AVAST gentle reader: mild SPOILER(S) and graphic depictions of shattered satellites ahead!
We recently had a chance to catch Oblivion, the first summer blockbuster of the season. The flick delivers on the fast-paced Sci-Fi action as Tom Cruise saves the planet from an invasion of Tom Cruise clones.
But the movie does pose an interesting astronomical question: what if the Earth had no large moon? In the movie, aliens destroy the Earth’s moon, presumably to throw our planet into chaos. You’d think we’d already be outclassed by the very definition of a species that could accomplish such a feat, but there you go.
Would the elimination of the Moon throw our planet into immediate chaos as depicted in the film? What if we never had a large moon in the first place? And what has our nearest natural neighbor in space done for us lately, anyway?
Earth is unique among rocky or terrestrial planets in that it has a relatively large moon. The Moon ranks 5th in diameter to other solar system satellites. It is 27% the diameter of our planet, but only just a little over 1/80th in terms of mass.
Clearly, the Moon has played a role in the evolution of life on Earth, although how necessary it was isn’t entirely clear. Periodic flooding via tides would have provided an initial impetus to natural selection, driving life to colonize the land. Many creatures such as sea turtles take advantage of the Full Moon as a signal to nest and breed, although life is certainly resilient enough to find alternative methods.
The 2000 book Rare Earth by Peter Ward and Donald Brownlee cites the presence of a large moon as just one of the key ingredients necessary in the story of the evolution of life on Earth. A Moon-less Earth is also just one of the alternative astronomical scenarios cited by Arthur Upgreen in his 2005 book Many Skies.
Contrary to its depiction on film, the loss of the Moon wouldn’t throw the Earth into immediate chaos, though the long term changes could be catastrophic. For example, no study has ever conclusively linked the Moon to the effective prediction of terrestrial volcanism and earthquakes, though many have tried. (Yes, we know about the 2003 Taiwanese study, which found a VERY weak statistical signal).
All of that angular momentum in the Earth-Moon system would still have to go somewhere. Our Moon is slowly “braking” the rotation of the Earth to the tune of about 1 second roughly every 67,000 years. We also know via bouncing laser beams off of retro-reflectors left by Apollo astronauts that the Moon is receding from us by about 3.8 cm a year. The fragments of the Moon would still retain its angular momentum, even partially shattered state as depicted in the film.
The most familiar effect the Moon has on Earth is its influence on oceanic tides. With the loss of our Moon, the Sun would become the dominant factor in producing tides, albeit a much weaker one.
But the biggest role the Moon plays is in the stabilization of the Earth’s spin axis over long scale periods of time.
Milankovitch cycles play a long term role in fluctuations in climate on the Earth. This is the result of changes in the eccentricity, obliquity and precession of the Earth’s axis and orbit. For example, perihelion, or our closest point to the Sun, currently falls in January in the middle of the northern hemisphere winter in the current epoch. The tilt of the Earth’s axis is the biggest driver of the seasons, and this varies from 22.1° to 24.5° and back (this is known as the change in obliquity) over a span of 41,000 years. We’re currently at a value of 23.4° and decreasing.
But without a large moon to dampen the change in obliquity, much wider and unpredictable swings would occur. For example, the rotational axis of Mars has varied over a span of 13 to 40 degrees over the last 10 to 20 million years. This long-term stability is a prime benefit that we enjoy in having a large moon .
Perhaps some astronomers would even welcome an alien invasion fleet intent on destroying the Moon. Its light polluting influence makes most deep sky imagers pack it in and visit the family on the week surrounding the Full Moon.
But I have but two words in defense of saving our natural satellite: No eclipses.
We currently occupy an envious position in time and space where total solar and lunar eclipses can occur. In fact, Earth is currently the only planet in our solar system from which you can see the Moon snugly fit in front of the Sun during a total lunar eclipse. It’s 1/400th the size of the Sun, which is also very close to 400 times as distant as the Moon. This situation is almost certainly a rarity in our galaxy; perhaps if alien invaders did show up, we could win ‘em over not by sending a nuclear-armed Tom Cruise after ‘em, but selling them on eclipse tours…
And a receding Moon also means that in approximately 1.4 billion years, the final total solar eclipse as seen from the Earth will occur. Conversely, the Moon was closer and appeared larger earlier in Earth’s history. About just under a billion years ago, the first brief annular eclipse similar to the one occurring next week on May 10th would have occurred. In the current epoch, annular eclipses constitute 33.2% of solar eclipses with total solar eclipses becoming ever rarer at 26.7%. (The remainder are hybrids and partials).
If the Moon was a necessary ingredient for life to take hold on Earth, then we may be a very rare occurrence in the universe indeed. The current theory for the formation of the Moon involves the Earth getting “wacked” by a Mars-sized body dubbed Theia early in its history. This would explain the relatively low density of our Moon compared to the Earth.
Oblivion isn’t the only science fiction to posit a moon-less Earth. Fans of 1970’s sci-fi will remember the TV series Space: 1999 which proposed an even more unlikely scenario of the Moon being “blown out of orbit” by a nuclear disaster. Of course, just how they managed to meet new alien civilizations every week was never explained, but hey, it was the 1970’s…
Oblivion did have one more glaring space science goof. Plutonium used for space travel and weaponized Plutonium are two different isotopes. It would not be possible (though it was a convenient plot device) to turn an nuclear-powered RTG such as one used on Mars to power the Curiosity rover into an explosive weapon.
But perhaps the greatest gift our Moon has to offer is its lessons to us as a species. The motion of the Moon provided early astronomers with a great lesson in Celestial Mechanics 101. Newton would have had a much tougher time deciphering the laws of motion and gravity were it not for the example provided by the Moon. Plus, it makes a great stepping stone for solar system exploration. Curse it or love it, the Moon is our celestial companion… let the sci-fi alien baddies be jealous!