Lunar 100 – L3 Mare/highland dichotomy

Mare/highland dichotomy

Full moon image
Mare/highland dichotomy

No binoculars or telescopes required!

Have you ever noticed how there are dark splotches on the Moon? Well, good news – you’ve already bagged L3! Nice one!

A simpler way to explain this Lunar 100 entry is to say that the Moon is not uniformly grey: there are dark bits of the Moon and lighter bits. The dark bits we see are maria (latin for seas) because early astronomers thought these dark, smooth regions were seas. In fact, they are low lying areas which used to be filled with molten rock. The molten rock solidified over time into the Luna maria we see today. They’re largely flat except for where they’ve been struck by meteors.

The lighter areas of the Moon are the highlands. They are older and more cratered, and the tallest peaks are over 6.5 miles above the Moon’s imagined sea level. That’s taller than Mount Everest!

Below is a closer shot showing the contrast between the dark and light areas. Notice how the right hand side of the image is markedly lighter as the landscape rises up from the relatively smooth dark ground of the maria.

Copernicus crater (the biggest one in this image) on the edge of Mare Imbrium

To give you a better idea of the topography of the Moon’s surface, here’s an image taken by NASA’s Lunar Reconnaissance Orbiter in 2017. The green areas are taken as the average surface level with the pink regions sitting 5.7 miles below the average and the highest peaks sitting 6.7 miles above the average. So the difference between the lowest and highest areas of the Moon’s surface is 12.37 miles.

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