New images from the New Horizons probe suggest that Charon once had a sub-surface ocean of liquid water.
The smaller moons of Pluto are now believed to wobble about unpredictably as they orbit the dwarf planet.
The New Horizons spacecraft has managed to capture the orbit of Pluto's largest moon like never before.
New Horizons will be looking for signs of a subterranean sea when it reaches Pluto and Charon next year.
NASA's New Horizons spacecraft has celebrated another milestone - its first photograph of Charon.