The moons of our Solar System
Sen—Our Solar System is home not just to eight planets, but 173 moons which orbit them.
There may be more to discover—several moons have been found in recent years. Two small moons (2km - 3km diameter) were discovered orbiting Jupiter in 2010 and confirmed in June 2011 by the International Astronomical Union. In 2013 another satellite was found to be orbiting Neptune, the 14th moon discovered around the ice giant.
We have discovered that not all moons are dead cratered worlds. Our exploration of the Solar System has instead found a diverse range of worlds, some with subsurface oceans, others geologically active with giant geysers and volcanoes, and Titan, with a thick atmosphere.
Starting with the planet closest to the Sun, the moons so far revealed are: Mercury - 0, Venus - 0, Earth - 1, Mars - 2, Jupiter - 67, Saturn - 62, Uranus - 27 and Neptune - 14.
Our own Moon is still the subject of exploration. Across the planet companies are competing to send a robot to the Moon to win the Google Lunar XPRIZE. Others are offering manned missions - Space Adventures and Excalibur Almaz will fly you to the Moon for US$150m per astronaut.
Further out at Mars are two small moons, Phobos and Deimos. Sadly in 2011 the Russian Phobos-Grunt probe failed in the early stages of its mission to the larger of the red planet's two moons. Phobos orbits closer to its parent planet than any other moon - at just 6,000km above the Martian surface. The rocky moon, which is the larger of the two, is still small measuring approximately 27 by 22 by 18 km in diameter.
Around the Solar System's largest planet, Jupiter, are the most moons. Jupiter's four largest moons - Io, Callisto, Europa and Ganymede - are known as the Galilean moons after Galileo Galilei who discovered them in 1610.
Io is the most volcanic place so far discovered in the Solar System. As a small frozen moon in the outer Solar System its geological activity came as a surprise to scientists, who reason that Io's internal heat is generated from a complex gravitational tug of war, the small moon being subjected to the gravitational forces of both Jupiter, Europa and Ganymede. The effect of this gravitational interplay is that Io is squashed and stretched creating an internal heat source. Io has huge volcanic eruptions that cover the surface in sulphur giving the volcanic moon it its yellow colour.
Callisto, Europa and Ganymede all have internal oceans where microbial life may exist. These moons will be explored by the European Space Agency's Jupiter Icy Moons Explorer which will arrive at Jupiter in 2030. We already have some knowledge of these moons gathered by NASA's Galileo probe between 1995 and 2003.
Jupiter's moons range in size from just 2km diameter to Ganymede, the largest moon in our Solar System with a diameter of 5,260 kilometres—larger than the planet Mercury. Ganymede has its own magnetic field which could be explained either by a liquid iron outer core around its solid heart or an ocean beneath the surface.
Callisto is the outermost of the four Galilean moons, orbiting the gas giant at 1.88 million km. It is Jupiter's second biggest satellite and the third biggest in the Solar System with a diameter of more than 4,800km.
Europa has fascinated scientists because its surface does not appear cratered like other moons, but instead has an icy white surface marked with cracks that resemble the ridges of ice found in the Arctic. Measurements of a magnetic field add to evidence that a salty subterranean sea is wrapped around the moon which could be more than 100km deep and a prime place to search for alien life. Europa is 3,140 km in diameter and lies 671,000 km from Jupiter.
Further from the Sun we come to Saturn where 62 moons have so far been found. Around Saturn, worlds that have created particular interest are Enceladus and Titan.
Enceladus is a very small icy moon, only about 500 kilometres (300 miles) across. Its icy surface is highly reflective of the sunlight that reaches it making it extremely bright. Enceladus is geologically active and in the south pole region are over 90 jets that spray water vapor, icy particles, and organic compounds into space. Cassini has flown several times now through this spray and found that there is salt in the icy particles. Scientists believe that the likely source of the jets which spurt out of Enceladus' tiger stripes—four parallel claw marks scratched into the smooth icy surface, about 120 km long—is a subterranean sea.
Titan is the second largest moon in the Solar System and Saturn's largest moon. Titan has a dense atmosphere made primarily of nitrogen and methane. The images of Titan sent back by Cassini show this atmosphere as a thick blue line surrounding the moon. Under the atmosphere we have explored the surface of Titan with the Huygens probe which landed on the surface in 2005, taking pictures as it descended through the atmosphere. The pictures taken by Huygens and Cassini have revealed a landscape similar to one that you can find on Earth, with mountains, lakes and river beds covered in rounded stones and pebbles that had clearly had their surfaces smoothed by flowing liquid.
Beyond Saturn, Uranus has 27 moons, named after characters from Shakespeare's plays. Oberon and Titania are the largest and were discovered by the astronomer who discovered Uranus itself - William Herschel. Three more moons were discovered before the Voyager 2 spacecraft identified ten further moons. The inner moons appear to be half water ice and half rock. Since Voyager 2 some very small moons - 8 to 10 miles across - have been discovered by Hubble.
Finally, the ice giant Neptune has 14 satellites, the largest being Triton, discovered in 1846 by brewer and astronomer William Lassall. The third largest moon, Nereid, was discovered in 1949 by Dutch astronomer Gerard Kuiper. The second largest moon, Proteus, is dark and close to Neptune and was discovered along with five other satellites by Voyager 2. Further moons were observed from Earth in 2002 and 2003, and Mark Showalter discovered a 14th moon in 2013 whilst studying images taken by Hubble between 2004 and 2009.
Beyond Neptune lies Pluto, with its five moons, and then the Kuiper Belt, ensuring there are a vast number of celestial bodies orbiting our star.