What If Earth Became Tidally Locked?

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There’s a reason we only ever see one side of the Moon. It’s tidally locked to the Earth, presenting only one side to us as it orbits around the planet. Tidal locking is a fate that befalls lots of planetary bodies, and it can wreak havoc on the surface.

Why does tidal locking happen? And more importantly, why hasn’t Earth become a tidally locked planet? And are we doomed to go that way eventually?

When the planet Zarmina was first reported as having been discovered, people got all excited about the idea of a planet existing in its star’s habitable zone — only to have their excitement fade a little when they learned that Zarmina was tidally locked to its star. This reduced the chances of life, in any complex form, existing on the surface. Tidal locking does a number on a planet, and not just its surface temperature. Everything from water composition to geography changes as one side starts getting all the sunlight, and the other slowly freezes.

How Tidal Locking Happens

When a planet orbits a star, it is being pulled by the gravity of that star. The different sides of the planet are pulled to different degrees, with the side closest to the star receiving a small but noticeably larger pull. This bends the planet out of shape, from a ball into an ellipse. No water is necessary for this to happen. Even solid rock stretched out — the surfaces of both the Earth and the Moon stretch toward each other. This stretching doesn’t happen immediately, though. It takes time for the planet to stretch its solid mass towards the sun and to settle back, and while it is stretching and settling, it is moving.

At first, it is moving in two different ways. It is rotating on its axis, the way the Earth does to produce night and day. It is also orbiting the star, as the Earth does to produce a year. Those two movements rarely sync up. For example, sometimes the rotation speeds past the orbit. In that case, instead of the bulges in the ellipse “pointing” directly at and away from the star, they turn past it.

Written By: io9
continue to source article at news.discovery.com

12 COMMENTS

  1. In reply to #1 by OHooligan:

    That seemed extremely badly explained. Anyone care to do a better job of it?

    Wikipedia is always a good place to start: http://en.wikipedia.org/wiki/Tidal_locking They even include the maths (unfortunately only the answer, not the derivation). Based on that, I estimate Earth will need another 30 billion years to become tidally locked to the Sun – time that it doesn’t have. Unfortunately, there’s a quantity called Q in the formula we don’t know every accurately, so I had to assume that its value was “typical”.

  2. In reply to #1 by OHooligan:

    That seemed extremely badly explained. Anyone care to do a better job of it?

    It needs more detail on conservation of angular momentum transferring energy because of tidal drag.

    Jos Gibbons – Wikipedia is always a good place to start: http://en.wikipedia.org/wiki/Tidal _locking

    There are different energy transfers from the effects of conservation of angular momentum

    As explained at the OP, gravity deforms a rotating sphere into an ovoid, by producing bulges on the sides facing and opposing the gravity, of the moon/planet/star causing the deformation. These bulges ripple through the planet/moon’s crust causing frictional heating, as energy is transferred between the two bodies. Energy can neither be created nor destroyed, so it is the kinetic energy of rotation which is converted into heat. This slows the rotation of the body (ies).

    The early Earth Moon system had both bodies spinning. The Moon has been slowed to near sychonicity, ( ie tidally locked) while the Earth’s day-length has been increased as it’s spin slowed from about 6 hours to 24 hours.

    Energy is at the same time, being transferred through tidal drag, moving our Moon into a higher orbit (confirmed by the Apollo laser reflectors) making months longer.

    Mercury has a strange eccentric orbit as it slows to approach sychonicity with the Sun. Its day is longer than its year. It is also incapable of retaining a moon, because of its proximity to the Sun.

    The Earth’s spin is also being reduced by tidal drag from the Sun. It is a combination of the gravity of the Sun and Moon which gives us monthly higher/lower, “Spring Tides” .

    There is a further gravitational effect of tidal drag, which is seen in the moons of Jupiter causing heating and cryo-volcanism. If an orbit is elliptical, the gravitational force increases or reduces as the moon becomes nearer or more distant from the planet. This also flexes it’s crust as the gravitational force increases and decreases with the position in the orbit.

    The interactions of the gravity of multiple moons/planets, can circularise (or destabilise) orbits, and move moons into Orbital-resonance – Examples are the 1:2:4 resonance of Jupiter’s moons Ganymede, Europa and Io, and the 2:3 resonance between Pluto and Neptune.

    These features are complicated, but hopefully I have separated them to avoid confusing different effects.

  3. In reply to #1 by OHooligan:

    That seemed extremely badly explained. Anyone care to do a better job of it?

    It does seem to lack details on conservation of angular momentum.

    Jos Gibbons – Wikipedia is always a good place to start: http://en.wikipedia.org/wiki/Tidal _locking

    As the OP explains, when a planet or moon is affected by the gravity of another body, it is deformed from a sphere into an ovoid, with bulges on the sides facing towards and away from the gravity source.

    If it is rotating, this produces a ripple in the crust as it revolves. This friction converts rotational or orbital energy into heat, slowing the rate of rotation.

    Earth’s Moon has been slowed to almost reach sychonicity (tidal-locking) so only rocks slightly. The Earth’s rotation has also slowed from about a 6 hour day to a 24 hour day as a result of tidal drag.

    The tidal drag, also transfers rotational energy from the Earth to lift the Moon into a higher orbit – as is confirmed by the Apollo laser reflector experiment. Days and months are getting longer!

    The Earth is also affected by tidal drag from the Sun, which is why we have extra high/low “Spring Tides”, when the Sun’s and Moon’s gravity combines in the same direction.

    If a planet/moon ( even a tidally locked one) is in an elliptical orbit, the gravity deforming its crust is greater when it is nearer its parent star/planet so it is heated by friction in an energy transfer from the parent planet/star. – In some examples causing cryo-volcanism.

    The gravity of planets or moons, can also interact to produce orbital resonance.

    Under some circumstances, a resonant system can be stable and self-correcting, so that the bodies remain in resonance. Examples are the 1:2:4 resonance of Jupiter’s moons Ganymede, Europa and Io, and the 2:3 resonance between Pluto and Neptune.

    The spin of the planet Mercury has slowed so much (as it nears sychonicity), that its day is longer than its year. Because of its proximity to the Sun it is also incapable of retaining a moon.

    • I hope this has separated some of the gravity related issues to make things clearer!
  4. What If Earth Became Tidally Locked?

    The oceans on sun-ward side would boil, and on the night side would freeze.
    There might be a bit of light and dark around the poles of a tidally locked Earth, if it wobbled on its axis.

    This is unlikely to happen before the Sun turns into a Red Giant and fries or absorbs the Earth in a few billion years time.

  5. Wont earth-moon become tidally locked? And wont that happen first? Maybe that is where the 30 billion years come in. Can earth really survive 30 billion years as the sun bloats into a red giant that may consume it? How could life even survive a close encounter?

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