FIGURE 2.1 An artist’s conception of a giant impact. The Moon is hypothesized to have been formed at ~4.5 Ga when a Mars-size body called Thea struck Earth. Courtesy of Don Davis.

FIGURE 2.2 Image of the western limb of the Moon obtained by the Galileo spacecraft during an Earth flyby. The Orientale multi-ringed basin is in the center. On the right is the nearside and extensive Mare Procellarum basalts. On the left is the farside, with the South Pole-Aitken Basin dominating the lower left half of the image. Orientale is the youngest basin on the Moon, and South Pole-Aitken Basin is the oldest. SOURCE: NASA Image PIA00077, obtained by the Galileo spacecraft.

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