Page 51 - Curriculum Visions Dynamic Book

Each satellite has a density that suggests they are about 60% ice and 40% rock. Miranda and the smaller satellites are less dense and so must have a higher ice content. The surfaces of all satellites appear to consist of dirty water-ice.
Oberon and Umbriel have surfaces that are greatly marked with impact craters, suggesting they formed at an early stage in the evolution of the solar system. By contrast, Titania and Ariel have relatively few craters and are thought to be much younger, although there is no convincing explanation for this as yet.
Large cracks appear on all of the satellites, especially Miranda, where they are up to 80 kilometres wide and 15 kilometres deep. The cracking of the crust may not be due to any geological activity inside the satellites, but rather more probably due to the original liquid core freezing and so expanding as it turned to ice. But again there is no convincing theory to explain how water could once have existed in liquid form in such cold parts of the solar system.
The rings were not discovered until 1977. They are made of a dark, sootlike material, with boulders a metre or more across together with a little dust.
It is believed that the dust particles are pulled quite quickly to the planet’s surface, so that they must be formed continually.
The inner two satellites, Cordelia and Ophelia, act as shepherd satellites for portions of the rings.
aurora A region of illumination, often in the form of a wavy curtain, high in the atmosphere of a planet.
crater A deep bowl-shaped depression in the surface of a body formed by the high-speed impact of another, smaller body.
gravitational field The region surrounding a body in which that body’s gravitational force can be felt.
magnetic field The region of influence of a magnetic body.
magnetism An invisible force that has the property of attracting iron and similar metals.
mass The amount of matter in an object. pressure The force per unit area.
radiation The transfer of energy in the form of waves (such as light and heat) or particles (such as from radioactive decay of a material).
satellite An object that is in an orbit around another object, usually a planet.
shepherd satellites Larger natural satellites that have an influence on small debris in nearby rings because of their gravity.
solar wind The flow of tiny charged particles (called plasma) outward from the Sun.
 Two shepherd moons (ringed) confine one of the rings around Uranus.
 The moons Titania (far left) and Oberon (left).
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