A new study says the Earth may have had two moons in the past. According to the study, the two moons crashed into each other to form the Moon we have today. Scientists say this may explain why the two sides of our Moon are so different from each other.
The study says the Earth had a second moon about 750 miles (1,200 kilometers) wide. What scientists call the global impact hypothesis suggests the second moon was formed from a collision between Earth and a Mars-sized object. According to BBC, the researchers say the Earth was struck, about four billion years ago, by another planet about the size of Mars. The resulting debris eventually coalesced to form two moons.
Space.com reports mathematical calculations indicate the second moon was formed at about the same time as a larger moon, from debris that coalesced after the Earth suffered impact with the Mars-sized body. The global impact theory, according to John Mills, an assistant professor of physics and astronomy at Anderson University, in Anderson Indiana, suggests that when the Mars-sized object impacted with the Earth, material lifted out was captured by its own gravity and held in ring shape in orbit around the Earth. The debris from the impact coalesced to form both a large and a second smaller moon about 4 percent the mass of the bigger moon.
The far side of the Moon remained a mystery to Earth-bound observers until 1959, when the Soviet Luna 3 spacecraft captured the first photographs of the surface. The photographs showed that the Moon's far side is very different from its near side. Scientists say that the Moon shows only one side from the Earth because gravitational attraction between the Earth and the Moon slowed the rate at which the Moon spins.
One of the major differences between the far and near sides of the Moon is the distribution of volcanic rock plains called "maria." Maria plains cover much of the near hemisphere but there are only a few maria plains on the far side. The terrains are also different. Space.com reports the surface of the near side is mostly flat but the far side is mostly mountainous. On the average, the far side is elevated about 1.2 miles (1.9 km) higher than the near side.
BBC reports scientists have for long wondered at the phenomenon of lunar dichotomy. Lunar dichotomy is the observation that the far side of the Moon looks so different from the near side. They have also wondered why the upper crust of the far side is thicker and more mountainous.The new study says that the difference in elevation between the near and far sides of the Moon suggests the second moon simply plastered itself against its larger companion.
Space.com reports calculations suggested to the scientists that the second moon was located at a Trojan point relative to the Earth and the larger moon for millions of years. A Trojan point is one in which gravitational forces balance out. An object at a Trojan or Lagrangian point can remain relatively stable in space. The Earth and Moon have two Trojan points known as L-4 point and L-5 points. But after millions of years, the Trojan point at which the second moon was located was destabilized after the larger moon expanded its orbit from Earth.
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