Why Earth may someday look like Mars

So when you look out
at the stars at night,

it's amazing what you can see.

It's beautiful.

But what's more amazing
is what you can't see,

because what we know now

is that around every star
or almost every star,

there's a planet,

or probably a few.

So what this picture isn't showing you

are all the planets that we know about

out there in space.

But when we think about planets,
we tend to think of far-away things

that are very different from our own.

But here we are on a planet,

and there are so many things
that are amazing about Earth

that we're searching far and wide

to find things that are like that.

And when we're searching,

we're finding amazing things.

But I want to tell you about
an amazing thing here on Earth,

and that is that every minute,

400 pounds of hydrogen

and almost seven pounds of helium

escape from Earth into space.

And this is gas that is going off
and never coming back.

So hydrogen, helium, and many other things

make up what's known
as the Earth's atmosphere.

The atmosphere is just these gases
that form a thin blue line

that's seen here
from the International Space Station,

a photograph that some astronauts took.

And this tenuous veneer around our planet

is what allows life to flourish.

It protects our planet from too many
impacts, from meteorites and the like.

And it's such an amazing phenomenon

that the fact that it's disappearing

should frighten you,
at least a little bit.

So this process is something that I study

and it's called atmospheric escape.

So atmospheric escape
is not specific to planet Earth.

It's part of what it means
to be a planet, if you ask me,

because planets, not just here on Earth

but throughout the Universe

can undergo atmospheric escape.

And the way it happens actually tells us
about planets themselves.

Because when you think
about the solar system,

you might think about this picture here,

and you would say, well,
there are eight planets, maybe nine.

So for those of you who are
stressed by this picture,

I will add somebody for you.

(Laughter)

Courtesy of New Horizons,
we're including Pluto.

And the thing here is,
for the purposes of this talk

and atmospheric escape,
Pluto is a planet in my mind,

in the same way that planets
around other stars that we can't see

are also planets.

So fundamental characteristics of planets

include the fact that they are bodies

that are bound together by gravity.

So it's a lot of material
just stuck together

with this attractive force.

And these bodies are so big
and have so much gravity.

That's why they're round.

So when you look at all of these,

including Pluto,

they're round.

So you can see that gravity
is really at play here.

But another fundamental
characteristic about planets

is what you don't see here,

and that's the star, the Sun,

that all of the planets
in the Solar System are orbiting around.

And that's fundamentally driving
atmospheric escape.

The reason that fundamentally stars
drive atmospheric escape from planets

is because stars

offer planets particles and light and heat

that can cause the atmospheres to go away.

So if you think of a hot air balloon,

or you look at this picture

of lanterns in Thailand at a festival,

you can see that hot air
can propel gasses upward,

and if you have enough energy and heating,

which our Sun does,

that gas, which is so light
and only bound by gravity,

it can escape into space.

And so this is what's actually causing
atmospheric escape

here on Earth and also on other planets,

that interplay between
heating from the star

and overcoming the force of gravity.

So I've told you that it happens
at the rate of 400 pounds a minute

for hydrogen and almost
seven pounds for helium.

But what does that look like?

Well, even in the '80s,

we took pictures of the Earth

in the ultraviolet

using NASA's Dynamic
Explorer Spacecraft,

so these two images of the Earth

show you what that glow
of escaping hydrogen looks like,

shown in red.

And you can also see other features
like oxygen and nitrogen

in that white glimmer

in the circle showing you the auroras

and also some wisps around the tropics.

So these are pictures
that conclusively show us

that our atmosphere isn't just
tightly bound to us here on Earth

but it's actually reaching out
far into space,

and at an alarming rate I might add.

But the Earth is not alone
in undergoing atmospheric escape.

Mars, our nearest neighbor,
is much smaller than Earth,

so it has much less gravity
with which to hold on to its atmosphere,

and so even though Mars has an atmosphere,

we can see it's much thinner
than the Earth's.

Just look at the surface.

You see craters indicating that it didn't
have an atmosphere

that could stop those impacts.

Also, we see that it's the red planet,

and atmospheric escape plays a role

in Mars being red.

That's because we think Mars used to have

a wetter past,

and when water had enough energy,
it broke up into hydrogen and oxygen,

and hydrogen being so light,
it escaped into space,

and the oxygen that was left

oxidized or rusted the ground,
making that familiar rusty red color

that we see.

So it's fine to look at pictures of Mars
and say that atmospheric escape

probably happened,

but NASA has a probe that's currently
at Mars called the MAVEN Satellite,

and its actual job

is to study atmospheric escape.

It's the Mars Atmosphere
and Volatile Evolution Spacecraft.

And results from it have already
shown pictures very similar

to what you've seen here on Earth.

We've long known that Mars
was losing its atmosphere,

but we have some stunning pictures.

See here, for example,
you can see in the red circle

is the size of Mars,

and in blue you can see the Hydrogen

escaping away from the planet.

So it's reaching out more than 10 times
the size of the planet,

far enough away that it's no longer
bound to that planet.

It's escaping off into space.

And this helps us confirm ideas,
like why Mars is red,

from that lost hydrogen.

But hydrogen isn't
the only gas that's lost.

I mentioned helium on Earth
and some oxygen and nitrogen,

and from MAVEN we can also look
at the oxygen being lost from Mars.

And you can see that because
oxygen is heavier,

it can't get as far as the hydrogen,

but it's still escaping
away from the planet.

You don't see it all confined
into that red circle.

So the fact that we not only see
atmospheric escape on our own planet

but we can study it elsewhere
and send spacecraft

allows us to learn about
the past of planets,

but also about planets in general

and Earth's future.

So one way we actually can learn
about the future

is by planets so far away
that we can't see.

And I should just note though
before I go on to that,

I'm not going to show you photos
like this of Pluto,

which might be disappointing,

but that's because we don't have them yet,

but the New Horizons mission
is currently studying atmospheric escape

being lost from the planet.

So stay tuned and look out for that.

But the planets that I did
want to talk about

are known as transiting exoplanets.

So any planet orbiting a star
that's not our Sun

is called an exoplanet,
or extrasolar planet.

And these planets,

that we call transiting,
have the special feature

that if you look at that star
in the middle,

you'll see that actually it's blinking.

And the reason that it's blinking

is because there are planets
that are going past it all the time,

and it's at special orientation

where the planets are blocking
the light from the star

that allows us to see that light blinking.

And by surveying the stars
in the night sky

for this blinking motion,

we are able to find planets.

This is how we've now been able
to detect over 5,000 planets

in our own Milky Way,

and we know there are
many more out there, like I mentioned.

So when we look at the light
from these stars,

what we see, like I said,

is not the planet itself,

but you actually see
a dimming of the light

that we can record in time.

So the light drops as the planet
decreases in front of the star,

and that's that blinking
that you saw before.

So not only do we detect the planets,

but we can look at this light
in different wavelengths.

So I mentioned looking at
the Earth and Mars in ultraviolet light.

If we look at transiting exoplanets
with the Hubble Space Telescope,

we find that in the ultraviolet,

you see much bigger blinking,

much less light from the star,

when the planet is passing in front.

And we think is because
you have an extended atmosphere

of hydrogen all around the planet

that's making it look puffier

and thus blocking more
of the light that you see.

So using this technique, we've actually
been able to discover

a few transiting exoplanets
that are undergoing atmospheric escape.

And these planets

can be called hot Jupiters,
for some of the ones we've found,

and that's because they're gas planets

like Jupiter, but they're so close
to their star,

about a hundred times closer than Jupiter,

and because there's all this
lightweight gas that's ready to escape,

and all this heating from the star,

you have completely catastrophic rates
of atmospheric escape.

So unlike our 400 pounds per minute
of hydrogen being lost on Earth,

for these planets, you're losing 1.3
billion pounds of hydrogen every minute.

So you might think, well, does this make
the planet cease to exist?

And this is a question that people
wondered when they looked

at our solar system,

because planets closer
to the sun are rocky,

and planets further away
are bigger and more gaseous.

Could you have started with
something like Jupiter

that was actually close to the Sun,

and get rid of all the gas in it?

We now think that if you start
with something like a hot Jupiter,

you actually can't end up
with Mercury or the Earth.

But if you started with something smaller,

it's possible that enough gas
would have gotten away

that it would have
significantly impacted it

and left you with something very different
than what you started with.

So all of this sounds sort of general,

and we might think about the solar system,

but what does this have to do
with us here on Earth?

Well, in the far future,

the Sun is going to get brighter,

and that happens, the heating
that we find from the Sun

is going to become very intense.

In the same way that you see
gas streaming off

from a hot Jupiter,

gas is going to stream off from the Earth,

and so what we can look forward to,

or at least prepare for,

is the fact that in the far future,

the Earth is going to look more like Mars.

Our hydrogen, from water
that is broken down,

is going to escape
into space more rapidly,

and we're going to be left

with this dry, reddish planet.

So don't fear, it's not
for a few billion years,

so there's some time to prepare.

But I wanted you to be aware
of what's going on,

not just in the future,
but atmospheric escape

is happening as we speak.

So there's a lot of amazing science
that you hear about happening in space

and planets that are far away,

and we are studying these planets
to learn about these worlds,

but as we learn about Mars
or exoplanets like hot Jupiters,

we find things like atmospheric escape

that tell us a lot more about our planet

here on Earth.

So consider that the next time you think
that space is far away.

Thank you.

(Applause)