For hundreds of years, “Are we alone in the universe?” has been the ultimate question
And while astronomers are busy searching for life beyond Earth, they’ve also started
asking another question:
If life seems so difficult to find, then why is our world so full of it?
One answer might be overhead right now: the Moon.
Moons are all over the solar system—bigger ones and smaller ones—but there’s something
unique about ours.
All of Jupiter’s moons combined are just two hundredths of a percent as massive as
A similar ratio holds for Saturn and Neptune’s moons, while the Uranian and Martian moons
are even less massive compared to their planets.
In comparison, our single moon is a whopping 1.2% of the Earth’s mass—which means its
effects aren’t exactly subtle.
In particular, the Moon’s gravity causes tides that affect huge swathes of the planet.
And while it’s hard to know anything for sure when you’re talking about billions
of years in the past, there are good reasons to suspect that those tides played a large
role in shaping life as we know it.
For one, tides likely played a key role in creating the basic conditions for life.
That’s because tides don’t just affect water.
Even though we think of tides as rising and falling ocean levels, tides actually affect
the entire surface of the Earth.
For example, New York City can rise and fall more than 35 centimeters in a day.
But while water can simply flow to its new shape every time the tides go up and down,
the Earth’s crust is full of rock that twists and grinds on itself, creating friction that
Earth already gets a lot of heat from radioactive decay in the mantle, and all of this heat
helps move around the pieces of the crust we know as tectonic plates.
As these plates move around, creating earthquakes, volcanos, and new mountains, they also release
elements critical to life, like phosphorus, copper, and zinc, that come from the Earth’s
And as old land gets pulled back into the mantle, it traps the greenhouse gas carbon
dioxide underground, which helps keep the planet cool.
What’s left is a delicate balance of temperature and nutrients that sets the stage for life
And the tides may have had a role in that next step as well.
We don’t know for sure how life got its start, but one of the most famous models describing
the origin of life is the primordial soup theory.
In this scenario, Earth’s early oceans were full of the basic building blocks of life,
like amino acids.
Under just the right circumstances, a very lucky combination of these ingredients could
have created the first life.
In particular, that cocktail would have had to include one important ingredient—a way
of copying itself, or making more life from life.
Today’s replicating molecules are DNA and RNA, and the backbone of these structures
is made of phosphate.
To make copies of genetic information, they have to come together and separate—a process
that’s sometimes described as zipping and unzipping.
Early life likely had replicating molecules that worked similarly and were also made of
The catch is, in normal, low-salt ocean water, phosphates repel each other and block one
strand of phosphates from connecting to another.
But in water with higher concentrations of salt, that effect is neutralized.
Strands of phosphates can suddenly come together and replicate.
And the ebb and flow of the tides could have helped make that possible.
As the tides wash out, seawater gets trapped in tidepools where it can start to evaporate,
leaving behind its salt and creating an extra salty environment where the miracle of molecule
replication can occur.
Once life got started — however it did — tides continued to shape its development.
Researchers debate whether tides would have helped stabilize or destabilize the Earth’s
climate over time, but the one thing they agree on is that tides definitely would have
See, while tides might seem like a simple in-out motion from any one point on the shore,
their movement around the globe is actually pretty complex.
Tidal motion helps set up enormous ocean currents that can redistribute the equator’s warm
water across the planet and have profound effects on the climate.
It’s hard to predict exactly what happened in the past because the motion of tides and
currents depends a lot on the location of the continents and the shape of the ocean
Like, just the appearance of a strip of land connecting North and South America—the Isthmus
of Panama—a few million years ago was enough to cut off circulation between the Atlantic
and Pacific Oceans and completely change the shape of ocean currents.
Because of that change, a new, Gulf Stream current started carrying warm water up north,
making Northern Europe as much as 10 degrees Celsius warmer than it used to be.
And there’s no doubt that shifts in the climate, affected by the tides, drove much
of life’s evolution.
Even today, tidal motion is increasing the melting of ice in the Arctic by carrying warmer
water up and under the sheets of sea ice.
Of course, Earth isn’t the only place that experiences tides, and tides have actually
become a signpost in the search for life.
Tidal heating has created an ocean twice the size of Earth’s under the surface of Jupiter’s
[en-SEHL-uh-duhs] And tides from Saturn inject enough heat into
its moon Enceladus to create giant geysers that shoot hundreds of kilometers into space.
Places like these—rather than dry, dusty Mars—might be our best hope for life in
the solar system.
After all, if the tides helped get life started once, maybe it could happen again.
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