Uranus- Planet of Diamonds

William Herschel portrait 1805 by James Sharples

This science article investigates the features and characteristics of planet Uranus.

William Herschel was the 18th century German born British astronomer who discovered Uranus. Starting out as a professional musician he soon developed a reputation as an accomplished amateur astronomer.

At a time when telescopes were largely ineffective he built his own reflector telescopes which he used to make astronomical discoveries. He sealed his reputation with the discovery of planet Uranus in March 1781 when he was observing the stars from his back garden at 19 New King Street, Bath, South West England. (Read Herschel-King’s Astronomer)

The discovery of Uranus led to his appointment as the king’s personal astronomer.

Herschel's house in New King Street Bath

19 New King Street, Bath

Characteristics of Planet Uranus

Uranus is 1.8 billion miles (2.9 billion kilometers) from the sun. It is the seventh planet from our star.

planets of solar system showing all planets and planetoids

A Uranian day, the time it takes for the planet to rotate once, takes 17 earth hours.

Uranus in infrared taken by Keck Observatory in 2007.

Image credit: Keck Observatory

Uranus is unique among the planets in our solar system in that it rotates horizontally on its axis.

This unusual axis of rotation is believed to have resulted from collisions with one or more planet-sized bodies early on in Uranus’s history, knocking the planet onto its side.

Obliquity of the Nine Planets in our solar system compared

This bizarre horizontal axis of rotation means that the planet has amazingly long seasons. By 1986 the south pole had been in permanent daylight for twenty one years! At the same time the north pole had been in permanent darkness for twenty one years!

Uranus and its orbit around the Sun showing the planet's tilt and seasons

In six earth years, between 2001 and 2007, Uranus moved only 24° counter-clockwise around the sun.

six years of a uranian orbit showing planet from 2001 to 2007

Only another 78 years and 336° to go before completing one full orbit of the sun!

six year orbit of Uranus around the Sun

In this image captured in 1998…

Uranus taken from Hubble showing raging storms

…..you can see the northern hemisphere coming out of its 21 year winter and state of frozen hibernation. As the sunlight warms up the northern hemisphere, methane gas rises up from the lower atmosphere. The methane gas then condenses in the higher atmosphere to form crystals of methane which become visible as white clouds.

With the warming up of the atmosphere massive storms develop with winds that reach incredible speeds of more than 300 mph! (563kph) The dark spot in the image below was captured in 2007 and shows a storm 1,900 miles long and 1,100 miles wide (3,000 km by 1,700 km).

Dark spot on planet Uranus believed to be a storm

Structure of Uranus- the rocky core

Uranus has a small solid core made of iron and nickel. Just the core alone is the size of planet earth! The core can reach temperatures of up to 4,700 C. (8,492F) This temperature seems very high but in actual fact is quite low when compared to Jupiter’s core temperature of around 35,500 C! (63,932F)

Internal structure of Uranus showing composition of upper atmosphere, mantle and core

Structure of Uranus- the ‘ice mantle’

The ‘ice mantle’ lies above the solid core. The ‘ice mantle’ is not formed from ice as we know it but from a hot and very dense mixture of fluids consisting of water, liquid ammonia and liquid methane. Inside the ice mantle water, ammonia and methane adopt a liquid form- and not a gas form- as a result of incredibly high pressures and temperatures that exist within.

These intense pressures and high temperatures provide the perfect conditions for the production of diamond crystals!!

diamonds falling simulating conditions in mantle of Uranus

Diamonds are formed after liquid methane, a form of carbon, is subjected to intense heat and pressure. The resulting diamond crystals sink slowly through the mantle towards the solid core.

It is thought that the larger diamond crystals come to rest as ‘diamond bergs’ at the base of the mantle just above the rocky core. The smaller diamond crystals do not stay in their solid state but melt under the intense heat near the core and revert to their liquid methane form.

Structure of Uranus- the atmosphere

On Uranus there is no definitive boundary between the mantle and the atmosphere. There is no way of knowing where the mantle ends and the atmosphere begins. As the pressure decreases with distance outwards from the core the solid ice in the mantle turn to less dense icy slush.

ice turns to slush with less pressure

At the point where the pressure is too low for water, ammonia and methane to exist in their liquid states, the icy slush completely disappears and the gaseous atmosphere begins.

The Uranian atmosphere is mostly composed of hydrogen, helium and methane gases. The atmosphere is believed to contain highly complex layers of clouds including water clouds lower down near the frozen mantle, methane clouds at the highest altitudes with what is believed to be ammonium hydrosulfide and hydrogen sulphide clouds somewhere in between.

Contrast Uranus with planet earth where it is entirely obvious where the land and ocean ends and the atmosphere begins.

Fjord near Longyearbyen, Spitzbergen, Norway

Force of gravity

The force of gravity on Uranus is 10% weaker than the force of gravity here on earth. The rate of acceleration for objects falling through the atmosphere of Uranus is 8.7m/s² compared to a rate of acceleration of 9.8m/s² on earth.

Considering that Uranus has a mass 14 times greater than the mass of the earth it appears strange that Uranus’s force of gravitational attraction is 10% weaker than the earth’s…..

Uranus,Earth size comparison

…. until you remember that Uranus is a gas giant with a massive frozen mantle of liquid ice and a comparatively small rocky core.

Earth has a dense outer core, a dense rocky mantle and a dense crust- all of which combine to give the earth a greater force of gravitational attraction than Uranus.

Atmospheric pressure on Uranus

At the altitude of the highest clouds- the methane clouds- the atmospheric pressure is believed to be equivalent to the atmospheric pressure at sea level on earth. (1 bar, 14.69psi or 101.3 kilopascals) The atmospheric pressure steadily increases with decreasing altitude towards the mantle. Where the atmosphere meets the mantle the atmospheric pressure is believed to reach a bone crushing 1,086 bars !! (15, 570 psi or 108,592 kilopascals)

For comparison purposes 1086 bars is equivalent to the pressure found on earth at the bottom of the deepest ocean- the Marianas Trench in the Western Pacific. The bottom of this trench lies 6.8 miles (11kms) below sea level.

Marianas Trench showing surrounding guyots

A ‘guyot’ is an isolated underwater volcanic mountain

If you look at this diagram of Uranus…

Cutaway of Uranus showing thicknesses of atmosphere, mantle and core

…you will notice that the atmosphere extends to an incredibly high altitude of 4460 miles (7,500 kms) above the mantle. At the point where the mantle meets the atmosphere the weight of gas molecules stacked on top of each other is truly massive. With thousands of miles of gas molecules stacked on top of each other comes an incredibly high atmospheric pressure;

In the same way, see how the downwards pressure of books at the bottom of this stack is greater than the pressure on the books at the top of the stack.

stack of books

Every molecule of gas has mass. It is the force of gravitational attraction that gives weight to each molecule in the Uranian atmosphere. The massive force exerted by the weight of the gas molecules in the Uranian atmosphere is the cause of the high atmospheric pressure. (See Understanding Gravity and Mass)

Atmosphere of Uranus explained

Compare the thickness of the atmosphere of Uranus with the thickness of the atmosphere here on earth. On earth most of the molecules of gas, that is 75% of our atmosphere, are found within the first 6.2 miles (16 kms) of the atmosphere above the surface of the earth.

The thinner atmosphere that surrounds our planet means that the force exerted by the weight of those gas molecules is less on earth. As a consequence the atmospheric pressure on earth is less despite the fact that the force of gravitational attraction is greater.

cross section of layers of Earths atmosphere

Temperatures on Uranus

As a result of its great distance from the sun (1.8 billion miles or 2.9 billion kms) Uranus receives 360 times less light and heat from the sun than earth receives.The much smaller amount of solar radiation that does reach Uranus is insufficient to heat up the planet’s atmosphere.Temperatures in the atmosphere routinely reach an incredibly low -224°C in places. These are among the lowest atmospheric temperatures found on any planet in the Solar System.

Uranus also lacks a powerful internal source of heat in its core; the planet is believed to generate about as much energy from its core as it receives in radiation from the sun. If Uranus had a powerful internal heat source which radiated more heat from the core, temperatures on Uranus would be far higher.

Contrast temperatures on Uranus with the lowest temperature ever recorded on earth; a ‘warm’ −89.2 °C (−128.6 °F) recorded in 1983 at the Vostok Research station in the Australian Antarctic Territory.

Lake Vostok Station, Antarctica shown on satellite image of Antarctica

Uranus- the inner moons

Uranus has 13 inner moons all of which orbit Uranus inside ‘Miranda’. All inner moons are probably composed of half water ice and half rock.

Uranian inner moons and rings shown on a diagram

Puck is the largest of the inner moons and has a diameter of about 90 miles. (150 km) From a distance of 36,039 miles (58 ooo kms) above the Uranian cloud tops a single revolution of Uranus takes 18 hours and 17 minutes.

Image of Puck taken by Voyager 2 in its fly past in December1986

The orbits of the inner moons around Uranus are influenced by each other’s forces of gravitational attraction. We use the word ‘perturb’ to describe how the inner moons constantly interfere with each other’s orbits. As a consequence of this ‘perturbation’ the orbits of the inner moons are chaotic and unstable.

Some moons may eventually cross orbits which will lead to collisions. It is thought that ‘Desdemona’ may collide with either ‘Juliet’ or ‘Cressida’ within the next 100 million years.

collision between two moons-relevance to possibility of two inner moons of Uranus colliding

Uranus- the major moons

Beyond the orbit of Puck Miranda Uranus has five major moons Miranda, Ariel, Umbriel, Titania and Oberon. These major moons are shown below in order of distance from Uranus. Titania has the largest diameter at 980 miles (1578 kms) and Miranda the smallest with a diameter of 293 miles (472kms).

Major moons of Uranus

These major moons may have been formed from the ‘accretion disk’ which existed around Uranus for millions of years after its formation.

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