Faced with the theory of the great collision against the eartha new study argues that the moon was captured in an encounter between a young Earth and a binary formed by the Moon and another rocky object.
In six missions to the Moon, between 1969 and 1972, Apollo astronauts collected more than 360 kilos of lunar rock and soil. The chemical and isotopic analysis of this material showed that it was similar to rock and soil on Earth: rich in calcium, basaltic and dated to about 60 million years after the formation of the solar system. Using that data, planetary scientists who met at the Kona Conference in Hawaii In 1984 they reached a consensus that the Moon formed from debris after a collision on the young Earth.
But that might not be the true story of the Moon’s origin, according to two Penn State researchers. New research published in The Planetary Science Journal by Darren Williams, professor of astronomy and astrophysics at Penn State Behrend, and Michael Zugger, senior research engineer at Penn State Applied Research Laboratory, offers another possibility: the encounter with a rocky binary.
“The Kona Conference laid the foundation for 40 years,” Williams said. But questions still remained. For example, a moon that forms from a planetary collision, taking shape as debris clumps together into a ring, should orbit above the planet’s equator. Earth’s moon orbits in a different plane. “The moon is more in line with the sun than it is with the Earth’s equator,” Williams said. In the alternative binary swap capture theory, the researchers said, Earth’s gravity pulled the binary apart, trapping one of the objects – the moon – and turning it into a satellite that orbits in its current plane.
There is evidence that this happens elsewhere in the solar system, Williams said, pointing to Triton, the largest of Neptune’s moons. The prevailing hypothesis in the field is that Triton was pulled into orbit from the Kuiper Beltwhere 1 in 10 objects is believed to be a binary. Triton orbits Neptune in a retrograde orbit, moving in the opposite direction to the planet’s rotation. Its orbit is also significantly inclined, at an angle of 67 degrees to Neptune’s equator.
Williams and Zugger determined that Earth could have captured a satellite even bigger than the moon (an object the size of Mercury or even Mars), but the resulting orbit might not have been stable. The problem is that the “capture” orbit (the one followed by the Moon) started as an elongated ellipseinstead of a circle. Over time, influenced by extreme tides, the shape of the orbit changed.
“Today, Earth’s tide is ahead of the Moon” Williams said. “High tide speeds up the orbit. It gives it a pulse, a little momentum. Over time, the Moon moves a little further away.” The effect is reversed if the Moon is closer to Earth, as it would have been immediately after capture. By calculating tidal changes and the size and shape of the orbit, 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 spin was locked in its orbit around Earth, as it is today. At that point, Williams said, the tidal evolution probably reversed, and the Moon began to gradually move away. Every year, he said, the Moon moves 3 centimeters away from the Earth. At its current distance from Earth – 384,000 kilometers – the Moon now feels a significant pull from the Sun’s gravity.
“The Moon is now so far away that both the Sun and the Earth compete for your attention” Williams said. “They both pull on it.” His calculations show that, mathematically, a satellite captured by binary exchange could behave like Earth’s Moon does. But he’s not sure that’s how the Moon formed. “No one knows how the Moon formed,” he said. “For the last four decades, we only had one possibility of how it got there. Now, we have two. This opens up a treasure trove of new questions and opportunities for further studies.
