[narrator] We have uncovered
a remarkable planet,
a world of extreme weather,
alien landscapes,
and bizarre phenomena.
It's the most complex system
we know of in the universe.
That planet
is Earth,
and we are only beginning
to unlock its secrets.
On planet Earth,
there's an invisible force...
Whoa!
[narrator] ...governing
everything.
[man] It gave me enough energy
to be able to go through
all the loops and twists
and turns of this amazing track.
[narrator] It's called gravity,
a phenomenon that
permeates the planet,
wielding secret power over
the continents and oceans,
plants and people.
But in a remote part
of North America,
some of it has
turned up missing,
and it could be gone forever.
If you suddenly
turn off gravity,
we would be flung
into outer space.
[second man] Without gravity,
there would be no you,
and there would be no me.
[narrator] The case of
the missing gravity
on Secrets of the Earth.
On this planet called Earth,
something strange is happening.
Gravity, a fundamental
force of nature,
is actually missing
in some places.
In fact, Canada's Hudson Bay
has lost a lot of gravity,
more than almost anywhere else
on the planet.
It is a mystery tied to one
of the most important forces
in the cosmos.
[man] Gravity is the glue that
holds the universe together.
Gravity is assisting us
in--in all of the actions
and things that we do
as human beings.
Without gravity,
there would be no galaxies,
there would be no stars,
there would be no planets.
There would be no you
and no me.
[narrator]
For over a half-century,
scientists on Earth
have been on a mission
to solve the case of
the missing gravity
and what secrets
it might reveal
about the planet.
It is a quest
that has taken them
to one of the most
remote outposts
along the Hudson Bay,
Churchill, Manitoba.
No roads lead here.
The only way in is by train
or small aircraft.
The subarctic community
of only 600 people
is the polar bear capital
of the world.
Tourists flock here
in the summer and fall
when the massive white beasts
move inland off the melted ice.
But for much of the year,
Churchill is a desolate town
where winter temperatures
can dip
to minus 30 degrees Fahrenheit,
and the modes of transportation
are snowmobile
and dogsled.
[man] We're in Churchill
in northern Canada,
one of the world's largest
gravity lows.
The missing gravity
in the Hudson Bay region
is so large that you can
observe it from space.
[narrator] Understanding what
caused this bizarre abnormality
in the Hudson Bay
requires decoding the secrets
of gravity on Earth
and in the universe.
Gravity is an attractive force
that causes
any two objects with mass to
be pulled toward each other.
This force is stronger
for more massive objects,
and it's stronger the closer
you are to that object,
and that's the way
gravity works.
[narrator] So really big things
like planet Earth
have lots of gravity
that pulls everything
towards their center.
[man] The rocks forming
this cliff here
attract me gravitationally,
but I don't feel an attraction
toward them.
Instead, I'm firmly planted
on Earth here because
Earth's mass
pulling me down
is so much greater
than the mass of those rocks
pulling me to the side.
[narrator] The Earth,
like all planets, was formed
when gravity pulled together
bits of rock, dust, and gas
over millions of years,
gradually forming a sphere.
Gravity holds the Earth
together
and keeps it orbiting
around the sun.
[man] To understand
how important gravity is,
let's turn it off.
If you suddenly
turn off gravity,
we would be flung
into outer space
at a thousand miles per hour.
[narrator] The power of gravity
isn't just reserved
for shaping the universe.
Humans sometimes harness it
for a thrill.
[Filippenko] Here I am
on the Xcelerator ride
at Knott's Berry Farm
in southern California,
and this incredible
roller coaster is gonna show us
the amazing power of gravity.
Here I go. Who-oa!
Whoo! Whoo!
Oh, man!
Ah!
And now gravity
is gonna take me down!
Whoa! Whoa!
[narrator] And the rest
of the ride is powered by
nothing but gravity.
[Filippenko] Oh, ho-ho! Whoo!
Oh, man!
So I got accelerated to
80 miles an hour,
and then launched
nearly vertically.
So I was doing work
against gravity.
Got to the top
of the track,
and then gravity
brought me down.
Whoa!
Gravity gave me enough energy
to be able to go through
all the loops
and twists and turns
of this amazing track.
[narrator]
Since the mass of an object
determines how much
gravity it has,
things weigh more
on massive planets.
On gigantic Jupiter,
a 180-pound human
would weigh about 470 pounds.
But on the surface
of the Earth's moon,
the same human would shed
a lot of weight.
The moon's
gravitational attraction
is less than that of Earth.
It's about six times less.
So a given person who weighs,
say, 180 pounds on Earth
would only weigh
30 pounds on the moon.
It's the same amount of mass--
that person is still
the same person--
but he or she is being
pulled down on the moon
only one-sixth as much
as on Earth.
[narrator] Back on Earth,
the effect the mass of
an object has on gravity
would expose an ancient secret
in the Hudson Bay region
of Canada.
In the 1960s,
scientists began taking
the first global measurements
of the Earth's gravity field
using simple
ground instruments.
This is a relative
gravity meter.
Essentially, it measures
the differences between gravity
at a spot where it's known
and where you want to measure.
So these meters
are typically placed
wherever you want
to observe gravity.
That could be out on the ice.
That could be on rock.
For the early measurements
in the Hudson Bay,
the gravity meter was lowered
to the bottom of Hudson Bay.
[narrator] These gravity meters
picked up an anomaly,
a peculiar change in gravity.
This change was due to a layer
of rock from the Earth's crust
that was missing.
It was about as thick
as the length
of three football fields
placed end zone to end zone.
Scientists also noticed
strange land features
surrounding the Hudson Bay.
These physical signs
pointed to a possible culprit,
a climate catastrophe
of epic proportions.
The disappearance of gravity in
the Hudson Bay region of Canada
has troubled scientists
for decades.
They are on a hunt to find out
what happened to it
and what this might mean.
Since the late 1960s,
scientists had noticed
weird geological features
along the shoreline of the bay.
The bedrock had deep gouges
and grooves that looked like
they had been made by
some colossal force.
A plausible theory about
the missing gravity surfaced.
Twenty thousand years ago,
Earth was suffering through
its last ice age.
The buildup of snow and ice
created
the Laurentide Ice Sheet,
a two-mile-thick glacier
that spanned most of
present-day Canada.
[Filippenko] It depressed
the crust of the Earth
and made it sort of
stretch outward
and squeeze out the sides,
meaning there was less there.
Gravity isn't as strong
there because some of the rock
was squeezed out.
[narrator] The continent-sized
glacier appeared to be
the smoking gun.
But in the 1990s,
new GPS surface instruments
picked up something else.
After the mega-glacier melted,
the Earth's surface
began to slowly rebound.
The phenomenon is known
as post-glacial rebound.
But the amount of gravity
recovering
from the last ice age
only added up
to about one-third of
the total amount missing.
So there has to be
something else,
something much bigger going on
in the Hudson Bay
to have caused this
gravity depletion.
And it's not the only place
where gravity is low.
In space, the gravitational
field is extremely small.
That's because gravity
becomes weaker
the farther apart one object is
from another object.
In this
microgravity environment,
astronauts appear to be
floating inside a spacecraft,
but they're actually in
a constant state of free fall,
or weightlessness.
Astronauts don't drop
to the floor
because
the floor is also falling.
[Kaku] So you have
the optical illusion
that there's no gravity
in your space capsule,
but actually there's
plenty of gravity.
You're just falling
at the same rate as the capsule.
Therefore, weightlessness
is an optical illusion
created by the fact
that gravity pulls objects
at the same rate.
[narrator] In low-gravity space,
the human body experiences
all sorts of side effects.
An astronaut experiences
muscle atrophy
and loses one percent
of bone mass per month.
[Aguilar] Muscle mass
starts changing,
our bones start deteriorating.
So consequently,
one of the things that
astronauts hate the most
is that they have to work out
two and three hours a day
to try and keep that body mass.
[narrator] With less gravity,
moisture from the bloodstream
accumulates
in the spinal discs,
which can lengthen the spine
almost two inches.
In fact, spacesuits
are sewn larger
to accommodate the stretch.
[Aguilar] Now, the interesting
aspect about that, too,
is when we're in space,
all the fluids move up
around your head and your brain
and your face area.
And one of the things
that the astronauts
complain about
is their faces get rounder
and fatter,
and you can't see
any facial gestures.
[narrator] While the weak
gravity field in space
has a profound effect
on humans,
the gravity on Earth
influences everything
big and small.
The case of the missing gravity
near Churchill,
along the Hudson Bay of Canada,
reminds us how central
this force is
to life on planet Earth.
It sets invisible limits
on even the tallest things
in the natural world.
[Filippenko] I'm standing here
among a bunch of
California redwoods,
and some redwoods can be
the tallest trees in the world,
almost 400 feet high.
Now, they can't get
much taller than that,
and that's because
the downward force of gravity
caused by the Earth
makes it hard for water
to go up toward
the tops of the trees.
[narrator] Even mountains
have a secret height limit
set by gravity.
[Filippenko] Mountains on Earth
can't become arbitrarily high
because eventually
Earth's gravity
is gonna be
pulling on them so much
that they'll sink
in Earth's crust.
[narrator] On Earth,
our tallest mountain, Everest,
reaches five and a half
miles high.
On Mars, where gravity is
roughly a third less powerful,
the largest mountain,
Olympus Mons,
rises three times higher
into the sky.
All creatures on Earth
have evolved
to deal with Earth's particular
gravitational pull.
Look behind me.
That animal right there weighs
about three and a half tons.
So gravity affects them,
but look at their legs.
They're like tree trunks.
They support all of this
massive weight.
They don't grow
much larger than that
because they need
to be able to move.
They need to be able
to forage and find food.
[elephant trumpets]
[narrator] In fact,
no land animal on Earth,
even a dinosaur,
has ever grown larger
than 100 tons.
But under water,
whales can grow to almost
twice that weight.
[Aguilar] Whales in the ocean
can use buoyancy of the water
to support themselves.
They've gotten around
the conundrum of gravity.
[narrator] Around the globe,
animals have been shaped by
Earth's gravity.
[elephant trumpets]
In the Hudson Bay,
could they provide a clue
to what made
the gravity disappear?
During the years
scientists have been studying
this gravity anomaly,
they've seen the weather
change drastically.
Global warming now keeps
the bay ice-free
for longer periods,
shortening the polar bear's
season to hunt for food,
and threatening
its very existence.
It turns out that
the melting of sea ice
also has the power to change
the planet's
gravitational field.
[controller] ...six, five...
March 17, 2002.
NASA and a German
space agency launch GRACE,
the Gravity Recovery
and Climate Experiment.
The satellites take
the most precise measurements
of the Earth's gravity field
that will provide
crucial information about any
changes in the planet's oceans,
geology, and climate.
[Killett] GRACE is
a dual-satellite system
made up of two satellites
in the same polar orbit.
One satellite is 140 miles
behind the other satellite,
and they're constantly measuring
the distance between them
using a very precise
radar system.
Suppose that GRACE flies over
a very heavy object
like a mountain,
represented by this rock.
The gravity of that mountain
will pull
the two GRACE satellites
towards it,
which will decrease the distance
between the satellites.
Conversely, if GRACE flies over
a region like Hudson Bay
where gravity is weaker,
that will result in
an increase of the distance
between the two satellites.
[narrator] For five years,
GRACE orbits the Earth
15 times a day at an altitude
of over 300 miles.
As the satellites monitor
variations of mass
within the Earth,
they uncover a long-held secret
about the missing gravity
in the Hudson Bay,
something that has been welling
deep inside the planet
for hundreds
of millions of years.
In the Hudson Bay
of northern Canada,
the case of the missing gravity
has been
an unsolved scientific mystery
for over half a century.
Scientists knew
that an ancient ice sheet
had deformed the rock
in the Hudson Bay,
leading to some of
the gravity loss.
But that couldn't account for
most of the missing gravity.
Then, in 2007,
scientists got
a new view from space.
The GRACE satellites generated
the most detailed maps
ever made of Earth's
gravitational field
and revealed something
astonishing.
The missing gravity
in the Hudson Bay of Canada
was caused not just
by the cold of the ice age,
but also by searing heat
from the depths of the Earth.
Hundreds of miles beneath
Earth's surface,
7,000-degree molten rock
continuously rises and falls
in a process called
mantle convection.
[Killett] We can see convection
in a pot of water
boiling on a stove.
As it's heated, it expands.
It becomes less dense,
so it rises.
When it gets to the top,
it cools off,
contracts and becomes
more dense, so it sinks.
This convective process
is the same in the mantle,
except it's a lot slower.
[narrator] Mantle convection
on Earth occurs in cycles.
In many places,
it's rising, or upwelling.
But under the Hudson Bay,
the flowing rock in the mantle
is sinking, or downwelling.
[Killett] In Hudson Bay,
it just so happens
that in that particular spot,
the mantle is going down
because it's more dense.
This is dragging every feature
of the Earth with it downward,
which ends up reducing gravity.
Mantle convection
is essentially permanent.
There is effectively going
to be no end to this
on a human time scale.
[narrator] After over 60 years,
the case of the missing gravity
in the Hudson Bay region
of Canada
has finally been solved.
Scientists now have
a deeper understanding
of how changes
in the gravity field
can reveal hidden secrets
about the planet
above and below its surface.
The quest has even led
to the discovery
of other gravity anomalies
that expose threats
to the global population.
[Killett]
The 2005 Amazon drought
left a gravity anomaly because
there wasn't as much water
as expected on the ground.
We also see that gravity
is decreasing
over most of the ice sheets
on the planet,
and this is because
those ice sheets are melting
and sliding into the ocean
primarily because of
the CO2 that we're dumping
into the atmosphere.
[narrator]
The journey to unravel
this secret of the Earth
shows how the elusive
force of gravity
has left its mark
on mountains and continents,
trees and animals.
It's an essential part
of the entire universe,
and something
we could not live without.