The moon is the simplest body in the solar system that we observe every day with our naked eyes. Do you ever wonder about the dark and light spots on its surface? Have you ever thought about what makes up our nearest neighbor?
The moon is thought to have formed from the debris of a small planet that collided with the Earth. Since the composition of other planets in the solar system differs from that of the Earth, it was expected that the moon’s composition would also differ from that of the Earth. Surprisingly, the composition of the Earth and the Moon are very similar.
Scientists had suggested many models for the moon origin, but since the 1980s there has been a focus on the most promising model, the so-called “giant impact” paradigm. According to the “giant impact” model, a collision between a small Mars-like planet (called Theia) and the ancient Earth gave rise to the Moon. Some of the debris from the collision fell back to Earth, some were scattered into space and the rest went into orbit around the Earth. This orbiting debris later combined to form a single object: the Moon.
Earlier it was believed that most of the material that eventually formed the Moon comes from the impactor, the small Mars-like planet called Theia, and only a smaller fraction originated from the impacted body i.e. Earth in this case. Therefore, according to the “giant impact” model, it was expected that the composition of the Moon should be very different from that of the Earth but should be similar to the other bodies in the solar system such as asteroids and Mars.
However, evidence indicates otherwise – in terms of composition, the Earth and Moon are almost twins and their compositions are almost the same, differing by at most few parts in a million. This contradiction challenges the “giant impact” model. Now, scientists have come up with a new answer to this mystery.
In contrast to the traditional studies which have focused on only the compositions of the final planets, the recent studies considered not only the final planets but also the composition of the impactors on these planets. Consequently, it is found that in many cases, the planets and the bodies that collide with them share a very similar composition, even though they formed independently. Thus, the similarity between the Moon and Earth stems from the similarity between Theia from which the Moon was formed and Earth.
The Earth and Theia were formed in the same region and have therefore collected similar material. They appear to have shared closely similar environments during their growth than just any two unrelated bodies. These similar living environments also led them eventually to collide; and the material ejected mostly from Theia, ultimately formed the Moon.
The Moon is made from many of the same things that we find here on Earth. Scientists studied the moon rocks brought back by the Apollo astronauts. Their tests showed that the rocks from the Moon are similar to three kinds of igneous rocks that are found here on Earth: Basalt, Anorthosites, and Breccias.
Scientists found three minerals on the Moon that are not found on the Earth. They are: Armalocolite, Tranquillityite, and Pyroxferroite.
The Surface of the Moon
Moon is not made of cheese like what we’ve heard in children’s fairytales. Like other celestial bodies in the solar system, the moon is made up of a rocky surface and is covered with dead volcanoes, impact craters, and lava flows.
Early in the solar system's history, all of the planets and moons suffered through a period of heavy bombardment of asteroids and meteorites which were captured by their gravity. Due to a sparse atmosphere, they weren’t burnt but ended up crashing into its surface, leaving numerous craters behind. Tycho Crater is more than 52 miles wide.
Over billions of years, these impacts have ground up the surface of the moon into fragments ranging from huge boulders to powder. The crust of the moon is covered with a rubble pile of charcoal-gray, powdery dust, and rocky debris called the lunar regolith. Beneath is a region of fractured bedrock called megaregolith.
The light areas of the moon are known as the highlands, and the dark stretches of the moon are known as maria (Latin for seas). They are sort of oceans, but rather than water they are made up of pools of hardened lava. Early in the moon’s history, the interior was molten enough to produce volcanoes, though it quickly cooled and hardened. When large enough asteroids broke through the crust, lava also burst from the surface.
The crust of the moon is about 38 to 63 miles (60 to 100 kilometers) thick. The regolith on the surface can be as shallow as 10 feet (3 meters) in the maria or as deep as 66 feet (20 meters) in the highlands.
Do you know why in moonwalks’ footage astronauts appear to almost bounce across the surface? This is because gravity on the surface of the moon is one-sixth of Earth’s.
The temperature reaches about 260 degrees Fahrenheit (127 degrees Celsius) when in full sun, but in darkness, the temperatures plummet to about -280 degrees Fahrenheit (-173 degrees Celsius).
Under the Surface
Like the Earth, the moon has a core, mantle, and crust.
Deep inside its interior, the moon has a solid iron core. The core is 149 miles (240 kilometers) in radius; it is proportionally smaller than the core of other terrestrial bodies. The solid, iron-rich inner core is surrounded by an outer layer of partially molten liquid. The outer core may extend as far out as 310 miles (500 kilometers). The inner core only makes up about 20 percent of the moon, compared to the 50 percent core of other rocky bodies.
The mantle extends from the top of the partially molten layer to the bottom of the moon’s crust. It is most likely made of minerals like olivine and pyroxene, which are made up of magnesium, iron, silicon, and oxygen atoms.
The outermost layer is the crust which has a thickness of about 43 miles (70 kilometers) on the moon’s near-side hemisphere and 93 miles (150 kilometers) on the far-side. It is made of oxygen, silicon, magnesium, iron, calcium, and aluminum, with small amounts of titanium, uranium, thorium, potassium, and hydrogen.
Most of the interior of the moon is made up of the lithosphere, which is about 620 miles (1,000 km) thick. As this region melted early in the lunar life, it supplied the magma necessary to create lava plains on the surface and formed active volcanoes. However, over time the magma cooled and solidified, thus, ending volcanism on the moon. Now, all the active volcanoes are dormant and have not erupted for millions of years.
Earth's moon is the second densest in the solar system, beaten by Jupiter's moon, Io. The separation of its interior into layers was likely caused by the crystallization of a magma ocean shortly after its formation.
The moon has a very thin and weak atmosphere, called an exosphere. It does not provide any protection from the sun's radiation or impacts from meteoroids.
Moon's near side and far side
Earth’s Moon has a ‘near side’ that is perpetually Earth-facing and a ‘far side’, which always faces away from Earth. The composition of the Moon’s near side is oddly different from its far side.
On the Moon's perpetually Earth‐facing near side, on any given night, or day, one can observe dark and light patches ('maria') with the naked eye. The far side is heavily cratered but has almost no maria. Only 1% of the far side is covered with maria compared with ~31% for the near side.
