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Repeated gravitational tugs by  passing miniplanets (foreground and upper right) could explain why the moon’s orbital tilt is unusually high today.

Repeated gravitational tugs by passing miniplanets (foreground and upper right) could explain why the moon’s orbital tilt is unusually high today.

Laetitia Lalila

How the moon got its tilt—and Earth got its gold

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°. And the tilt would have been more pronounced, 10° or so, immediately after the moon formed 4.5 billion years ago, before Earth started to smooth the moon’s orbit out a bit. This significant discrepancy between prediction and reality has been dubbed “the lunar inclination problem.”

Scientists have proposed a few solutions to this conundrum. Other large objects may have slammed into the moon and jostled its orbit, they say, or perhaps the strange orbit was caused by repeated tugging from the sun.

But 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 their models, the researchers began with a common view of the early solar system: one populated with lots of mini-planets that had coalesced from dust, ranging in size from 1 lunar mass down to 0.1 lunar mass. Each simulation started just after the moon formed and ended when all of the mini-planets orbiting near Earth either fell into the sun, crashed into another planet or was ejected from the solar system entirely—an interval that typically lasted about 100 million years in simulated time.

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, which many scientists have tried to explain with models of an impact-delivered “late veneer” that came after Earth’s formation. 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.

Those impacts were small individually, but together the objects would have totaled between 60% and 120% the mass of the moon, the researchers report online today in Nature. Based on the known average cosmic abundance of various elements, that’s plenty enough material to explain the anomalous concentrations of the metals now present in Earth’s crust, they say.  

The findings are compelling, says Robin Canup, a planetary scientist at the Southwest Research Institute in Boulder, Colorado: “They provide a simple and elegant solution to the lunar inclination problem.”

If all of the mini-planets whizzing through the early solar system hadn’t existed—or if the moon-generating impact had occurred much later than it did, after those objects had been cleared from the inner solar system—the moon’s orbital plane would be very close to Earth’s, Canup says. That would cause our satellite to block the sun each time it orbited Earth, thus providing a total solar eclipse every month, Canup notes in an accompanying News & Views perspective in Nature. The trade-off might be unappealing jewelry, however: Rather than platinum and gold, we’d be adorning ourselves with tin and copper.