Which is the most interesting world of all? Well, we don't really know what other worlds will still reveal. If moons like Jupiter's Europa or Saturn's Enceladus are found to having oceans of life supporting water beneath their icy surfaces, or if Mars is found to support life even the smallest of microorganisms then the interest value of these worlds will increase hugely. 

For now though, we must content ourselves with the known, and Mars has plenty of interest even in the absence of life. Mars has a volcano, Olympus Mons, which is 27 kilometres high nearly three times the height of Mt Everest! It also has a giant canyon, the Vallis Marineris, which stretches for over 4,000 kilometres and is up to 7km deep. 

Many other worlds have equally extraordinary features the planet Neptune is known to be buffeted by 1200 mph winds. The moon Io will surely be a geologist's dream if ever we can explore it first hand.

But there are other worlds in the Solar System on which the geology and chemistry and meteorology are all incredibly complex and alien. The first of these is Triton which orbits Neptune. Triton is a bizarre world the coldest in the Solar System at -235°C, and the only moon in the Solar System which orbits in the opposite direction to the planet's rotation. 

The lack of extensive cratering suggests the moon is very active, and the strangest feature of all also suggests this gigantic geysers are believed to be spewing forth liquid nitrogen or methane gas, several kilometres into the atmosphere. 

Venus is an extraordinary planet - it is our nearest planetary neighbour in space, and quite similar in size to our own planet, and for that reason has been described as our sister world. But it is a very ugly sister. Venus has thousands of volcanoes. It is also the hottest planet in the Solar System with a temperature of nearly 500°C at the surface. It's surface pressure is 92 times that of our planet. 

Some of these effects are the consequence of an incredibly thick carbon dioxide atmosphere, creating a greenhouse gas effect. If you stepped out on to the surface of Venus, you would be simultaneously and instantaneously crushed, fried, suffocated and poisoned - that is, if you ever got to the surface past the corrosive clouds of concentrated sulphuric acid which exist in the atmosphere! Venus really is the personification of Hell (but still interesting for all that). 

Saturn's largest moon Titan is remarkable. It is the only moon (as opposed to planets like Venus) with a thick cloudy atmosphere, composed of nitrogen and hydrocarbons such as ethane, methane and hydrogen cyanide. Unfortunately, this bright and orange atmosphere prevented much useful information being discovered about the surface of Titan until 2004 when the Cassini mission released the Huygens probe to pass through the atmosphere to land on the surface. 

Titan has since been revealed to be quite like our planet in appearance, though not of course in terms of its chemistry. Titan has mountains and volcanoes, and it is also the only other body in the Solar System with liquid lakes and rainfall, but there's a difference they are lakes of ethane and the raindrops are probably ethane and methane. All this, and as mentioned elsewhere. Titan is another of those tantalising worlds which may just possibly support some alien form of life. 

The worlds of Triton, Venus and Titan have a complexity of activity which - when better known - will fill several text books. There is one object in the Solar System however which has a more diverse geology on and under its surface than any other, including huge tectonic plates, volcanic activity, and sedimentation. It also has an extremely complex atmosphere with clouds and rain and lightning and with liquid water on its surface. 

But this is just a tiny fraction of what makes this planet interesting. A whole different branch of science - the science of biology - is what makes this the most fascinating of planets. Even as we visit, set foot on, and one day become fully familiar with the exotic features of other worlds, still more could probably be written about this one body and its complex physical, geological and biological systems than all the others put together. It is of course planet EARTH.


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