Our young Earth may have collided with a body similar to the planet Mercury , says a new paper . The dramatic event could explain why our planet has a hot core that gives it its magnetic field . And it could also explain why certain rare-earth elements in Earth ’ s mantle do not seem to originate from the planet ’ s other building blocks - meteorites . Oxford scientists say a Mercury-like body struck the young Earth ( artist 's illustration shown ) . The object would have been the heat source for our planet 's core . The same object could have been responsible for creating the moon . It also explains where some rare-Earth elements came from . The study , by two scientists from the University of Oxford , was published in the journal Nature . Our current theory of Earth ’ s formation involves basically more and more asteroids and other objects accumulating into one body . However , this model can not explain where the heat source for Earth ’ s core - and ultimately its magnetic field - came from . In addition , Earth ’ s crust and mantle appear to have a higher ratio of the rare-earth elements samarium and neodymium than most meteorites - suggesting there was more at play in Earth ’ s formation than first thought . To address this discrepancy , scientists believe that Earth was hit by a body that was the size of Mars - but like Mercury in its composition - early in its life . This likely would have happened about 4.5 billion years ago . The body would have been rich in sulphur but very poor in oxygen - just like Mercury . Many researchers believe the moon formed after Earth was hit by a planet the size of Mars billions of years ago . This is called the giant impact hypothesis . The hypothesis claims the moon is debris left over following an indirect collision between our planet and an astronomical body approximately 4.5 billion years ago . The colliding body is sometimes called Theia , after the mythical Greek Titan who was the mother of Selene , the goddess of the Moon . But one mystery has persisted , revealed by rocks the Apollo astronauts brought back from the moon - why are the moon and Earth so similar in their composition ? Several different theories have emerged over the years to explain the similar fingerprints of Earth and the moon . Perhaps the impact created a huge cloud of debris that mixed thoroughly with the Earth and then later condensed to form the moon . Or Theia could have , coincidentally , been isotopically similar to young Earth . A third possibility is that the moon formed from Earthen materials , rather than from Theia , although this would have been a very unusual type of impact . Our current theory of Earth ’ s ( left ) formation involves basically more and more asteroids and other objects accumulating into one body . Now scientists think that Earth was hit by a body that was the size of Mars , but like Mercury ( right ) in its composition , early in its life . In the young solar system 4.5 billion years ago , illustrated , various planetesimals and other small bodies coalesced together to form larger bodies like Earth . As it collided with Earth , it would have produced radioactive uranium and thorium , generating the heat needed to drive the ‘ dynamo ’ of molten iron in Earth ’ s core . And they do not rule out the possibility that this is the same Mars-sized body that formed the moon . ‘ We think that that is quite conceivable , ’ said study co-author Bernard Wood , reported the LA Times . ‘ It 's kind of exciting to think that this reduced body could actually be the thing which caused the moon. ’ In an accompanying News and Views article for Nature , Dr Richard Carlson of the Carnegie Institution for Science in Washington , DC said the theory could open up new avenues for thought.. For example , if true , it would raise further questions about ‘ how Earth ended up in its present oxidised state , which it has apparently retained for more than three billion years . ’
Oxford scientists say a Mercury-like body struck the young Earth . The Mars-sized object would have been the heat source for our planet . The same object could have been responsible for creating the moon . It also explains where some rare-Earth elements came from .