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.
BBC reports the destabilization led to a collision at about a relatively slow rate of 4,500 to 6,700 miles per hour (7,200 to 10,800 kph). Instead of forming a crater, the smaller moon splattered itself across the larger moon, causing a thick extra layer of solid crust tens of miles thick. Space.com reports that Erik Asphaug, planetary scientist at the University of California, Santa Cruz, and lead researcher in the study, said: "It is entirely plausible for a Trojan moon to have formed in the giant impact, and for it to go unstable after 10 million to 100 million years and leave its imprint on the moon." He suggested to Space.com, "imagine a ball of Gruyere colliding into a ball of cheddar."
According to BBC, one of the authors, Dr Martin Jutzi of the University of Bern, Switzerland, explained: "When we look at the current theory there is no real reason why there was only one moon. And one outcome of our research is that the new theory goes very well with the global impact idea." According to Jutzi, the impact between the two moons was a low velocity crash. He said: "It was a rather gentle collision at around 2.4km per second; lower than the speed of sound - that's important because it means no huge shocks or melting was produced."
The researchers say the remains of the smaller moon make up the highlands on the Moon's far side. The theory, according to the study, also explains why the near side of the Moon has an underground sea of magma containing phosphorus, rare-earth metals, radioactive potassium, uranium and thorium.
There are, however, competing theories explaining why the Moon's far side has highlands. One theory is that the near side of the Moon suffered less asteroids and comets impact. But this is not a popular theory because the terrain on the far side cannot be satisfactorily explained by impacts. Besides, the near side of the Moon is also highly cratered, suggesting it also suffered impacts.
Most researchers argue in favor of the gravitational tidal forces theory as alternative to the two moons collision theory. The gravitational tidal forces theory proposes that gravitational forces exerted by the Earth on the near side is greater than on the far side. Mills said: "The result is that the closer side gets thinned out as the inner bulk of the Moon is pulled ever so slightly toward earth, allowing the far side to remain thicker."
Space.com reports that Francis Nimmo at the University of California, Santa Cruz, also said that gravitational forces could explain lunar dichotomy. Nimmo argued there is not enough data to say which of the two proposals (two moons collision and gravitational tidal forces theory) is more likely. He said: "As further spacecraft data and, hopefully, lunar samples are obtained, which of these two hypotheses is more nearly correct will become clear."
According to Mills, for now there are no obvious flaws in the two moons collision theory, but the problem is that there is not enough information to provide direct proof of its validity. Mills said: "The largest hurdle this theory faces is that, while it explains well how the Moon could have formed the way that it did, it doesn't necessarily provide any conclusive evidence for universal acceptance."
BBC, however, reports that proponents of the two moons collision theory say one way of proving it is to compare models with details of internal structure of the Moon that will be obtained by Nasa's Lunar Reconnaissance Orbiter. VOA reports proponents also say high resolution gravity mapping set to be carried out next year by the Gravity Recovery and Interior Laboratory (GRAIL) mission may also provide evidence. But Dr Jutzi said the best method of proving the theory would be for scientists to obtain samples from the far side of the Moon. He said: "Hopefully in future, a sample return or a manned mission would certainly help to say more about which theory is more probable."
Asphaug and his colleague Martin Jutzi, at the University of Bern in Switzerland, published their study findings in the August 4 issue of the journal Nature.