By Sabine Stanley, Ph.D., Johns Hopkins University
What does it mean when we say that Uranus is an ice giant? And how does that differ from planets that are referred to as gas giants? Does the structure of Uranus has an impact on the nature of night and day and seasons as we know them on Earth? Let’s explore these and other questions about Uranus.
In the 1970s, scientists had an opportunity that only comes around once every 176 years. A rare planetary alignment meant that launching a spacecraft would allow for a mission that could fly by all four giant planets, using gravity boosts from each to shorten the total travel and make the course corrections needed to reach all four planets.
This opportunity could not be passed, and so NASA decided to launch two spacecrafts at this time: Voyager 1 and 2. Ultimately, it was decided that Voyager 1 would skip going to Uranus and Neptune in favor of getting close to Saturn’s moon, Titan. That left Voyager 2 to visit Uranus and Neptune. A lot was riding on Voyager 2 as it approached Uranus in January 1986.
Since the 1940s, scientists knew that Uranus and Neptune had a larger ice fraction than Jupiter and Saturn, but the term ‘ice giant’ seems to have been coined in the late 1970s as a way to describe Uranus and Neptune. So Voyager 2 was to be the first mission to explore a different type of planet: an ice giant.
Learn more about Uranus, a water world on its side.
What Is an Ice Giant?
For planetary scientists, ice does not mean that Uranus has a frozen solid surface. Rather, ice is a compositional type. It refers to materials made up of combinations of elements slightly heavier than hydrogen and helium—things like carbon, oxygen, nitrogen, and sulfur.
These elements, when combined with hydrogen, can form molecules like water, ammonia, methane, and other hydrocarbons.
This is similar to how Jupiter and Saturn are called ‘gas giants’, even though most of their hydrogen is under extreme pressure and exists as a liquid metal.
Gas was just shorthand for hydrogen and helium. So ice here is just shorthand for a compositional mix, regardless of how much of those elements are solid, liquid, or gas.
Learn more about mighty Jupiter, the ruling gas giant.
Differences Between Ice Giants and Gas Giants
The most important difference between the ice moons encountered at Saturn and Jupiter, and an ice giant like Uranus, is that the planet is big. The diameter of Uranus is four times larger than Earth.
The bulk of an ice giant is icy materials, just like an ice moon. But because it’s so much more massive, it’s also able to capture some of the hydrogen and helium gas in the protosolar nebula. This gives the ice giant an atmosphere similar in composition to that of Jupiter or Saturn, but just not as much atmosphere.
Like gas giants, ice giants also don’t have a surface the way terrestrial planets do. Instead, the ice giants have hydrogen-helium rich atmospheres which transition directly to a sort of magma, that’s a flowing mixture of water, ammonia, and methane—all possibly surrounding a core of rock-rich materials deep in the planet.
In fact, Uranus and Neptune both sort of look like what one would expect Jupiter and Saturn to look like, if one added much less of the surrounding hydrogen and helium layer. So one can think of the ice giants as looking something like the cores of the gas giants.
Learn more about how the solar system family is organized.
The Structure of Uranus
This sort of structure makes sense when we think of how planets formed. In the early solar system, while all the planets were forming, they started out by creating protoplanets from the condensed materials available at their location. In the inner solar system, the temperatures only allowed rocks and metals to condense. But in the outer solar system, ices also condensed.
So all four giant planets had additional resources for creating larger protoplanets. But Jupiter and Saturn are closer to the crowded center of the protoplanetary disk and so move faster in their orbits.
Moving fast in a crowded part of space meant that they were able to collect protoplanetary material faster than Uranus and Neptune. In fact, they did it so fast that they grew large enough to start gravitationally attracting the hydrogen and helium gas from the solar nebula.
Out at the distances of Uranus and Neptune, there was less protoplanetary material to collect, and the slower movement around the solar system meant that these planets formed slower.
It is thought that Uranus and Neptune don’t have large gas envelopes because their cores didn’t get big enough, fast enough, to attract a large amount of gas from the solar nebula before the Sun got large and bright enough to create solar winds that blew away the remaining free gas in the nebula.
This is a transcript from the video series A Field Guide to the Planets. Watch it now, Wondrium.
Uranus Doesn’t Experience Normal Days or Seasons
In 1986, 4.5 years after Voyager 2 flew by Saturn, the spacecraft approached Uranus. Out here, we are twice as far from the Sun as Saturn, and 20 times farther than Earth.
Uranus takes 84 Earth years to complete one orbit; that means Uranus has completed less than three orbits since it was discovered in 1781. It also means that only about two seasons of a single year have elapsed since Voyager 2 encountered the planet back in 1986.
Because of how the planet’s axis is tilted, seasons on Uranus are quite different than on other planets. Uranus’s rotation axis is about 98° away from its orbital axis. This means that Uranus kind of looks like it’s rotating on its side. But Uranus is not only rotating on its side, it’s also traveling along its orbit. Thus, this planet doesn’t experience normal days and nights, or normal seasons.
If one lived in the northern hemisphere on Uranus, at latitudes similar to the US or Europe, then in the summer, he would constantly face the Sun, experiencing no night. It would be a single summer day that would last 21 Earth years. And if one didn’t head south for winter, he would go through a single night lasting another 21 years.
Since the axis tilt isn’t exactly 90°, there is a small swath near the equator where one could live and experience day-night cycles during these times, but the Sun would be very low on the horizon.
The story is quite different in spring and fall. As Uranus moves along its orbit, different latitudes begin to face the Sun. Near the spring and fall equinoxes, most latitudes experience typical day-night cycles, similar to those on Earth.
Learn more about Saturn’s moons.
Why Is Uranus Tilted Sideways?
Planets like Earth, Mars, and Saturn have tilts around 25°, while some planets have little to no tilts, like Jupiter and Mercury. Then there’s Venus whose upside down tilt is a whopping 177°. But Uranus is the only planet rotating on its side.
The most common explanation for the tilt of Uranus is that while the planet was forming, it suffered a glancing blow from an Earth-sized protoplanet. Such an impact could have torqued the planet, changing its rotation vector. In some ways, this makes the most sense considering that the same mechanism is used to explain other planetary tilts.
Common Questions about the Structure and Seasons of Uranus
NASA decided that the Voyager 1 spacecraft would be dedicated to getting close to Saturn’s moon Titan, while the Voyager 2 spacecraft would visit Uranus and Neptune.
Uranus is known as an ice giant because its compositional mix contains materials made up of combinations of elements slightly heavier than hydrogen and helium, such as carbon, oxygen, nitrogen, and sulfur. The term ice here is a shorthand for the compositional mix, regardless of how much of those elements are solid, liquid, or gas.
Uranus’s rotation axis is titled. It is about 98° away from its orbital axis, which means Uranus appears to be rotating on its side. It is the only planet rotating on its side.
Uranus has strange seasons due to its sideways tilt. So, in its northern hemisphere, during summers, you would constantly face the Sun, experiencing no night. It would be a single summer day that would last 21 earth years. And during winters, you’ll have a single night lasting another 21 years.
