-
I'm really glad to be here.
-
I'm glad you're here,
-
because that would be a little weird.
-
I'm glad we're all here.
-
And by "here," I don't mean here.
-
Or here.
-
But here.
-
I mean Earth.
-
And by "we," I don't mean
those of us in this auditorium,
-
but life,
-
all life on Earth --
-
(Laughter)
-
from complex to single-celled,
-
from mold to mushrooms
-
to flying bears.
-
(Laughter)
-
The interesting thing is,
-
Earth is the only place
we know of that has life --
-
8.7 million species.
-
We've looked other places,
-
maybe not as hard
as we should or we could,
-
but we've looked and haven't found any;
-
Earth is the only place
we know of with life.
-
Is Earth special?
-
This is a question I've wanted
to know the answer to
-
since I was a small child,
-
and I suspect 80 percent
of this auditorium
-
has thought the same thing
and also wanted to know the answer.
-
To understand whether
there are any planets --
-
out there in our solar system or beyond --
-
that can support life,
-
the first step is to understand
what life here requires.
-
It turns out, of all of those
8.7 million species,
-
life only needs three things.
-
On one side, all life
on Earth needs energy.
-
Complex life like us derives
our energy from the sun,
-
but life deep underground
can get its energy
-
from things like chemical reactions.
-
There are a number
of different energy sources
-
available on all planets.
-
On the other side,
-
all life needs food or nourishment.
-
And this seems like a tall order,
especially if you want a succulent tomato.
-
(Laughter)
-
However, all life on Earth
derives its nourishment
-
from only six chemical elements,
-
and these elements can be found
on any planetary body
-
in our solar system.
-
So that leaves the thing
in the middle as the tall pole,
-
the thing that's hardest to achieve.
-
Not moose, but water.
-
(Laughter)
-
Although moose would be pretty cool.
-
(Laughter)
-
And not frozen water, and not water
in a gaseous state, but liquid water.
-
This is what life needs
to survive, all life.
-
And many solar system bodies
don't have liquid water,
-
and so we don't look there.
-
Other solar system bodies
might have abundant liquid water,
-
even more than Earth,
-
but it's trapped beneath an icy shell,
-
and so it's hard to access,
it's hard to get to,
-
it's hard to even find out
if there's any life there.
-
So that leaves a few bodies
that we should think about.
-
So let's make the problem
simpler for ourselves.
-
Let's think only about liquid water
on the surface of a planet.
-
There are only three bodies
to think about in our solar system,
-
with regard to liquid water
on the surface of a planet,
-
and in order of distance from the sun,
it's: Venus, Earth and Mars.
-
You want to have an atmosphere
for water to be liquid.
-
You have to be very careful
with that atmosphere.
-
You can't have too much atmosphere,
too thick or too warm an atmosphere,
-
because then you end up
too hot like Venus,
-
and you can't have liquid water.
-
But if you have too little atmosphere
and it's too thin and too cold,
-
you end up like Mars, too cold.
-
So Venus is too hot, Mars is too cold,
-
and Earth is just right.
-
You can look at these images behind me
and you can see automatically
-
where life can survive
in our solar system.
-
It's a Goldilocks-type problem,
-
and it's so simple
that a child could understand it.
-
However,
-
I'd like to remind you of two things
-
from the Goldilocks story
that we may not think about so often,
-
but that I think are really relevant here.
-
Number one:
-
if Mama Bear's bowl is too cold
-
when Goldilocks walks into the room,
-
does that mean it's always been too cold?
-
Or could it have been just right
at some other time?
-
When Goldilocks walks into the room
determines the answer
-
that we get in the story.
-
And the same is true with planets.
-
They're not static things. They change.
-
They vary. They evolve.
-
And atmospheres do the same.
-
So let me give you an example.
-
Here's one of my favorite
pictures of Mars.
-
It's not the highest resolution image,
it's not the sexiest image,
-
it's not the most recent image,
-
but it's an image that shows riverbeds
cut into the surface of the planet;
-
riverbeds carved by flowing, liquid water;
-
riverbeds that take hundreds or thousands
or tens of thousands of years to form.
-
This can't happen on Mars today.
-
The atmosphere of Mars today
is too thin and too cold
-
for water to be stable as a liquid.
-
This one image tells you
that the atmosphere of Mars changed
-
and it changed in big ways.
-
And it changed from a state
that we would define as habitable,
-
because the three requirements
for life were present long ago.
-
Where did that atmosphere go
-
that allowed water
to be liquid at the surface?
-
Well, one idea is it escaped
away to space.
-
Atmospheric particles
got enough energy to break free
-
from the gravity of the planet,
-
escaping away to space, never to return.
-
And this happens with all bodies
with atmospheres.
-
Comets have tails
-
that are incredibly visible reminders
of atmospheric escape.
-
But Venus also has an atmosphere
that escapes with time,
-
and Mars and Earth as well.
-
It's just a matter of degree
and a matter of scale.
-
So we'd like to figure out
how much escaped over time
-
so we can explain this transition.
-
How do atmospheres
get their energy for escape?
-
How do particles get
enough energy to escape?
-
There are two ways, if we're going
to reduce things a little bit.
-
Number one, sunlight.
-
Light emitted from the sun can be absorbed
by atmospheric particles
-
and warm the particles.
-
Yes, I'm dancing, but they --
-
(Laughter)
-
Oh my God, not even at my wedding.
-
(Laughter)
-
They get enough energy
to escape and break free
-
from the gravity of the planet
just by warming.
-
A second way they can get energy
is from the solar wind.
-
These are particles, mass, material,
spit out from the surface of the sun,
-
and they go screaming
through the solar system
-
at 400 kilometers per second,
-
sometimes faster during solar storms,
-
and they go hurtling
through interplanetary space
-
towards planets and their atmospheres,
-
and they may provide energy
-
for atmospheric particles
to escape as well.
-
This is something that I'm interested in,
-
because it relates to habitability.
-
I mentioned that there were two things
about the Goldilocks story
-
that I wanted to bring to your attention
and remind you about,
-
and the second one
is a little bit more subtle.
-
If Papa Bear's bowl is too hot,
-
and Mama Bear's bowl is too cold,
-
shouldn't Baby Bear's bowl be even colder
-
if we're following the trend?
-
This thing that you've accepted
your entire life,
-
when you think about it a little bit more,
may not be so simple.
-
And of course, distance of a planet
from the sun determines its temperature.
-
This has to play into habitability.
-
But maybe there are other things
we should be thinking about.
-
Maybe it's the bowls themselves
-
that are also helping to determine
the outcome in the story,
-
what is just right.
-
I could talk to you about a lot
of different characteristics
-
of these three planets
-
that may influence habitability,
-
but for selfish reasons related
to my own research
-
and the fact that I'm standing up here
holding the clicker and you're not --
-
(Laughter)
-
I would like to talk
for just a minute or two
-
about magnetic fields.
-
Earth has one; Venus and Mars do not.
-
Magnetic fields are generated
in the deep interior of a planet
-
by electrically conducting
churning fluid material
-
that creates this big old magnetic field
that surrounds Earth.
-
If you have a compass,
you know which way north is.
-
Venus and Mars don't have that.
-
If you have a compass on Venus and Mars,
-
congratulations, you're lost.
-
(Laughter)
-
Does this influence habitability?
-
Well, how might it?
-
Many scientists think
that a magnetic field of a planet
-
serves as a shield for the atmosphere,
-
reflecting solar wind particles
around the planet
-
in a bit of a force field-type effect
-
having to do with electric charge
and those particles.
-
I like to think of it instead
as a salad bar sneeze guard for planets.
-
(Laughter)
-
And yes, my colleagues
who watch this later will realize
-
this is the first time in the history
of our community
-
that the solar wind has been
equated with mucus.
-
(Laughter)
-
OK, so the effect, then, is that Earth
may have been protected
-
for billions of years,
-
because we've had a magnetic field.
-
Atmosphere hasn't been able to escape.
-
Mars, on the other hand,
has been unprotected
-
because of its lack of magnetic field,
-
and over billions of years,
-
maybe enough atmosphere
has been stripped away
-
to account for a transition
from a habitable planet
-
to the planet that we see today.
-
Other scientists think
that magnetic fields
-
may act more like the sails on a ship,
-
enabling the planet to interact
with more energy from the solar wind
-
than the planet would have been able
to interact with by itself.
-
The sails may gather energy
from the solar wind.
-
The magnetic field may gather
energy from the solar wind
-
that allows even more
atmospheric escape to happen.
-
That's an idea that has to be tested,
-
but the effect and how it works
-
seems apparent.
-
That's because we know
-
energy from the solar wind
is being deposited into our atmosphere
-
here on Earth.
-
That energy is conducted
along magnetic field lines
-
down into the polar regions,
-
resulting in incredibly beautiful aurora.
-
If you've ever experienced them,
it's magnificent.
-
We know the energy is getting in.
-
We're trying to measure
how many particles are getting out,
-
and if the magnetic field
is influencing this in any way.
-
So I've posed a problem for you here,
-
but I don't have a solution yet.
-
We don't have a solution.
-
But we're working on it.
How are we working on it?
-
Well, we've sent spacecraft
to all three planets.
-
Some of them are orbiting now,
-
including the MAVEN spacecraft
which is currently orbiting Mars,
-
which I'm involved with
and which is led here,
-
out of the University of Colorado.
-
It's designed to measure
atmospheric escape.
-
We have similar measurements
from Venus and Earth.
-
Once we have all our measurements,
-
we can combine all these together
and we can understand
-
how all three planets interact
with their space environment,
-
with the surroundings.
-
And we can decide whether magnetic fields
are important for habitability
-
or not.
-
Once we have that answer,
why should you care?
-
I mean, I care deeply ...
-
And financially as well, but deeply.
-
(Laughter)
-
First of all, an answer to this question
-
will teach us more
about these three planets,
-
Venus, Earth and Mars,
-
not only about how they interact
with their environment today,
-
but how they were billions of years ago,
-
whether they were habitable
long ago or not.
-
It will teach us about atmospheres
-
that surround us and that are close.
-
But moreover, what we learn
from these planets
-
can be applied to atmospheres everywhere,
-
including planets that we're now
observing around other stars.
-
For example, the Kepler spacecraft,
-
which is built and controlled
here in Boulder,
-
has been observing
a postage stamp-sized region of the sky
-
for a couple years now,
-
and it's found thousands of planets --
-
in one postage stamp-sized
region of the sky
-
that we don't think is any different
from any other part of the sky.
-
We've gone, in 20 years,
-
from knowing of zero planets
outside of our solar system,
-
to now having so many,
-
that we don't know
which ones to investigate first.
-
Any lever will help.
-
In fact, based on observations
that Kepler's taken
-
and other similar observations,
-
we now believe that,
-
of the 200 billion stars
in the Milky Way galaxy alone,
-
on average, every star
has at least one planet.
-
In addition to that,
-
estimates suggest there are somewhere
between 40 billion and 100 billion
-
of those planets
that we would define as habitable
-
in just our galaxy.
-
We have the observations of those planets,
-
but we just don't know
which ones are habitable yet.
-
It's a little bit like
being trapped on a red spot --
-
(Laughter)
-
on a stage
-
and knowing that there are
other worlds out there
-
and desperately wanting to know
more about them,
-
wanting to interrogate them and find out
if maybe just one or two of them
-
are a little bit like you.
-
You can't do that.
You can't go there, not yet.
-
And so you have to use the tools
that you've developed around you
-
for Venus, Earth and Mars,
-
and you have to apply them
to these other situations,
-
and hope that you're making
reasonable inferences from the data,
-
and that you're going to be able
to determine the best candidates
-
for habitable planets
and those that are not.
-
In the end, and for now, at least,
-
this is our red spot, right here.
-
This is the only planet
that we know of that's habitable,
-
although very soon we may
come to know of more.
-
But for now, this is
the only habitable planet,
-
and this is our red spot.
-
I'm really glad we're here.
-
Thanks.
-
(Applause)