A new study carried out by researchers from the Observatory of the Côte d'Azur suggests that gravitational forces arising from massive, nearby-passing rocks that eventually slammed into the Earth billions of years ago may have perturbed the orbit of the moon, and that might be the reason behind moon orbit's current tilt of 5 degrees with respect to the Earth.
According to researchers, these giant rocks also added vast amount of metals onto the Earth.
It is believed that collision between Theia (a Mars-size rock) and Earth about 4.5 billion years ago resulted in birth of moon, which emerged from the debris that resulted from this collision. Some earlier studies also suggest that moon's current orbit, which is tilted about 5 degrees with respect to Earth, should actually be 10 times smaller – a mystery known as lunar inclination problem.
The latest study suggests that gravitational forces operating between moon and nearby-passing rocks could solve this mystery and also provide an explanation to why gold, platinum and many other metals are found in Earth's outer layers.
In this study, researchers ran computer models of newborn solar system, and performed computer simulation of the movement of moon around the orbital plane of the Earth. They also performed simulation of rocks with a total mass equivalent to 0.75% to 1.5% of the mass of Earth. Each of these giant rocks had experienced a large number of close passes with the Earth, which probably brought them much closer to Earth or the moon and strongly perturbed the orbit of the moon with gravitational pull. The study suggested that gravitational forced due to these close passes could be the reason behind the tilt in moon's orbit seen today.
"The most surprising thing about these results is the ease with which the moon's orbital trajectory can be tilted, or excited, by gravitational interactions with passing objects." Kaveh Pahlevan, lead author of the study and a planetary scientist at the Observatory of the Côte d'Azur in Nice, France told Space.com.
"If the moon-forming event had occurred earlier, when more massive bodies were around, the moon's orbit would have been much more excited and likely destabilized, with the moon colliding with the Earth or escaping to interplanetary space." Pahlevan said.
"The relative lateness of the moon-forming event during Earth formation can be understood as a necessity for its survival. Earlier moons were simply lost."
These findings also suggest that giant rocks added a "late veneer" of metals onto Earth.
Scientists suggest that because the newborn Earth was extremely molten, most of the iron in the planet sunk to the core. Metals like platinum, gold and iridium are known to have a high chemical affinity for iron, and therefore they should also have sunk with iron to Earth's core. However, they are today found on the outer layer of the Earth and were probably brought by gigantic rocks that collided with Earth after the formation of the Earth's core.
The findings of this study have been published in the journal Nature.