Download What If Earth Became Tidally Locked?

What If Earth Became
Tidally Locked?
FEB 2, 2013
IO9.COM
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.
The problem is, the near bulge is closer to the
star than the rest of the planet, and it feels a
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.
gravitational pull dragging it backwards — so
it's once again aligned with the center of the
star. It doesn't necessarily get pulled all the way
back, but it gets shifted a little bit. That shift
happens every time the planet rotates. If the
rotation is too slow and the orbit is fast, the
bulge lags behind as the planet orbits forward,
and the gravitational pull of the star drags it
forward. No matter what, the planet gets a tug
until its rotation is exactly the same period of
time as its orbit. When that happens, it's tidally
locked. It shows one face to the sun at all times.
Life on a Tidally Locked Planet
The immediate disadvantage of a tidally locked
planet is obvious. One side of the
planet cooks while the other
freezes. Water on one side is
vapor, and on the other side is ice.
If there is any appreciable amount
of life on the surface of the planet,
it has to be in the twilight strip of
land between the two halves.
But it's not as simple as getting the
temperature right. If our
atmosphere permanently lost the
heat of the sun, it would first turn
into a denser gas, then condense
into a liquid, and then further
condense into solid ice.
Although it's doubtful that 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
livable, but it would also lead to hellish storms,
as the atmosphere from the light and dark side
of the planet essentially switched sides
continually.
And those winds may bring some very, very
nasty things with them. Even geologists foresee
major problems with tidally locked planets.
Rock and soil erodes differently when it's
exposed to different levels of light. The cool
side of the planet is preserved fairly well, but
the lit side of the planet is stripped of its oceans
and made to face burning sun and scrubbing
wind every day. It will erode faster, and rocks
that might have turned to
terrestrial sand in a climate with
night and day may be vaporized,
picked up by the wind, or dissolved
in water vapor to go airborne. Life,
if it manages to struggle along on
such a planet, will either be
underground or very, very hardy.
So why are some planets and
moons tidally locked while others
are not? All planets bulge towards
their stars, and all of them have
their orbit slightly out of sync with their
rotation. The mechanics of how it happens are
the same in every case — but whether tidal
locking actually happens depends on things like
orbit distance, the mass of both bodies, and the
malleability of the orbiting object.
Generally, closer objects are more likely to
experience tidal locking. 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.
Nervous scientists have speculated that the
Earth might eventually be a tidally locked
planet, but it appears that such a fate is not in
store for us. At least not with the sun. The
Moon, which we've already ensnared, might
turn the tables on us. The Earth's rotation
actually gets slowed down by the Moon a little
bit each year.
There are many factors involved in calculating
how slow we'll get. For example, we have to
consider whether the Earth will continue
slowing at a steady rate, the fact that the Moon
drifts a little away from us each year, and the
question of whether or not the sun will
eventually swallow both the Moon and the
Earth when it goes red giant. But someday, the
Earth might be tidally locked to the Moon. In
that case, only half of the Earth will ever see the
Moon. So think of how lucky you are to see it
nearly every night, even though because it's
tidally locked you can only see half of it.