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Understanding Uranus

by Perry Pezzolanella

Uranus is an oddball planet that remains poorly understood even though Voyager 2 flew past it on January 24, 1986. In fact, Voyager 2 raised more questions than it answered. Trying to explain its observations oftentimes has led to competing theories, some totally bizarre. The problem is that Uranus lies about 1.8 billion miles away making it difficult to study from Earth. As it grudgingly gives up its secrets, its complexity and importance is just being realized.

Uranus was discovered by Sir William Herschel in Bath, England on March 13, 1781. Nothing much could be seen on it and for decades Uranus was just a featureless turquoise disc, too small to see any detail. In the early years, any fuzzy feature, whether bright or dusky, was always questionable. It is known that the turquoise hue is due to a small amount of methane, around 2%, in its atmosphere. From Earth Uranus never appears larger than 4.2 arcseconds across and shines no brighter than magnitude +5.4. Uranus is four times larger than Earth at 31,700 miles in diameter and it is tipped over on its axis at 98º for reasons still unknown. It has a day of 17 hours 14 minutes and takes about 84 years to orbit the Sun. Five dark, thin rings were discovered on March 10, 1977 with eight more since, all unlike any found around Saturn. It is a cold world hovering at -330ºF where daylight is no brighter than twilight on Earth shortly after sunset.

Uranus has 27 known moons, most less than 100 miles in diameter. The original five moons: Oberon, Titania, Umbriel, Ariel, and Miranda are mid-sized worlds from 300-1000 miles in diameter. This is large enough for gravity to mold them into spheres. The moons are rich in water ice and thought to be too cold for any geological activity and therefore only covered with craters. Voyager 2 proved otherwise.

Oberon, discovered by Sir William Herschel in 1787, 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, may have erupted from the heat of the impacts. Several craters also have bright white ejecta indicating areas of fresh ice beneath the surface. A lonely mountain along the limb of Oberon is nearly four miles high! Titania, also discovered by Herschel in 1787, is the next moon closer to Uranus and is the largest of the moons at 990 miles in diameter. This moon is covered by craters, but is also cut by rift valleys several hundred miles long indicating great geological activity in the past. It is possible that a vast underground ocean may exist as tidal flexing from Uranus and heat from radioactive decay of rocks in its interior may be intense enough for liquid water to exist aided by ammonia which acts like antifreeze. Closer to Uranus is Umbriel at 740 miles in diameter, which was discovered by William Lassell in 1851. It is unusually dark compared to the other four moons and it is also covered with craters. A single, bright ring near the limb might be an impact that uncovered fresh ice. Ariel, also discovered by Lassell that same year, 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 the result of a younger event and active geysers could be suspect.

Miranda, discovered by Gerard Kuiper in 1948, was the 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, which may indicate that they formed more recently. Miranda has 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! One 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 with 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. Another theory that may seem more plausible is that Miranda was in a highly eccentric orbit around Uranus, which caused intense tidal heating and the interior to churn, deforming the surface enough to create the ovoids and grooves. As Miranda settled into a more circular orbit, the tidal flexing eased and it cooled, thus freezing these features in place.

The bland appearance of Uranus may be due to the unusual 98º tilt of its axis. At the time of the Voyager 2 encounter the south polar region of Uranus 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 to Jupiter, Saturn, and Neptune, thereby resulting in less convection, fewer spots and belts, and therefore fewer storms. Uranus passed its equinox in 2007 and there has been a proliferation of white spots due to the changing seasons as its equator has been pointing towards the Sun and experiencing more uniform heating causing it to look more like Neptune. Uranus, like Neptune, is known as an ice giant because of the vast amount of water it contains in its interior compared to the more gaseous hydrogen worlds of Jupiter and Saturn.

There is still much to learn and follow-up missions are finally in the works. NASA is planning a spacecraft with an orbiter-probe combination, hopefully to be known as Herschel, which could launch on May 25, 2031 and arrive at Uranus on May 17, 2043. It would deploy a probe that would descend deep into the clouds of Uranus to thoroughly study the chemistry and weather. The orbiter would orbit Uranus for at least four years and study the clouds, magnetic field, rings, and moons. All five moons would be encountered several times at close range. If there are geysers on any of the moons there would be opportunities to explore them at very close range. The European Space Agency (ESA) is also planning a similar orbiter/probe with similar goals and it might be possible for both NASA and ESA to combine their efforts to keep costs down so a mission to Uranus can finally fly.

Uranus will be encountered with sophisticated state of the art equipment unlike any ever flown. It will be like seeing Uranus with a fresh set of eyes so scientists can gain a new understanding and appreciation of this under-rated world. Planets similar to Uranus and Neptune are being discovered around other stars and are turning out to be the most common type of planet. Understanding Uranus will yield a better understanding of similar worlds in our galaxy, and planet formation in general.