Mohawk Valley Astronomical Society

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Frigid Turquoise Tranquility

by Perry Pezzolanella, MVAS

From the moment of discovery by Sir William Herschel on March 13, 1781, Uranus was a very difficult planet to observe. It rarely revealed anything more than a small, featureless turquoise disc and through the years five moons were eventually discovered. Sir William Herschel discovered the first two, Oberon and Titania in 1787, followed by Umbriel and Ariel in 1851 by William Lassell, and finally Miranda in 1948 by Gerard Kuiper. Uranus yielded its secrets slowly and on March 10, 1977 five thin, dark rings were discovered. The advent of the space age finally provided a means to acquire further knowledge of this distant planet - send a spacecraft past it.

Fortunately there was an opportunity to fly one of the two Voyager spacecraft that were launched during the late summer of 1977 past Uranus. It was not a sure deal as the Voyagers were specifically designed to study Jupiter and Saturn along with their moons. Uranus was a long shot, as the spacecraft would have to last years past its designed lifetime. If Voyager 1 succeeded at Jupiter, Saturn, and especially Saturn’s strange moon, Titan, then Voyager 2 could be programmed to fly past Uranus sometime in late-January 1986. Voyager 1 fulfilled all its objectives and the flyby past Uranus was set for January 24, 1986. Voyager 2, however, had a sticky scan platform and a faulty radio receiver that threatened the encounter. Pointing a camera at Uranus or its rings and moons required the scan platform to move quickly in order to track the objects as the spacecraft sped by. The amount of data required for a single picture was enormous and to transmit a single picture at the speed of light to Earth required at least 2 hours and 45 minutes. This would put a strain on even the healthiest radio receiver.

Technology advanced swiftly in the 1980’s. Scientists grew familiar with Voyager 2’s capabilities and were able to program the whole spacecraft to turn itself fast enough to track an object while taking a picture, thereby restricting the use of the scan platform to only slow panning. New programming also allowed images to be compressed before being transmitted to Earth. This reduced the risk of losing precious photographs and allowed for more photographs to be taken. The balky radio receiver was left on for the rest of the mission to eliminate the risk of it not surviving being switched on and off repeatedly. The fuel supply was never an issue. So very little was known about Uranus that anything would be a discovery and scientists waited for the surprises. Voyager 2 delivered.

The best views of Uranus before Voyager 2 arrived were nothing more than a fuzzy turquoise disc that hinted at cloud belts and spots. The five known moons were nothing more than star-like points of light and the rings were too dark to photograph properly. The Hubble Space Telescope would not be launched for years yet. Uranus is around 1.8 billion miles from the Sun where daylight is no brighter than twilight on Earth shortly after sunset and temperatures are as low as -330ºF. Uranus is about four times as large as Earth with a diameter of 31,700 miles. It never appears larger than 4.2 arcseconds across and no brighter than magnitude +5.4 as seen from Earth. Distance has always been the challenge in trying to wrestle out the secrets from Uranus, but in early 1986 it was time to tease out some of them.

As Voyager 2 journeyed closer, Uranus was still stubborn in revealing any detail. It was not until one week before the flyby that a few white clouds were finally seen and only with computer enhancement could a vast hazy hood be seen covering a huge area surrounding the South Pole. The blandness may be due to the unusual 98º tilt of its axis. At the time of the Voyager 2 encounter Uranus’ south polar region had been in constant sunlight for nearly 21 years. Due to the tipped over orientation, sunlight can last up to 42 years on either pole of its 84-year orbit around the Sun. The continuous sunlight reacting with the chemicals in the atmosphere may allow smog to form. Another reason may be the lack of internal heat, compared with Jupiter, Saturn, and Neptune, thereby causing less convection, fewer spots and belts, and therefore fewer storms. The unusual tilt remains a mystery; one theory is that an object as large as Mars may have struck Uranus. The rings around Uranus are as dark as coal and very thin compared to the vast icy rings of Saturn. Voyager 2 discovered a thin sheet of dust between the rings and a new ring, which makes a total of ten rings. Several small moons were discovered within the rings, which help to keep the rings from dispersing. These moons are known as shepherd moons.

Uranus has 27 known moons, most less than 100 miles in diameter. The original five known moons: Oberon, Titania, Umbriel, Ariel, and Miranda are mid-sized worlds from 300 to 1000 miles in diameter. This is large enough for gravity to mold them into spheres, but too small for geological activity, or so scientists thought. The moons are rich in water ice and were thought to be dull and covered with craters. In the -330ºF cold there could not possibly be any geological activity, but Voyager 2 proved otherwise and the moons stole the show from Uranus.

Oberon is the outermost of these moons and is 960 miles in diameter. It is covered with craters, but several of these craters have dark floors where something, perhaps water, could have erupted from the heat of impacts. Several craters also have bright white ejecta indicating areas of fresh ice beneath the surface. A lonely mountain was seen along the limb of Oberon and is nearly four miles high! Titania is the next moon closer to Uranus and is also the largest of all the moons at 990 miles in diameter. This moon is covered with craters, but is also cut by rift valleys several hundred miles long indicating great geological activity in the past. Closer to Uranus is Umbriel at 740 miles in diameter. It is unusually dark compared to the other four moons and is also covered with craters. A single, bright ring near the limb might be an impact that uncovered fresh ice. Ariel is the next moon closer to Uranus and is 725 miles in diameter. It is covered with craters, scarps, and faults, but there are extensive areas where water or ice flows from the interior may have erupted within the valleys and resurfaced part of the moon by flooding the lowlands. The scarcity of craters on the flow features indicates that this is a younger event.

If there were a more bizarre moon, Miranda would be it. It turned out to be the real highlight of the Voyager 2 flyby with geology almost beyond explanation. It is only 292 miles in diameter and was thought to be too small for anything more than craters to be seen. It is the closest of the five main moons to Uranus at a distance of 80,000 miles. Voyager 2 flew within 17,500 miles of Miranda to reveal a world broken up with huge winding swaths of parallel fractures or grooves. There are three large areas called ovoids with one of them near the South Pole appearing like a bright V and nicknamed the “chevron”. These ovoids are either lighter or darker than the rest of the moon and have few craters. This indicates that they formed more recently. On Miranda resides the most dramatic cliff in the solar System, called Verona Rupes. The crust split and slumped at a 45º angle creating a cliff dozens of miles long and 12 miles high! The best theory as to what happened to Miranda is that it may have been shattered to pieces by a giant impact. Initially, Miranda differentiated where the darker heavier rock settled towards the core and the brighter, buoyant ice rose towards the top. Then the impact shattered the moon and through time it was able to reassemble itself into a jumbled moon of dark rock and bright ice pieces. It started to differentiate again, but there was not enough internal heat to complete the process. The result is a lumpy sphere with chunks of ice and rock barely squeezed into a sphere.

Recently, the Hubble Space Telescope has revealed an increase in Uranus’ spots and belts due to the changing seasons as its equator now points towards the Sun and it experiences more uniform heating. The bland haze lingers, but there is increasing detail and Uranus is starting to take on the appearance of Neptune. Much mystery remains and there is so much more that follow up mission could discover. The most logical mission would be an orbiter, probably called Herschel, which would deploy a probe into the atmosphere while the orbiter would investigate Uranus, its rings, and its moons for several years. Unfortunately no such mission is planned for the foreseeable future.

A hazy turquoise globe will shine through our telescopes this summer reminding us of the tranquil appearance of Uranus. Someday man’s robotic spacecraft will return to this frigid realm that we dare not consider boring and uncover more secrets than we can ever imagine.