Read out to the most eccentric of the planets. Pluto, with its enigmatic giant moon Charon, is the only planet space probes have yet explored up close. NASA engineers and scientists are currently working on a project to visit the sunniest satellite and elicit its secrets. A bitter cold, icy world - that's the only thing we know about Pluto so far. Otherwise, the outsider in the solar system is a fairly blank slate. No space probe has ever explored him close up. All the information we have about Pluto and its giant moon Charon, discovered in 1978, comes from terrestrial observations or, in the case of the Hubble Space Telescope, measurements from Earth orbit - that is, from a distance of well over four billion kilometers.

Pluto is also an outsider in terms of his size: he is by far the smallest among the planets, much smaller than our moon. Aside from the earth, he would not even cover Europe or the US.

Whether Pluto has an atmosphere was first ascertained in 1988. At that time there was a rare opportunity for the planet to pass just in front of a star. During the "covering", the star did not darken abruptly, but darkened immediately before. From this it could be concluded that the planet is surrounded by a thin atmosphere. It consists of nitrogen, carbon monoxide and methane, as spectral studies have shown later. According to theoretical considerations, it may also contain some photochemically produced molecules (hydrocyanic acid, nitriles, ethyne and other hydrocarbons) as well as atomic hydrogen.

The high point of Pluto research so far was in March 1996, when American astronomers published a map of the surface. It is based on observations made by the Hubble Space Telescope with the Faint Object Camera of the European Space Agency ESA. The images were taken in June and July 1994, when Pluto was only 4.8 billion kilometers from Earth, during a Pluto day lasting 153 earth hours. Each pixel of the photo corresponds to an area of ​​160 square kilometers. After lengthy image processing procedures, a Pluto map was created showing the largest light-dark contrasts of all planets in the solar system except Earth. display

Probably the most exciting question concerns the genesis of Pluto and its extraordinarily large moon. The physical properties, as well as the different internal structure and chemical composition of Pluto and Charon rule it out that Charon once split off from Pluto, or that both bodies arose together from the gas and dust cloud of the protosolaren Urnebels. The hypothesis that Pluto was a leaked Neptune's Moon, which was frequently mentioned earlier, has also been refuted by Charon's discovery. That Pluto has simply captured his moon, is also unlikely for heavenly mechanical reasons.

The most plausible assumption is that Pluto once collided with a body many hundreds, maybe even a thousand, kilometers in size. From the rubble, in an orbit around Pluto, Charon could have formed. However, soon after the discovery of the Moon, an important objection was raised against this collision hypothesis: If Pluto and Charon's forerunners were the only objects in this remote region of the solar system, such a collision would be extremely unlikely.

=== Rüdiger Vaas

© science.de

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