Metis is Jupiter’s closest moon, and is located within that planet’s tenuous main ring. For this reason, it is still an object of some interest, even though its small size makes it otherwise quite unremarkable.
Jupiter possesses 63 known moons, more than any other planet in the solar system (Saturn, in second place, has 62, but all the other planets have far fewer). However, the vast majority of these – including Metis itself – are relatively small and unremarkable. Astronomers have been, for the most part, much more interested in its far larger siblings, the Galilean moons (Europa, Io, Ganymede, and Callisto). This lack of interest by astronomers is understandable: those moons have much more striking features, and Europa is even believed to have a massive underground ocean potentially capable of supporting primitive life. Metis, by contrast, is just 60 kilometres across in its widest dimension, and is too small to have been compressed into a sphere by its own gravity.
Because of its size, Metis was undiscovered until Voyager 1 passed by Jupiter in 1979, after which the newly discovered body was named after one of Zeus’s wives in Greek mythology. Zeus was also known as Jove or Jupiter, in the Roman pantheon; it is this name which has been applied to the giant planet since the time of the Roman empire. Although it has subsequently been imaged by the Galileo probe, which spent years orbiting Jupiter and studying its moons, the tiny moon is still not fully understood. Its mass seems much too low even for its small size, leading astronomers to suspect that much of the moon, beneath its red rocky surface, is water ice. It has never been seen except from a great distance, but photographs suggest that the rocks on the surface of the leading hemisphere have been worn away over time by impacts, exposing the comparatively shiny ice which lies beneath.
There is, however, one element to Metis which makes it more notable than the many other moons of its size which orbit Jupiter and Saturn: it is located directly within Jupiter’s main planetary ring. Jupiter’s rings are thin and themselves unremarkable compared to the massive disk surrounding Saturn, but are still more visible than the tenuous rings of Uranus and Neptune. Rocky material blown off the surface of Metis is believed to account for a large proportion of the dust in the ring, although some of the material would have been contributed by Jupiter’s other inner moons, as well.
Like all of Jupiter’s small inner moons, and Earth’s own Moon, Metis is tidally locked. This means that its day and its orbital period around Jupiter last the same amount of time – in its case, about seven hours.
