So I'm Geoff Bonning
and I am a PhD candidate
at the Research School
of Earth Sciences at ANU.
Most of the time, what I like
to study are the chondrites.
And they're these really
primitive meteorites
and the building
blocks of the planets.
Particularly for me, I like
the carbonaceous chondrites,
which are really our only
pieces of the outer solar system
that we have that we
can actually look at.
A while ago-- last year, it was
the hundredth anniversary of--
or the 90th anniversary
of the discovery of Pluto.
And so we were asked
to talk about Pluto.
So I developed
this talk that I'd
like to share with
you all now, which
is just the geology of Pluto.
So that's what we're
going to go over today.
Just really fun.
And Pluto, as we'll see, is a
really exciting, really bizarre
world.
Sometimes you want to
[INAUDIBLE] with other planets
like Mars.
You can make
analogies to things.
You can talk about--
you know, we've
got the atmosphere.
Got a rocky crust.
Maybe there was an ocean.
There's ice caps.
With Pluto, things
are so different.
The chemistry is so different
that it's really hard
to draw those analogies.
So we'll look at especially
one called Sputnik Planum.
And we'll go over all that.
So start off with--
let's go here.
Here's a little overview
of what we're going over.
To start with, I'm
going to be talking
about our intrepid little
robot explorer, New Horizons,
the spacecraft that
actually went to Pluto.
Before New Horizons
got there in 2015,
Pluto was a blurry haze
in the Hubble telescope.
We could barely see anything.
But now we've got all these
beautiful high-resolution
images, thanks to New Horizons.
Then we'll go over a little
bit of the basics of Pluto.
So Pluto 101-- just things about
how big is Pluto, how far away.
And then we'll look
at the geography
and look at some of the
specific features, which
you can see right here
in that map just there.
And then we'll talk
briefly about the moons.
And then very
quickly, we'll look
at the follow-up to
the Pluto mission
itself, which was New
Horizons-- because it's
going extremely fast.
It's at escape velocity, one of
the few spacecraft to do that.
It's on its way out
of the solar system
right now after
its flyby of Pluto.
And on the way out, it's stopped
by a little tiny, tiny world
called Arrokoth.
That's the most primitive
object we've ever seen.
Basically, a clump
of nebula as a world.
So we're going to look at
that at the very end, too.
And then we'll talk about the
facts of interstellar space,
and time for
questions at the end.
So New Horizons
launched in 2006.
And you can see it's
going right out here--
up and intercepted the
Pluto system in 2015.
Now we say the Pluto system
here, because Pluto--
even though it's not
technically a planet,
we call it a dwarf planet--
is actually a binary
planetary system.
Because the center of
mass of it and its moon--
Charon-- if you were to
balance them on a stick,
it's actually not
inside either of them.
So we call it a double
binary planetary
system or double
planet system, even
though they're [INAUDIBLE].
And then yes, so New Horizons
is off to-- these KBOs
are what we call
Kuiper Belt Objects.
They're really icy things.
So we talk about the asteroid
belt in the inner solar system
between Mars and Jupiter.
But you go farther
out than that,
and there is a lot
more stuff out there
that we are only just beginning
to discover and understand.
So New Horizons was
basically a spacecraft.
It's basically a camera
and a spectrometer.
Those are its main instruments,
for taking pretty pictures
and looking at what the things
in those pictures are made of.
So a spectrometer--
for those who
don't know about
them-- a spectrometer
is an instrument that we use.
It splits up the
light coming into it.
And using that, we can
see-- so it splits it up
into different colors.
And then looking at the
intensity of different colors,
we can use that to figure
out what things are made of,
especially by comparing
things to things in the lab.
So that's how we figure out.
So when I talk about chemical
information and chemical
composition stuff in this,
what we're talking about
is information that came
from the spectrometer.
Oh, and one thing you might
notice about New Horizons
here is a lot of our solar
system exploration craft
you might have seen, a lot
of them have solar panels.
But this one doesn't, because
it's going so far away,
it cannot rely on that.
So it's actually relying on a
nuclear generator, a small one.
So it's basically just a
bunch of radio isotopes
in there emitting heat.
And it's getting
power from that.
One of the first strange
things about Pluto
is it's really eccentric.
And by that, we mean
a really oval orbit.
So most planets have
very circular orbits.
It's got a much more
oval-shaped one.
It's also inclined.
So most of the planets
are in a single plane
in the solar system.
But Pluto is on this
really, really high orbit.
That's the case with a
lot of things out there.
They're smaller and they
are much further away.
So they're less tightly-bound,
in some ways, to the sun.
So it's 30 to 50
times farther away
than the Earth is from the sun.
And at that point,
the sun is still
the brightest object in the sky,
but it's only a point of light.
So you couldn't
actually resolve it.
On Earth, the sun
is about the size
of the moon in terms
of angular size.
You can look at them and
they're [INAUDIBLE] [? disk. ?]
Out there, it's
a point of light.
And it is 250 times
brighter than the full moon.
I don't know exactly what
that is to look like,
but it might hurt
your eyes to look at.
And out there, of
course, it is extremely
cold, minus 229 degrees
Celsius being typical.
But there are a lot of
variations, because of this.
So Pluto is extremely tilted.
So not only is it itself
inclined, eccentric,
but Pluto itself is at a really
strange angle to its orbit.
So we're at about a 25 degree--
24 degree angle, roughly--
to our orbit.
And the Earth's spin
is something like--
I can't really do that
with my hands very well.
Something like that, on its
orbit to give us the seasons.
Pluto is like this.
And it's doing
something like this.
It's doing these
huge oscillations.
What that means is that its
summers and winters last
an extremely long time.
So on Earth, if you're
at the south pole--
or either of the poles--
the day times in the summer
are six months long.
And the winters-- the
nights in the winter
are also six months long.
On Pluto, that's true, but
they last almost a century.
So extremely long
periods of day and night.
And because of that
really eccentric orbit,
if a summer happens when
Pluto is close to the sun--
so the pole pointing
towards that.
If that lines up,
that's what we call
these extreme seasons on Pluto.
Pluto has extreme
seasons that happen,
as well as the seasons
themselves, when
that eccentricity lines up.
So then you've got-- and those
happen every 800,000 years.
There's been a large
cycle there that
changes the temperature
of the poles dramatically.
So Pluto is very small.
Here are all the facts here.
0.2% the mass of the Earth.
And its surface gravity
is less than 10%.
So you could kind
of walk around.
You know, it's not too
dissimilar from the moon.
But it wouldn't be easy.
And it's about the
size of Russia,
or a little bit over two
times the size of Australia.
If you've walked
across Australia,
you could probably
walk across this.
And just as a comparison
in the composition,
so whereas we've got a rocky--
I mean, an inner metallic
core inside the Earth,
so these two bits are metallic--
the core of Pluto is rocky.
And instead of a rocky
mantle, like we have--
so this is where a
really dense, hot,
plastically-deforming
really gooey [INAUDIBLE]
as the mantle--
instead of that, it's got
water ice acting as its mantle.
And above that, it's
got nitrogen. And again,
I want to use the
analogy that nitrogen
is like the water on the Earth.
But it's not exactly,
because it's still
a solid most of the time.
So it's not exactly
like water on the Earth.
But it does form
this outer layer.
Nitrogen is what
dominates that bit.
And it's also got an atmosphere.
That's something that we kind
of knew about from Hubble.
But it's not until we
got out with New Horizons
that we could actually
see how complex and thick
that atmosphere is.
So you can see it here.
We've got these haze layers.
So these are layers of actual
haze in the atmosphere.
That haze is made of
organic compounds.
So there's a little
bit of methane on Mars.
That's part of the atmosphere.
When the solar radiation
hits that methane,
it reacts with itself.
So that methane
reacts and you form
more complex organic chemicals.
And as that continues,
those organic compounds
from the methane start to get
bigger and bigger and bigger
and bigger, until they slowly
rain out of the atmosphere.
So there's a really steady
accumulation of rain or haze
on the surface of Pluto.
You can see it again here.
It's actually blue.
And that's the
nitrogen. It's actually
blue for the same reason that
the Earth's atmosphere is blue,
which is the nitrogen
in the atmosphere.
As I was saying before, this
is one of the first things
that shocked us.
So this is one of the nice
beautiful pictures of Pluto.
It's an extremely
diverse surface.
That was one of the
first things that
struck the scientists
working on the mission
was that it's not
just some dead body.
One of the first
things we can see
is that you've got
these really, really
cratered ancient terrains,
like this down here.
And then you've also got these
really fresh-looking terrains,
like this.
This is Sputnik
Planum just here.
And then you've got kind of
intermediate-looking terrains,
things that are a bit older
but not quite ancient.
So there's actually
resurfacing going on on Pluto.
Pluto is an active world.
Mars, for example-- you
can compare it to Mars.
And Mars is pretty
much an extinct world.
It's a dead world.
The surface itself isn't
turning over any more.
On Pluto, this is
clearly still happening.
And just a reminder,
there's going
to be question time at the end.
So don't forget to ask
any questions if I've just
glazed over something and you're
like, hang on, what was that?
Don't just throw that
away, what do you mean?
Please ask me.
So yeah.
With these organics here--
we were talking about a
haze in the atmosphere.
And that slowly accumulates.
And what that
means is that we've
got kind of an indicator for
the surface age on Pluto.
So you can see these
redder terrains.
The more red a terrain is, the
more ancient it is, generally.
So we've got these really
ancient red terrains,
fresh white surface
on Sputnik Planum.
And something kind of
intermediate over here.
This is over, I
think, Tartarus Dorsa.
So this intermediate age--
a little bit red, little bit
cratered, but not quite as red
and cratered as this over here.
So this is our overview
of the geography.
This is our geography
of Pluto session.
And so we have Sputnik
Planum in that middle here.
This is this
footprint-shaped one.
And Cthulhu Regio here.
And this is one of the
most ancient turns.
So we found out on--
so this is what this is what we
call the encounter hemisphere.
This is the bit
that New Horizons
got close to as it flew by.
As it flew around
the other way, we
did get to see the
other side, but we
didn't get to see it as well.
But we did see that this
Cthulhu Regio type band extends
all the way around the equator.
So you can imagine this thing
extending all the way around.
And the north polar region.
We're going to
start looking at--
first, we're going to go
over the compositions.
So nitrogen, like I said,
dominates the surface of Pluto.
It's the most abundant chemical
[? layer. ?] Most of it's
as ice.
Most of it's solid.
And most of that ice is
in that Sputnik Planum.
So that's where you kind of want
to use the analogy sometimes
that nitrogen is like
the water on Earth.
So nitrogen is kind
of like the water
and it flows into this plane.
So it does flow into it.
And it's also the dominant
gas in the atmosphere.
So again, you want to
use the analogy to water.
But this plane, as
we're going to see,
isn't quite analogous
to an ocean.
It's very solid.
It's very different to an ocean.
Methane is also a feature here.
That's also mostly
as ice and also
as a gas in the atmosphere.
And maybe even
sometimes as clouds.
So we see some
tenuous things that
look like clouds there, and
also in Hubble images as well.
Things that kind
of look like clouds
that might be the methane.
Also carbon monoxide, but that's
mostly sunk into that Sputnik
Planum there.
And water-- as we know, water
is what the bulk of Pluto is
or bulk of the mantle of it.
But that's only
exposed at the equator.
This high, high-altitude
equatorial region.
So that's where we get the
water actually being exposed,
because the nitrogen isn't
actually stable there.
You can see it actually--
you see there's
no nitrogen there.
Just a little overview
of some of the geography
and the names that
we've got on Pluto.
So what's kind of
fun about it is
that it's got three main
things that it's drawing on.
It's got things that are
after underworld deities.
So Pluto itself, of course,
is one of the Greek and Roman
deities of the underworld.
But then we've drawn
on a number of others,
as well as fictional ones.
So we've got Cthulhu Regio here,
the Balrog Macula, as well as
some others I'm
[? individually ?] not
as familiar with, like
Krun, Ala, Vucub-Came.
And then other things that
are named after are explorers.
And that's a mixture of
human and robotic explorers.
So we've got Viking,
which was a Martian rover.
Hayabusa, which was
a Japanese spacecraft
going to the first asteroid.
We've also got the
al-Idrisi Montes here.
So al-Idrisi was an Arab
explorer that went into Europe
and [? mapped ?] all that.
So now we're going to
zoom in on Sputnik Planum
here and see it in a
little bit more detail.
Again, you want to
call it an ocean.
But it's not.
It's solid, but it's
still doing convection.
It's a a very strange thing.
It's got these cells.
So we call this
cellular terrain.
And so the middle bit is
where the hot material-- now
convection, if you're not
familiar with the term,
is what happens in your pan
when you're boiling water.
Or when you're
cooking a stew, maybe.
And you've got hot stuff
rising up in the middle
and then pushing out
to the side and then
sinking down at the edges.
And that's exactly what we've
got going on in these cells.
So we've got things push up
and rise up in the middle,
and then sink down
at these edges.
So those are our
convection cells.
And we can see that these
things are quite young,
because there's no
craters on them.
We don't see almost any
craters on Sputnik Planum.
So we expect that these things
are surfacing pretty fast
in a geological timescale,
which means maybe they
take 100,000 years, maybe a
million years, to turn over.
You've also got-- on the
edges of Sputnik Planum,
we've got these mountain-sized
icebergs made of water ice.
So there's glaciers
actually on the edges
[? because ?] the nitrogen
flows into Sputnik Planum.
Glaciers form at
higher altitudes.
They flow into Sputnik Planum.
And as they do that, they
break off the water ice
that forms the basement rock.
And these things end up floating
out into Sputnik Planum.
You can see a little bit
on the past one here.
So these things, like this.
And these are giant icebergs.
But they're the size
of mountains on Earth.
So these things can to be
up to five kilometers tall.
They're gigantic,
floating mountains.
We can also see dunes on
top of our convection cells.
So that, again, is evidence
that the atmosphere can
become quite thick on Pluto.
So that's something
else to remember.
The nitrogen is acting
as the atmosphere.
It's acting as one
of the things that's
moving around on the surface.
So it condenses out
of the atmosphere
like water does on Earth.
It can condense out as an
ice or it can be a gas.
And on Pluto, during those
really intense summers,
so much of the
nitrogen. is possible
that it becomes a gas that
the pressure at the surface
becomes way greater.
So you might actually have
liquid nitrogen flowing
on the surface of Pluto.
And wind able to
actually blow grains
of ice across these dunes--
across these convection
cells, forming these dunes.
So during those
intense summers, you
have the pressure of the
atmosphere increase enormously.
And the opposite is
true during the winter.
Most of the atmosphere
condenses out.
And you're in basically
a vacuum again.
And it just kind
of forms a snow.
During the springtime,
when things are heating up,
you've got these
sublimation pits.
So these things are
hundreds of meters across.
And a sublimation
is not a process
that happens much on the Earth.
But it's where a solid
goes straight to a gas
without going through
a liquid state.
Now most of the time, we're used
to seeing ice melt in spring.
But in the rest of
the solar system,
especially where pressures
are lower on Neptune--
sorry, [? not Neptune. ?] But
Neptune's moon Triton has it.
Pluto and Mars, as well.
Things go from solid to
gas pretty regularly.
So this is what these pits are.
You have a whole piece of
the ground just evaporate.
And next, we're going to look
briefly at Tartarus Dorsa
here, which is what we
call the bladed terrain.
So you can't quite see
the bladed texture here.
But we're going to
zoom in on it now.
And so these things are ridges.
And they can be kilometers tall.
And they're basically
oriented north-south.
So it's called bladed terrain.
The description from some of the
scientists that worked on this
is that if you got all the
knives in your drawer and just
kind of stacked them up this
way and just did it like that.
So there's these alleys
in between the knives
and then these just
sharp cliffs of methane.
And so this is a really bizarre
terrain that doesn't really
have an analogy on the Earth.
And the best
thinking about it is
that during those extreme
summers and extreme winters,
the nitrogen evaporates
from one of the poles.
The nitrogen and the
methane evaporate
from the current summer pole
and migrate to the colder winter
pole.
And while it's
doing that, they're
forced up the
high-altitude equator.
So the equator is at
quite a high altitude.
And just like snow on
Earth-- so if you've
got moist air being forced up
a mountain, you get rainfall
and you also get snow
coming out of the gas.
Same thing happens here.
As these nitrogen
and methane winds
blow from the summer
pole to the winter pole,
the methane crystallizes out
and forms these massive ridges.
And then the nitrogen
winds are kind
of blowing through the channels
between those huge ridges.
So it's really bizarre
terrain that we
don't have a huge amount of
analogies for on the Earth.
And yes, next we're going to
look at the Cthulhu Regio here.
So this is our most
ancient terrain.
So again, just that comparison.
We've got almost no
craters on Sputnik Planum,
and then we've got this very
heavily-cratered Cthulhu Regio.
Lots of craters there, which
tells us it's very old.
It's been around to
experience a lot of impacts.
And because it's at
that high altitude,
that nitrogen isn't
actually stable there.
So we see the water ice,
that bedrock, exposed.
Water ice is exposed here.
And because it's so
ancient, that water ice
is then coated in
what we call tholins.
And those are our
organics that we
were talking
about-- those things
that rain out of the atmosphere
from the methane getting
complex.
And then once they're
out on the surface,
they become even more complex
still by solar radiation.
So you get these really--
on Earth, if you
were to melt it,
it maybe would look like a tar.
But out here, it would be
like a solid substance.
Yeah, so we've got
this really red layer
of these organic compounds.
And then these white bits--
this is not water ice snow,
obviously.
This is methane snow
forming on those.
So it's a little bit maybe
like the Tartarus Dorsa,
where the winds kind of blow
over here and precipitate out
on the mountaintops.
And again, just remember if
you have questions, please
let me know.
I know I'm rushing
through everything here,
so let me know.
So one of the other things
that we see in Virgil fossae--
in Cthulhu Regio, sorry--
is the Virgil fossae.
So that's this crack
you can see here.
So this is a sign of
something like tectonism,
so maybe that there is actually
a liquid layer in between.
So we have that icy shell
beneath, water ice and nitrogen
ice on the outside.
But in between those two, there
might be a liquid layer inside.
And in fact, most of that
water layer might be liquid.
And so this crack, for one
thing, is evidence of it,
because it means that it's
kind of shifting around.
And as that solid
nitrogen and water ice
layer on the outside shifts
around, it might crack a bit.
But the stronger evidence
that we have for that
is that around that crack,
you see a lot more water
than you do elsewhere.
So it looks like-- so
we've got cryovolcanism,
which is just cold volcanoes.
There's this cold water
being squirted out
of this crack, where that liquid
ocean beneath might have just
pushed that water out.
And part of the
way that it stays
that cold is all those
organic compounds actually
form a bit of an
antifreeze in there,
which unfortunately
puts a bit of a damper
on the idea of astrobiology.
Not to say it is impossible.
It's just that any organic
chemistry that we know
would find it very hard to
live in a liquid water ocean
with that much antifreeze
kind of substances,
with that chemistry.
So a brief look
at the moons now.
So we've got Charon.
And then these
other little moons.
We've got Styx, Nix,
Kerberos, and Hydra.
Now Charon itself is a really
similar composition to Pluto.
And it looks like it also had
a liquid layer inside at one
point, but that froze over.
And when it froze over, what
you can see is this crack here.
If you've ever tried putting
a waterbottle in your fridge--
or in your freezer, sorry.
And if you screw
the lid on too tight
and that water can't go
anywhere-- the pressure
can't go anywhere
as it expands--
your waterbottle might crack.
That's basically what
happened to Charon.
The whole planet basically
just rifted around the middle,
where the water ice--
the liquid ocean inside--
expanded and
cracked the surface.
And you also have
this weird layer--
this is the Mordor Macula--
on Charon, which seems to be
some of the tholins coming--
so what it might be is that
between these two planets
as they're orbiting
each other, Charon
might be stealing some of
that methane atmosphere.
So there might be some of some
of Pluto's tenuous atmosphere
kind of trailing over to
Charon and depositing on there.
And as they deposit, they
form those tholins again
once that methane's on there.
And then this is our final view.
So this was our
final view of Pluto
as New Horizons
flew past around.
So we've got the encounter
hemisphere, just everything
that we've seen so far.
Sorry, I should have said.
Forgot to mention these guys.
So these are just
tiny little worlds.
So these didn't really
get to differentiate.
They're just little
clumps of ice.
And so they might--
it's still argued about
whether they accreted along
with these other two.
So you have a disk of
ice as Pluto and Charon
are forming around each other.
These little moons might
have formed from that disk.
Or they might be captures from
other Kuiper Belt objects.
They might have just pulled them
in and attracted them to it.
So this here is the
other hemisphere of Pluto
that we didn't get
to look closely at.
But what you can see is that
dark equatorial band again.
That Cthulhu Regio.
So that is where that very
ancient high-altitude terrain
goes fully around the planet.
And New Horizons, as we
showed before, kept going.
It would be extremely
hard to stop.
So once New Horizons was
going at those speeds
to get out here, to get to
Pluto in a reasonable timeframe,
it's very hard to stop.
It would have had to carry a lot
more fuel to slow itself down.
So it was just going to fly by.
It flew past.
And then it went to this guy.
So they didn't actually
know exactly where they
were going to go afterwards.
But they knew these things were
out there and figured that--
they being NASA and
the New Horizons team--
could deflect it to one
that they selected later.
And they ended up with Arrokoth.
Now, it was originally
called Ultima Thule,
which was the most distant
place known in Greek mythology.
But unfortunately,
that name is also
used by neo-Nazis as
the Aryan homeland.
And the NASA scientists decided
that was not a good name
to keep.
And they chose Arrokoth, which
is a Powhatan word from North
America.
So it's where the original
settlers in Virginia landed.
That's one of their
thunder deities.
So it was discovered in 2014.
So yes, for one thing, it was
discovered after New Horizons
actually launched.
And it is the most
primitive object
we've ever seen up close.
And this is-- if you
were to imagine what
would come out of a nebula.
When you see this beautiful--
I should have put a picture
of a nebula in here.
But you see this
beautiful picture
of the clouds in
interstellar space.
If you were to take a
bunch of ice and dust
and just kind of push it
together like a snowball,
this is effectively what
that would look like.
This is a nebular
solid, basically.
It's never been heated.
It is in the same orbit that
it was in 4.5 billion years
ago when the solar
system formed.
It's barely moved.
So if we're looking
at the asteroids
in the inner solar system
as a counterexample,
they have moved around.
Jupiter and Saturn moved
in their orbits over time.
And that kind of flung around
all the smaller bodies.
And so none of them are really
where they originally formed.
Everything's been mixed up.
But out here, things
stayed really still
and quiet and stable.
So this is exactly where it was.
So you can see it's another
kind of a double planet.
What happened here is
these two tiny bodies--
I should say it's about
39 kilometers long--
slowly, slowly
orbited each other.
Slowly, slowly,
slowly, until they
just bumped into
each other and stuck.
So they're mostly made of
water ice and organics.
So a similar
composition to Pluto,
just they didn't
differentiate at all.
They're just all mixed up.
If you were to go there, you
could grab a fistful of this.
And you would have basically
pristine inter-stardust.
That's the stuff between stars.
When you look at the
dark band of the galaxy--
when you look at the Milky Way--
that dark band is the
dust between stars.
And you would basically
be getting a sample
of that in this kind of object.
And as I said before, New
Horizons is on the way out.
I haven't added it to this one.
But these are the other four
probes that have done it.
Voyager 1 and 2, and
Pioneer 10 and 11.
So New Horizons is
the fifth spacecraft
to achieve escape velocity
from the solar system.
So it is our fifth
interstellar probe.
So it's on the way out,
which is very exciting.
Don't know how long it will
last out there though, actually.
Any questions about it?
Yes.
I've got one question already.
If you have a question, just
please keep sending them in.
I've got someone who's
forwarding them on to me.
The first question was are
there some places on Pluto
that get more craters
than other places?
And as far as we have any
reason to believe, no.
And it's the same
on most bodies.
We kind of expect
that the impactors
come at all directions,
come from all directions.
And on Earth, as
far as we can tell,
that's exactly what
happens here too.
Things impact the surface from
all directions. and all angles,
all the time.
So it's fairly random scatter.
What the different ages--
what the different
amounts of cratering tells
you is much more so about
the freshness of the surface.
So a surface that hasn't
been changed at all
gets a lot more craters on it.
So as another example,
Australia is a bit more
of a cratered continent than
some other fresher, younger
places.
So if you were to take note on
the average size of Indonesia--
which is a freshly
put-up mountain chain--
it'll have a lot less
craters than Australia,
which is 3 billion years
old, some parts of it.
It's a very ancient terrain.
So it's got more craters on it.
And likewise here.
This is a very ancient terrain,
very fresh and young terrain.
Second question.
Does accretion require heat?
How do those two lumps
of ice get stuck together
if it's never melted enough to
create some kind of adhesion?
So the adhesion on bodies
like this is just gravity.
They're just stuck
together, two particles.
So it's very weak.
You know, they're
not very big bodies.
The gravity isn't very strong.
And you can see that--
well, there's actually a
little bit of a crater there.
But a significant impact
would break them apart,
possibly very
easily and quickly.
But it's just a very
slow gravitational kind
of settling onto each other.
But then that gravity
wasn't enough to deform
their original shape into a
ball, which is what happens
when you form a larger body.
So when you get a larger
planet, the heat doesn't come--
so the heat in accretion
on larger bodies
is often because these things
are coming and flying in hot
and bringing a lot
of energy with them.
Whereas this was a very cold
collision to start with.
Yes, so accretion
doesn't require
heat is the answer to that.
But it definitely can
involve a lot of heat,
as it did with the Earth.
The Earth had massive
violent impacts.
And especially because we were
in the inner solar system,
where things were
being mixed up.
So we've got Jupiter and
Saturn moving around,
which moved all the
planetary building
blocks around and threw a
bunch of these asteroids
at the Earth.
And created these
violent impacts,
which melted the surface--
melted anything that was here.
Whereas out here is a very
slow, steady, stately, graceful
affair.
Things just generally coalesced.
What is the-- and
so another question.
What is the size of Pluto
compared to the Earth?
I've got that back here.
It's 0.2% the mass.
And you can see the surface
area will also be quite small.
So it's very small.
It's actually smaller
than the moon.
Smaller than the Earth's moon.
Significantly smaller
than the Earth.
And yeah, the area of it
is about twice the size
of Australia.
That's all the questions that
I've got coming in so far.
Here's another one.
How can such a small body
hold onto an atmosphere?
That's a good question.
Sorry, the others were
good questions, too.
I just mean that's a
very perceptive one,
because when you go
to places like Mars,
it's lost a lot
of an atmosphere.
So how could this tiny
body have anything on it?
And part of the answer to that
is because it's so dang cold.
That nitrogen-- [INAUDIBLE].
Because it's so
cold, the nitrogen--
even though it is a gas--
is just not kind
of vibrating enough
to bounce out of the atmosphere,
even though it's barely
being held on.
It's simply because
it's so cold.
And actually, that haze--
one thing that was kind
of confusing from Hubble
and the measurements
that were made from Earth
was that Pluto is actually
colder than you would expect.
So if you were to just imagine
how cold an airless body is
at different distances
from the sun,
you can kind of estimate
their temperatures,
based on just how much
light is coming onto them.
But Pluto is colder than
you would expect for a body
without an atmosphere.
And part of the reason is
because of that methane haze.
So because of its
atmosphere-- its atmosphere
actually effects
to cool it down,
because the methane in that
haze reflects more light.
So another question is how far
away is Voyager 1 from Earth
now?
The answer to that one,
I don't actually know.
I don't have the answer to that.
I can look that up.
And I can google that
in just a second,
but I don't actually know
the answer to that one.
How many moons, does Pluto have?
What was it, five?
It's five.
Yeah.
So we've got-- yeah,
Charon's its major one.
Although I could say--
maybe we should actually
kind of say four moons around
a double planetary system.
So you put on Charon--
Charon's not really technically
a moon, because it's--
the center of mass
of these bodies
is outside of Pluto, which
makes it a double planet system.
Not a planet-moon
system, like the Earth.
So four little moons.
Possibly more smaller ones.
New Horizons did look for
more, but it didn't see any.
But that doesn't mean
that there aren't any.
It flew by very
fast, so it didn't
get to spend a long time
out there looking for more.
Any other questions?
That's all we've got right now.
Any other questions about Pluto?
I hope I didn't speak too fast.
I hope you could all hear me.
This is my first time
doing this over Zoom,
public talk like this.
I normally get to see
the audience's face
and see how they're responding.
OK, so I've got
another question.
How do you know
the center of mass
is in the middle of
Pluto and Charon?
It's by knowing their mass,
their individual masses.
I'm sorry, partly by
knowing their densities.
But we can estimate them
based on their composition
and how fast they're
spinning around each other.
So we know that they're made of
a lot of ice and a lot of rock.
And by knowing how fast they're
spinning around each other,
we can estimate their
respective masses
from the amount of angular
momentum they've got.
Yeah, once you know
those respective masses,
then it's just a matter of
kind of balancing those.
Just taking the
distance between those.
And it's almost like--
in the model, if you're
trying to imagine it,
you just have a stick
between the two.
And you're trying to kind of
put your finger on a balance
in the middle of it.
And on the Earth, in
the Earth-moon system,
that balancing point
is inside the Earth
if you do that calculation,
whereas with Pluto and Charon,
it's on the outside of Pluto.
Who discovered Pluto?
Gosh.
I should know this,
because we talked about it.
Was it Clyde Tombaugh?
Because that was--
sorry, this feels
like Who Wants To
Be a Millionaire
and now I'm on the hot seat.
Sorry, guys.
I am googling this.
You just saw that.
I think it's Clyde Tombaugh.
Yes, it's Clyde Tombaugh.
And extra fun fact was that it
was named by a school girl who
just kind of wrote into them--
from what I understand--
and had been reading
about mythology in a book.
And was like, Pluto would
be a good name for that,
in terms of the thought
of the underworld
and this dark, cold place.
So it was a child that
actually named it.
That was in [INAUDIBLE].
Any more questions?
OK, so we've got no more
questions coming in.
So I'm going to wrap up.
Thank you all for coming.
Hope you enjoyed it.
Hope it made sense.
Yeah.
Thanks.
Stay safe, everyone.
