Without the Moon, the Earth would be a dead planet
The role of our satellite in the existence of the Earth and its inhabitants is much more
important than we think. The Earth is linked to the Moon by a very close relationship, by a bond
that begins from the moment of their formation. When the Earth was still a newborn, just a few
tens of millions of years old, the surrounding space was full of insidious objects of various
sizes, from small pieces of rock up to very massive bodies, which scampered here and there
attracted by the Sun and the various planets in formation. Collisions were therefore very
frequent, sometimes destined to drastically change the appearance and size of the bodies involved.
It happened then that a large protoplanet of the mass of Mars, then named Theia,
instead of choosing a quiet orbit around the Sun ended up falling over the young Earth.
Theia hit at a speed of a few kilometers per second... The impact was inelastic, with the
almost total transformation of kinetic energy into heat and the ferrous core of the planetoid sank
into the Earth's mantle until it merged with the Earth's core. At the same time, the portion of the
mantle of Theia farthest from the Earth was spared from the impact and continued on its way, but
perturbed by Earth's gravity changed direction and entered the orbit of our planet, where powerful
tidal forces broke it into minute fragments. In a few hours, eighty percent of Theia was completely
assimilated, while the remaining fraction went to form a disk of debris around our planet.
The energy of the collision had generated a frightening amount of heat,
so that the disc was eventually composed of gas, small particles and molten rock:
almost all material from the Earth's mantle, which is notoriously poor in iron.
Other light elements evaporated and were lost in space, leaving behind rocks apparently
devoid of volatile substances but which, as the Apollo missions later demonstrated,
still retained minimal traces of water. After that. the disk began to radiate
heat into space, cooling. In a few decades, the particles solidified, so that the Earth found
itself surrounded by a ring of rocks that extended up to 30,000 km from its center.
The rock particles began to collide and aggregate, becoming larger and larger, but the conditions
for forming aggregates were only optimal where the tidal force of our planet was weakest, i.e. beyond
the so-called "Roche Limit", located about three Earth radii away. The solid matter located beyond
that limit quickly began to form larger and larger planetoids, which then, again by coalescence,
in a few thousand years ended up giving shape and mass to a single celestial body: the Moon.
If not a sister, then, the Moon is definitely a half-sister of our planet! But to understand
if and how much this half-sister has influenced the evolution of the Earth,
we will have to perform a little mental experiment: to imagine its hypothetical absence.
What would become of our world if the Moon was not real, or better, did not exist at all?
After all, it would be enough to remove the initial brick, to see collapse and then disappear
all the construction on which the history of the Earth-Moon system has been built... If Theia,
4.5 billion years ago had never hit the Earth, if it had only grazed it, how would things have gone?
Well, the first consequence is easily imaginable: if the Moon had never formed,
the Earth would have continued to rotate on itself with the speed it had 4.5 billion years ago,
when its day did not last more than 4 or 5 hours. It was the Moon itself, with the gigantic tides it
produced, that slowed down the Earth's rotational motion. How? Well, simply with the friction that
the tides produced on the seabed, moving in the opposite direction to the rotation!
Would such a fast rotation have been compatible with the onset of life?
Maybe yes, life would have made it anyway, but if at this point I told
you that such a rotation would have favored the formation of winds that
would have traveled the Earth at a speed of at least three hundred kilometers per hour...
would you still be convinced that some form of complex life could have formed?
The fact is that without the Moon (which in those times was ten times closer to the Earth
than today), instead of being shaken by giant tides, our oceans would have been "caressed"
only by the very weak tides generated by the gravitational force of the Sun. Absolutely not
capable of stopping the rotation and bringing the length of the day to its current length.
Nor can they perform another important function,
that of vigorously stirring the water of the oceans... a condition that biologists consider
necessary for the first cells to appear on Earth to receive the right combination of elements
with which to build themselves. In fact, it is believed that the tides
have played a very important role in promoting and protecting the birth of life on our planet,
and that life can develop better and faster in regions with critical environmental conditions,
able to stimulate genetic selection. In practice, as biologists like to repeat: weak tides,
slow biological evolution... strong tides, explosive biological evolution!
It is also well known that the inclination of the rotation axis of a planet with respect
to its orbital plane is a factor of utmost importance for long-term climate stability.
This inclination for the Earth is currently 23.44 degrees, a decidedly optimal value,
but over time it could be strongly affected by the gravitational perturbations of the other planets.
Fortunately for us, the presence of such a massive Moon can confine this variation within very narrow
limits: just over two degrees. The extreme values are approximately 22.1 and 24.5 degrees
over the course of a 41 thousand year cycle! But even such a small change can heavily
influence climate. A greater tilt causes the poles to point alternately in the direction of
the Sun and makes for more extreme seasons, with hotter summers and colder winters.
For comparison, Mars' polar tilt, which is currently, and quite coincidentally, similar
to Earth's, varies chaotically between 0 and 60 degrees on a time scale of about 10 million years.
Such a wide range is capable of producing incredible climate changes, hampering any
attempt by life to take root. The consequences for the red planet have been in fact dramatic,
having suffered over time the cancellation of entire ecosystems and total desertification.
The Moon, therefore, plays an important role in stabilizing the obliquity of our
planet by providing the Earth with long-term climate stability. Without
the Moon and with the current 24-hour rotation period, the Earth's obliquity
would vary chaotically from 0 to 85 degrees on time scales of tens of millions of years;
most likely our planet would not have been able to enjoy the climatic continuity necessary for
the emergence of complex life forms, and we would not be here now to talk about it.
But the list of "misfortunes" that would have fallen on our planet
if there had not been the Moon does not end here! Without the presence of a large and massive
natural satellite, able to attract on itself a good part of large and small asteroids,
the cosmic bombardment suffered by the Earth in certain periods of its formation would have
been much more intense. In short, the Moon has done and still does as a shield to our planet,
just as Jupiter protects the inner solar system from the impacts of comets from the Kuiper Belt.
In addition, many astronomers think that certain processes of renewal of the Earth's crust,
such as plate tectonics and the consequent drift of the continents, would not have
been activated without the tidal influence of the Moon. If tectonics didn't force the
rock plates to plunge under each other and then back up again, in a process called subduction,
the seafloor would have remained completely barren and devoid of interesting chemistry, meaning first
and foremost that life at the microbial level would have had much more difficulty
establishing itself, while life above would have been completely prevented from doing so.
And again... even the existence of a magnetic field must say "thanks" to the gravitational
influence of the Moon... The Earth's magnetic field, which originates more than three thousand
kilometers below our feet, extends for tens of thousands of kilometers around us.
We can imagine it as a shell that surrounds our planet and we can represent it through lines,
called lines of force, that go out from the South Pole of the Earth and return to the North Pole.
The magnetic field is an essential component for the development of life on this planet.
Such a magnetic bubble surrounds the Earth and constantly protects it from the flow of
particles coming from the Sun. Our star is in fact an immense thermonuclear furnace,
which continuously emits into space huge amounts of ionized solar plasma: the so-called solar wind.
After a few days of travel, the solar wind reaches the Earth... and the impact deforms the Earth's
magnetic field confining it in a region, called magnetosphere, that manages to divert it and
keep it away from our planet, making it flow like water from a river around the pillar of a bridge.
Which is good, because the flow of high-energy particles would be able to damage the DNA of
living things and "blow away" our atmosphere. Now, at the origin of the Earth's magnetic field
is almost certain that there is the so-called geodynamics: the action produced by the rapid
movement of huge amounts of liquid ferrous material in the outer core of our planet.
Much more debated is instead the origin of the energy that has fed - and continues to
feed - during billions of years the geodynamics. Well, the most recent researches suggest that at
least part of the necessary energy may come from the Moon, and in particular from the deformation
of the mantle induced by tidal effects. This mechanical tension applied to the whole planet
causes strong currents in the external core, constituted by a liquid iron alloy of very low
viscosity. Currents that are sufficient to generate the Earth's magnetic field.
If we didn't have the Moon and its rotational energy exchange, the Earth's core would stop
rotating and then solidify. And with the geodynamo now "off," the planet's magnetosphere would
vanish, allowing the solar wind to completely devour the atmosphere. Without an atmosphere, any
water supply on Earth's surface would evaporate and solar radiation would turn our world into a
barren desert. Again, Mars is the perfect example of such a planetary tragedy. When it was young,
the red planet had an atmosphere and oceans, just like Earth does today. Four billion years ago,
however, its magnetic field began to weaken until it disappeared. And gradually, within
a few million years, the solar wind has taken away its atmosphere, and consequently its oceans.
An effect of gravitational forces on the magnetic field of a planet, already widely documented for
two moons of Jupiter, Io and Europa, and for a good number of extrasolar planets.
But beyond all the missed gravitational influences, the absence of the Moon,
if our species had been able to emerge, would have involved a last and perhaps
more serious consequence, this time of psychological nature.
In fact, I think we can safely say that the Moon played a key role in motivating
our species to raise its eyes to outer space and to ask the first questions about the meaning of
what was happening around it; as well as, much later, the presence of a celestial body so close
certainly helped us to imagine being able to take that first tiny step towards the stars.
If in fact we had not been enticed by the presence of a target close enough
to be reached in a few days of travel, what future would space exploration
have had? Would we have invested as much in technology? I don't think so.
In short, without the Moon, for a lot of good reasons, this world would have been
entirely different. With us or without us. And this obviously raises questions
about the requirements for a planet to succeed in producing higher life forms,
and imposes heavy statistical constraints on the number of such planets in our galaxy.
In short, the presence or absence of a large satellite is certainly a factor that must be
taken into account in directing programs to search for life among extrasolar planets.