Miranda doesn’t seem like a particularly important moon. It is one of five main satellites orbiting Uranus, travelling along the innermost orbit. It is only 472km in diameter - the smallest of the five major moons. While its surface appears bright, it is in fact second to Ariel in brightness. But Miranda’s unassuming statistics hide a secret. As Voyager 2 passed by Uranus in 1986, it sent back images of an unexpectedly fascinating world.
Nicknamed the ‘Frankenstein moon’, Miranda’s surface merges together like mismatched patchwork. Smooth areas appear alongside heavily cratered ones, and gigantic cliffs give way to deep canyons. However, the most interesting features of this bizarre coalition are the three giant coronae. In this image, the coronae are the “chevron” figures enclosed by concentric lines. Each corona measures around 200km across, and is surrounded by parallel grooves and concentric ridges. They are shaped roughly like trapezoids, giving the impression that Miranda doesn’t quite fit together.
Scientists have proposed several different explanations for Miranda’s curious features. One reason could be that at some point in its history, Miranda was smashed apart in a violent collision, and then gravity caused the fragments to reassemble in a haphazard manner. As rocky material submerged into the newly reformed moon, it produced concentric creases, forming the coronae. Another possibility suggests that the coronae are impact sites from large meteorites. Miranda is thought to comprise of roughly even amounts of silicate rock and water ice. As these meteorites collided with the moon, they partially melted the ice beneath the crust, causing water to make its way to the surface where it would then refreeze.
However, the most likely scenario to explain Miranda’s exotic features places Uranus squarely into the picture. Given Miranda’s small size, it would have cooled quickly after its formation. This particular structure doesn’t possess the radioactive materials like Earth has to keep its centre hot. Instead, through three-dimensional computer simulations, researchers have demonstrated how Uranus influenced the structure of the moon.
As a result of its gravitational pull, Uranus generated tidal forces, melting the moon from the inside out. In this model, Miranda once had a more oval-shaped orbit. As its proximity to Uranus shifted, the tidal effects constantly stretched and compressed Miranda, enough to generate a considerable amount of heat. This heat was transmitted to the icy mantle so that the crust receded, allowing new material to rise up in its place and form the coronae. As Miranda’s orbit gradually stabilized, the moon stopped producing heat and cooled down into its present form.
Miranda has proved to be a fairly complex world. While we have only seen the southern hemisphere, as Voyager 2 only imaged the one half, we have still been able to view one of the most diverse landscapes ever found on an extra-terrestrial object.