We only see one side of the moon, because it is tidally locked to the planet Earth (tidal locking the situation when an object’s orbital period matches its rotational period). What if the Earth was tidally locked to the Sun?
In the video below, published by “What If” channel and presented by Presented by INSH and Hashem al Ghaili, you can find answers to that question.
What if one side of the Earth always faced the Sun, while the other side was forever shrouded in darkness? Just how hot would it be on the bright side? And how cold would the dark one get? What part of the planet would still be able to sustain life? And will this be the eventual fate of the Earth? Here’s what would happen if the Earth was tidally locked to the Sun.
Is this scenario likely?
No, there’s not enough time. Our planet will be gone long before tidally locking to the Sun. In general, closer objects are more likely to experience tidal locking. If the Earth was too close to the Sun, the answer might be yes – but in this case, there would be no life on it.
Far-away objects are less likely to experience dramatic differences in gravity between their two sides, resulting in smaller bulges, and the bulges themselves will feel less of a pull. For many stars, the habitable zone – the ring of space within which planets are able to sustain life – overlaps partially with a zone that makes planets likely to be tidally locked to their star, making them significantly less habitable – but it appears that such a fate is not in store for the Earth.
Earth will never be tidally locked to the Sun, though. Image: NASA – ISS013-E-78960 (9 Sept. 2006) – A rising sun and Earth’s horizon are featured in this image photographed by an Expedition 13 crew member on the International Space Station.
Tidal Locking
Tidal locking results in the Moon rotating about its axis in about the same time it takes to orbit Earth. Except for libration effects, this results in the Moon keeping the same face turned toward Earth, as seen in the figure on the left. (The Moon is shown in polar view, and is not drawn to scale.) If the Moon were not rotating at all, it would alternately show its near and far sides to Earth, while moving around Earth in orbit, as shown in the figure on the right. By Stigmatella aurantiaca – Own work, CC BY-SA 3.0, Link
The Moon is tidally locked to the Earth, which means that it always shows one face to our planet. In fact, this is the case for the most large moons in the Solar System. What’s the process going on to make this happen?
Life on a tidally locked planet
Atmospheric system of a tidally‐locked planet
On a tidally-locked planet, a single region is consistently close to the star. Known as the substellar point, this region continuously receives more direct sunlight, and thus more heat. Water on one side may be in a vapor form while on the other side it may be frozen into ice. Strong constant heating of a planet on one side can change or even control how much weathering occurs on the planet, which can lead to significant and even unstable climate changes. These dramatic climate effects could make a planet that otherwise has the potential for life to instead be uninhabitable. An unstable climate on a tidally-locked planet could create a runaway greenhouse effect that could result in an atmosphere like that of Venus.
And it’s not just the extreme temperatures. Permanently devoid of the heat of the star, the atmosphere on the dark side would first turn into a denser gas, then condense into a liquid, and then perhaps further condense into solid form. Meanwhile, air that is constantly exposed to light – or that is heated by a ground that is constantly exposed to light – will heat up and expand. Although it is doubtful whether the atmosphere on the dark side of the planet would get to solid form, it would certainly keep condensing and leaving a vacuum to suck in the expanding hot air from the other side. This might make for circulation of atmosphere that would make the planet liveable, but it will also lead to hellish storms, as the atmosphere from the light and dark side of the planet essentially switched sides continually. The presence of an atmosphere can help distribute the heat across the planet, equalizing the temperatures. But tidal locking could result in wide climate variations, a result that could threaten the evolution of life on the surface of these planets.
The far side of the planet would be frigid since it would never see the star. Its only source of warmth would be winds from the warmer half of the planet. If there was no heat transport from the hot side to the cold side then we could expect the temperatures to be below −100 °C on the cold side and more than 100 °C on the hot side. However, if the planet has an atmosphere then it will transport heat from the hot side to the cold side, and this might make the temperature difference much more moderate.
Perhaps some of the water could be found in a liquid state near the boundary between the hot and cold regions and one could expect some kind of water cycle, with something like glaciers being continually melted by the warm air blowing in from the hot side, with the melted water flowing in gigantic rivers to the hot side, where it evaporates and cycles back around to fall as snow on the cold side.
Could life be sustained on a tidally‐locked planet?
Consider what life would be like for those living on a planet tidally locked to its star where while one side is permanently bright and boiling hot, the other side is permanently dark and freezing cold. The lit side of the planet will be stripped of its oceans and made to face burning star and scrubbing hot winds all the time, while the dark side will be covered with frozen oceans and biting cold winds howling all the time. Three would never be any relief from the extreme temperatures so it may be difficult to sustain life on such an inhospitable world. There would be zones of different climates, in concentric rings, depending upon how far away from the sub-stellar point. In the center there would probably be scorching hot deserts but farther away from the sub-stellar point, as the star would get lower in the sky, there would be gradually cooler climates. Different places on the planet would be confined to either day or night, or even dusk at regions in between. However, at a given place any change from day to night or vice versa will not occur.
Life, if it manages to struggle along on such a planet, will be very hard or perhaps be underground. More likely a circular belt between the two sides – a sort of “twilight zone” – could be the place for life to evolve and flourish. In this dusk band around the planet, where star will be permanently hanging very low near the horizon or perhaps the stellar disc partially peeking above the horizon, with an ever-colourful red, yellow sky due to scattered light, the temperatures would be more moderate, right in between the hot and cold sides. However the heat on one side would cause the air to rise, creating a low pressure system, while the cold on the other side would cause the air to sink, creating a high pressure system. This would cause the planet to experience a constant and violent circulation of air, or, essentially a planet-wide hurricane. The constant air circulation would actually circulate the temperatures extensively and extremes in temperature would mitigate. Water cycles with huge rivers crossing from cold to hot side might make living there possible.
One important issue is the concept of time. With no day-night cycles, concept of time will be difficult to come. On Earth right from birth, we notice that many phenomena in nature are repetitive. This is due to our most basic natural clock, viz. the rotation of the Earth, causing the rising and setting of the Sun, giving rise to alternative periods of light and darkness. All human and animal life has evolved accordingly, keeping awake during the day-light but sleeping through the dark night. Even plants follow a daily rhythm. Of course some crafty beings have turned nocturnal to take advantage of the darkness, e.g., the beasts of prey, blood-sucking mosquitoes, thieves and burglars, and of course astronomers.
At least there might be no astronomers on a tidally-locked planet, as starry sky may not be known (except for some rumours about it by adventurer fellows daring to venture deeper into darker side of the planet). Secrets of the Universe − planets, stars, Milky-way, other galaxies − all these might remain very difficult, if not outright impossible, to unravel. Just imagine, it took us humans thousands of year to figure out that a few “wandering stars” are heavenly bodies (planets) in just our neighbourhood and all this happened in spite of the fact that a starry sky is daily visible most places for about half of the time (night). How will the inhabitants of a tidally locked planet ever know about it when their everlasting “hot summer afternoons” never turn into cool evenings and dark nights?
