the earth's relatively large natural satellite, the moon, is unique. during the apollo program, rocks from the moon's surface were brought to earth. radiometric dating of these rocks has shown the moon to be 4527 ± 10 million years old, about 30 to 55 million years younger than other bodies in the solar system. new evidence suggests the moon formed even later, 4.48±0.02 ga, or 70–110 ma after the start of the solar system. another notable feature is the relatively low density of the moon, which must mean it does not have a large metallic core, which other terrestrial bodies in the solar system have. the moon has a bulk composition closely resembling the earth's mantle and crust together, without the earth's core. this has led to the giant impact hypothesis: the idea that the moon was formed during a giant impact of the proto-earth with another protoplanet.