of the moon to temperatures that are warm enough to produce subsurface water.

The Cassini spacecraft began to study Enceladus with a first pass in 2005 and then, due to the amount of interest that the data generated, returned several times later. Some discovery highlights include:

  • The discovery of an atmosphere, possibly arising due to outgassing from the moon.
  • Large, dark cracks, called "tiger stripes". These cracks have been determined to be relatively young, and are likely to be continually supplied with fresh ice from below the surface.
  • A warmer than expected south pole, suggesting an internal heat source.
  • Material ejecting from fissures on the moon. The material is identified as being mostly ice, which raises the possibility that these plumes are fed by underground liquid water, similar to geysers found here on Earth (see above image of the plumes taken from Cassini in 2009).

The Cassini mission will continue to study Enceladus and will hopefully one day answer the question of whether liquid water does indeed exist below the surface.

NASA pages for the moons discussed here: Titan, Europa, Enceladus