Uranus, the ice giant.
This cold, bluish grey marble seems like a desolate waste in the far reaches of the solar
system, but in actuality there are some fascinating facts about this planet which makes it unlike
anything else in the solar system.
I’m Alex McColgan, and you’re watching Astrum.
Stick with me on this journey and we will explore everything you could want to know
about Uranus.
The first unique aspect of Uranus is its name.
All the planets are named after Roman gods, except for Uranus.
It’s named after the Greek god of the sky, Ouranos.
The Latinised version of this word is what we use today, Uranus.
Had they just kept the Greek version, it might have saved a bit of embarrassment as people
stumble over saying Uranus in a polite way.
It even has two ways to pronounce it as no-one has been able to definitively agree on the
matter, Uranus and Uranus.
Uranus is also very special in the way it rotates and orbits.
It is the 7th planet from the Sun, the second from last planet.
It orbits on average around 19.2 AU from the Sun, which means it is over 19 times further
away from the Sun than our Earth.
This varies throughout its year by 1.8AU, the biggest difference of any planet.
Being this far away from the Sun means it is freezing cold.
As in at the upper atmosphere, -220c cold, the coldest planet in the solar system.
Its year lasts 84 Earth years.
When it was first observed, astronomers predicted its orbit.
After some time though, they realised it hadn’t followed their predictions, and concluded
the reason was because there was another planet that had a gravitational influence on it.
They predicted where this planet should be and as a result, Neptune was discovered.
Interestingly, the same theory surrounds this “Planet X”.
Some far out objects in our solar system are not where they should be, and theory suggests
this is because of another planet that has a gravitational influence on them.
The hunt is now on to actually find this planet!
Now, what’s really interesting about Uranus is its rotation.
Most planets rotate like a spinning top on the table of the solar system plane.
Not Uranus, no Uranus has fallen over and is rolling instead.
You see, its axial tilt is 97 degrees.
This means its seasons are crazy in comparison to the rest of the planets.
During its solstice, the time of year when the Sun is highest or lowest in the sky, one
side of the planet always faces the Sun while the other is complete darkness.
It kind of looks like the planet is rolling forward along its orbit.
Only a very narrow strip near the centre of the equator of the planet experiences day
and night during this time, but the sun only rises just above the horizon.
The poles get 42 years of continuous darkness followed by 42 years of daylight.
During its equinox, the opposite of a solstice, the planet has more of a normal day-night
cycle.
Uranus rotates once every 17 hours and 14 minutes.
Because its surface is not solid though, some parts of the atmosphere rotate faster than
others, and due to high winds, some parts can make a full rotation of the planet in
only 14 hours.
This strange rotation means it is the only planet that gets more energy from the sun
at its poles on average than its equator.
For some reason though, the equator it hotter than at the poles, and no-one really knows
why.
Speculation also exists as to why Uranus rotates the way it does in the first place, although
it is generally accepted that a large earth sized planet crashed into Uranus, knocking
its rotation on its side.
How big actually is Uranus?
Well it is the least massive of the gas giants, at 14.5 Earths compared to Neptune’s 17
Earth masses.
Its diameter is just bigger than Neptune’s though at 50,700km, about 4 times more than
Earth’s.
Because this mass is spread out over a large area, the gravity on Uranus is only slightly
less than on Earth, at 8.7m/s2 or 0.89g.
Quite comfortable!
And what is it made of?
Well it has a rocky silicate material smaller than earth at the core, which is surrounded
by a mantle of water, ammonia and methane ices.
Although it’s referred to as ices, this mantle is in fact very hot, reaching almost
5000c, and is more like a liquid ocean surrounding the core.
The atmosphere is in comparison very insubstantial, only consisting of a total of 0.5 Earth masses,
most of the mass of Uranus being in the core and mantle.
The atmosphere is comprised of mostly helium, hydrogen and 2.3% methane and then a cloud
layer on top.
It’s this methane that give Uranus it’s aquamarine or cyan colour.
Very interestingly, some models suggest that the pressure at the base of the mantle on
Uranus is enough to break apart the methane molecules apart, which then compresses the
carbon atoms into diamonds.
These diamonds rain through the mantle like hailstones.
Unbelievably, the very base of the mantle could be a layer of liquid diamond, with solid
“diamond-bergs” floating in it.
To me, that's a bit difficult to wrap my head around!
We’ll fly away from the planet just a little bit now to have a look at its planetary ring
system.
Uranus, much like the other larger planets in our solar system, has rings.
It has thirteen, very dark and young rings.
Most are not bigger than a few kilometres wide and they are thought to only be 600 million
years old, much younger than Uranus.
They are comprised of extremely small particles, the biggest being only a few meters across,
made of water ice and dark radiation-processed organics.
Their albedo doesn’t exceed 2%, or in other words, darker than wet soil.
As we’ll see shortly, Uranus has a lot of moons, and the rings are thought to be the
result of high impact collisions of some moons in the past.
It is unclear why some of the rings are kept so narrow, the usual explanation being that
the rings are being kept in line by shepherd moons, but this is only the case for one of
the rings.
Uranus was first discovered to have rings in 1977 when an occultation of a star occurred.
The star dimmed a few times on either side of Uranus as Uranus moved in front of it,
confirming the presence of rings.
Uranus has only been visited by spacecraft once, and that was in 1986 by Voyager 2.
Voyager 2 discovered a lot of the rings and moons of Uranus, giving us close up shots
of the faint ring systems.
When Voyager flew by though, this only brought the total of known rings to eleven.
When Hubble was launched, it also had a look at Uranus, discovering two additional rings
that had never seen before.
The outermost ring is twice as far away from Uranus as the previously thought outermost
ring.
And as promised, here is a look at the many moons!
Unusually, the moons are named after figures in English literature.
Overall, Uranus has 27 known moons divided into three categories, the thirteen inner
moons, five major moons, and nine irregular moons.
The inner moons are connected with the rings of Uranus, some of which may have provided
the rings materials.
The largest of these moons is called Puck, at only 162km in diameter.
It is the only of the inner moons to be captured in detail by Voyager 2.
Interestingly, these inner moons constantly perturb each other and the system seems very
unstable.
There’s a good chance some of them may collide again in the future.
The five biggest moons, in order of distance from Uranus starting on the left are Miranda,
Ariel, Umbriel, Titania, and Oberon.
Titania is the largest moon of Uranus and the eighth largest moon in the solar system,
at 1,600km.
Again, as can be seen, these are very dark objects, Umbriel being the darkest.
With the exception of Miranda, which is comprised mainly of water ice, the rest are thought
to be a mix of water and rocky materials.
These moons may have differentiated interiors, meaning a core of rocky material with a mantle
of ice.
Between the core and the mantle could well be a layer ocean of liquid water.
Interestingly, the axial tilt of the large moons is the same as Uranus, meaning that
during solstice, if you were to look at the Sun, it would only ever move in a circle in
the sky, never setting.
During solstice, only one side of the moon faces the Sun, meaning a constant daytime.
The final nine known moons are irregular moons.
They are likely to be captured objects and are much further out than the last of the
big moons, Oberon.
They vary in size from 20km to the biggest, Sycorax, which is about 200km in diameter.
Finally, let’s explore Uranus’ climate and magnetosphere.
Uranus’ seasons are quite unique in the solar system due to its exceptional axial
tilt.
We only have data for about one Uranian year though, which means it’s difficult to be
able to say with certainty differences between the years.
What has been observed is that as the planet approaches solstice, the pole brightens and
a collar forms.
Moving away from solstice, the pole and collar dim.
This brightness is thought to be because of thickening of methane clouds, although the
cause is not clear.
Seasons also affect storms in the upper atomshpere.
Storms are relatively rare on Uranus compared to the other gas planets, but are thought
to be caused by changes in the seasons.
And another unique feature of Uranus is its unusual magnetosphere.
Usually magnetospheres originate from the geometric centre of the planet, but that’s
not the case with Uranus.
Also, it’s not in line with the rotational axis, but is 59 degrees off.
This unusual placement means the magnetosphere is much stronger at the north pole than at
the south.
One theory for this is the liquid diamond ocean could deflect the magnetosphere, or
even that it is not the core of the planet that produces the magnetosphere but rather
the water mantle.
The magnetosphere is about as strong as Earth’s, and because of its unusual rotation, the magnetotail
corkscrews off for millions of km into space.
Thank you so much for watching, I hope you’ve learned more about this rather mysterious
planet.
I was dreading making this video a bit because I was wondering what I can actually talk about.
Having done the research though, I found it one of the most interesting planets I’ve
covered so far.
If you want to support what I’m doing here, then I have social media links that you can
follow, and I’ve also started a Patreon page if you wanted to donate for the videos
I’m making.
Finally, I have made a lot of other videos on space and other planets you may be interested
in, so be sure to check them out!
See you next time!