shaped by the chaotic collisions and bombardments

that marked its earliest days.

But as the planets finally eased into stable orbits

3.8 billion years ago, new threats arose.

And in some cases, they were as bad as anything

bearing down from outer space.

ALEX FILIPPENKO: Catastrophes in the solar system

didn't end with the era of Late Heavy Bombardment.

There have been catastrophes since that time.

Asteroids and comets can pummel into planets.

Rampant volcanism can occur on planets.

NARRATOR: The Red Planet has been ravaged at least five

times by episodes of catastrophic volcanism,

giving Mars a unique complexion unlike anything

else in the solar system.

MICHAEL MISCHNA: In some respects,

the shield volcanoes on Mars are similar to some volcanoes here

on Earth, such as at Hawaii.

You've got a large magma chamber beneath the surface

that's erupting lots of lava onto the surface,

creating the volcano.

However, on Mars, these types of volcanoes are 10 to 100 times

larger than they are on Earth.

So all across the planet, you've got

these eruption events that are occurring here,

and then here, and then here.

And over time, the whole surface gets filled in with this lava.

NARRATOR: Mars is home to about 20

major volcanoes, including Olympus Mons, the largest

in the solar system.

This towering relic provides an eerie glimpse

into the planet's fiery past.

When we look at Mars's geologic record,

we can tell how old things are by looking at how many

craters are on the surface.

If the surface is relatively young,

then there has been relatively little time for craters

to build up, and so we see a surface that's

largely flat and unmarked.

If a surface is very old, then we

see large numbers of craters.

And so, through crater dating and through careful mapping

of the Martian surface, it looks like there

were perhaps five very major episodes

of volcanic activity on Mars.

NARRATOR: Just as with shield volcanoes on Earth,

magma chambers rose to the surface of Mars,

broke through, and spewed basaltic lava

in centuries-long eruptions.

But there was a key difference between Earth's shield

volcanoes and the behemoths on Mars.

GREG LAUGHLIN: On Mars, there was

never large-scale plate tectonics of the type

that we have on Earth.

And so in Mars, when a volcano gets going,

it just sits there and keeps piling lava out and out

and out, and it builds up enormous shield

volcanoes like Olympus Mons.

On Earth, there's plate tectonics.

So, for example, the volcanic islands of Hawaii

are in a chain because the plate is moving north,

the hot spot is relatively fixed,

and new islands keep on popping up.

But on Mars, it's the same island all the time,

and it just keeps getting bigger and bigger.

NARRATOR: Massive volcanism radically changed the surface

of Mars time and time again.

But it wasn't the only terrestrial planet

that fell victim to episodes of extreme homegrown violence.

Our closest planetary neighbor, Venus, may

have begun quite Earth-like.

It was born at roughly the same time

and made with a same cosmic materials.

But something transformed Venus into Earth's evil twin.

ALEX FILIPPENKO: If you could penetrate

through its thick atmosphere, you'd see that about 90%

of the surface of Venus is covered by solidified lava

from previous volcanism.

And the thick, noxious atmosphere consists mostly

of carbon dioxide, and it has an atmospheric pressure about 90

times that on Earth's surface.

Wow, that's like being 3,000 feet

below the surface of the ocean.

That's what you'd feel.

Moreover, the temperature on Venus

is nearly 900 degrees Fahrenheit.

That's huge.

It's enough to melt lead.

So if a human were suddenly placed on the surface of Venus,

he would be baked and very quickly totally crushed.

NARRATOR: The hellish conditions on Venus

could have been caused by an extreme runaway greenhouse

effect over three billion years ago.

ALEX FILIPPENKO: But what happened

to the oceans of Venus, if they were there to begin with?

Well, the sun gradually grew brighter,

and that led to more evaporation of the oceans,

increasing the water vapor content of the atmosphere.

Well, water is a greenhouse gas.

So that led to a greater increase in temperature, which

led to more evaporation, more greenhouse gases,

a runaway greenhouse effect that eventually evaporated

away the oceans of Venus.

NARRATOR: Evidence of these lost oceans

may exist above the cloud decks of Venus.

We know from sampling Venus's atmosphere that there

is a high concentration of the form of heavy hydrogen called

deuterium.

Most of the hydrogen escaped to space,

and the small dregs of hydrogen that did remain

are this special heavy form called deuterium.

Venus provides a great example of what can happen to a planet

when the climate changes dramatically.

If we are able to understand what happened on Venus,

we can apply those lessons learned here on Earth.