The Moon had a Really Rough Day

The familiar face of the Moon, facing Earth, is covered in vast regions of large smooth basin, called maria, remains of lava flows which took place on the lunar surface in the distant past. The far side of the Moon, however, is largely devoid of these large dark basins, a difference which has puzzled astronomers for decades. Now, an international team of astronomers believe they have found the answer to this mystery of our planetary companion. Billions of years ago, the Moon may have been the target of a dwarf planet, which struck our planetary neighbor, forever altering its geology.

The Moon itself was likely formed through the impact of a semi-molten Earth with a solid body the size of Mars in the early ages of our solar system. However, this theory of lunar formation also comes with its own conundrum — if this were the case, the chemistry of the Moon should be nearly identical to the Earth. However, chemical differences between the pair of planetary siblings is seen in samples of the lunar crust brought to Earth by the astronauts of Apollo. A single event — the collision of the Moon with a body the size of Ceres — the largest asteroid in the inner solar system today — could also explain differences seen between the chemistry of Earth and the Moon.

“The giant impact model also provides a good explanation for the unexplained differences in isotopes of potassium, phosphorus and rare-earth elements like tungsten-182 between the surfaces of the Earth and Moon, the researchers explain. These elements could have come from the giant impact, which would have added that material to the Moon after its formation,” researchers with the American Geophysical Union explain.

It’s a Split Decision…

Numerous theories have been put forward in an effort to explain differences between the two hemispheres of the Moon. One idea postulated that the Earth once had two moons, and these two bodies merged, at a time when the Moon was still largely molten, forming the singular natural satellite we see today.

A second major idea is that a dwarf planet, orbiting the Sun, may have collided with the Moon when the solar system was still young. Data supporting that idea has been found in data from the The Gravity Recovery and Interior Laboratory (GRAIL) mission, launched in 2012.

The GRAIL mission revealed the crust covering the far side of the Moon is thicker than it is on the side facing Earth. An extra layer of material, not present on the side of the Moon facing Earth, is also present on the far side, GRAIL data revealed.

It’s all in the Simulation, Ya Know…

Researchers ran 360 computer simulations of impacts between the Moon and various bodies, attempting to find conditions which would have resulted in the geological conditions we see today on our planetary companion. Simulations revealed that a body 780 kilometers (480 miles) in diameter, striking the near side of the Moon at 22,500 kilometers per hour (14,000 MPH), would have provided the impact needed to create the conditions observed on the far side of the Moon. This event would be roughly equivalent to a body slightly smaller than the dwarf planet Ceres hitting the Moon at around one-quarter of the speed of shooting stars entering the atmosphere of the Earth. Simulations also suggested a similar geology would have resulted from the impact of a smaller body, with a diameter of 720 km (450 miles), hitting the Moon at a slightly higher velocity — 24,500 kph (15,000 MPH).

Such a cataclysmic event would have kicked up enough material on the far side of the Moon to cover that hemisphere in a layer of rock five to 10 kilometers (three to six miles) deep, creating the extra layer of material revealed by GRAIL.

The computer simulations showed the object which impacted the Moon was, likely, orbiting the Sun before impact, rather than existing as a second moon prior to the event.

“Understanding the origin of the differences between the nearside and the farside of the Moon is a fundamental issue in lunar science,” said Steve Hauck, professor of planetary geodynamics at Case Western Reserve University.

The Moon’s a Nice Place to Visit, but…

“When I look at the moon I do not see a hostile, empty world. I see the radiant body where man has taken his first steps into a frontier that will never end.”

- David Scott, Commander Apollo 15

In October 1959, the Luna 3 spacecraft, launched by the Soviet Union, was the first vehicle to record images from the far side of the Moon.

“The spacecraft returned very indistinct pictures, but, through computer enhancement, a tentative atlas of the lunar farside was produced. These first views of the lunar far side showed mountainous terrain, very different from the near side, and only two dark regions which were named Mare Moscovrae (Sea of Moscow) and Mare Desiderii (Sea of Dreams),” NASA explains.

The Lunar Reconnaissance Orbiter (LRO), launched in June 2009, created global elevation maps and photographic mosaics of the Moon in far greater detail than ever possible prior to that mission.

The Chang’e-4 lander, designed in China, became the first spacecraft to successfully touch down softly on the far side of the Moon. This spacecraft, named for a Chinese goddess of the Moon, released a rover, the Yutu-2, to explore this mysterious half of the lunar surface.

As we learn more about the Moon, science is entering a third age of understanding our planetary companion. Once thought to be dry and dead, the Moon is now known to be home to water ice (some of which may form on the lunar surface), and an active interior, resulting in moonquakes. The American space agency, NASA, has plans to place humans, once more, on the lunar surface by 2024, and several other nations and agencies have their own plans to send people to the Moon.

Originally published at on May 20, 2019.

The Cosmic Companion

Exploring the wonders of the Cosmos, one mystery at a time

The Cosmic Companion

Written by

James Maynard is the author of two books, and thousands of articles about space and science. E-mail:

The Cosmic Companion

Exploring the wonders of the Cosmos, one mystery at a time