How the moon got its tilt—and Earth got its gold
Source: By Sid Perkins, Science
Excerpt: Miniplanets zooming through our early solar system passed close to our moon and tugged it into the strange, tilted orbit it has today, according to a new study. The findings solve a longstanding mystery and may also explain why Earth’s crust is unexpectedly rich in gold and platinum: When some of these small planets slammed into Earth, they delivered a payload of precious metals. Scientists have long debated the origin of the moon. The prevailing idea, first proposed decades ago, is that a Mars-sized planet collided with Earth, flinging material into space that then coalesced into our only natural satellite. According to current models of that collision, the ring of debris that eventually became the moon should have ended up in a plane tilted no more than 1° from the ecliptic, the plane in which Earth orbits the sun, says Kaveh Pahlevan, a planetary scientist at Université Côte d’Azur in Nice, France. But in fact, the moon’s orbital inclination today is 5°. ...Pahlevan and university colleague Alessandro Morbidelli, also a planetary scientist, realized that for every cosmic collision, there would likely have been dozens of close calls—and the closer the encounter, the more the moon’s orbit would have been influenced. In their new study, the pair used thousands of computer simulations to estimate the cumulative effects of such close encounters on the lunar orbit. ...In a substantial fraction of the team’s simulations, the moon’s orbital tilt ended up being 10° or more, the amount that planetary scientists estimate the nascent moon would have had based on today’s orbital tilt. What’s more, says Pahlevan, some of the mini-planets crashed into Earth at some point in those simulations—impacts that would have delivered iridium, gold and platinum, among other elements. The proportions of those metals are unusually high in Earth’s crust, .... According to some models of planetary formation, the metals would have sunk to Earth’s core when much of the planet’s iron did, which means that new supplies had to come later in order for them to be found in the crust in such relatively high abundances....