Triton is most like Jupiter's moon Europa. Like Europa, the interior of Triton may have been warm enough at one time to contain a liquid layer just under the surface. On Earth, we know that there are some creatures which can survive in an environment of very cold water, such as under the ice of the north pole.
It is very cold on Triton, about -300 degrees. There is almost no atmosphere, but what there is is like Saturn's moon Titan because there is Nitrogen. Nitrogen is one of the waste products left behind by lifeforms.
Unfortunately Triton is inside the magnetosphere of Neptune, which is very harmful to life.
Overall, the environment sounds unfriendly to life as we know it on earth, because of the magnetosphere and the cold. However, if the interior is warm enough, and if the conditions are just right, there may be living creatures on Triton under the icy surface.
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