Voyager 2 image of Uranus, enhanced to reveal atmospheric structure.

THE INTERIOR OF URANUS

Uranus at a distance of 2,875 million kilometres from the Sun might be expected to comprise the flimsiest materials, spun out towards the far edges of the Solar System as the pre-planetary disk was formed. In fact, Uranus has a far higher density than Saturn. It is far smaller in size than Jupiter, but because Uranus has a comparable density (1320 kg m-3), it must therefore comprise heavier materials. Jupiter's terrific mass compresses its lighter constituents, while Uranus has less overall mass and heavier component materials.

Uranus is assumed to contain silicate rock, ices, and gases. The ices include water (H2O), methane(CH4)and ammonia (NH3) . The gases present are hydrogen (H2) and helium (He). Deeper within the interior these are compressed and exist in a liquid state. As Uranus is far less massive than both Jupiter and Saturn the pressure within is not sufficient to compress hydrogen into a metallic form.

Uranus is also colder within. This is because it is less massive and not self-compressed or squeezed by its own gravity to the same extent as Jupiter, and because it does not have helium "rain-out" generating heat by friction as may happen within Saturn.

In some models of the interior, rock and ice are thought to comprise as much as 80% of Uranus' mass. The upper layers of hydrogen are liquefied. The current best estimate of the interior layout of Uranus ascribes 20% of Uranus' radius to a core of rock, which may also contain some metal. Moving outward, encasing the core, is a mantle (50% of the planets radius) of methane, ammonia and water ice. The outermost 30% of the planet's radius is a layer of liquid hydrogen and helium which towards the surface grades into the atmosphere of gaseous hydrogen and helium.

Cross-section of Uranus.

Evolution

Uranus formed at a distance from the Sun where lighter elements were more abundant during the early history of the Solar System. These condensed forming ices, contributing to Uranus' density. Even after the formation of the Solar System, Uranus may have had a significant amount of cometary material colliding with it, adding further water ice and rock to its bulk.

Unlike its larger counterparts, Uranus doesn't radiate very much heat, and it reflects back into space more energy than it emits. The amount of internally generated energy is a small fraction of that which is absorbed by the planet. Though the output is just 6% of that received, it is orders of magnitude greater than the energy emitted by the rocky planets of the inner Solar System.

The simplest explanation for Uranus' presently low heat output is Uranus' small size. Most of its heat was lost quickly, but the rate of cooling slowed and Uranus' cool surface helped the planet to maintain a small reserve of energy. The 98o axial tilt, (Uranus appears to move about the Sun on its side) has been a cause of a great deal of speculation. It is still very much a mystery how Uranus ended up like this. One popular idea, in the absence of a better explanation, is that it was involved in collision in its early history. A very large impact, of which there is now no trace, left Uranus toppled over. A stable ring system would not survive such a catastrophe. The rings could have been formed by the episode itself. Alternatively, they developed afterwards, perhaps the result of smaller planetary impacts and later on, the break-up of poorly consolidated satellite material.