Empty sky, empty earth? – If there was no moon

WAXING AND WANING from invisible to full-beam full and back, month after month, the Moon is notoriously fickle. But appearances deceive. Its appearance in the sky can change; the brutal fact of 73,000 billion tons of rocks in orbit at a distance of about 380,000 km is not. All humans with eyes to see have seen. It has raised tides in the Earth's oceans since the formation of the oceans.

The moon is old. But this is not necessary. Look at the other rocky planets of the Sun. Venus, the one closest to the Earth, has no moon. Neither little Mercury. Mars has a couple, but they are irritable. The Moon has more than 1% of the Earth's mbad; March overrides Deimos, the largest of its moons, more than 10m to 1.

is, scientists believe, an accident of history – the result of a collision, and incredibly violent, between Earth and another young planet, perhaps on the size of Mars , shortly after the formation of the solar system. And some think that it could have been a happy accident: perhaps, in a rather literal, vital sense.

Scientists have long wondered what aspects of the earth made it a suitable cradle for life and, 4 billion years later, intelligence. It's a question that influences their way of thinking about the rest of the cosmos. If there is nothing special about the Earth, then it seems reasonable to bet that life, and perhaps even civilizations, are quite common elsewhere in the universe. But if the Earth is a spatial singularity, life can be rare, even unique.

Donald Brownlee, a Palaeontologist, in "Rare Earth", a book published in 2000. They argued that, although microbial life can grow quite easily on all kinds of planets, the evolution of life complex depended on various aspects of the Earth and the solar system. so. One of these aspects was the unlikely existence of a large moon.

The idea that the Moon might have relevance for life beyond providing a nighttime illumination was not new. Some have argued that without the Moon, the Earth would have a stifling atmosphere like that of Venus. Fortunately, this theory suggests that the impact that formed the Moon has been helpful in helping the volatile chemicals that formed the Earth's original Venus atmosphere, allowing a much thinner gas layer to form. in its place. Others have suggested that the lunar tides – which were much more pronounced on the primitive Earth, because the moon was then much closer – were a prerequisite for the origin of life. By occasionally sinking seawater into the tidal pools, from which it would then evaporate, they would have provided a means of concentrating the chemicals from which life came into being. developed. This is not an idea that many people take seriously right now: smart money on the origin of life places it today in the deep hydrothermal vents of the oceans, not in the tide pools. But ideas on the subject have already changed and may change again. Hypotheses, like tidal pools, come and go.

All of these ideas are intriguing, but hardly compelling. Dr. Brownlee and Dr. Ward have instead opted for another lunar effect: a dampening of the Earth's oscillation. The planets do not sit directly in their orbits: they lean over. The axis of rotation of the Earth is currently at an angle of 23.4 ° to the vertical, as measured from the plane in which the planet rotates. The planet is slowly sitting upright; but once it reaches about 22.1º, in just over 10,000 years, it will begin to lean again. Its obliquity oscillates from 22.1º to 24.5º regularly every 41 000 years. The effect of this nodding on the seasons of the planet is one of the things that determine the rhythm of the ice ages.

On moonless Mars, things are very different; the skew moves both more and more in a chaotic way. Sometimes Mars stands upright, with no seasons to mention. During the eons, its obliqueness can reach 60º, a situation in which the inhabitants, if any, would know the midnight sun far in the tropics, producing intolerably hot hyper-seasons. In the 1990s, Jacques Laskar, one of the astronomers who discovered the role of chaos in the seemingly stable solar system, showed that the difference between the gentle shaking of the Earth's head and the wild Mars's oscillations could be explained by the moon. A constant lunar tug on the equatorial bulge of the Earth – a painful distortion of the sphericity of the planet caused by its spin – keeps things in order. Move the Moon away and the obliquity of the Earth becomes even less stable than that of Mars, oscillating up to 85º. Such episodes pointing to the sun would make uninhabitable continents at presently temperate latitudes. In "Rare Earth", Dr. Ward and Dr. Brownlee argued that the double whammy of sometimes very high obliquities, which could also change dramatically, would mean that a moonless Earth would have such a chaotic and disaster-prone climate. A complex life would be hard

Subsequently, however, the story came to light a little more complex. The way in which the obliquity of a planet is chaotic depends on its speed of rotation. It turns out that the Earth and Mars have days of very similar lengths, which is why, if it were not for the Moon, the axis of the Earth would move forward and backward. backward as Mars does. But while Mars may have had something like a 24-hour day for almost all of its history, the Earth does not have – because of the moon. Immediately after the collision that created the Moon, the Earth shot much faster than today. Since then, it has been slowing down again, thanks, once again, to the Moon. The tides it lifts into the oceans act as a slight brake to the Earth's rotation.

So yes, without the Moon, an Earth with 24-hour days would undergo radical chaotic changes in its obliquity. Yet no one has any idea of ​​what would be the period of rotation of the Earth if it had not been beaten strong enough to form the Moon in the first place.

But there are other ways to demonstrate that complex life is much more likely on a planet with a big moon. David Waltham, a British geophysicist, suggests in his book "Lucky Planet", published in 2014, that complex life requires both a fairly stable obliquity and a fairly long day. On planets with significantly shorter days, he argues, the heat transfer from the equator to the poles would be less effective. The winds and currents responsible for this transfer are diverted from the direct trajectory from the equator to the pole that you might expect from the Coriolis effect, which shifts them to the east. The faster a planet rotates, the stronger the effect, and the harder it will be to warm the poles. Dr. Waltham argues that the Moon is just the right size to allow the Earth to have a stable obliquity and poles hot enough to keep ice ages relatively minor. It's a cunning argument, but not convincing.

It may be that making significant progress on the issue of the importance of the Moon for life will have to wait for planets inhabited by complex life forms to be discovered around other stars. If there are many terrestrial planets that do not boast of complex life or large moons, the idea can be said to have some merit, as if those who were looking for signs of life. extraterrestrial intelligence found such signals from giant planets

. While the world is waiting for such discoveries – an expectation that will likely be measured in decades or even centuries – it is worth keeping in mind an intriguing argument from Isaac Asimov, a science fiction writer, in "The Tragedy of the Moon", an essay published in 1972. He argues that rather than being crucial for the development of complex life, and therefore humans, the moon could be a factor in their downfall.

Asimov claimed that if humans had evolved on a moonless planet, they might never have imagined that their home was the center of the universe. The fact that the moon really turns around the Earth, and that we can show that it does so, made it natural to badume that all the other things in the sky do too. Even real appearances deceive when they lead to false generalizations. But the easiest way to explain the different movements of the different planets is that the Earth and everything else gravitate around the Sun. According to Asimov, if there was no Moon in orbit around the Earth, this idea centered on the Sun would have been much easier to accept. Thus, a heliocentric solar system might well have been a constant of astronomical thought of Babylon and clbadical Greece

That would not have simply meant that the world would have been better explained earlier (although Astimov, still the pedagogue, who was indeed a good in itself). Civilizations for which the Earth was not the center of the universe could, he suggested with a kind of atheistic piety, develop less the kind of monotheism that sees humans as the center of the world. Universe and the summit of creation. Such broader perspectives, he continued in an eco-dark way in the 1970s, could have led civilizations to be less suspicious of their environment and less likely to think that their way of seeing the world deserved to be preserved even if it brought them to nuclear war. Such civilizations, he imagined, could last longer than his own.

Barely a stunning argument. But it seems useful to warn against the arguments that seek to make the Moon important in terms of evolution by emphasizing how humans tend to think of themselves and their planet as special. And that also suggests a new prediction. If humans come into contact with extraterrestrials who have evolved under their own moon, they will also tell all kinds of wonderful stories about its meaning. Whether they are real or not, who knows?