Earth had 2 moons?

Earth once had two moons, the one visible today and a much smaller 'twin' that crashed to form one lunar body, scientists claim. The second moon would have formed early in Earth's history from the same giant impact thought to have given rise to the surviving moon, according to the theory by Professor Erik Asphaug, from the University of California at Santa Cruz. "The second moon would have lasted for only a few million years; then it would have collided with the moon to leave the one large body we see today," he was quoted as saying by 'The Sunday Times'. Evidence of the 'twin' is to be found on the dark side of the moon permanently turned away from Earth which has mountainous landscapes very different from the smooth face visible on Earth, Asphaug said. He believes that those mountains are actually the remains of the smaller moon, which would have been around one-thirtieth the size of the larger body. "It would have orbited Earth at the same speed and distance and just got slowly sucked in until they hit and then coalesced," he said. Scientists believe that at one time the inner solar system may have had up to 20 planet-sized bodies that collided into each other until only the eight we see today remained. Earth and its moon are thought to have been formed between 30 million and 130 million years after the birth of the solar system when a proto-Earth was struck by a smaller planet the size of Mars. Asphaug will present his theory at a conference on the moon to be held at the Royal Society in London in September. A total of nine ‘super-Earths’, planets between one and 10 times the mass of Earth have previously been found. Scientists from Harvard put forward a theory last year that suggested the Moon was once part of Earth that spun off after they collided with another body. The study was published in journal Science. Last month astronomers reported discovered three planets, similar to Earth, orbiting around a single star which may be able to support life. Researchers estimate there could be as many as 100 billion planets similar to the Earth in our galaxy, the Milky Way. The researchers involved hope that data from two US space agency (Nasa) lunar missions will substantiate or challenge their theory within the next year. For decades, scientists have been trying to understand why the near side of the Moon - the one visible from Earth - is flat and cratered while the rarely-seen far side is heavily cratered and has mountain ranges higher than 3,000m. Various theories have been proposed to explain what's termed the lunar dichotomy. One suggests that tidal heating, caused by the pull of the Earth on the ocean of liquid rock that once flowed under the lunar crust, may have been the cause. But this latest paper proposes a different solution: a long-term series of cosmic collisions. The researchers argue that the Earth was struck about four billion years ago by another planet about the size of Mars. This is known as the global-impact hypothesis. The resulting debris eventually coalesced to form our Moon. But the scientists say that another, smaller lunar body may have formed from the same material and become stuck in a gravitational tug of war between the Earth and the Moon.
Far side of the Moon
It takes the Moon about the same amount of time to rotate on its axis as it does to complete an orbit of the Earth
This is known as "synchronous rotation" and explains why the Moon always presents its familiar near side to Earth
The near side is covered in smooth, dark lunarmaria (Latin for "seas") created by magma erupting on the surface
The far side is more rugged, with a thicker crust pock-marked by impact craters; the highest point on the Moon is located on the far side
In 1959, the USSR's unmanned spacecraft Luna 3 became the first to photograph the far side; many of the features have been given Soviet names

After spending millions of years "stuck", the smaller moon embarked on a collision course with its big sister, slowly crashing into it at a velocity of less than three kilometres per second - slower than the speed of sound in rocks. Dr Jutzi says it was a low velocity crash: "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. At the time of the smash, the bigger moon would have had a "magma ocean" with a thin crust on top.The scientists argue that the impact would have led to the build-up of material on the lunar crust and would also have redistributed the underlying magma to the near side of the moon, an idea backed up by observations from Nasa's Lunar Prospector spacecraft. In a commentary, Dr Maria Zuber from the Massachusetts Institute of Technology (MIT) in Cambridge, US, suggests that while the new study "demonstrates plausibility rather than proof", the authors "raise the legitimate possibility that after the giant impact our Earth perhaps fleetingly possessed more than one moon". The researchers believe one way of proving their theory is to compare their models with the detailed internal structure of the moon that will be obtained by Nasa's Lunar Reconnaissance Orbiter. They will also be looking to high resolution gravity mapping set to be carried out next year by the Gravity Recovery and Interior Laboratory (GRAIL) mission. But according to Dr Jutzi the scientists would prefer to get their hands on samples from the far side of the Moon to prove their theory. "Hopefully in future, a sample return or a manned mission would certainly help to say more about which theory is more probable." Earth once had two moons, which merged in a slow-motion collision that took several hours to completeBoth satellites would have formed from debris that was ejected when a Mars-size protoplanet smacked into Earth late in its formation period. Whereas traditional theory states that the infant Moon rapidly swept up any rivals or gravitationally ejected them into interstellar space, the new theory suggests that one body survived, parked in a gravitationally stable point in the Earth–Moon system. Several such 'Lagrangian' points exist, but the two most stable are in the Moon's orbit, 60° in front or 60° behind. Traces of this 'other' moon linger in a mysterious dichotomy between the Moon's visible side and its remote farside, says Erik Asphaug "By definition, a big collision occurs only on one side," he says, "and unless it globally melts the planet, it creates an asymmetry." Asphaug and Jutzi have created a computer model showing that the Moon's current state can be explained by a collision with a sister moon about one-thirtieth the Moon's mass, or around 1,000 kilometres in diameter. Such a moon could have survived in a Lagrangian point long enough for its upper crust and that of the Moon to solidify, even as the Moon's deeper KREEP layer remained liquid. Meanwhile, tidal forces from Earth would have been causing both moons to migrate outward. When they reached about one-third of the Moon's present distance (a process that would take tens of millions of years), the Sun's gravity would have become a player in their orbital dynamics. "The Lagrange points become unstable and anything trapped there is adrift," Asphaug says. Soon after, the two moons collided. But because they were in the same orbit, the collision was at a relatively low speed. "It's not a typical cratering event, where you fire a 'bullet' and excavate a crater much larger than the bullet," Asphaug says. "Here, you make a crater only about one-fifth the volume of the impactor, and the impactor just kind of splats into the cavity." In the hours after the impact, gravity would have crushed the impactor to a relatively thin layer, pasted on top of the Moon's existing crust. "You end up with a  pancake," Asphaug says. The impact would have pushed the still-liquid KREEP layer to the Moon's opposite side. Apshaug's theory isn't the only attempt to explain the lunar dichotomy. Others have invoked tidal effects from Earth's gravity, or convective forces from cooling rocks in the Moon's mantle. Peter Schultz of Brown University in Providence, Rhode Island, calls it "interesting" and "provocative", despite his own theory involving a high-angle collision at the Moon's south pole, which he believes would have pressed crustal material northward to form the farside highlands³. "All this is great fun and tells us that there are very fundamental questions that remain about the Moon," he says. NASA's upcoming GRAIL mission, designed to probe the Moon's interior using precise measurements of its gravity, may help figure out what happened billions of years ago. "But in the end," Schultz says, "new lunar samples may be necessary."