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How Earth and the Moon interact

The Earth is unique amongst the terrestrial planets in having a large satellite, the Moon, which, relative to the Earth, has the largest mass of any satellite-parent system. Numerous lines of evidence indicate that the Moon was derived from the Earth as the result of a singular impact event soon after the initial formation of the Earth. As a result, the subsequent evolution of the Earth and the emergence and development of life has been strongly influenced by the presence of the Moon.

This article will highlight and explain the key areas in which the Moon has both directly and indirectly influenced the emergence and evolution of life on the Earth, a process that has culminated in the development of an intelligent, technologically advanced species.


Perhaps the most obvious manifestation of the influence of the Moon on the Earth are the ocean tides, particularly the spring tides where the gravitational pull of the Sun and Moon combine to give the greatest effect. The regular rise and fall of sea level creates an unique environment in the Solar System, where life is exposed to both immersion in water and exposure to air in the space of a few hours. This interface between two distinct ecological niches is thought by many to be crucial in evolutionary terms.

This is an environment in which organisms can experience the stresses and strains of an alien world before safely returning to their aquatic habitat, such changes possibly promoting the alteration and/or migration of organisms from one environment to the other. Hence the presence of the Moon to cause tides may well have sparked the spread of organisms from the sea to the land.

Earth Moon System

The Moon also raises tides in the solid body of the Earth and in the past, when the Moon orbited much closer to the Earth than at present, these tides are estimated to have produced displacements in the Earth's solid surface of up to a kilometre. This would have produced intense stress and deformation within the Earth which, coupled with the decaying heat of accretion and the higher content of radioactive (U, Th and K) elements, would have greatly promoted melting of the early Earth. This melting may well have had an important role in the early differentiation of the Earth, in particular producing the earliest evolved crust, which would then be available for recycling by nascent plate tectonic processes.

Stable Axial Tilt

It is considered likely by many authors that the current circa 23.5 degree tilt of the Earth's axis of rotation is a relic of the oblique collision that produced the Moon. Furthermore it is argued that the presence of the orbiting Moon has, through a large part of geological time, stabilised this axial tilt or obliquity of the Earth. This has had important ramifications for life on the Earth as major and frequent shifts in this obliquity would have led to significant and rapid changes in the Earth's climate due to changes in insolation values at the poles and equator. A similar mechanism has been suggested to explain the apparent contradictions in the climate record of Mars.

The current relatively moderate axial tilt of the Earth ensures that the difference in heating between the poles and equator is sufficient to promote a healthy and diverse range of climatic zones without veering from one extreme to another (e.g. Snowball Earth hypothesis). In particular, the stability of the Earth's axial tilt produced by the Moon, coupled with the break up of the Pangean supercontinent in the late Mesozoic, produced a diverse set of climate zones (with their associated ecological niches) compared with what had gone before during the time of the dinosaurs. This helped set the stage for the rise of the mammals, including Man.


Perhaps one of the least obvious but most significant contributions from the Moon to life on Earth has been the gift of workable metal deposits at the surface of the planet. Ever since the first samples of lunar rock were returned by the Apollo astronauts and the geochemical data were made available, scientists have been intrigued by the relatively high abundance of siderophile and chalcophile metals in the silicate Earth compared with the Moon. Current theory suggests that if the Earth had once been entirely molten then these metals should have been locked up in the Earth's metallic core as the Earth cooled. The current abundance of these elements in the Earth's mantle should be much lower, similar to those of the Moon (part of which was derived from the Earth's original mantle).

Computer modelling of the collision between the Earth and the Mars-sized impactor shows that the bulk of the mantle of the impacting object and a proportion of the Earth's silicate mantle were ejected into Earth orbit and coalesced to form the Moon. However, the metallic core of the impactor was not ejected into orbit but instead fell into the main body of the Earth. This impacting core material, in some models, is the 'wedding ring' of metals deposited into the Earth's silicate mantle after collision and subsequently recycled into workable ore deposits by plate tectonic processes over geological time. Without this gift of metals, the so called 'late veneer', it is very unlikely that a technological civilization could have developed on the Earth.

Maths, Art and Eclipses

The Moon has also encouraged the development of intelligence in less quantifiable ways. What did primitive man make of the luminous orb that lit the night sky for half a month and changed its phase and brightness over 28 days? Did the regular procession of the lunar cycle combined with the wanderings of the planets and the stately progression of the stars and Sun with the seasons underpin and prompt thinking about the nature of the Universe? Certainly the earliest astronomers observed and calculated calendars based on those observations and planned their agriculture accordingly. The lore, mythology and literature of the Moon, from illuminating star-crossed lovers to turning men into werewolves, permeates human culture and society. When Galileo turned his telescope towards the Moon and recognized mountains, craters and 'seas', did this not spur humankind into thinking about the 'plurality of worlds'? In a perhaps more direct fashion, the Moon has spurred technological development. Clearly the 'Race to the Moon' which, arguably, helped prompt some of the most rapid advances in tracking, propulsion, electronics, life support and other high-technology industries as well as transform humanity's view of itself, could not have happened without a Moon!

Perhaps the most beautiful and eloquent symbol of our dependence on the Moon is that of the total solar eclipse, that chance coincidence of distance and angular size which not only allows us to see and understand the true extent of our star but also allowed the first observational confirmation of Albert Einstein's theories, which transformed our understanding of the Universe.

The conclusion reached is that the Moon, itself born in a unique and random event, has been essential for the emergence of intelligent life on Earth and as a result such intelligence is probably a very rare occurrence itself.

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Author: Paul J. Henney

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