Would life be possible without the moon?
In considering the characteristics of the Earth that make it unique among planets in supporting life, the influence of our satellite is often ignored. However, the presence of a large moon is one of the more conspicuous characteristics distinguishing the Earth from its potentially life-bearing sisters, Mars and Venus. Is this coincidence, or has the moon played a role in bringing or keeping the Earth within the thresholds of habitability?
Why do we ignore the moon?
There are valid reasons why the moon has usually been neglected from discussions of the Earth ’s habitability. When considering astronomical influences upon the terrestrial environment, the moon is in most respects feeble. This is particularly apparent in comparison with the sun. The energy the moon radiates upon the Earth is negligible. Despite its tug on our oceans, the moon ’s gravitational pull on individual organisms on the Earth ’s surface is minute. Most disappointingly of all, despite the imaginations of humankind, the moon has been revealed to be a barren ball of debris, a hopelessly uninhabitable environment. In line with its traditional associations with the intuitive and the mystic, the moon ’s influence on us has often been relegated to psychological and pseudo-psychological effects, sometimes dropping out altogether from the realms of science.
What’s special about OUR moon?
Nearly all the moons of our solar system are satellites of the gas or ice giants. Of the moons of the terrestrial planets, Mercury and Venus have none, and Mars has in Phobos and Deimos two tiny, irregular lumps.
No moon is so large in comparison to its “mother” planet as ours is. The outer giants, not surprisingly, host larger satellites, but they orbit worlds far larger than our own. The moon has only 2% of Earth ’s volume and 1.2% of its mass. Yet no other planetary moon approaches that relative size.
Among other features, our moon also stands out in the solar system for its unusually high angular momentum and its unusually high orbital tilt (about 10°). (Whitehouse, 254)
How does the moon influence life on Earth?
Of course, the most unusual and striking feature of the Earth-moon system is the self-evident fact that it supports life. Can any connection be drawn between this feature and the moon ’s other distinguishing characteristics?
Tides
The lunar influence is at its most tangible with the tides. These are caused by direct gravitational influence, as first proposed in Newton ’s Principia of 1687. The moon ’s gravitational pull causes the Earth ’s oceans to swell towards it (and, counterintuitively, also equally in the opposite direction on the other side!).
The amplitude of tides in the opean sea does not much exceed 76 cm, but when amplified by coastal topographical features these can be experienced as up to 10 m variations in water level. The tides also exert their influence upon the Earth ’s atmosphere and crust, though to a vanishingly small degree. (Morgan, 280-281)
The moon was once much closer to the Earth
By the reflection of laser beams off the lunar surface, it has been shown that the moon is moving away from us at a rate of 3.82 cm per year. (Mackenzie, 86) This is slower than the rate of continental drift, yet over a multi-billion-year timeframe it infers a moon much closer to us in the distant past. Its tidal influence would have been correspondingly much greater. In fact it is the tidal forces themselves acting between these two massive and proximate bodies that dissipated some of the moon ’s orbital energy, causing it to recede from us. Reconstruction of the moon ’s distance in the far past shows that the tidal forces on the young Earth would have been particularly strong. (Whitehouse, 250-256)
Solar tides
With no moon, we would still have solar tides following the sun, with high tide every noon and midnight. They would be much weaker than even present-day tides, and nothing like the tides from the earliest days when Earth and moon were closer. But it ’s an enigma how important tidal mixing was to the emergence of life, or how big the tides would have needed to be to do the job.
Do we need tides?
The influence of tides on the Earth is undeniable, for the nature of their influence upon life is problematic. It is difficult to assert that tides are essential to life today, except to a narrow class of coastline-dwelling organisms. But since Darwin in 1871 speculated about life ’s origins in “some warm little pond with all sorts of ammonia and phosphoric salts” (Benn, p.63), the tidally-regulated pool has often been considered an ideal setting for the beginnings of life.
“Some warm little pond”
Tidal draining and refilling are an ideal way to encourage the formation of complex organic molecules. Every time the water drains out, some organic sediment is left behind, and when water returns, the proto-organic “soup” will have slightly increased in concentration. Over millions of repetitions, the tidal pool would have a density of organic precursors far higher than that of the adjacent ocean. The increased concentration makes contact between molecules, and therefore their polymerization, much more common.
Earth’s cosmic “chain”
A far less obvious, yet potentially more significant effect of our large moon is its role as a stabilizing force. The gravitational influence of our satellite keeps the Earth ’s axis of rotation near-fixed at a tilt of 23.5°, protected from the chaotic “wobble” that would otherwise occur. Simulations have shown that without the influence of a moon, the gravitational alignments of the giant planets would cause the Earth ’s obliquity (axial tilt) to vary up to 60° at certain times, resulting in dramatic alterations to our climate. This is essentially the case on Mars, whose two small moons are not sufficiently massive, leaving its axis “unchained”. (Whitehouse, 257)
Does the moon affect the seasons?
Climatically, the Earth would be a much different place without the moon ’s axial stabilization. The terrestrial climate is a sensitive equilibrium of numerous factors, among which axial tilt plays a prominent role. Even slight changes in tilt have dramatic repercussions related to global convection patterns and precipitation cycles. It has been suggested that the desertification of the Sahara region was the result of an axial shift of less than 1°. (Whitehouse, 257)
Sources
Barbieri, Cesare, and Francesca Rampazzi, editors. Earth-Moon Relationships: Proceedings of the Conference held in Padova, Italy at the Accademia Galileiana di Scienze Lettere ed Arti, November 8-10, 2000. Kluwer Academic Publishers, 2001.
Benn, C. R. “The Moon and the Origin of Life”. Earth, Moon and Planets 85-86, p.61-66. Netherlands: Kluwer Academic Publishers, 2001.
Broecker, Wallace S. How to Build a Habitable Planet. New York: Eldigio Press, 1988.
Camuffo, Dario. “Lunar Influences on Climate”. Earth, Moon and Planets 85-86, p.99-113. Netherlands: Kluwer Academic Publishers, 2001.
Cirkovic, Milan M. “On the First Anthropic Argument in Astrobiology”. Earth, Moon and Planets 91, p.243-254. Netherlands: Kluwer Academic Publishers, 2003.
Drury, Stephen. Stepping Stones: The Making of Our Home World. Oxford: Oxford University Press, 1999.
Head, James W. “Lunar and Planetary Perspectives on the Geological History of the Earth”. Earth, Moon and Planets 85-86, p.153-177. Netherlands: Kluwer Academic Publishers, 2001.
Mackenzie, Dana. The Big Splat, or How Our Moon Came to Be. United States: John Wiley & Sons, 2003.
Morgan, Elfed, “The Moon and Life on Earth”. Earth, Moon and Planets 85-86, p.279-290. Netherlands: Kluwer Academic Publishers, 2001.
NASA. Planetary Fact Sheet.
San Diego Natural History Museum. Geologic Timeline.
Whitehouse, David. The Moon: A Biography. Great Britain: Headline Book Publishing, 2001.
Could we live on an “unchained” Earth?
An “unchained” planet would certainly be less hospitable, but it is difficult to argue that it would be less habitable. Extreme shifts in climate do not preclude life, as long as there is liquid water somewhere on the planet. It has however been suggested that the stabilization of the Earth ’s axis was probably essential for the evolution of life on land. (Benn, 64)
Does the moon affect the weather?
The moon has, of course, been credited with countless areas of influence on the Earth, scientific and otherwise. Studies of lunar rhythms and weather have shown that while certain effects – atmospheric tides, Sun-Eath-moon syzygy, monthly weather fluctuations, the lunar nodal precession – have risen above the level of statistical significance, they are minute to the point of irrelevance when integrated with other factors: the solar input, ocean surface variations like El Niño, volcanic emissions, etc. (Camuffo, 99)
Our celestial sister
In our search for patterns and order in the universe, there is something satisfying in the idea that our exceptional moon has something to do with our exceptional planet. We are also motivated by a simple, irrational fondness for our celestial sister; we want it to be more than just a lifeless rock. If it is indeed a giver of life on Earth, its own lifelessness would become somehow noble, a sacrifice so that we might live.
The mystery of life
Is our ubiquitous celestial partner more than just a coincidental companion? Scientifically, it is as yet impossible to support the claim that without the moon, life could not have evolved on the Earth. Solar tides, though comparatively weak, might have been sufficient for the concentration of organic solution. Terrestrial life might have survived the wild climatic variation of an unchained axis, as indeed it has survived the acute climatic changes brought on by meteor impacts and solar variation.
The weak anthropic principle:
“conditions that are observed in the universe must allow the observer to exist” (Merriam-Webster)
The mediocrity principle:
“You aren’t special.” (P.Z. Myers)
Keep an eye out for big moons!
There is simply too little knowledge of the ultimate origins of life to know exactly what its prerequisites were. Thus we cannot establish whether the moon ’s influence was decisive in allowing this process to occur. However, the anthropic principle – that the properties of the observed universe must support our existence within it (Cirkovic, 243) – and conversely the mediocrity principle both lead us to draw a tentative connection between the properties of the Earth and the properties of any habitable planet. Until our understanding of life ’s origins has improved to the point that we know precisely what is required for it to emerge, we must consider all the characteristics that go into making something “Earth-like”. In our search for another planet like the Earth, we must keep our eyes open for a moon like our own.
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