MIRANDA
The chiselled satellite
Mass: 6.6 x 1019kg (1.5 x 1020lbs) Diameter: 470km (292 miles) Parent planet: Uranus Discovered: 1948, Gerard Kuiper
Miranda is one of the strangest worlds in the Solar System. Voyager images revealed an extraordinary patchwork of terrains, seemingly put together at random. Some parts are heavily cratered and some relatively uncratered – indicating their youth, as they have been less exposed to bombardment. One prominent feature is a pattern of concentric ovals resembling a race track, while elsewhere parallel V-shapes form a chevron-like scar.
An early theory to explain Miranda’s jumbled appearance is that it is a Frankenstein world – a collection of fragments from a predecessor moon that coalesced in orbit around Uranus. Astronomers wondered whether Miranda’s predecessor might have been shattered by an interplanetary impact, and whether this cataclysmic event might somehow be linked to Uranus’ own extreme tilt. Further studies, however, have shown that such a theory comes up short when trying to explain Miranda’s mix of surface features, and the right kind of impact is unlikely. Instead it seems plausible that tidal forces are to blame.
Today Miranda follows an almost-circular orbit, but in its past its orbit was in a ‘resonant’ relationship with larger moon Umbriel. This brought the two moons into frequent alignments that pulled Miranda’s orbit into an elongated ellipse that experienced extreme tidal forces. Pushed, pulled and heated from within, its surface fragmented and rearranged itself before the moons moved again and Miranda’s activity subsided.