Torn Apart at Birth: 70 Years Since the Discovery of Miranda, Mysterious Moon Viewed by Voyager 2

Uranus’ moon Miranda shows signs of past geological activity, with deep canyons and other chaotic terrain. Photo Credit: NASA/JPL-Caltech

Seventy years ago, the existence of a new world was added to the growing vault of human knowledge. More than 1.8 billion miles (2.9 billion km) from Earth, circling the gaseous planet Uranus, its small size and closeness to its giant host had hitherto prevented it from being directly observed. Yet in the small hours of 16 February 1948, perched at the controls of the 82-inch (2.1-meter) telescope at the McDonald Observatory in West Texas, astronomer Dr. Gerard Kuiper recognized its significance almost immediately. Since the discovery of Uranus in 1781, four moons had already been discovered. “This exposure was intended to provide data on the relative magnitudes of the four known satellites,” Kuiper later wrote. “The close companion to the planet was noticed at once, but no opportunity to establish its nature occurred until 1 March 1948, when two control plates showed it to be a satellite and not a field-star.”

It was the eighth planet or satellite to be detected in the Solar System in the 20th century—following on the heels of six small moons of Jupiter and the dwarf world, Pluto—but it would be several more weeks before further observations confirmed its orbital parameters and over a year before its discovery was published in June 1949. The telescope used by Kuiper, located at an altitude of 6,800 feet (2,100 meters) at the peak of Mount Locke, was later named in honor of astronomer Otto Struve. At the time of Miranda’s discovery, it was the second-largest of its kind in the world, after the 100-inch (2.5-meter) Hooker Telescope at Mount Wilson Observatory in Los Angeles, Calif.

Visible only as a crescent, Uranus recedes from Voyager 2 in the days after Closest Approach. Photo Credit: NASA

Yet Kuiper was not looking for new moons that evening. Rather, he sought to gather data on the relative magnitudes of Uranus’ four known satellites, Titania, Oberon, Ariel and Umbriel, the most recent of which had been discovered almost a century earlier. His four-minute exposure of the giant planet on 16 February 1948 was tantalizing, but it would several weeks before it could be confirmed through subsequent observations. On the night of 1 March, the control plates showed it to be a satellite and, later that same month, eight more images revealed its orbital parameters, suggesting that it occupied a roughly circular path around Uranus, in-plane with the other moons, and requiring about 33 hours and 56 minutes to complete a single revolution.

“Miranda was chosen as the name for the fifth satellite,” Kuiper wrote in Publications of the Astronomical Society of the Pacific in June 1949. “Uranus’ own children, the Titans, are not suitable for mythological reasons; they have been assigned to the son of Uranus—Saturn (Cronus)—who gained supreme power after wounding his father.” He noticed that the other four Uranian moons honored characters from William Shakespeare’s Midsummer Night’s Dream and Alexander Pope’s Rape of the Lock and it therefore the name Miranda, daughter of the magician Prospero in the Bard’s play, seemed fitting. It broke with convention, however, because the other four moons had been named for fairies, whereas Miranda was a human. And when humanity gained its first close-range glimpse of Miranda, less than four decades after Kuiper’s discovery, the diminutive moon would indeed prove itself to be an oddball, unique in the Solar System.

The Voyager 2 adventure began more than 40 years ago, in 20 August 1977. Image Credit: NASA/JPL-Caltech

Launched in August 1977, NASA’s Voyager 2 spacecraft was originally targeted to fly past the giant planets Jupiter and Saturn, but the scope of its journey was expanded to perform the first-ever close encounters with Uranus and Neptune. In January 1986, just a few days before the loss of shuttle Challenger, it swept just 50,640 miles (81,500 km) over Uranus’ aquamarine clouds and revealed a misleadingly quiescent world. Unlike the other gaseous planets, there appeared to be no violent storms, no multi-colored bands, no rotating spots or eddies and a puny system of rings. Several members of the Voyager 2 imaging team wryly nicknamed themselves “The Imagining Team”.

The moons, however, had other stories to tell. Gaseous signatures from their surfaces revealed a composition of 50 percent water-ice, 30 percent rock and 20 percent “other elements”, including carbon and nitrogen. Additionally, they all proved intrinsically dark-gray in color, the consequence of a powerful radiation “sting” in Uranus’ magnetic field. It was later postulated that the intensity of this radiation would have quickly broken down and darkened any methane on the surfaces of Titania, Oberon, Ariel, Umbriel and Miranda, within only about 100,000 years, to leave a thick carbon “dust” in its wake.

Uranus and its six largest moons, from left Puck, Miranda, Ariel, Umbriel, Titania and Oberon. Image Credit: NASA

Due to the nature of the spacecraft’s inbound trajectory, it wound up passing closer to Miranda—about 18,000 miles (29,000 km)—than any of the other moons. “Uranus’ gravity increased the velocity of Voyager 2 by about 1.24 miles per second (2 km/sec),” explained planetary scientist Dr. Ellis Miner of the Jet Propulsion Laboratory (JPL) in Pasadena, Calif. “The flyby distance was determined by the need to go on to Neptune and was close to the orbital distance of Miranda. It was for that reason that Voyager 2 was able to obtain high-resolution images of Miranda.”

The tiny moon measures only about 310 miles (500 km) in diameter, but its surface turned out to be a veritable jigsaw of activity, the result of a history of geological upheaval. Traces of internal melting and the occasional “upwelling” of internal icy material were identified by Voyager 2, with vast, fault-like canyons, up to 12 miles (20 km) deep, as well as oval, racetrack-like features, running like systems of scratches across the surface. These were juxtaposed against strange, “terraced” regions, with old and young, bright and dark, heavily and lightly cratered terrain types. Inverness Corona, in particular, exhibited a pale, chevron-like feature, betwixt darker layers, suggestive of a re-aggregation of fragments of Miranda’s original surface having been pulled apart and forcibly hammered back together.

Further clues will have to await a future mission to the Uranian system, although Voyager 2’s data led to speculation that the coronae developed either through tectonic activity or icy volcanism, following an initial spate of heavy micrometeoroidal bombardment. More than three decades have now passed since human eyes last saw Miranda up close. And in spite of concerted efforts to implement a Uranus orbiter and atmospheric probe concept into a future Planetary Science Decadal Survey, it remains to be seen when another mission will visit the planet or tiny Miranda itself.

 

 

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