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- Pluto repeats an anomaly when compared with the terrestrial planets
- (Mercury, Venus, the Earth and Mars) or the giant planets (Jupiter, Saturn,
- Uranus and Neptune). This icy world is more like a huge asteroid or the
- satellites of the outer planets; it has even been suggested that Pluto was
- a satellite that escaped from Neptune's pull.
-
- The small size of Pluto combined with its distance make it very difficult
- to observe; however, two important discoveries have just been made.
-
- In 1976 new spectroscopic observations revealed that the surface of Pluto
- was covered, at least partially, by frozen methane. The infrared spectrum
- enabled this methane to be identified, besides other kinds of ice, such as
- ice of ammonia or water. The surface of Pluto, covered by ice, is therefore
- bright and its reflective power (or albedo) is much greater than we would
- suppose, thinking that it was all covered by rocks. Knowing its distance
- from the Earth and its reflection coefficient we can deduce a diameter of
- between 2000 and 2700 kilometers; its mass is thus much less than first
- estimates.