Video summary
The outer planets, also known as Jovian planets, possess atmospheres primarily composed of hydrogen and helium, mirroring the composition of the Sun and the rest of the universe. When observing these giants, we are essentially viewing their cloud layers rather than a solid surface, as the surrounding hydrogen gas is clear and invisible. These clouds form distinct patterns of rising and sinking convective currents: lighter-colored zones represent areas where material rises higher in the atmosphere, often consisting of ammonia clouds, while darker belts indicate regions where material sinks deeper down. Although the composition of these lower layers is less well understood, spacecraft data from missions like Galileo have provided valuable insights into the specific atmospheric conditions of Jupiter.
Saturn displays a similar banding structure but with less distinct features due to its greater distance from the Sun and cooler temperatures, which bury cloud layers under thicker haze. A particularly fascinating feature found on Saturn is the polar hexagon, a rare geometric pattern swirling around the pole that defies the typical circular or spherical shapes common in space, likely resulting from unique fluid dynamics near the pole. In contrast, Uranus presents a very bland and stable atmosphere with almost no visible structure because it lacks an internal heat source to drive cloud formation. Neptune, however, exhibits more dynamic activity with visible banding and storms. The distinct blue-green color of both Uranus and Neptune is caused by methane in their atmospheres, which absorbs red and orange light from the Sun while reflecting blue and green wavelengths back to our eyes.
Large-scale storms are a defining characteristic of all giant planets, though their nature and longevity vary significantly. Jupiter's Great Red Spot is a massive high-pressure storm larger than Earth that has been observed for centuries but is currently shrinking, potentially disappearing within decades if its energy source diminishes. Unlike low-pressure systems like hurricanes on Earth, this spot is a persistent high-pressure region. Neptune once hosted a similar feature called the Great Dark Spot, which was visible when Voyager 2 visited in 1989 but had vanished by the mid-1990s, suggesting these storms may last anywhere from a few years to thousands of years depending on the planet's specific conditions. Uranus also experiences dark spots and global storms, likely influenced by its extreme axial tilt which creates unusual seasonal patterns where one pole faces the Sun for decades at a time.
In summary, the complex structures seen in the atmospheres of the Jovian planets are determined by their temperatures and internal heat sources. While Jupiter has an active interior driving long-lasting storms, Uranus remains relatively quiet due to its lack of internal heat, despite having a similar atmospheric composition to Neptune. Observations reveal that large storms occur on every giant planet, but their visibility and behavior depend heavily on factors such as cloud depth, atmospheric haze, and seasonal heating variations. As we continue to study these distant worlds through enhanced imaging from telescopes like Hubble, scientists hope to better understand the energy sources that power these massive weather systems and determine whether shrinking storms will eventually fade away or stabilize at a smaller size.
Read the full video transcript
Greetings and welcome to the
introduction to astronomy.
In this lecture, we are going to
continue talking about the outer planets
and specifically focus on their
atmospheres. And of course, the
atmosphere is really what we see when we
are looking at the giant planets. So,
let's go ahead and look at their general
properties again. And they are made up
primarily of hydrogen and helium. And
that's really just like everything else
in the universe. While we've
concentrated on other materials,
especially in the inner solar system,
most things like the Sun and the outer
planets and most of the rest of the
universe is also composed of hydrogen
and helium.
Also, many of the hydrogen compounds,
note that each of these contains
hydrogen, as well as other
elements such as oxygen, carbon, and
nitrogen.
And we had the Galileo spacecraft that
visited Jupiter and sent a probe into
its atmosphere, which is the one chance
that we have to actually learn something
about the atmosphere. So, for at least
that position, that part of Jupiter's
atmosphere, we have a pretty good idea
of what things like the composition of
the atmosphere is.
Now, so what do we see when we look at
Jupiter? Let's go ahead and take a look
here.
And Jupiter, here's a nice image of
Jupiter. We see it has a number of
different belts and zones. So, you will
see different things labeled there. And
the belts are the darker areas, and
these zones are the lighter colored
areas. Now, when we look at Jupiter, we
are really looking at clouds in the
atmosphere. Above that, there is
hydrogen, but hydrogen is clear and
invisible, just like our own atmosphere.
So, it is specific clouds that we are
seeing, and the actual layers of
hydrogen around it are completely
invisible to us.
We see, as I said, there's two parts.
There's the zones, and we see for
example, labeled the equatorial zone
here.
And then we have the belts, which are
dark colored, and you can see a whole a
number of belts here that are labeled in
terms of the temperate belts and
equatorial belts here and down below.
So, the zones are lighter colored and
are rising convective currents. They are
up higher in the atmosphere and often
consist of ammonia clouds.
The belts are dark colored, which is
where the material is sinking down. They
are lower in the atmosphere.
And their composition is less well
known. Now, when we look at this,
uh we we look at Jupiter and we look
like we're looking at a surface, that
everything there is at the same
distance, but they're not. The zones are
much higher up above and the belts are
further down below, so there are areas
that are higher up and lower down. And
we don't really get that kind of
perspective when studying it here.
We also do see the Great Red Spot, which
we will talk about later on.
Well, let's take a look at Saturn. And
what we see with Saturn is the similar
type of structure. We see but we see
them less distinctly. You can see
lighter and darker bands there, but
because Saturn is further from the sun,
temperatures are are cooler and the
cloud layers are buried deeper in the
atmosphere, under more haze, and it
makes it harder to see them.
One thing we do see when looking at it
in more detail, looking at the polar
regions, is the polar hexagon. So, we
can see the polar hexagon here in this
image. And the north the north or south
pole would be at the center. And then
the hexagon of material swirling around
it, which is a very interesting
combination. Again, we don't normally
get patterns like this. Things like
squares are relatively rare in space. We
tend to get everything tends to be
circular or spherical or
squashed versions of those things like
ellipses and ellipsoids.
So, the fact that it forms here is
something very interesting and may have
something to do with the fluid dynamics
in the atmosphere near the pole.
Now, the other planets, Uranus is much
a very bland, very featureless
atmosphere and is a very stable
atmosphere. If you recall when we talked
about the interiors, this is the one
that has no interior heat source. So,
the clouds are essentially invisible to
us.
Neptune, on the other hand, has some
more structure to it. So, when we look
at images of Neptune, has a little bit
more structure. We can see some banding.
We can see signs of storms and clouds
that are present here in the atmosphere.
If you note, there's also a difference
in coloring that they appear blue and
that is Uranus and Neptune. Now, why do
they appear blue? Well, it's a
concentration of methane in the
atmosphere. So, the atmospheres are not
made of methane but have enough methane
and methane is very good at absorbing
red and orange light and not so good at
absorbing blue light. So, it's very good
at absorbing those short those longer
wavelengths of light and that means
what's left to be reflected back is
blue. So, it absorbs the red and orange
lights coming from the sun, reflects
back the blues and greens and gives us
this bluish-green color that we see for
Uranus and Neptune and it all has to do
with the composition of their atmosphere
and especially the methane present.
Now, we can also look at the
uh these in terms of a temperature
profile, and this is where we can see
the clouds on Jupiter. Note where the
clouds on Jupiter are forming. Here's
the altitude. Altitude of zero is where
what we see as the surface. So, those
three layers of clouds, which are
ammonia, ammonium hydrosulfide, and
water,
are all within about 100 km of the
surface.
When we look at Saturn, we see that
these are much further down, going to
200 or even close to 300 km below the
surface.
And that means that they're much less
visible when we look at them.
Uranus and Neptune very similar, but
remember that Uranus did not have the
internal heat source, so maybe that's
some reason that even though they have
very similar compositions
in their atmospheres, they're not we're
not able to see much structure in
Uranus. It might also have to do with
the fact of the way Uranus is heated
because it is tilted on its side.
Now, we mentioned one great storm, which
was the Great Red Spot. Let's take a
look at that in a little bit more
detail.
Here we see the Great Red Spot, which is
a storm that is bigger than Earth. And
interestingly enough, this is a
high-pressure region, not a low-pressure
region. When we talk about things like
hurricanes and tropical storms on Earth,
they are low regions of low pressure.
This the Great Red Spot is instead a
high-pressure region, which is what we
generally associate with good, clear
weather.
Now, this has been present for a long
time. And in fact, we have images of it
going back to the
later part of the 1600s,
and we don't know for sure that it's
always been there. It hasn't been It's
only been observed continuously for the
last 150 years or so, but we kind of
suspect that it probably has been
present for hundreds of years.
However, we do know that it is
shrinking. It is changing in size,
especially over the last few decades.
So, as we look at it, and here we see
some images taken from
um the Hubble Space Telescope from 2000
from 1995 through 2014,
and it's definitely has shrunk a bit,
but even going back a little earlier, we
can see that it has probably gone down
almost by a factor of two in size. If
this rate continues, it could disappear
in just decades. So, in a few decades,
the storm could be gone. Now, of course,
we don't know exactly what powers it,
why it is what is the energy source? So,
is it losing an energy source? Is it
just dying out as storms do here on do
here on Earth? You know, after a month
or two, a tropical depression that
started off of Africa and came up
through over the Atlantic Ocean, will
[snorts] then eventually fizzle out and
be gone and will dissipate. So, are is
this just a much longer-lasting storm,
and we're just seeing the very end of
it? Or will some kind of energy source
recur? Will it either stabilize at a
smaller size, or will some energy source
be reappear, and therefore it comes
back, and will start to grow again?
These are all good questions, and I
can't really tell you anything except
that we've seen that it is slowly
shrinking. And that's all we know for
sure. What might happen with it in the
future, we are just going to have to
watch and wait and see.
Now, this is not the only giant storm in
the solar system. We also
have the Great Dark Spot, which was
visible on Neptune. That was seen in
1989 when Voyager 2 visited the planet.
Now, we didn't have the resolution to be
able to see Neptune, so we couldn't
study it again. But, by the time in the
mid-90s when Hubble was up and able to
look at Neptune, it was gone. Again, we
don't know how long these storms last.
They could last hundreds or thousands of
years, and maybe we caught this one
right at the end, or they may only last
a few years, and it may vary from planet
to planet. So, we really don't have
enough data yet to be able to say
anything in detail about these storms.
But, they do occur on all of the
planets. Saturn has had global storms,
as we see a great storm stretching
across the planet here.
And Uranus has a dark spot similar to
Neptune. But, again, remember, in order
to see any structure on Uranus, we have
to have a highly processed image. And
here we're looking at this little
section right here, which is then
enhanced to see a very a storm there.
Again, is it something seasonal because
of the tilt of Uranus? It has very
extreme seasons. For [snorts]
several decades, one pole is pointed
toward the sun, and then it will have
then the equatorial regions will go
through their turn, and then the other
pole will be pointed toward the sun.
So, [snorts] it has very unusual seasons
compared to any of the other planets.
But, we do see that these storms are
present every place we look in the giant
planets in the solar system.
So, let's go ahead and finish up with
our summary. And what we've looked at
this time is that the Jovian planets
have complex atmospheres.
The structures are determined by the
temperatures and internal heat sources
of the planets.
And large storms have been seen on every
single one of the Jovian planets, so not
just the Great Red Spot on Jupiter, but
other planets as well.
So, that concludes this lecture on
atmospheres of the giant planets. We'll
be back again next time for another
topic in astronomy. So, until then, have
a great day, everyone, and I will see
you in class.