March 22, 2004

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Double Planet


Image Credit: Galileo Mission

Double Planet

The origin of the Moon was long a mystery. Most scientists now accept the giant impact model but one of the earlier ideas was that the Earth and Moon formed at the same time from the same region of the solar nebula. However, this sister planet or double planet theory could not explain why, if the two worlds formed of the same material, the Earth has a massive iron core and a density of 5.5 g/cc while the Moon has only a small amount of iron and a resulting density of only 3.3 g/cc. The double planet theory is dead, but the Earth-Moon system does look like a double planet because the Moon is relatively large compared to the Earth: their diameters are 3,476 km and 12,756 km. And like a contrasting double star pair, the colors of our two worlds are strikingly different - we must be one of the most wonderful sights in the solar system!

Technical Details:
The Galileo spacecraft took this image in 1992 while swinging close to Earth for a gravitational push towards Jupiter. The Moon is actually in the foreground moving from left to right around the Earth. The JPL web site says that contrast and color of both Earth and Moon were enhanced, but the Moon looks yellower to me than it actually is. However, the image was taken not through a green filter that mimics our eyes sensitivity, but with violet, red and infrared filters and may be correct for this combination.

Related Links:
Galileo mission overview
The Origin of the Moon

Yesterday's LPOD: Fold a Moon

Tomorrow's LPOD: Magnificent Moon!

Author & Editor:
Charles A. Wood



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