The biggest moons in the Solar System
Sen—While many think of Earth's Moon as a large satellite, there are many others in our Solar System that are much bigger. A glance at the ten largest satellites of Earth, Jupiter, Saturn, Uranus and Neptune show a varied group of moons. Here's a snapshot of what each one of them is like.
Ganymede (diameter: 5,262.4 km)
Gigantic Ganymede is so large that it eclipses the sizes of Mercury and Pluto, and even rivals the size of Mars. According to NASA, Ganymede is made up mostly of ice and likely includes a bit of rock. Pictures of the surface show roughly 40 per cent is cratered and dark, with 60 per cent grooved and lighter.
It was discovered by Italian astronomer Galileo Galilei on Jan. 7, 1610 and was probed in the most detail by the Galileo spacecraft, which orbited Jupiter between 1995 and 2003. Notable discoveries include a thin oxygen atmosphere, lumps (possible rock) below the ice, and very flat craters on its surface.
Ganymede, a moon of Jupiter. Image Credit: NASA
Titan (diameter: 5,152 km)
Titan was an enigma when the first spacecraft (Voyager 1 and 2) flew by it in the 1980s. It was completely shrouded by orange haze, making it impossible to see the surface. More detailed close-up observations would need to wait until the early 2000s, when the joint European Space Agency-NASA Cassini mission began regular flybys of the moon and also sent a probe, Huygens, to the surface.
Titan was discovered on March 25, 1655 by the Dutch astronomer Christian Huygens. Notable features include evidence of a subsurface ocean, lakes of methane and ethane, and an atmosphere of nitrogen, methane and hydrocarbons. With its lakes and seas Titan is the only other world in the Solar System with liquid on its surface.
Titan, a moon of Saturn. Image Credit: NASA/JPL/Space Science Institute
Callisto (diameter: 4,820.6 km)
Callisto is a moon that is heavily pockmarked with craters and that likely died (geologically speaking) long ago, which makes it unusual among other moons in the Solar System. This means that its surface is likely the oldest among the moons.
It was also discovered by Galileo and is nearly the size of Mercury. Despite its unchanging surface, it's possible (based on measurements of its magnetic field) that Callisto may contain an underground ocean, leading to speculation that it could contain microbial life.
Callisto, a moon of Jupiter. Image Credit: NASA/JPL/DLR(German Aerospace Center
Io (diameter: 3,643.2 km)
Io is yet another Galilean moon, and is best-known for being volcanically active. Its blotched surface bears evidence of extensive eruptions that have been caught by spacecraft that travelled near the moon, such as NASA's Galileo and New Horizons spacecraft. The volcanoes are believed to be caused by the gravitational forces exerted on Io by Jupiter.
Other notable features of Io include a "plasma torus" of material that extends well beyond its surface, auroras in the atmosphere, and a surface likely made of sulfur or silicate rock. It was last examined regularly by Galileo, which looked at Jupiter and its moons in the 1990s and 2000s.
An eruption on Io, a moon of Jupiter. Image Credit: NASA
Moon (diameter: 3,475 km)
Our planet's Moon is the only one that has been touched by humans. In the 1960s and 1970s, 12 people walked upon its surface and gathered hundreds of pounds of rocks. This helped scientists formulate a leading theory for the Moon's formation; the thinking is a Mars-sized object crashed into Earth during our planet's formation. The pieces from the collision eventually coalesced into our Moon.
In more recent years, extensive evidence of ice has been found in the Moon's regolith. This means it could be possible for future lunar colonies to live off the land to an extent, making it somewhat easier to do exploration. Other interesting features include dust that hovers at the day-night boundary, and a tenuous exosphere.
Earth's moon. Image Credit: NASA
Europa (diameter: 3,121.6 km)
Europa is an icy cueball-like moon with a cracked surface, overlying a probable global ocean below. In 2013, observations from the Hubble Space Telescope detected water vapor above Europa's surface, suggesting that water plumes might be erupting from the moon. These findings, however, give more credence to theories that there could be life in the waters below.
Europa was also discovered by Galileo. It appears to have a very young surface (only 40 to 90 million yeras old) and also is covered with reddish-brown streaks, possibly salty compounds. Like Io, it is greatly warped by interactions with Jupiter.
NASA are planning a mission to Europa to launch in the 2020s to try and discover if the moon has conditions suitable for life. The European Space Agency is also working on a probe to study Europa, as well as Ganymede and Callisto—its Jupiter Icy Moons Explorer, or JUICE, is due to arrive at the Jovian system in 2030.
Europa, a moon of Jupiter. Image Credit: NASA/JPL-Caltech/SETI Institute
Triton (diameter: 2,706.8 km)
Triton orbits opposite to the direction of Neptune and also has a partially melted and reformed surface. This leads scientists to suggest it could be a captured object from an icy collection of small bodies in the Kuiper Belt. When more information is gathered about Pluto with the New Horizons mission this year, it will be compared closely to Triton to see how similar the two bodies' histories are.
Triton was discovered in October 1846 by William Lassell just over two weeks before Neptune itself was found. So far it has only been seen up close by one spacecraft, Voyager 2. It is believed to have an atmosphere largely composed of nitrogen, with a rocky interior. There also are volcanoes on the surface, as shown by the portions of methane in its nitrogen atmosphere.
Triton, a moon of Neptune. Image Credit: NASA/JPL/USGS
Titania (diameter: 1,577.8 km)
Little is known of Titania since the information we have comes from a brief flyby by Voyager 2. The moon of Uranus was discovered in January 1787 by British astronomer William Herschel, but remained a point of light until the spacecraft flew by nearly 200 years later.
Pictures from Voyager 2 showed several large fault valleys on the moon, suggesting extensive geologic activity in the past. There also is some sort of reflective material, perhaps frost, that is scattered on parts of the surface.
Titania, a moon of Uranus. Image Credit: NASA/JPL
Rhea (diameter: 1,529 km)
Rhea was found in December 1672 by Giovanni Cassini, and the spacecraft bearing its name has done several observations of the small moon in recent decades. Like many other moons in the Solar System, it's likely mostly made up of ice and rock. It also is very reflective.
There are two major types of terrain on Rhea, including heavily cratered zones and less-cratered areas that have larger pockmarks. This may show evidence of some kind of major resurfacing event in Rhea's past.
Rhea, a moon of Saturn. Image Credit: NASA/JPL/Space Science Institute
Oberon (diameter: 1,522.8 km)
With close-up views only obtained by Voyager 2, Oberon still has many mysteries left to probe. The moon, discovered by William Herschel in January 1787, is one of the more extensively cratered among its neighbours at Uranus, and has a composition of roughly equal parts ice and rock. A tall mountain (6 km high) also was spotted on its surface.
Oberon, a moon of Uranus. Image Credit: NASA/JPL