Study suggests Moon may have been captured from space rather than formed from colliding particles

During six missions to the Moon, from 1969 to 1972, Apollo astronauts collected more than 800 pounds of lunar rocks and soil. Chemical and isotopic analysis of this material showed that it was similar to Earth’s rock and soil: calcium-rich, basaltic, and dating to around 60 million years after the formation of the solar system.

Using this data, planetary scientists meeting at the Kona Conference in Hawaii in 1984 reached a consensus that the Moon had formed from debris after a collision with the young Earth.

But that may not be the true story of the Moon’s origin, according to two Penn State researchers. New research published in The Journal of Planetary Science by Darren Williams, professor of astronomy and astrophysics at Penn State Behrend, and Michael Zugger, senior research engineer at Penn State’s Applied Research Laboratory, offers another possibility: that the Moon was captured in an encounter close between a young Earth and a terrestrial binary: the moon and another rocky object.

“The Kona Conference set the script for 40 years,” Williams said. But questions persisted. For example, a moon that forms following a planetary collision, taking shape when debris clumps together into a ring, would be expected to orbit above the planet’s equator. Earth’s Moon orbits in a different plane.

“The Moon is more aligned with the Sun than with Earth’s equator,” Williams said.

In the alternative binary swap capture theory, the researchers said, Earth’s gravity pulled the binary apart, snagging one of the objects — the moon — and making it a satellite that orbits in its current plane.

There is evidence that this is happening elsewhere in the solar system, Williams said, pointing to Triton, the largest of Neptune’s moons. The leading hypothesis in the field is that Triton was orbited from the Kuiper Belt, where one in ten objects is thought to be binary.

Triton orbits Neptune in a retrograde orbit, moving in the opposite direction of the planet’s rotation. Its orbit is also significantly inclined, at 67 degrees relative to Neptune’s equator.

Williams and Zugger determined that Earth could have captured a satellite even larger than the Moon – an object the size of Mercury or even Mars – but that the resulting orbit might not have been stable.

The problem is that the “capture” orbit – the one the moon follows – started out as an elongated ellipse rather than a circle. Over time, under the influence of extreme tides, the shape of the orbit has changed.

“Today, the Earth’s tide is ahead of the Moon,” Williams said. “The high tide speeds up the orbit. It gives it a boost, a little boost. Over time, the moon moves away a little.”

The effect is reversed if the Moon is closer to Earth, as it would have been immediately after its capture. By calculating tidal changes and the size and shape of the orbit, the researchers determined that the moon’s initial elliptical orbit contracted on a time scale of thousands of years.

The orbit also became more circular, rounding its path until the lunar rotation locked into its orbit around Earth, as it does today.

At this point, Williams said, the tides likely reversed and the moon began to gradually recede.

Every year, he says, the Moon moves 3 centimeters away from Earth. At its current distance from Earth – 239,000 miles – the Moon now feels a significant pull from the sun’s gravity.

“The Moon is now so far away that the Sun and Earth are competing for its attention,” Williams said. “Both are shooting at each other.”

His calculations show that, mathematically, a satellite captured by binary exchange could behave like Earth’s Moon. But it is not certain that this is how the moon was born.

“No one knows how the moon formed,” he said. “Over the past four decades, we’ve only had one avenue to explain how it got here. Today, we have two. This opens up a treasure trove of new questions and opportunities for study more in-depth.”