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Triton is the biggest moon of the Planet Neptune. It is the seventh biggest moon in the Solar System.  Seven reasonably big moons have been discovered in our Solar System.  After that there is a big gap in sizes, and Triton is more massive than all the moons smaller than Triton put together.

Triton is 2700 kilometres in diameter and its mass is about 21,400,000,000,000,000,000,000 Kilograms.  If you prefer, this is 21,400 quadrillion tonnes.


There are probably colder places in the Solar System, but the coldest ever recorded was on the surface of Triton.  This was minus 235 degrees C (391degrees F) so it is reasonably cool. However there are definite indications that the inside of Triton could be warmer.  One of these indications is geysers, apparently of Nitrogen gas and dust.


Triton has a very thin atmosphere probably containing Nitrogen and Methane.  Clouds, possibly of frozen Nitrogen, form in the atmosphere.


The surface of Triton is mainly rocky, but there is evidence of areas of ice, so, although it does not have as much water as some of the moons of Jupiter, it appears to have some.


To consider life of such a cold place may sound strange, but remember that we really have little idea of life forms outside our own planet, so although I would say that any life on the surface of Triton is extremely unlikely, the idea of life in liquid water, or of water and Ammonia, in the interior of Triton is something that has been seriously considered. 

Picture of Triton made by Voyager 2 in 1989.
Picture from NASA
Cold Volcano on the surface of Triton
Picture taken by Voyager 2 on August the 25th 1989 from a distance of 190,000 kilometres.
Photo from NASA