Another big surprise of the Voyager mission was the discovery that the coldest place in the solar system is also home to a considerable amount of geological activity. The moon appears to be widely populated by a large amount of cryovolcanos which were observed directly by Voyager in 1989. These volcanos can be seen in this image as small black smudges mostly located running across the center of the disk. It also has a tenuous atmosphere almost entirely composed of nitrogen.

One of the most interesting bits of information about Triton to me is that the orbit runs in the opposite direction of Neptune’s rotation. This suggests that at one time, Triton may have been a dwarf planet captured by Neptune’s pull. This would also explain the lack of numerous moons which we see at many of the other gas giant planets. Triton would have swept through the Neptune system and probably either collided with or ejected whatever moons may have existed in the system. Triton also has a very slowly decaying orbit and will likely collide with Neptune or shatter into pieces to form a new ring system in 3.6 billion years.

Image Note: This is the orignal Triton portrait image that was posted to this site. The newer and improved main top image was recently generated by Ted Stryk and posted to unmannedspaceflight.com.

Titan is without a doubt, not only one of the most interesting moons, but it is easily one of the most interesting places in all the solar system. It has been determined that Titan is host to the only other lakes ever to be discovered off Earth! Data also supports the idea that Titan has rain and other kinds of weather as well as cryovolcanism (a type of cold volcano). Even with all this, it is near impossible to eek out an interesting image of the place as its thick atmosphere hides everything taking place on the surface. Additionally, similar to Venus and Uranus, the clouds fail to show much in the way of structure or detail as well.

Titan passed in between Cassini and the sun giving us this opportunity to see the haze of the atmosphere lit from behind.

WALLPAPER NOTE: The original image only revealed half the disc. To complete the full image the top half was taken, flipped and overlapped with another lesser impressive eclipse image to avoid obvious repeat. So, in reality, the lower half of the image is “faked” but it is composed entirely of real references.

During my run through posting images of the major moons of the solar system, I felt compelled to include this tiny 600km moon in the mix as it has suprised most scientists to be quite an active little place. Usually any body as small as Enceladus would fail to have any geological activity, but in 2006 the Cassini spacecraft discovered that there are several active geysers spewing water and ice into the space around it. These geysers have even established a tenuous atmosphere of mostly water vapor at the southern poles where the geysers are located.

WALLPAPER NOTE: I have to be honest… those stripes in the southern region with the geysers… they aren’t really blue. They would be bright gray like all the rest. Its just irresistable to use the blueish tones because they outline where all this geological activity is going on. I usually don’t use false-color.

With 3 other facinating large moons in orbit around Jupiter, asking for yet another in Callisto would have been expecting too much. This body is one of the most heavily cratered bodies we have yet seen. The heavy cratering record tells us that little has changed here on Callisto since early in the solar system’s history. So, not unlike our moon, not very much happens here… ever.

Although there are some theories being floated that Europa may not be alone in its candidacy for an internal ocean of water. Some think that Ganymede and even Callisto may also host such environments but to a lesser degree.

The largest of all the moons in the solar system is actually larger than the planet Mercury. The moon also shows evidence to tectonic plates (the same process that causes earthquakes on Earth) and there may even be some underground ocean as on Europa though it is less likely to be nearly as extensive. Recent observations from the Hubble Space Telescope suggest that there may be a tenuously thin atmosphere of pure oxygen at Ganymede due to the radiating of water ice into oxygen and hydrogen. In this scenario the hydrogen is lost to space while the oxygen (being a heavier element) is retained at the surface.

Ganymede in the best “disk” detail I have ever seen. There are some great close up images of Ganymede, but for some reason most of the images that show the moon from a distance are usually blurry and lack the detail you see here. This composite is by Ted Styrk but the trick is that his original was only in black and white, so I ham-handedly used other images of Ganymede to colorize it. So… the color here is interpretive and not based upon actual data.

Europa has become a focal point of interest in our solar system. Subject to the same tidal forces that tear Io apart, Europa is less severely effected by this and here the stress takes the form of internal heating which keeps the moon from being frozen throughout. It has more-or-less been confirmed that below its icy crust lies an almost global ocean of 100% water. Considering that the center of the moon is quite warm and the icy shell is of course frozen rock-solid, somewhere in between must lie a zone with temperatures similar to that of earth deep within this watery underground ocean. As any marine biologist will tell you, the Earth’s oceans are riddled with life. Even in places of extreme heat and cold, life still somehow has survived the ages and has adapted to such extreme environments. Leading to conjecture that no-matter the obstacles to life ever developing on the surface of such a place… the mere presence of earth-like conditions deep within Europa’s dark underground water ocean may have been just enough for some if not many forms of life to evolve and survive to this day. There are many missions on the drawing board right now to find out more answers including a Europa orbiter, various landers and one truly aggressive mission that would involve melting through the crust and “injecting” a submersible probe into the watery core. For a new and improved portrait image of Europa see here.

Awesome detail of Europa on this half phase image by Ted Styrk. The cracks and ridges which suggest the internal watery ocean are clearly evident and are Europa’s most notable features.

Io is the first large moon of Jupiter and is the most geologically active body in the solar system. Its close proximity to Jupiter and tidal forces from the giant and its 3 other giant moons push and pull the moon apart internally. This causes Io to turn itself inside out and fuels its many active volcanos. Due to this constant volcanic activity the surface of Io is quite young, continually being reshaped with not a crater to be found.

Image Note: The image itself is largely original and complete. I added the “dark side” details and completed disk. The the two plumes showing at the edges of the dark side of the disk are also additions. The plumes are taken from other real images and they do appear in true scale, however some brightening of these plumes were likely applied to these original images to show detail, so I darkend them a bit and reduced the color saturation that usually results from these manipulations.

Galileo, on its way to the Jupiter system, looked back at the moon (Luna) and returned this image which largely shows the side of the moon we never see from Earth. The far side of the moon was first photographed by the Soviets in 1959 with the Zond-3 spacecraft.