Space & Innovation

Baby Moon-Birthing Collision Slapped Earth Sideways

The impact that formed the moon was a very violent and dizzying time for our planet.

<p>NASA</p>

While we think of the moon as a familiar, unchanging sight, when compared with the other moons in the solar system it's a bit weird. Earth's moon is relatively big compared to our planet, its orbit is far from Earth and its tilt is huge.

All of this could be explained by a big collision that took place between the Earth and a Mars-sized object early in the evolution of our solar system, according to University of Maryland researchers. After the big crash about 4.5 billion years ago, Earth's spin would have spun up. Our planet also would have tilted to an extreme degree, lying practically on its side.

Today, the system we have is much smoother thanks to gravitational interactions between the Earth, moon and sun. But these characteristics could be leftovers of what happened in ancient solar system history.

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"Despite smart people working on this problem for 50 years, we're still discovering surprisingly basic things about the earliest history of our world," said Matija Cuk, a scientist at the SETI Institute and lead researcher for the simulations, in a statement. "It's quite humbling."

A mosaic of images the NASA Galileo spacecraft took of the moon in 1992, on its way to Jupiter. Credit: NASA/JPL/USGS

The previous scenario suggested that after the proto-moon smashed into the Earth, debris spun from our planet out into space and eventually created a ring. Over time, the individual rocks, dust and gas particles coalesced and created the moon we know and love today. The only trouble is that wouldn't work if Earth's tilt was the same as today, which is 23.5 degrees, said the University of Maryland, which also participated in the research.

Here's the challenge: according to physics, the debris ring (which also represents the moon's orbit immediately after it was formed) should have been in Earth's equatorial plane. Over time, tidal interactions between the Earth and the moon pushed the moon further away.

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At that point, the moon's orbit should have shifted to the ecliptic, which is the path the Earth takes around the sun. In reality, however, the moon's orbit has a five degree tilt away from the ecliptic. But a new model by the science team suggests an alternate scenario, where the Earth spun very fast and the impact changed Earth's tilt to somewhere between 60 and 80 degrees.

The Earth and the moon captured in a single image by the Galileo spacecraft in 1992, eight days after it made a close flyby of Earth. Credit: NASA

"When the planet's equator and its orbit are nearly perpendicular, the satellite becomes confused about which way is 'up', and its orbit becomes elongated due to sun's meddling. In the case of our moon, the varying distance from Earth on its eccentric orbit then triggered strong tidal flexing within the moon, which fought back against the efforts of Earth's tides to push it outward, resulting in a stalemate," the SETI Institute added.

"Such a stalemate can last for millions of years, during which Earth kept losing its spin while the moon did not go into a wider orbit. Instead, its orbit became more tilted. Once the Earth had lost enough of its original spin, the moon broke out of this stalled state and continued its outward journey."

Eventually, the moon's gravity was enough to move our tilt back to about 23.5 degrees, and tidal interactions between the Earth and the moon changed the moon's orbital inclination to the ecliptic, they added.

A study based on the research was recently published in the journal Nature.

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