Lunar Origin

November 28, 2016

Just two decades ago, no planets were known outside our Solar System. In 1995, the exoplanet, 51 Pegasi b was found in orbit about its host star, 51 Pegasi. At this writing, there are more than 3300 confirmed exoplanets existing in about 1200 planetary systems. Advancing observational technologies will reveal many more exoplanets in the future.

Many such exoplanets resemble planets in our own Solar System, but only one of these planets is known to harbor living organisms. Earth exists in the Goldilocks zone of our Sun, more technically called the Sun's circumstellar habitable zone. In this zone, the conditions for the origin and evolution of life are "just right."

What was surprising to me was that the original "Three Bears" story didn't feature a little girl, but an old woman who "could not have been a good, honest old Woman," since she entered the Bear's house without permission. She was also reviled for her profanity.

"So first she tasted the porridge of the Great, Huge Bear, and that was too hot for her; and she said a bad word about that. And then she tasted the porridge of the Middle Bear, and that was too cold for her; and she said a bad word about that too. And then she went to the porridge of the Little, Small, Wee Bear, and tasted that; and that was neither too hot nor too cold, but just right; and she liked it so well that she ate it all up."[1]

Goldilocks running from the three bears in the Three Bears story that I remember. (From The Book of Knowledge, Arthur Mee and Holland Thompson, eds., The Grolier Society (New York, 1912) via Wikimedia Commons.)

This "just right" condition for a planet is an orbit about its host star that would allow the presence of liquid water on its surface, provided that other environmental requirements, such as an atmosphere to contain the water, are present. Life, as we know it, needs water, but was this the only important factor for the genesis of life on Earth?

The Earth is unique among the planets in having a huge, solitary Moon, and such moons are likely rare elsewhere in the galaxy. It's conjectured that the presence of the Moon may have been important to the genesis and evolution of life on Earth. I reviewed one particular influence of the Moon on the Earth, the lunar tides, in a previous article (Synchronizing to the Moon, April 11, 2016).

The lunar tides would have been a mechanism for concentrating ocean chemicals in littoral ponds as a way to generate early proto-biotic molecules such as the polymer precursors to nucleic acids. The tides would have aided thermal equilibration of our planet to smooth climate fluctuations.[2]

Is this any way to treat a neighbor? (Still image from Georges Méliès' 1902 film, "Le Voyage dans la lune" (A Trip to the Moon), colorized Wikimedia Commons image.)

However, the Moon acted to create a stable environment for life long before the creation of Earth's oceans by stabilizing its axis of rotation and establishing the tilt to the orbital plane (its obliquity) that's responsible for our mild seasonal variation. An early analysis of the Earth's obliquity without the presence of the Moon showed that our axial tilt would have chaotically shifted between 0 degrees and about 85 degrees. The torque exerted on the Earth by the Moon keeps our axial tilt at 23.3° ±1.3° and acts as a climate regulator.[3]

Astronomers have attempted to extrapolate the dynamics of the Earth-Moon system backwards in time to discover the origin of our unique planetary system. The giant-impact hypothesis, that the Earth-Moon system formed from a coalescence of the matter left after a collision of an early Earth with another planet, is the preferred theory, since it explains so many things. The giant-impact hypothesis explains why the Moon has an iron core, its surface was once molten, and Earth rocks and Moon rocks seem to have had a common origin.

One thing that's not explained by the giant-impact hypothesis is why the Moon's orbit is tilted five degrees from the ecliptic, which is the plane of Earth's orbit around the Sun. A new study by astronomers at the SETI Institute (Mountain View, California), the University of Maryland (College Park, Maryland), Harvard University (Cambridge, Massachusetts), and the University of California, Davis, (Davis, California) presents evidence that a more energetic collision of the proto-Earth with the planet-sized body, called Theia, would explain the present features of the Earth-Moon system.[4-5]

The energetic impact model starts at the same place as previous models, with the planetary impact, but the impact was so energetic that the planetary bodies were both reduced to a cloud of vapor and molten material. Both the Earth and Moon would have condensed from this cloud, thus their similar compositions.[5] Conservation of angular momentum is the fundamental principle that guides our understanding of planetary dynamics. According to the energetic impact model, some of the angular momentum of the Earth-Moon system was transferred to the Earth-Sun system.[4-5]

Evolution of the Earth/Moon system from 1-55 million years. After formation of the Earth-Moon system, Earth's axis points towards the Sun. Over the course of millions of years, the orbit of the Moon (red) becomes more distant, and Earth's day gets longer, while a transfer of angular momentum causes Earth's axis to tilt to its modern value. (Still images from a SETI Institute video by Matija Ćuk. Click for larger image.)

This transfer of angular momentum happened when the Moon's orbit enlarged to a point called the "LaPlace plane transition." This is the point at which the gravitational force caused by the Earth became less important than the gravitational force of the Sun.[5] This caused the Earth's axis to flip upwards from its orbital plane. Eventually, the Moon went from an orbit of high inclination to the ecliptic to its present 5 degree inclination.[5]

This energetic impact model explains the similarity in composition of the Earth and the Moon, while also reproducing its present orbit.[4-5] This research was funded by NASA.[5]

References:

1. The Story of the Three Bears, Chapter 18 of "English Fairy Tales" by Joseph Jacobs, G. P. Putnam's Sons (New York, 1890).
2. Bruce Dorminey, "Without the Moon, Would There Be Life on Earth?" Scientific American, April 21, 2009.
3. J. Laskar, F. Joutel, and P. Robutel, "Stabilization of the Earth's obliquity by the Moon," Nature, vol. 361, no. 6413 (18 February 18, 1993), pp. 615-617, doi:10.1038/361615a0.
4. Matija Ćuk, Douglas P. Hamilton, Simon J. Lock, and Sarah T. Stewart, "Tidal evolution of the Moon from a high-obliquity, high-angular-momentum Earth," Nature, Advanced Online Publication (October 31, 2016), doi:10.1038/nature19846.
5. Andy Fell, "New Theory Explains How the Moon Got There," University of California Davis Press Release, October 31, 2016.
6. Origins of Earth and Moon, Sarah Stewart's web site.