Earth is pretty special.
We know of no other planet that is able to match the set of characteristics that make Earth habitable.
In this video we are going to explore the conditions that allowed for the development of complex life on Earth.
Whoa, wait a sec. What do you mean by “complex” life?
For example, things like multi-cellular organisms; what we would typically call animals today.
We are going to look at a set of eight different conditions that contribute to life on Earth.
Let’s start with an obvious one.
Earth is just the right distance from the sun to ensure temperatures
that result in liquid water over most of the planet’s surface.
Venus is closer to the sun and its original water has evaporated.
In contrast, any water that remains on Mars is present as ice.
So, if Earth had been located just a little closer to the sun or a little farther away, we wouldn’t be talking today?
That’s it.
Just a few percent either way would mean no water.
Remember, most organisms need water for their basic cellular processes.
And the first life on Earth lived in the oceans.
And...
the average adult is about 60% water.
Condition number 2 is that Earth orbits the right type of star.
More than 90% of stars are actually smaller than our sun and would provide less energy.
In contrast, larger stars have a much shorter lifetime,
maybe just a few million years, not nearly long enough to support the evolution of complex life.
Condition number 3 is related to our moon.
The Moon stabilizes the angle of tilt of Earth’s axis, ensuring that it varies within a range of just a few degrees.
So, without the Moon, Earth’s axis would have wobbled all over the place?
Just how weird could things get?
Well, if it increased to more than 50 degrees, there would have been freezing
temperatures along the equator and tropical conditions at the Poles.
And if the tilt was moving around that much,
there may not have been the stable environments necessary to support the development of complex life.
Condition number 4 is about Earth’s mass.
The presence of an atmosphere is essential to protecting life
and Earth is large enough to generate sufficient gravity to hold
a blanket of gases close to its surface.
While the atmosphere is thin compared to the size of the planet, it is thick enough to block harmful solar radiation
and cause most incoming asteroids and comets to burn up before reaching the planet’s surface.
Internally, Earth can be divided into three compositional layers,
the crust, mantle and core.
Earths deep hot interior is over 6000 degrees Celsius.
This internal heat is responsible for the next two conditions that support life:
plate tectonics and a strong magnetic field.
Condition number five is related to the fact that the outermost layer of Earth, is divided into rigid tectonic plates.
These plates ride on top of giant convection cells
where hot material from Earth’s interior rises through the mantle
toward the surface before cooling and descending.
The convection cells are slowly moving the plates across Earth’s surface,
causing the opening and closing of ocean basins and
building up mountain ranges and lines of volcanic islands.
The range of physical environments that result supports a great diversity of life
that has allowed the biosphere to bounce back from several major extinction events in Earth’s history.
Condition number 6 and the second way that Earth’s heat engine supports life
is through the action of the magnetic field.
We see evidence of Earth’s magnetic field where charged particles from the sun
generate the brightly colored lights that form aurora around the Poles.
The magnetic field is produced by convection currents in the hot, molten metallic outer core
due to the daily rotation of the planet.
Without the magnetic field, the solar wind would strip away our atmosphere exposing us to
harmful radiation and removing our protection from collisions with near Earth objects.
It’s not just the presence of the atmosphere that is important,
as we will see in condition number 7,
the composition of the atmosphere also plays a significant role in supporting complex life on Earth.
Among other gases, Earth’s ancient atmosphere contained higher levels of
like carbon dioxide and methane but no oxygen.
Microbes living in Earth’s early oceans, gradually consumed much of the
carbon dioxide and other gases and produced the oxygen, nitrogen and trace gases of the air we breathe today.
For the final condition, let’s look more closely at these trace gases.
In particular, we are interested in the small fraction of gases made up of
carbon dioxide and other greenhouse gases.
People often overestimate how much carbon dioxide is present in our air.
It’s just about 0.04% of the atmosphere,
but that tiny fraction is vitally important to regulating our climate and supporting life on Earth.
The greenhouse gases absorb just enough heat to maintain an average global temperature of
15 degrees Celsius or 59 degree Fahrenheit.
This is what we refer to as the greenhouse effect.
Without this heat trapping blanket of gases in the lower atmosphere,
temperatures on Earth would be a frigid -18 degrees Celsius.
Wait a second, how does that work?
Well, the solar radiation that reaches the surface heats the land and oceans
and infrared radiation is reflected back toward space.
Some of this infrared radiation escapes
but part of it is trapped and some of that is emitted back down toward the surface
from carbon dioxide, water vapor and other greenhouse gases.
Otherwise, all of this heat would be lost and Earth would be a frozen rock in space.
So to recap, there are at least 8 characteristics that contribute to the development of complex life on Earth.
Four of these are related to Earth’s position in space and its relationship with the sun and moon,
two are related to the hot interior of the planet,
and another couple are tied to the composition of Earth’s atmosphere.
So thats it for us
Here are the learning objectives for today
How confident are you that you could successfully respond to each objective?