Imagine a tiny dusty moon, about one-seventh the size of our planet’s, with polygonal topographical features extending for hundreds of miles, and cliffs taller than any you’d find anywhere on Earth. This tiny celestial ball orbits the sideways planet Uranus. That moon, Miranda, the smallest of Uranus’ major moons — just 235 km across — was discovered 75 years ago, today.
Uranus is a weird planet for more reasons than just its name, which is the name of the Greek deity of the sky. It is the only planet in the Solar System that lies on a rotational axis that is almost perpendicular to its orbital plane, so it orbits the sun on its side. The planet’s satellites and rings are all oriented sideways too, so to make matters stranger, extreme seasonality is the norm. It also, true to its name, smells like farts. Uranus is perhaps the most unusual planet in our Solar System — and Miranda may be the Solar System’s strangest moon.
Gerard Kuiper identified the elusive Miranda on February 16, 1948 while observing the skies from the Otto Struve Telescope, an optical telescope at McDonald’s Observatory in West Texas; the second largest telescope in existence at the time. Kuiper was considered by all standards a pioneer in planetary science. He proposed the existence of an icy belt outside of Neptune, now called the Kuiper belt, and even proved in 1956 that Mars’ ice caps contained mostly frozen water as opposed to mostly carbon dioxide ice, as was previously believed.
Until the discovery of Miranda, more than 100 years had passed since the last discovery of one of Uranus’ moons. The first two, and the largest, Oberon and Titania, were discovered in 1787 by William Herschel, an astronomer and composer – who also identified the planet Uranus itself in 1781. In 1851, William Lassell discovered Umbriel and Ariel. Each moon of Uranus was named after a character in Shakespeare, and Miranda, for whom the moon is named,is the protagonist of The Tempest, Prospero’s obedient and virtuous daughter who was exiled with only her father from the age of three, never interacting with other humans until her adolescence.
Fittingly, the moon’s namesake is quite a good metaphor for the moon itself. Like Shakespeare’s Miranda who spends the majority of her youth obediently in her father's shadow, Uranus' Miranda is the closest of the planet's moons. Orbiting about 129,000 km above Uranus, its close proximity to the planet is part of why it took so long to discover the last major moon.
Why Miranda is important (and strange)
Chloe Beddingfield, a research scientist affiliated with the SETI Institute and NASA Ames Research Center, whose main studies include investigating the geology of icy bodies, particularly the Uranian satellites, explains to Inverse why there was such a delay in discovering Miranda compared to its “siblings.”
“Miranda was relatively difficult to discover because it is smaller and much fainter than the other classical Uranian moons. As the innermost of Uranus’ five largest moons, Miranda orbits fairly close to the planet, just beyond the ring system,” Beddingfield says. As a result, Miranda can be difficult to discern from Uranus’ bright disk using telescopes, especially using the older generation telescopes available to Gerald Kuiper when he made the discovery in 1948.”
In January 1986, when Voyager 2 became the first — and only — spacecraft to explore Uranus, it made its closest approach to any planetary or satellite body when it passed about 29,000 km from Miranda, taking numerous detailed photos of the moon, enough to see a literal smorgasbord of features. Voyager 2 identified an additional eleven moons that were not round and were closely related to the planet’s ring system, as well as an additional two rings.
Voyager 2 made an up close and personal (by outer space standards) exploration of Miranda, which revealed one of the strangest celestial bodies spotted in the Solar System. Scientists have yet to fully explain what they saw. While like Earth’s moon, Miranda is tidally locked so the same side of the moon always faces Uranus, there are countless strange characteristics across the surface of the satellite, which is thought to be equal parts silicate rock and water ice, wildly unique.
This teensy weensy moon, is home to the largest known cliff in the Solar System, Verona Rupes — a 20 km cliff face that would evidently take 12 minutes to fall down, accounting for Miranda’s gravitational differences. Miranda also has three regions that have minimum widths of greater than 200 km, which are gargantuan on a moon with a radius just over that.
“Miranda’s surface displays three large regions of deformation that appear polygonal, termed coronae. These coronae are massive and are approximately centered on the leading and trailing hemispheres and near the south pole,” says Beddingfield. “Miranda’s coronae are unlike resurfaced regions that we see on icy bodies elsewhere in the Solar System because of their overall polygonal shape and the strange morphologies of the densely packed systems of ridges observed within them.”
Although, Beddingfield does think the coronae bear some similarity to the some features on Saturn’s sixth largest moon.
“Terrains that are somewhat similar to Miranda’s coronae are three large, resurfaced regions on Enceladus that include its south polar terrain where plume activity is occurring, and regions of deformation centered on its leading and trailing hemispheres,” she says.
Miranda also has a Global Rift System that is observed primarily in the corona, but extends globally. These tectonic structures vary in size, but they cut through and across the moon’s terrain.
To top it all off, literally, Miranda has one of the thickest layers of regolith (loose, unconsolidated, deposits that cover a solid surface) in the Solar System. It is scattered across the satellite’s cratered surface between coronae.
“This regolith has buried small ancient craters and fault scarps. This thick regolith is mysterious, and the source is unknown. It may be sourced from ejecta from a giant impact or fallout from plume activity, like Enceladus’ plumes that are jetting water vapor and other material into orbit in the Saturnian system,” explains Beddingfield. “Alternatively, Miranda may have formed in and/or migrated through Uranus’ ancient rings, and perhaps it accumulated a large amount of ring material along the way, as it migrated outward to its current orbit.”
Beddingfield believes the explanation for some of these features may be interrelated and ultimately a product of diapirs, a type of intrusion that can come from the melted material of a planetary body’s interior toward the surface.
“The reason the coronae are polygonal may be due to the presence of the Global Rift System cutting across Miranda’s surface, which may have existed before the coronae formed. The Global Rift System’s linear and cross cutting sets of faults may have acted as planes of weakness,” says Beddingfield. “Once the upwelling diapirs resulted in deformation on Miranda’s surface, the resulting faulting and fracturing may have followed these pre-existing planes of weakness, causing the shapes of the coronae to appear polygonal.”
The upwelling of melted material may give rise to the question of whether there is a subsurface ocean concealed on Miranda, as is the case in many other icy moons, such as Saturn’s Europa. But Beddingfield is skeptical.
“While the other large Uranian moons may host a residual ocean, it is unlikely that Miranda still has a liquid layer in its interior, unless it experienced a good amount of tidal heating in the geologic recent past,” says Beddingfield.
She doesn’t rule out the possibility, though.
“There are other ways an interior liquid ocean can be preserved, including the presence of antifreeze agents like ammonia and ammonium and carbonate salts. Also, the thick layer of regolith across much of Miranda’s surface could act like a blanket, trapping heat in Miranda’s interior, extending the life of a subsurface ocean,” explains Beddingfield. “We need to learn more about the properties, extent, thickness, and age of this blanketing regolith to better understand how effective it has been at trapping heat inside Miranda, and therefore possibly extending the longevity of a subsurface ocean.”
Why we should visit Uranus (and Miranda)
Better understanding of Miranda’s geology may be essential to unlocking the history of Uranus and, in particular, its four other round satellites. It is thought that these moons may have experienced orbital resonance, which means they can have significant gravitational influence on each other — and thus their orbits and subsequently their geologic features.
“Some of these resonances may have dumped a substantial amount of tidal energy into Miranda, heating and melting its interior, and possibly explaining its highly deformed surface we see today. So, geologic features on the other large moons may also have been driven by the tidal heating associated with these same orbital resonances,” says Beddingfield. “The geologic features on the other moons may have formed while Miranda’s surface was being reshaped, connecting the geologic histories of the Uranian moons in intricate ways.”
Miranda may also reveal clues about Uranus itself.
“If Miranda formed out of Uranus’ rings, then understanding the compositions and properties of material on Miranda’s surface may allow us to gain insight into the properties of Uranus. Additionally, if Miranda has or had a subsurface ocean in the past, then investigating its surface geology may provide insight into the compositions and properties of icy moon oceans in general.”
Beddingfield is an advocate for further exploration of the Solar System’s most eclectic moon, and fortunately, NASA has a project called the Uranus Orbiter and Probe (UOP) in the works. Every decade NASA asks the National Academy to determine priorities in planetary science for the decade to come. Though this was the first time this exchange was conducted over the Covid epidemic, over 176 meetings, 500 white papers, and 300 presentations later, it was determined that deepening understanding of the ice giants was a primary concern, and the UOP is proposed as a major interdisciplinary undertaking to address this paucity of knowledge. (A paper published today in Science advocates for a Uranus mission.)
“To gain a fuller understanding of Miranda, it is critical that we obtain new images and data of Miranda’s northern hemisphere, which was previously shrouded by winter darkness during the Voyager 2 flyby in the 1980s,” she says. “To accomplish this, it is critical for a new spacecraft to arrive at the Uranian system before Miranda’s unimaged northern hemisphere falls back into darkness. In other words, the Uranus Orbiter and Probe should be developed quickly to extract the maximum amount of science from the mission when it arrives in the Uranian system.”
The UOP, if greenlit, will address questions about planetary formation and evolution, and also the collision that is thought to have knocked Uranus on its side.
The UOP isn’t likely to reach the Uranus for two more decades; in the meantime, Beddingfield remains interested in promoting and contributing to the development of science requirements for the Uranus Orbiter and Probe mission. Fingers crossed the mission makes it to Uranus before part of Miranda goes dark.
