Is the “Dark Side of the Moon” Actually Dark?

The short answer is no.

In fact, the side of the Moon we cannot see directly from Earth is brighter than the side we do see! This dark side is also known as the farside (and the side facing Earth is called the nearside). The phrase dark side of the Moon is used to refer to something mysterious or unknown. For most of human history we did not know what the farside of the Moon looked like. It wasn’t until 1959 when the Soviet Union launched the Luna 3 spacecraft that we got our first look at the farside.

The United States and the Soviet Union were racing to space, hoping to showcase their dominance through space-based technology. The Luna 3 mission was the third Soviet spacecraft to reach the Moon. The first mission to the Moon was the Luna 1 mission in January 1959. Not to be outdone, the US sent the Pioneer 4 mission in March 1959. Both Lunar 1 and Pioneer 4 flew by the Moon and are considered to be partial failures because they did not complete their expected objectives. The Soviet Luna 2 was the first successful mission to the Moon, (purposefully) impacting into the lunar surface in September 1959. Luna 3 quickly followed in October 1959 and (purposefully) flew from Earth around the Moon and back towards Earth.

Admittedly, the first farside image of the Moon is not much to look at now. But at the time it was collected, this image was exciting. It was published in newspapers and magazines around the world. The farside is very different from the nearside. There are only a few dark spots whereas the side we see from Earth is nearly covered with patches of dark material known as maria (pronounced mar-ree-ah):

Maria are ancient lava flows that erupted into the holes created by massive impact craters. Given that Luna 3 first observed the farside, scientists in the Soviet Union got to name newly observed features. One of the largest new maria identified on the farside was named Mare Moscoviense (MM), which translates to ‘Sea of Moscow’. Tsiolkovskiy crater (T) is named after Russian rocket scientist Konstantin Tsiolkovsky (don’t ask me why the crater and scientist’s names are spelled differently). Tsiolkovskiy crater is an interesting location on the Moon, as it was proposed as one of the landing sites for the Apollo 17 mission. Unfortunately, this site was deemed too risky—being on the farside would make communication with Earth more challenging. Instead, the Apollo 17 mission landed in the Taurus-Littrow valley on the southeastern edge of Mare Serenitatis on the lunar nearside.

Back to the mare. These dark lava flows make the nearside appear darker than the farside. Over time, the dark mare material gets mixed with the lighter material, known as the highlands, darkening the whole surface. You can get a taste for how this process works by looking at a large impact basin on the farside. Take a look at the bottom right of the farside in the image below. Does it look darker than the rest of the farside? It should! That is because there is a high concentration of small mare deposits in this large basin relative to the rest of the farside. The mixing of mare soil and highlands soil will, on average, darken the surface. The rest of the farisde doesn’t have a similar concentration of mare so it doesn’t have much dark material to mix with.

The lack of mare on the farside actually makes it mysterious (living up to the meaning of the phrase “dark side of the Moon”). Scientists are still trying to fully understand why there isn’t a lot of mare on the farside. The largest confirmed impact crater on the Moon, the South Pole-Aitken Basin (named because its rim extends from the south pole of the Moon to Aitken crater), carved a big hole in the Moon ~1500 miles (2500 km) across. That hole removed a lot of crust, setting up a scenario that favors eruption of magma from the Moon’s mantle. Mare should have filled that hole, but it didn’t. Why? Well, that is a question for another blog. Until next time!