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The Man in the Moon's Missing Twin

June 27, 2014

Most people, when they hear the term, "far side," think of the popular Far Side comic strip by Gary Larson. Larson stopped drawing the Far Side in 1995, but many of the strips are so memorable that they can be found now, twenty years later, in an Internet search. Of course, none of those images should be there, since these are copyrighted works. Larson has a web site at thefarside.com.

There's another "far side," and that's the far side of the Moon. I wrote about the far side of the Moon in a previous article (Far Side of the Moon, February 6, 2012). Many people refer to it as the "dark side" of the Moon, since they think that it's always dark, when it actually gets about as much insolation, on average, as the side that's visible from the Earth.

The side facing the Earth, however, gets a bit more light because of sunlight reflected from the Earth, a phenomenon called Earthshine. Because of Earthshine, the New Moon is never perfectly black. The darkness of the New Moon is actually a function of how much snow and reflecting clouds the Earth has at a given time.

We only see one side of the Moon, since the Moon is tidally locked to the Earth. Until 1965, it was thought that the planet, Mercury, was tidally locked to the Sun; instead, it rotates three times for every two revolutions around the Sun, so it had appeared to be tidally locked in prior observations. Because of libration, a consequence of the slightly elliptical orbit of the Moon, and parallax, we can actually observe about 59% of the Moon's surface from Earth.

Many men have walked on the Moon, but none have walked on the far side. In the early days of manned spaceflight, such an expedition would have been dangerous, since the bulk of the Moon would have blocked communication with Earth. Today, it's easy to put a communications satellites at the Moon to solve this problem. Still, we might still use the term, "dark side of the Moon," if by "dark" we mean an unexplored territory.

Until 1959, there wasn't even an image of the Moon's far side. Finally, in October, 1959, the Soviet spacecraft, Luna 3, transmitted low resolution photographs of about 70% of the far side (see image). These images revealed a few large craters, as could have been expected. However, subsequent satellite reconnaissance in preparation for manned landing revealed some major differences between the Moon's near and far sides.

Luna 3 First Image

The first image of the far side of the moon, taken on October 7, 1959, by the Soviet Luna 3 spacecraft (North is up in this image).

The Luna 3 images, low resolution though they were, were a sensation when they were published.

(Via NASA Web Site))

The Moon's near side has large, smooth areas, while the far side does not. The smooth areas of the near side are called maria, from the Latin word for sea, since they resemble the oceans of Earth. The Moon's maria, which cover about a quarter of the near side, are not made of water; rather, they are remnants of lava outflows from volcanic eruptions. Maria are darker than their surroundings because they are iron-rich. The far side has very little maria coverage (see image, below).

Far side of the Moon, NASA Lunar Reconnaissance Orbiter composite image, June, 2009

Far side of the Moon.

This NASA Lunar Reconnaissance Orbiter composite image was made from more than 15,000 images acquired between November, 2009, and February, 2011.

(NASA/Goddard/Arizona State University image.)

Now, astrophysicists from Pennsylvania State University (University Park, PA) have published a theory of why there is a lack of far side maria. They believe that it's because of Earthshine of the hot, early Earth.[1-2] The paper, published in The Astrophysical Journal Letters, is authored by Arpita Roy, a graduate student in astronomy and astrophysics at Penn State and the lead author, along with assistant professor Jason Wright and professor Steinn Sigurdsson.[1-2]

The far side is nearly devoid of maria because it has a thicker crust than the near side, and that's a consequence of how the Moon was originally formed.[2] The Moon was formed by the impact of a Mars-sized object's hitting the proto-Earth with a glancing impact, as conjectured in the Giant Impact Hypothesis. The outer layers of both the Earth and the striking object were thrown into space to become the Moon.[2]

The early Moon was about 10-20 times closer to Earth than it is now, and it quickly became tidally locked to have one hemisphere always facing Earth. The Earth and Moon were both initially molten, and there was also a debris disk of rock, magma and vapor around the Earth.[2] The proximity of the hot Earth caused a thermal gradient that affected the coalescence of the material in the disk. Crust-forming refractory materials were preferentially deposited on the cooler farside, giving rise to its thicker crust.[1]

Earthshine at that time was quite a bit more energetic at that time, since the temperature of the Earth was more than 2500 °C. Says study coauthor, Steinn Sigurdsson, "When rock vapor starts to cool, the very first elements that snow out are aluminum and calcium," and these elements are found in high concentrations in the Moon's crust. These elements would have preferentially condensed on the cooler, farside of the moon, since the nearside was still too hot. Eventually, these elements crystallized into plagioclase feldspars, which migrated to the Moon's surface to form the crust.[2]

Although the Moon is presently solid throughout, it still had a molten interior during earlier meteor bombardments. These meteors were able to punch through the crust to release the lava that formed the maria, but only on the near side. The far side crust was too thick to be penetrated, so the far side topography has valleys, craters and highlands, but very few maria features.[2] This research was funded by the NASA Astrobiology Institute.[2]

Selene, the Moon goddess

Equal time for the Woman of the Moon, Selene, the Moon goddess.

The Roman god, Apollo, is also associated with the Moon, thus the eponymous Apollo Program, successful in landing the first humans on the Moon in July, 1969.

(Oil on canvas painting, 1880, by Albert Aublet (1851-1938), via Wikimedia Commons.)


  1. Arpita Roy, Jason T. Wright and Steinn Sigurdsson, "Earthshine on a Young Moon: Explaining the Lunar Farside Highlands," The Astrophysical Journal Letters, vol. 788, no. 2 (June 9, 2014), doi:10.1088/2041-8205/788/2/L42.
  2. A'ndrea Elyse Messer, "55-year-old dark side of the moon mystery solved," Penn State University Press Release, June 9, 2014.

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