From Earth, one can look towards the night sky and observe that the moon appears to have areas that are lighter, and areas that are considerably darker. These dark areas are referred to as lunar maria, which are basically land basins on the moon in varying sizes. These basins were formed when hot, molten lava spewed out from the moon’s surface through crevices and cracks caused by previous collisions.
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By looking at lunar maria rock samples retrieved in two Apollo missions, scientists found that the rock samples from the moon shared a great deal of similarity with the volcanic rock, referred to as basalt, found in the Icelandic and Hawaiian volcanoes on Earth. Similar to the basalt rocks found on our planet, the mare, which is singular for maria, is made up of fine grains and is crystal like in its composition. Mare is generally dark in appearance, usually black or grey, and can be categorized under the rock types called pyroxene and plagioclase feldspar. Elaborating on composition, some mare is known to contain amounts of limonite and olivine, both of which are minerals made of a mix of oxygen, titanium and iron. Scientists and astronomers placed the age they believe the basalts, the youngest lunar rocks, at 3.1 to 3.8 billion years ago. Interestingly, astronomers found that the gravity of the moon fluctuated as one was to explore various areas of the moon, and saw that the force of gravity was much greater just above the more round and circularly shaped maria. The reason as to why the gravitational pull in close vicinity to the circular maria was significantly stronger is that there were mass concentrations of solidified molten lava like material stored up under the surface, which did not make its way all the way through the fractures in the moon’s crust. This is the same material that originally spewed out of the moon’s surface, and melted the moon’s crust into large dome shaped basins and ditches which now cover the moon, referred to as lunar maria.
- Maria bedrock contains a unique feature, which is referred to as rille. Basically, the molten hot lava running over the surface of the moon left behind streaks and deep tunnels as it burned and melted through the crust. This molten lava left the moon pockmarked with hollow tunnels and large trenches and valleys due to the intense heat of the lava, and over time, these rivers of hot lava would cool off and harden, leaving an exterior shell of the moon’s crust.
- The Hadley Rille is the most commonly known, which is found on the perimeters of a distinct basin called Mare Imbrium, and is said to have been formed by rivers of lava that flowed over the moon’s surface, left trenches in the crust, then eventually drained back into the moon, leaving a hollow tunnel of which the roof collapsed on. Here is a picture of the Hadley Rille:
- Lunar Maria was given its name from the Latin word “mare” which means “sea”, because early astronomers figured that these large basins were full of water, thus their dark appearance.
- Until the 20th century, astronomers did not have much thought on lunar maria apart from the misconception that they were large, dark, ocean like bodies of water.
- The true reason as to why the lunar maria appear to be darker is due to their iron rich composition
- Lunar maria is simply hot lava that spewed over the moon’s surface, and eventually melted shapes into the crust, and over time hardened into rocks which were found to be very similar to basalt. (Inong, Freedman 10)