How the Universe Works

3.7 billion years ago,

early life was on the brink of extinction.

Colossal impacts...

Ferocious climate change...

And total atmospheric collapse have turned paradise into hell.

But this isn't Earth...

It's Mars...

And this is the violent history of perhaps

the first life forms in our solar system.

Could these martians still exist today?

Could they even be living among us?

Captions paid for by discovery communications

scientists suspect that life may once have thrived on Mars...

That the barren world we see today

swarmed with martians long ago.

If I had to bet something incredibly valuable to me...

If I had to bet my car... On whether or not

there's life on Mars,

or evidence of past life on Mars,

I'd take that bet.

The building blocks for life

are widespread in the universe,

and early Mars could have been the perfect place

to pull these ingredients together.

If you had a recipe book for everything you need for life,

you'd have things like water, organic chemistry,

a stable surface, a thick atmosphere...

Well, Mars had all of those.

Mars satisfies every specific requirement

for letting life get started.

Life on Mars

may have been inevitable,

and we've sent a robot army to hunt for signs of it.

But so far, it's been elusive.

Our rovers, landers, and satellites

haven't found life yet,

but they have found evidence of something extraordinary.

Mars was the victim of a devastating series

of extinction-level events

that rocked the red planet to its core...

Leading us to wonder,

if life could have started over multiple times,

with generation after generation of martians

rising and falling through Mars' violent history.

It seems likely that a first Genesis of life

could have occurred very early on in Mars' history,

just as soon as the crust was cool enough

to give it a solid foothold.

The secret to this early life would have been

a crucial ingredient, one shared by the young Earth.

Dry, harsh Mars once had oceans.

For life to get started,

you need some carbon, an energy source,

it needs nutrients like nitrogen...

But those are likely to be present on Mars,

they are present and widespread on Earth.

The essential requirement is really the liquid water.

Picture Mars

4.5 billion years ago.

Molten rock has cooled to form a solid crust.

Water collects on the surface, forming primitive oceans.

Rain clouds sweep across

the steaming, volcanic landscape,

and in shallow pools of water,

the martians begin to emerge.

But these first aliens are simpler

than Sci-Fi would have us believe.

We're probably not talking about little green men

or women, or whoever.

Walking around on the surface of Mars, we...

We're talking about something probably

much, much smaller and simpler,

single-celled life.

If would look familiar,

it would look just like bacteria on Earth.

Little, tiny, round, rod-shaped organisms.

If this generation

of bacterial martian life really did exist,

it was the first life to grace our solar system.

Multiplying inside the rock pools of Mars

long before life took hold on Earth,

Mars' small size would've given it a head start.

Because Mars is smaller

than the Earth it would have cooled

a little bit faster than we did.

So early on in the life of the solar system,

Mars may have been more like Earth

than Earth was at the time.

Let's go back 100 million years

after the sun formed.

The surface of the Earth is still a molten lake.

But martian life could be thriving

on the smaller, cooler world.

But these first martians won't have long

to enjoy their time in the sun.

Inescapable death is already on its way from space...

A cosmic bomb so huge,

it would completely alter the shape of the planet,

leaving it lopsided,

the northern hemisphere crushed.

Most planets are round,

and that's just something you don't really

give much thought to,

but it turns out Mars isn't as round as it could be.

The Southern hemisphere, on average,

has a higher elevation than the northern hemisphere.

So, in other words, if you were to start on the north pole

and walk all the way around to the south pole,

you'd be walking uphill, essentially, the whole time.

So, we call this difference between the northern

and the Southern hemisphere,

we call this the crustal dichotomy on Mars,

and it's been one of the biggest mysteries of the planet,

you know, it's the first thing that you see about it,

and you say, well, how could this possibly have happened?

In 2008, scientists mapping

the surface of Mars came up with an explanation

for the massive dent in the planet, shown in blue.

This basin, the biggest in the solar system,

had to be the result of a massive impact.

Called the Borealis impact,

it blasted out a crater 6500 miles wide and five miles deep,

big enough to swallow the entire United States

with room to spare.

Something really big happened.

In fact, the entire top half of the planet

seems to have practically been blown off.

The only thing that could do that is a huge collision,

and we're talking a collision

with something the size of Pluto, perhaps.

You're talking about an impact

that makes the dinosaur killer impact

65 million years ago look pretty much like a wet firecracker.

4.5 billion years ago,

the early solar system is filled

with planetesimals and protoplanets...

Asteroid-like leftovers from a planet building process

that created Mars and the Earth.

One of these asteroids is huge,

and it's on a direct collision course with Mars.

Any microscopic martians have just seconds to live.

If this impact was happening today

and we were so unlucky as to be there to witness it,

what you first would have seen is another moon in the sky...

And then you would've looked back and seen,

oh, it's getting bigger.

As it came down it would have filled the entire sky,

from horizon to horizon,

and as it struck,

the top would have still been well out into space.

The impactor

is 1200 miles across,

the size of Pluto,

and as it hits, the energy of the impact

shakes Mars to its core.

The entire planet wobbles like jell-o.

As it came down,

it would have been hitting into the surface of Mars

as fast as a bullet out of a gun,

and it would have slammed into the surface

and sent a shockwave out

that would've been bigger than any earthquake ever recorded.

It would have been like a Tsunami of rock

coming out and tossing us out of the way.

The impact is catastrophic.

It blows nearly half the planet's surface into space

and turns what crust remains into a boiling lake of lava.

You can't have an impact of that scale

without almost melting the planet.

It's not literally a planet breaking event,

but it's a planet melting event,

and it is, it is the sterilization

of the planet at that point.

The surface of Mars was molten,

its atmosphere blown into space,

the oceans boiled away.

If Mars was home to the first generation of life

in our solar system,

that life didn't stand a chance.

It would take the surface of Mars 50 million years

to recover from the impact.

But what sort of planet will rise from the ashes?

Compelling new evidence suggests that the conditions for life

may have returned to Mars,

but did life itself make a comeback?

This impact was only a blip in time,

and there was possibilities for life

and the planet to recover.

4.5 billion years ago

an asteroid the size of Pluto

slammed into the surface of infant Mars.

It melted the surface of the planet,

it blew the atmosphere into space,

and it boiled away the oceans.

If life had gotten a foothold on the planet,

that life would have been completely exterminated.

But some scientists believe this extinction

could have been brief,

and that life could have started again from scratch.

One of the wonderful things