Observing Project 5B Plato A Volcanic Blast

Located well to the north of the lunar equator is the Moon's Alps mountain range. Many of the mountains here tower over 15,000 feet above the Imbrium basin to the south that the mountains surround. In the middle of these bright highlands is a huge dark floored crater that rivals Tycho in size. Oddly enough, Plato is not listed in Charles Wood's Lunar 100. The monstrous crater is 99 kilometers across and its floor is some 2,200 meters below the rim, about half as deep as the floor of Tycho. While Tycho's floor is bright and rough, Plato's is dark and flat suggesting that some part of this crater's history involves volcanic activity rather than being purely an impact event. The rock that makes up the floor of the crater is the same type of basalt that forms the floor of the nearby Imbrium basin and other lunar seas. While Tycho is very young, Plato is very old, between some 3.2 and 3.8 billion years old. It was formed during the Moon's early geologically active period. Exactly what process formed the monstrous crater is unclear but what is obvious is that whatever cataclysm created Plato bore deep enough to flood the massive crater with a layer of lava thousands of feet deep. Another issue of interest as we look at Plato is that even after billions of years, the crater floor is still almost perfectly flat. Assuming that the Moon has been geologically dead for billions of years, what force of nature then has been keeping the crater floor smooth for billions of years?

Plato is an anomaly in that it is so different from the surrounding terrain, the event that created it having crushed the mountain area around it. Many of the mountains surrounding the crater formed long before Plato itself did. The crater is some 3.8 billion years old, but the mountains surrounding it were thrust up hundreds of millions of years before during the Moon's formative period. What cataclysmic disaster blew this enormous hole out of the lunar surface, leveling mountains that stood some three or four miles tall? One of the things that are not commonly found on the lunar surface is a volcanic mountain such as those found

Figure 5.3. Crater Plato. Celestron Super C8 Plus and Meade DSI CCD. Photograph by author. The DSI's higher resolution chip provides a much sharper image.

Figure 5.3. Crater Plato. Celestron Super C8 Plus and Meade DSI CCD. Photograph by author. The DSI's higher resolution chip provides a much sharper image.

on Earth or Io. But this is as close to a major volcanic feature that can be clearly viewed on the Moon's visible side. Did a volcanic catastrophe blow a mountain apart and leave this crater behind? Or might it be an impact crater that penetrated so deep as to flood the crater with lava? Nearly four billion years ago, the Moon had an active interior that would support either type of activity. When Tycho formed within the last half billion years, the Moon was already geologically dead, thus is was not possible to flood the area with lava.

Plato sits at approximately the same lunar longitude as does Tycho so your best opportunities to view it occur at about the same time. Local sunrise occurs when the Moon is around eight days old and the viewing geometry is very favorable for the next three days. Sunset occurs at about day 23 or about one day after last quarter, so the best time to look begins about three days prior to sunset.

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