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Near the magnetic poles, these particles sometimes leak into our atmosphere, causing the gases to glow. This is known as the aurora. The most obvious feature on the Moon’s surface are its many craters. Most of the Moon’s craters are round, implying that they were formed from impacts by high-speed meteoroids. Craters include features such as central peaks, collapsed terrace walls, and ejecta blankets. 1 Close examination of moon rocks reveal tiny microcraters less than one millimeter in diameter. The Moon’s surface also has large, dark gray plains called maria. Since these regions have few impact craters, the lunar surface here is relatively young compared with the lighter surrounding terrain. From Earth, maria appear to be smooth regions on the lunar surface. However, close-up photographs reveal small craters and occasional cracks called rilles. CLOSE-UP VIEWS FROM A LUNAR ORBITER APOLLO 15 MISSION 2 A surprising find from lunar orbiters is that the side of the Moon facing away from the Earth has only one small mare, while the rest is covered with craters. Missions to the Moon have revealed much about its history. An enormous boulder fractured while sliding down a mountain The Moon’s surface is covered with a layer of powdered rock called regolith. Lunar Rocks from Different Regions MARE BASALT contains many holes, suggesting that gas was dissolved in the lava and solidified to create this rock. ANORTHOSITE from the highlands is an ancient rock believed to be part of the Moon’s original crust. IMPACT BRECCIAS are created from debris fused together under high temperature and pressure created by impacts of space debris with the lunar surface. 3 Seismic experiments revealed that the main regions of the Moon’s interior mimic those of the Earth, but in different proportions. Water ice may exist in the polar craters, where the energy received from the Sun is insufficient to melt it. The similarity in materials making up the Earth and Moon suggest that the Moon may have been created from Earth material ejected after a collision with a large planetesimal. MOON EARTH This computer simulation shows how the Moon could have been formed in a collision between the Earth and a large planetesimal.. planetesimal The Earth and Moon orbit around their center of mass, called the barycenter. barycenter. A wrench sliding across a table rotates around its center of mass. 4