5. Regular satellites in close up

Regular satellites of the giant planets have been described as ‘worlds in their own right’. ‘Regular satellites in close up’ describes the fascinating physical features and chemistry of Jupiter’s ‘Galilean moons’—Io, Europa, Ganymede, and Callisto—before considering Saturn’s moons, Titan, Enceladus, and Iapetus, as well as Miranda and Ariel, the moons of Uranus, and Triton, a moon of Neptune. These moons have very distinct characteristics and some are widely regarded as better candidates than Mars for hosting extraterrestrial life. It concludes with a look towards future space missions to observe and examine these distant moons.

3. The Moon’s influence on us

The Moon’s presence in the sky has long pervaded human culture in many ways. ‘The Moon’s influence on us’ considers the influence on timekeeping and how the orbits of the Moon and Earth are the origin of our calendar. Ocean tides are caused by the gravitational pull of the Moon and Sun on ocean water with the Moon’s influence being twice as strong as the solar tide. The elliptical nature of the Moon’s orbit affects lunar and solar eclipses; these are explained along with orbital recession and day-length changes. The Moon’s influence on human behaviour and wildlife is also considered, along with the potential of a more sustained lunar presence in the future.

8. Moons in other planetary systems

The first definite discovery of a planet around another star—an ‘exoplanet’—was made in 1995. We now know of more than 1,000 stars with exoplanets. In our Solar System, moons are considerably more numerous than planets, so it would be surprising if exomoons did not outnumber exoplanets. Only a few exceptional exoplanets have been seen by direct imaging and any exomoons are presently well below the visibility threshold. ‘Moons in other planetary systems: exomoons’ considers why exomoons matter. If hydrothermal vents on ocean floors really are a good place for life to begin, then icy exomoons with internal oceans throughout the galaxy could host microbial life.

6. The moons of Mars

The moons of Mars—Phobos and Deimos—were first discovered in 1877 by the American Asaph Hall. ‘The moons of Mars: captured asteroids’ explains that Phobos and Deimos are small rocky bodies in synchronous rotation. They are far too small for their own gravity to pull their shapes into hydrostatic equilibrium. Their densities, just less than twice that of water, are too low for them to be solid rock. It is more likely that below the surface regolith, their interiors consist of chunks of loosely packed rubble. Spectroscopically, both Phobos and Deimos resemble asteroids that are believed to equate to a class of meteorite known as carbonaceous chondrites.

1. The discovery and significance of moons

‘The discovery and significance of moons’ begins with the long-established Earth-centred (geocentric) view of the cosmos that was eventually replaced by Nicolaus Copernicus’ heliocentric model in the early 16th century, which had the planets including the Earth going round the Sun and only the Moon going round the Earth. The discovery of more planets and their moons by Galileo Galilei, Christiaan Huygens, William Herschel, and others is described along with the process of naming moons. There are 190 known moons of our Solar System’s planets, but is knowing of them any use? The ability to determine longitude and to measure the mass of the object about which the moons orbit was important.

7. Moons of small bodies

The small bodies of our Solar System consist of: asteroids (rocky or carbonaceous objects that are mainly concentrated in the space between the orbits of Mars and Jupiter); trans-Neptunian objects (icy bodies beyond Neptune’s orbit); comets (small icy bodies with strongly elliptical orbits that can come close to the Sun); and centaurs (asteroid-like bodies of dominantly icy rather than rocky composition, whose orbits lie beyond that of Jupiter, but inside Neptune’s). Of these, only comets are devoid of known moons. ‘Moons of small bodies’ describes some of the 165 asteroids known to have moons, as well as the moons of centaurs and trans-Neptunian objects, including the dwarf planet Pluto.

2. The Moon

The Earth’s Moon is the fifth largest moon in our Solar System and is made almost entirely of rock. The virtual absence of an atmosphere leads to a very large day–night range of temperature at the Moon’s surface of about 270°C. ‘The Moon’ outlines the phases, orbit, and rotation of the Moon and goes on to describe the face of the Moon seen from Earth and the farside of the Moon first observed in 1959 from the Soviet probe Luna 3. The impact craters observed on the Moon are explained, and the results of lunar exploration and dating of rock samples brought back are described, but what are the Moon’s origins?

4. The moons of giant planets

The giant planets Jupiter, Saturn, Uranus, and Neptune each have an extensive entourage of moons: small inner moonlets, closest to the planets, mostly less than a few tens of kilometres in radius and irregular in shape; next are large regular satellites exceeding about 200 km in radius; and then there are the irregular satellites mostly less than a few tens of kilometres in radius. ‘The moons of giant planets’ describes these different types of moons, the space missions to find them, their orbital resonance and tidal heating, as well as the spectacular and complex rings and shepherd moons of Saturn and the other giant planets.