location. And finally, since Jupiter travelled to a much more distant position than it had been before, where lots of icy objects lived, the massive planet deflected some of these icy objects toward the sun and into the asteroid belt. As a result, the asteroid belt contains rocky objects from the inner solar system and icy objects from the outer solar system, all in the position where we see them in the belt today.


9.2.4 Our Moon and its Violent Origin


Te disc of the full moon has inspired story-telling for centuries. Some see the lunar maria – the large, dark, basaltic plains formed by ancient volcanic eruptions – as the shape of a person’s face, making up stories about the mysterious “man in the moon”. Te plains were dubbed maria, which is Latin for “seas”, by early astronomers who mistook them for actual seas. Others, especially children, compare the Moon’s heavily cratered highlands to cheese and dream of an entire sphere made of that delicious product. In Greek-Roman mythology, Sun and Moon are represented as male (Helios/Sol) and female (Selene/Luna).


9.2.4.1 How Did the Moon Form?


Figure 9.33: During its flight in 1992, the Galileo spacecraft returned separate images of the Earth and Moon, here combined to generate this view. We see a partial view of the Earth centred on the Pacific Ocean about latitude 20°S, with the west coast of South America and the Caribbean. The swirling white cloud patterns indicate storms in the south-east Pacific. The distinct bright ray crater at the bottom of the Moon is the Tycho impact basin. The dark areas are lava rock-filled impact basins.


formed later than Jupiter, also got pulled in and eventually caught up with Jupiter at about the current orbital distance of Mars. When the two planets came close, their fates became linked. Locked in a gravity dance, their sun-bound death spiral came to a halt and the planet pair turned and moved away from the sun. Tey kept migrating outward until the disc ran out of gas and the planets were stranded somewhere not too far from their current orbits. We refer to Jupiter’s path as the Grand Tack, since this


change in Jupiter’s direction is like the course that a sailing boat takes when it tacks around a buoy. Jupiter’s wild migration now explains the puzzles. As


Jupiter migrated inwards, it pushed most of the rocky material inwards. Dust and material that was meant to make Mars the size of Earth and Venus was swept away. Jupiter completely cleared out the Mars zone, Mars got too little material, and ended with a tenth the mass of Earth. Earth and Venus formed within a region with lots of rocky material. Everything Jupiter met on its way was strongly affected.


Starting its migration inwards and passing the asteroid belt which at the time consisted of a loose collection of rocky objects, the Grand Tack model indicates that Jupiter pushed the whole belt farther out. Essentially, Jupiter deflected the objects, and switched places with the asteroid belt. Much later, when Jupiter turned and moved away from the sun, the big gas planet nudged the asteroid belt back inward to its present


316


One speculative theory claims that the Earth used its gravity to capture the Moon from Venus, giving our planet its big satellite. Te leading theory for the Moon’s origin, however, is the Giant Impact Hypothesis, first proposed in the 1970s. Tis suggests that the Moon was formed about 30–50 million years after the origin of the solar system from debris thrown into orbit by a violent cosmic crash between the just-forming proto-Earth and a Mars-size planetoid called Teia. Tis orbiting debris, primarily derived from Teia, later coagulated to form a single object: the Moon. It is still a puzzle as to why the composition of the Earth


and the Moon is so alike. Te Moon’s bulk density and composition is similar to that of the Earth’s mantle and it has been established that the composition of the oxygen isotopes of the Moon and the Earth is identical. A study by Mastrobuono- Battisti et al. (2015) suggests the compositional resemblance stems from the similarity between Teia and Earth. Tey are said to have grown up together in the same ‘neighbourhood’ and therefore collected similar material. Living in a similar environment also led them eventually to collide. However, Touboul et al. (2015) suggest that Teia did not


have the same composition as Earth. Instead, they believe that the material from Teia and Earth mixed so thoroughly that the Earth’s mantle and the moon each received a similar portion, explaining their resemblance.


9.2.4.2 What If the Moon Did Not Exist?


Te Earth would be quite a different place if the Moon did not exist. At the time of the Moon’s formation the Earth rotated much faster than it does today, so a day on early Earth was only a few hours long. Since then, the Earth has been slowing its rotation due the Moon’s gravitational influence, which also produces the ocean tides. Tere is a little bit of friction between the tides and the rotating Earth, causing the rotation to slow down just a little. As Earth slows, it lets the Moon creep away. Today, the Moon is slowing down the rotation of the Earth by about 15 microseconds every year, gradually


NASA/JPL/USGS


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