This in many ways could be the hardest question to judge. Do we mean unique within the Solar System, or unique in the Galaxy? 

In the Solar System there are plenty of planets and moons objects which are unlike any other. There is for example, no planet with the superb ring system that Saturn possesses, though other gas giants are now known to have faint rings, and rings may be quite common elsewhere in the Galaxy. 

Saturn's largest moon Titan has a denser atmosphere than any other known moon in this Solar System, whilst Neptune's largest moon Triton has been discovered to have a seemingly unique geology. It is, of course, unknown whether the conditions which create these moons might also exist in other star systems. Titan and Triton may be unique - they are certainly intriguing, and both will be featured within the next category.

If geology and chemistry are uncertain, appearance is not. The smallest and innermost moon of Uranus is Miranda. Miranda's surface posesses a curious patchwork of light and dark grooves, arranged almost haphazardly; it almost looks as if the moon has been shattered and reconstituted, but the true nature is unknown. 

Iapetus, the third largest moon of Saturn, has an even more unique appearance with very dark and very light surfaces. One hemisphere of the moon is dark reddish, and may be composed of liquid methane which has flowed out from within the moon. There are very few craters here too, which suggests the surface is constantly changing. The light side of Iapetus has hundreds of craters, including one big one. The causes of this strange dichotomous appearance are largely speculative at present. 

Mimas, another Saturnian moon, is really small, only 393km in diameter, and yet it has a uniquely large impact crater on its surface the Herschel Crater is 130km across. Astronomers believe that If the meteor which struck had been even slightly bigger, Mimas would have been totally destroyed. That surely makes Mimas a very rare object, even in the Galaxy as a whole.

Jupiter's moon Io has already been mentioned on this page, in connection with its bizarrely colourful surface a surface pockmarked with hundreds of active volcanoes which constantly smother it with lava flows and yellowish and reddish sulphur compounds. It is the only moon to have erupting volcanoes of this kind, and it is by far the most geologically active body in the Solar System. 

The reason for Io's absolutely unique geology is clear  Jupiter's closest large moon is caught in a gravitationally induced tug-of-war between the great planet and other moons such as Europa and Ganymede Io is continuously being distorted and heated through this process of gravitational friction. It seems unlikely even in other star systems that there can be many moons positioned so strategically to create a world like Io.

But there is one heavenly body which has so many unique features. It may well be the only planet in the Solar System with life on it. It is in my opinion the only planet in the Galaxy with intelligent life on it (that may be the subject of a future page). If so, this fact alone makes it by far the most unique astronomical body we will ever find. This is planet EARTH.


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