Charon: The last moon
Charon is such a large moon in relation to its dwarf planet that with Pluto they are considered to be a dual planetary system. When NASA's New Horizons probe performed its historic flight in the Pluto system in 2015, it accumulated a wealth of data about this amazing moon.
By NASA link [Public domain]
Pluto was discovered in 1930, its largest moon, Charon, wasn't noticed until 1978. He was discovered by the astronomer James Christy at the Navy Observatory in Washington. The moon circles Pluto every 6.4 Earth days, it takes the same rotation of Pluto the same time, so they are all one side to each other. The Pluto System - Charon is considered to be a dwarf double planet, the only one in the Solar System. The satellite is 1200 km in diameter and is therefore half the size of Pluto. The center of mass of two bodies is outside the surface of Pluto. The pair probably formed at the same time. The Pluto-Charon system is tilted to the side, and the dwarf planet Pluto has a retrograde orbit compared to other planets, suggesting a rapid origin, perhaps a cosmic collision. As we can see in the comparison below, Charon is a tiny world. Even our moon is huge with it.
Earth: NASA, Moon: Gregory H. Revera, Pluto: NASA/JHUAPL/SWRI link [Public domain]
South Pole Charon entered the polar night in 1989 and will not see the sunlight again until 2107. The New Horizons probe was able to study some of the night landscapes because they were very poorly illuminated by the light reflected from Pluto. Charon's moonlight also helped scientists explore Pluto when the spacecraft left the day side. The probe revealed a mysterious red formation at Charon's North Pole. This reddish color comes from the atmosphere of Pluto. Pluto alone is too small to maintain its atmosphere, so nitrogen, methane and carbon monoxide leave its surface. Charon takes a part of this matter, which is then directed to its surface. As the material accumulates, the cosmic rays and ultraviolet rays from the Sun cooperate, creating organic compounds with such a color. This satellite is extremely cold, the temperature on its surface ranges from -258 to -230 degrees Celsius and consists of water ice. Charon has more craters than Pluto, which suggests that its surface is older.
By NASA/JHUAPL/SwRI link [Public domain]
Charon also has a spectacular canyon that extends over 1,600 km. This canyon is therefore at least four times longer than the Grand Canyon in Arizona and twice as deep in some places! To the south of the huge canyon there is a smoother part of the satellite. There are noticeably fewer craters than in the northern hemisphere. Grooves and small ridges suggest that volcanism could have occurred here. The internal ocean could have frozen long ago, but scientists are almost certain that it existed. Because of the huge distance the last major satellite in our solar system will remain a mystery until the exploration of the cosmos for the next generations.
Greetings to lovers of Astronomy!