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The Unknown Realm of the Ice Giants
The Unknown Realm of the Ice Giants

Orion is proud to partner with BBC Sky at Night Magazine, the UK's biggest selling astronomy periodical, to bring you this article as part of an ongoing series to provide valuable content to our customers. Check back each month for exciting articles from renowned amateur astronomers, practical observing tutorials, and much more!

The Unknown Realm of the Ice Giants: Uncharted Territory — Uranus & Neptune

In January 1986 and August 1989, Voyager 2 flew past the outer giants Uranus and Neptune — becoming the first spacecraft to visit either. In its brief visits, these cold worlds almost beyond the reach of the Sun's warmth were revealed to be every bit as mysterious as their closer cousins.

Uranus as seen by NASA's Voyager 2

Uranus as seen by NASA's Voyager 2. By NASA (http://photojournal.jpl.nasa.gov/catalog/PIA18182) [Public domain], via Wikimedia Commons

Perhaps the toughest kind of exploration is studying a pair of planets about which virtually nothing is known with certainty. In February 1984, several dozen scientists gathered in Pasadena, California, to consider the scant level of knowledge about Uranus and Neptune. These two giants, which reside on the fringe of our planetary system, had only been discovered in the preceding two centuries. And as Voyager 2 journeyed towards them for humanity's first-ever visits, neither had revealed itself as much more than a fuzzy blob in an ocean of emptiness.

Worth a visit

The desire to travel to Uranus and Neptune originated with NASA's Grand Tour programme, but a subsequent rescoping of the mission led to a revised focus on Jupiter and Saturn. Nevertheless, in 1976 NASA approved an extension to Uranus on the condition that all primary objectives were met. A modified infrared detector was built to achieve the necessary resolution at Uranian distance — 2.6 billion km from Earth — but when the Uranus flyby formally began it did so with a greatly reduced budget and a leaner workforce.

By that time Uranus was known to possess five moons, named after characters from the works of William Shakespeare and Alexander Pope. The first pair, Titania and Oberon, were found in 1787 by Uranus's own discoverer, William Herschel, with Ariel and Umbriel observed by William Lassell in 1851 and tiny Miranda identified by Gerard Kuiper in 1948. Some 1.6 billion km beyond Uranus, Neptune was attended by Triton and Nereid, fittingly named after classical deities of the sea. All were barely detectable with Earth-based instruments before the Voyager 2 probe arrived.

Following Uranus's discovery in 1781 and the finding of Neptune thanks to the mathematical predictions and telescopic observations of Johann Galle, Urbain Le Verrier and John Couch Adams in 1846, there existed only the most general and sweeping awareness of either planet. They were known to be near-twins in size, with equatorial diameters around 50,000km, four times bigger than Earth and over 15 times more massive. The probe confirmed hydrogen and helium as their predominant constituents. Uranus's plain aquamarine colour and the richer, sky-blue fašade of Neptune also betrayed the presence of ammonia, methane and hydrogen sulphide.

Data gathering

Travelling at 64,000km/h, Voyager 2 had only six hours on 24 January 1986 to make its close-range observations of Uranus. But to maximise the scientific yield of its flyby, the probe took measurements of the planet continually for 16 weeks from November 1985 until February 1986. A similar campaign was adopted at Neptune from June to October 1989. "We had a prediction of where the spacecraft would be at each point in time," recalled Voyager imaging team member Andrew Ingersoll. "We told the engineers what latitude and longitude we wanted to look at and they told the camera to take a picture. These commands had to be worked out and radioed to the probe weeks in advance."

Due to Uranus's 98° axial tilt, Voyager 2 approached the planet's sunward-facing south pole and generated time-lapse movies to track cloud movements and wind speeds. Unlike on Jupiter and Saturn, there was little evidence of storms or latitudinal banding, which led the imaging team to wryly dub themselves 'the imagining team'. However, infrared data revealed clouds beneath a high-altitude layer of hydrocarbons and the probe's radio-science and ultraviolet instruments revealed uniform temperatures throughout the atmosphere of around -216°C.

Theories abounded that this atmosphere might extend more than 3,000km beneath the cloud tops, perhaps terminating in a slushy ocean of water, ammonia and methane, girdling an Earth-sized rocky core. A similar situation is also thought to exist at Neptune. Voyager 2 was unable to prove the existence of such oceans, but did detect radio signals induced by interactions between the solar wind and electrons in the planets' magnetic fields. This enabled its magnetometer to measure Uranus's day at 17.25 hours and Neptune's day at 16.1 hours, which in turn helped provide wind-speed estimates.

The dark of the moons

To great surprise, Uranus's magnetic field extended only 600,000km sunward, but wound backwards, like a giant corkscrew, 10 million km beyond the planet. Ultraviolet observations of polar aurorae showed that the field was tilted at 59░ to Uranus's rotational axis — a curiosity that, on Earth, would be equivalent to having our north magnetic pole in the Florida Keys — and bore a powerful sting in the guise of trapped, high-energy radiation. This clearly manifested itself on the surfaces of Uranus's moons.

All five moons appeared intrinsically dark, suggesting that radiation had broken down any methane on their surfaces within a few tens of millions of years, darkening them and leaving a thick, charcoal-like dusting. Umbriel is by far the darkest, although it does exhibit a few splotches of bright material, including an enigmatic 'Cheerio' in the crater Wunda at its equator. As for its siblings, Titania — the largest, at 1,580km across — is marred by huge faults and winding canyons, pointing to a violent tectonic past. Oberon revealed bright and dark regions, not unlike our Moon, indicating meteoroid bombardment and perhaps the volcanic extrusion of subsurface material.

But it was Miranda and Ariel to which Voyager 2 devoted the most attention. The latter is the brightest Uranian moon, with an ancient and heavily cratered terrain that features rolling plains, parallel ridges and troughs lying tens of kilometres apart. Miranda, less than 500km in diameter, was the most closely inspected, principally due to the flyby geometry needed by Voyager 2 to reach Neptune.

It revealed unmistakable evidence of billions of years of impacts, which tore Miranda apart, then hammered it back together, gouging out 20km-deep canyons, at least three enormous, oval-shaped 'coronae' and broad terraces of old and young, bright and dark, lightly and heavily cratered terrain. One area, the 200km-wide Inverness Corona, showed a bright chevron-like feature between dark layers, possibly a result of reaggregated bits of Miranda's original crust, poking out from the present surface.

New moons

Voyager 2 also found 10 new moons, ranging from 160km-wide Puck to diminutive Cordelia, about an eighth as large. A small subset that share similar orbits, surface colouration and generally elongated shapes was classified as the 'Portia Group' (named for its biggest member). Another object was photographed by the probe, but went unrecognised as a moon until 1999. It was fittingly named 'Perdita', the Latin word for lost.

Something that had already been seen at Saturn was the pivotal role tiny 'shepherd' moons play in anchoring ring material. Several narrow rings had been detected around Uranus by ground-based observers in the 1970s, but Voyager 2 uncovered another pair. The probe revealed the new pair to be relatively insubstantial, although the brighter 'Epsilon ring' achieved a maximum extent of 96km.

Up to 18 shepherd moons were predicted to exist at Uranus, but only two — Cordelia and Ophelia — were detected, lying astride and 'binding' the inner and outer edges of the Epsilon ring. The general darkness of the rings underscores their extreme youth, for they are probably no more than 600 million years old. Data from Voyager 2's photopolarimeter and other instruments revealed them to be so sharp that the Epsilon component can't be greater than 150m thick.

Rings were also eagerly anticipated at Neptune, with ground-based studies between 1968 and the early 1980s suggesting that incomplete 'arcs' might run part-way around the planet. Numerous theories postulated that the arcs were held in place by tiny shepherd moons or maybe new rings were in the process of forming. With just two weeks to go until Voyager 2's arrival, their true nature was revealed. A pair of incomplete arcs did appear to exist, with three shepherd moons (Galatea, Larissa and Despina) interacting with them.

Building rings

As the probe drew nearer, it became apparent that more rings extended around the planet. Their uneven 'clumpiness' and irregularly distributed particles offered an early explanation for why arcs had been suspected for so long. Indeed, Neptune's outermost 'Adams' ring revealed several clods of material, up to 50km wide. It was argued that debris from ancient moons could have contributed to this unequal distribution of mass and a pair of tiny moons, Thalassa and Naiad, could themselves someday be torn apart and incorporated into the system.

Voyager 2 confirmed the existence of six new moons at Neptune, including potato-shaped and heavily cratered Proteus, which is thought to be almost big enough for gravity to pull it into a spherical shape. Eccentric-orbiting Nereid, discovered by Gerard Kuiper in 1949, was also seen by the probe, but at a distance of 4.7 million km it was still too far away to resolve any surface detail, much less measure its rotational characteristics.

Before it found any rings, it was hoped that Voyager 2 would fly within 10,000km of the planet's largest moon, Triton, which ground-based observations had shown to possess nitrogen ices on its surface. To avoid the risk of colliding with ring particles, the probe's trajectory was altered to carry it 4,950km over Neptune's north pole — the closest planetary encounter achieved by either Voyager craft — on 25 August 1989. It then plunged south, passing within 39,800km of Triton, five hours later.

Circling Neptune in a highly inclined 'retrograde' orbit, the moon proved smaller than predicted, at just 2,700km across, and a stellar occultation allowed Voyager 2 to measure its 800km-deep atmosphere, all the way down to its surface, the coldest known in the Solar System at a frigid -236°C. In fact, Triton's tenuous mix of gases and particulates is virtually a vacuum, barely capable of supporting thin nitrogen-ice clouds and haze at an altitude of 13km.

Voyager 2 strongly hinted that these constituents originated from the evaporation of surface ices, with winds transporting dust particles up to 50km across its terrain. Triton is the most spectroscopically diverse object in the Solar System, reflecting over 85 per cent of incident sunlight — eight times more than our Moon — and this extreme brightness was a key reason why such a tiny, distant body was found telescopically by William Lassell in October 1846, just weeks after Neptune itself.

Freeze-thaw effect

The probe imaged a third of Triton's surface, uncovering a greenish landscape, nicknamed 'cantaloupe', due to its similarity to the scaly skinned melon. It was crisscrossed with circular depressions, each around 25km wide, and long, interconnecting ridges were thought to be the result of epochs of melting and refreezing. This reinforced the notion of Triton as a captured Kuiper Belt object and that the tidal heating from Neptune's gravity had left its interior fluid for a billion years, underpinning these complex internal processes.

Voyager 2 also revealed a pinkish southern polar cap, abutted by a blue-tinged crustal region, indicative of the presence of methane, nitrogen and water ices. And it was from within the polar caps that a moderate greenhouse effect could have been nurtured, forcing exotic ices to 'de-gas' and build pressure, before prompting one of the most surprising discoveries at Triton: erupting geysers.

In August 1989, only Earth and Jupiter's moon, Io, were known to harbour active volcanism, but dark streaks across Triton's southern polar cap indicated that such phenomena were commonplace, even in this far-flung corner of the Solar System. One geyser was observed to hurl carbonaceous material to an altitude of several thousand metres, while other measurements allowed local wind speeds to be clocked at 54km/h, as strong as a moderate gale on Earth.

Providing a backdrop to these discoveries was magnificent Neptune itself, whose outward similarity to Uranus belied a far more active world. Despite its greater distance from the Sun, infrared data showed it to radiate 2.6 times as much heat from incident sunlight as Uranus. And although the near-twins are thought to have similar compositions, Neptune is marginally more massive, which influences its magnetic field and internal heat. "Wow," exulted one planetary scientist, breathlessly, as Voyager 2 became the only spacecraft in history to visit four planets. "What a way to leave the Solar System!"

Beefing up the Voyagers from the ground

Orbiting billions of kilometers beyond Saturn, the outer giants Uranus and Neptune inhabit a gloomy region of the Solar System, requiring Voyager 2 to examine worlds where high noon is dimmer than dusk on Earth. One scientist likened the problem to photographing a pile of charcoal briquettes lying at the foot of a Christmas tree, lit by a single-Watt bulb.

Stability was crucial, but even turning its tape recorder on and off was enough to induce a disruptive 'nodding' effect in Voyager 2. Its gyroscopes could keep the instruments reasonably steady, but engineers had to halve the duration of thruster firings to allow the probe to settle after manoeuvres. At Neptune, longer exposures of 96 seconds and thruster firings under four milliseconds became necessary. Image motion compensation allowed Voyager 2 to resolve finer detail, but at the expense of picking out irritating optical flaws, including dust on its lenses.

Back on Earth, the three tracking stations that make up NASA's Deep Space Network (located in Canberra, Australia, the remote foothills west of Madrid in Spain and in California's Mojave Desert) received a $100 million facelift to boost Voyager 2's ever-weakening signal, which by January 1986 was a billion times fainter than a watch battery. All three stations had their 64m antennas augmented for Uranus, electronically synchronising them to strengthen the signal. Further upgrades to 70m were implemented for Neptune and two additional tracking stations in Japan and New Mexico were called into duty.

Due to the position of Uranus in Earth's skies in the winter of 1985-1986, Canberra was the main tracking station, following Voyager 2 for 12 hours per day and allowing a 21.6kbps downlink rate. In support, a 400km microwave connection was established with the Parkes radio telescope in New South Wales, bolstering it by 25 per cent and allowing up to 50 extra photographs to be returned every day.

The Bullseye Planet

Uranus is unique among the planets in our Solar System thanks to its extraordinary rotational tilt of 98° — Uranus presents itself to observers as a world tipped on its side. Its poles lie where its equator should be and receive a correspondingly higher level of incident sunlight. Situated 2.8 billion km from the Sun, Uranus circles its parent star every 84 years, with each pole rhythmically illuminated, before being plunged into frigid darkness, every four decades.

When Voyager 2 viewed the planet only the southern pole was in direct sunlight. Its five main moons orbit their giant host within its equatorial plane, placing their southern halves at the height of Uranian summer in January 1986 and casting their northern extremities into a 21-year-long winter season.

How Uranus's axial tilt came about remains a mystery, although a collision with an Earth-sized impactor has been proposed. The fact that the moons circle within its equatorial plane implies that they formed much later from debris placed into orbit by this impact. Moreover, Uranus radiates hardly any heat into space — its temperatures dip as low as -224°C, giving it the coldest planetary atmosphere in the Solar System — and it's possible that whatever hit the planet caused it to expel much of its primordial heat.

A World Of Wild Weather

Four-and-a-half billion kilometres from its parent star, recipient of half as much sunlight as gloomy Uranus and with temperatures as low as -218°C, Neptune should be an inactive world. Yet Voyager 2 revealed it to be surprisingly energetic, with the oval-shaped Great Dark Spot observed at a latitude of 22°S. This counter-clockwise-rotating vortex bore many uncanny parallels with Jupiter's Great Red Spot, in terms of relative size, motion and position within the atmosphere.

As the probe drew closer, a second, smaller dark spot was found, together with a chevron-shaped, westward-moving cloud feature, whose rapid 16-hour transit around Neptune's atmosphere generated the nickname of 'Scooter'. The Great Dark Spot, in keeping with its name, was 10 per cent darker than its surroundings and hustled northwards through the atmosphere at 1,100km/h. At its edge was a hovering, shape-shifting 'bright companion' cloud. The spot lay 50km below Neptune's main cloud deck, with the companion at a slightly higher altitude, creating analogies with lenticular cloud formations on Earth.

Elsewhere in the sky-blue atmosphere were cirrus streaks of methane-ice, which cast shadows, tens of kilometres long, on Neptune's cloud deck at low northern latitudes. How such wild weather can manifest itself on such a cold planet must be related to its dense interior and the fact that it emits 2.6 times as much heat as it receives from incident sunlight. It has been suggested that temperature differences between Neptune's internal heat and its cold atmosphere could trigger instabilities and induce large-scale meteorological phenomena.

Copyright © Immediate Media. All rights reserved. No part of this article may be reproduced or transmitted in any form or by any means, electronic or mechanical without permission from the publisher.

Details
Date Taken: 12/06/2017
Author: Ben Evans for BBC Sky at Night Magazine
Category: Astronomy

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