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 make it unlike anything else we have seen before. I’m Alex McColgan, and you’re watching
Astrum. Stick with me on this journey and we will
explore almost 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. -220°c cold, which makes it the coldest planet
in the solar system. Its year lasts 84 Earth years. When it was first observed, astronomers attempted
to predict its orbit. After some time though, they realised it hadn’t
followed their predictions, and concluded that the reason was because there was another
planet that had a gravitational influence on it. They mathematically predicted where this planet
should be and as a result, Neptune was discovered. Interestingly, the same theory surrounds this
as yet undiscovered “Planet X”, or “Planet Nine”. 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 for large portions of its year. 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, or the time of year when
the Sun is highest or lowest in the sky, one hemisphere of the planet always faces the
Sun while the other is in 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 does
only rise just above the horizon. The poles, on the other hand, get 42 years
of continuous darkness followed by 42 years of daylight. During its equinox, which is the opposite
of a solstice, the planet has more of a normal day-night cycle. Uranus is currently leaving its equinox, having
passed it in 2007, and is now heading back towards a solstice. Uranus rotates once every 17 hours and 14
minutes. Because its surface is not solid, however,
some parts of the atmosphere rotate faster than others, and due to high winds, some sections
can make a full rotation of the planet in only 14 hours. This strange rotation and axial tilt means
it is the only planet in the solar system that gets more energy from the Sun at its
poles than at its equator on average. For some reason though, the equator is 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 though is just bigger than Neptune’s
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. That would feel quite comfortable! And what is it made of? Well, it is believed to have a core just smaller
than Earth, of rocky silicate material, 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. So, to call Uranus a gas giant is a bit disingenuous,
it certainly is not gaseous all the way through. The atmosphere is in fact very insubstantial
in comparison, only consisting of a total of 0.5 Earth masses, with most of the mass
of Uranus being in this 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 the methane molecules
apart, which then compresses the carbon atoms from the methane into diamonds. These diamonds rain through the mantle like
hailstones. The very base of the mantle could be a layer
of liquid diamond or carbon, with solid “diamond-bergs” floating in it. 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 with 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 been 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 inner moon 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 that 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 an ocean layer 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. Telescope technology has only allowed us to
resolve details on the surface of Uranus for the last few decades, which means it’s difficult
to be able to say with certainty if there are changes between Uranian years. What has been observed though 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 due to the thickening
of methane clouds, although the cause is not clear. Seasons also affect storms in the upper atmosphere. Storms are relatively rare on Uranus compared
to the other gas planets but are thought to be caused by changes in the seasons. With the improvements in telescope technology,
we have also been able to observe bands stretching around the planet, much like the other gas
planets. However, these bands are mainly visible in
the infrared, which is why Voyager was only able to show us this in visible light. In these infrared images, you can also see
small storms dotted all over. 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 at all, but rather the liquid 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. So, there we have it, almost everything you
could want to know about the fascinating world of Uranus. Thank you so much for watching, I hope you’ve
learned more about this rather mysterious planet. Interested in learning about the other planets
in our Solar System? Check out this playlist here and be sure to
subscribe for future updates. All the best and see you next time!
Stop lying to yourself, we all thought about that joke when we saw if.
U-Uranus by Man or Astroman?
https://youtu.be/RFWFN-lzZ48