Lunar Mare: It's Not Water!

October 23, 2022

What are those dark regions on the Moon?

The lunar mare are large, dark, basaltic plains on our Moon, formed by ancient volcanic lava on the near side that faces us. Early astronomers mistook them for actual Seas. They are less reflective than the "highlands" as a result of their iron-rich composition, and hence appear dark to the naked eye. The mare cover about 16% of the lunar surface, mostly on the side visible from Earth. The few maria on the far side are much smaller, residing mostly in very large craters. The traditional nomenclature for the Moon also includes one oceanus (ocean), as well as features with the names lacus ('lake'), palus ('marsh'), and sinus ('bay'). The last three are smaller than maria, but have the same nature and characteristics.

Major Mares (Or Seas) on the Moon

You can see those sees with your naked eye! I put my favorite ones in this photo above, and those are:

Mare Tranquillitatis (Sea of Tranquility). The 876 Km in diameter sea was the first lunar region to be visited by humans!
Mare Serenitatis (Sea of Serenity). The 674 Km sea is located above the Sea of Tranquility and to the East of the Sea of Showers.
Mare Crisium (Sea of Crisis). This 556 Km sea is located to the north east of the Sea of Tranquility, the reason behind the naming is not clear, it was named by an Italian astronomer Giovanni Riccioli and it has since been used as a standard name.
Mare Imbrium (Sea of Showers/Rain). This 1146 Km in diameter sea is actually one of the largest craters in our solar system. It's the second largest mare on the Moon after the Oceanus Procellarum.
Mare Fecunditatis (Sea of Fertility). This 840 Km sea is located to the South east to the Sea of Tranquility.
Mare Cognitum (Sea that has become Unknown). This 350 km in diameter sea is located southern to Oceanus Procellarum and Copernicus Crater.
Mare Humorum (Sea of Moisture). A 420 Km sea that is located to the south of Mare Cognitum, to the west of Tycho Crater.
Oceanus Procellarum (Ocean of Storms). This 2592 Km sea is the largest of the mare and hence called "the Ocean".

What are these colors on my Moon photos?

Many have wondered and asked me about the blue/rusty colors of the Moon in my photos.

The blue areas you see are the lunar Mare, and are generally ancient lava flows with high iron and titanium content. The redder/rusty areas have high magnesium content.

While these colors aren't immediately evident to our eye, the camera sensor + saturation boosts can reveal them with great clarity. It's an excellent way to look at some of the formation boundaries on the surface.

Chemical composition

The term of mare constituents is called lunar basalts. Mare basalts are generally grouped into three series based on their major element chemistry: high-Ti basalts, low-Ti basalts, and very-low-Ti (VLT) basalts. While these groups were once thought to be distinct based on the Apollo samples, global remote sensing data from lunar missions now shows that there is a continuum of titanium concentrations between these end members, and that the high-titanium concentrations are the least abundant. Titanium oxide abundances can reach up to 15 wt.% for mare basalts, whereas most terrestrial basalts have abundances much less than 4 wt.%. A special group of lunar basalts is the KREEP basalts, which are abnormally rich in potassium (K), rare-earth elements (REE), and phosphorus (P). A major difference between terrestrial and lunar basalts is the near-total absence of water in any form in the lunar basalts. Lunar basalts do not contain hydrogen-bearing minerals like the amphiboles and phyllosilicates that are common in terrestrial basalts.

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