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1 7 Uranus and Neptune

t THE URANUS SYSTEM Uranus has 27 known and 13 rings (blue). The inner 18 moons the ’s equator; the others follow highly inclined paths, suggesting they’re captured objects.

Nor do we know whether the are layered. If they are, each planet may have a -iron core, a water-- ammonia , and a hydrogen-helium envelope. But it’s also possible that the various substances mix together so that no sharp boundaries exist. Clues from Clouds Despite their different distances from the , Uranus and Neptune have the same effective of 59 Kelvin (−353° Fahrenheit). This similarity actually betrays a key dif- ference: Uranus radiates no more heat than it receives from the Sun, whereas Neptune emits more than twice as much, presumably leftover heat from its birth. and also give off more heat than they receive. No one knows why Uranus is unique. Perhaps it lost its heat of formation soon after birth, or, conversely, perhaps its interior is so stratifi ed that the heat can’t escape. The unexplained contrast in heat fl ow does explain another difference between the two ice giants: “Neptune is a lot more dynamic than Uranus,” de Pater says. Voyager showed Uranus to be so bland that the planet made news when astronomers later detected clouds and storms there. In contrast, Nep-

MIRANDA tune has plenty of both, thanks to its vigorous internal heat. Although the planet’s best-known , the that saw, has vanished, similar storms have erupted

TITANIA since. Fast winds race across both planets. High white clouds often fringe the dark storms and prob- ably consist of methane ice. These clouds resemble orographic clouds on , where air rises over mountains and cools, causing water vapor to condense and form clouds. Likewise, methane-laden air on Uranus and Neptune rises over the storms and condenses, creating the high white clouds. Below lies the main cloud deck. In 2018 Patrick Irwin (University of Oxford, UK) and his colleagues reported the MARGA RET signature of hydrogen sulfi de, a poisonous gas that smells like rotten eggs. This work confi rmed earlier sugges-

FE tions indicating that the main cloud deck consists of hydrogen RDIN TR AND INCULO sulfi de ice. CO IS C N Why hydrogen sulfi de? Beneath the hydrogen sulfi de A S R E F T S EB YC clouds, ammonia (NH3) and hydrogen sulfi de (H2S) join OS OR AX to form yet another compound, ammonium hydrosul-

OBERON fi de (NH4SH). Because each molecule has one atom

O of nitrogen and one of sulfur, whichever element is HAN STEP less abundant gets used up, leaving the other to form a gas. An overabundance of sulfur explains the hydro-

gen sulfi de in the air above. T & S

/ The discovery means Uranus and Neptune have more N A M

sulfur than nitrogen. That makes them unlike Jupiter and R E D N

Saturn and unlike the overall galaxy, which has nearly fi ve I D

G

times as much nitrogen as sulfur. “This observation is quite G E R

important,” de Pater says. “It really does show that the forma- G

18 DECEMBER 2019 • SKY & TELESCOPE tion of the planets isn’t as simple as mers saw hints of one in the 1980s people initially had thought.” (S&T: June 1989, p. 606), but Voyager If instead nitrogen were more com- URANUS STATS defi nitively discovered fi ve. A sixth mon in the atmosphere, ammonia ring, which may not fully wrap around clouds would form, as they do on Jupi- AVERAGE MEAN the planet, also appears among the ter and Saturn. Perhaps, at the greater DISTANCE FROM (WATER = 1.00) others in the Voyager data. In addition, THE SUN 1.27 distance and colder of 2.87 billion km the saw enhancements of Uranus and Neptune, water ice in the (19.2 a.u.) KNOWN MOONS material in four sections of the outer- 27 planetesimals that built the two worlds YEAR most ring. How these ring arcs arose trapped more sulfur-bearing gases 84 Earth years RINGS is unknown, but they may owe their than nitrogen-bearing ones. 13 existence to gravitational resonances ESTIMATED Far below all these clouds, tempera- 17 14 minutes DISCOVERED with various satellites or to collisions tures become warmer, and scientists March 13, 1781 in the ring. Since Voyager’s visit, two expect good old-fashioned water clouds 97.8° of the four ring arcs have disappeared; to exist. But no one has yet seen them. perhaps new ring arcs will form in 14.54 the future. Rings and Things Whereas Saturn’s stunning RADIUS Both Uranus and Neptune spin fast 4.0 Earth radii rings glisten with water ice, but have very different axial tilts. Voy- those of Uranus and Neptune ager found that Uranus spins every 17 are dark, probably due to carbon hours or so and Neptune about every compounds. Small moons tug on 16 hours — faster than Earth but more slowly than Jupiter the rings and actually spawn others. and Saturn. These fi gures constrain models of planetary inte- For example, the moons Cordelia and Ophelia orbit Uranus riors; however, with only one spacecraft measurement, some on either side of its brightest ring, named Epsilon, their grav- scientists have questioned the numbers. ity keeping that ring narrow, while Neptune’s moon Although Uranus and Neptune have similar spin peri- probably sprinkles material along its orbit and creates the ods, their rotation axes are another story. Early in Uranus’s diffuse ring it inhabits. life, an object roughly twice as massive as Earth might have slammed into the planet, knocking it over. Or that object It’s a Mab Mab World may have whizzed by the planet, twirling it around via grav- Indeed, both planets have lots of moons. When Voyager fl ew ity. Either way, Uranus now lies on its side as it spins, with past, it tripled the number known at Uranus from 5 to 15 and an axial tilt of 98°. In contrast, Neptune’s axis tilts only 28°, quadrupled the number known at Neptune from 2 to 8. This similar to Earth’s. created the pleasing coincidence that the eighth planet from Both Uranus and Neptune have rings, though they’re the Sun had eight known moons. much darker than those around Saturn. Astronomers fi rst But Hubble and ground-based telescopes have nearly detected Uranus’s rings in 1977, when the planet passed in doubled the numbers again. Today, Uranus has 27 known front of a star and 5 narrow rings blocked the star’s light moons and Neptune 14. before and after the planet did. Subsequent discoveries have On its visit to Uranus, Voyager passed closest to Miranda, boosted the total number of known rings to 13. Because of the smallest and innermost of the fi ve classical Uranian its drastic axial tilt, Uranus points one pole almost directly moons, and astonished scientists with pictures of radically sunward during summer and winter solstices, giving observ- different terrain types, some ancient, others young. “It’s a ers on Earth a face-on view of the rings, fi rst from the top, kind of schizophrenic world,” McKinnon says.

S U

then decades later from the bottom. Thirteen additional moons, most found by Voyager, lie N D A N R U Neptune has six rings. inside Miranda’s orbit. All are smaller than Miranda. One U O

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Institute) and Jack Lissauer (NASA Ames) found in 2003. S , O A T T & N t MIRANDA This 1986 image

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D from Voyager 2 shows the É T N B T the moon , which Saturn and is about the A U U

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/ size of Miranda, lies in that planet’s E ring. The E ring comes ;

moon Miranda. Features that R H E E C V T I

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P But there’s a problem. “Mab is tiny,” Showalter says. L

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N It’s much smaller than Enceladus. How can a moon that’s G G The moon itself is less than A A A R M M I I U 500 km wide. roughly a dozen miles across be geologically active? No one

skyandtelescope.com • DECEMBER 2019 19 Uranus and Neptune

t THE NEPTUNE SYSTEM Neptune has 14 known moons and six rings (blue, four combined here due to scale). The planet likely captured from the , as well as fi ve far-out satellites that travel on wildly tilted orbits. knows — so perhaps the ring comes instead from that strike the moon and kick up dust. But dusty rings are GALATEA reddish, not blue. The puzzle therefore remains. Nine of Uranus’s inner moons constitute the most tightly packed satellite system ever seen. Planetary scientists have long recognized that the moons are in danger, as gravitational tugs among the moons may make them swerve into the wrong lane. In 2017, Robert Chancia (University of Idaho) and his colleagues measured the mass of a small named Cressida based on how its gravity distorts Uranus’s Eta Ring. Knowing the mass, the scientists then predicted that the TRITON moon might crash into its neighbor, Desdemona, in just a million years. The debris from that collision should encircle the planet in a new ring. All the moons near Uranus — the 13 innermost moons plus the 5 satellites known prior to Voyager — are so-called regular satellites, because they follow fairly circular orbits close to the planet’s equatorial plane. In contrast, at much greater distances lie 9 additional moons, all found since Voyager, on elliptical and inclined orbits. These “irregular” satellites did not form with Uranus but instead were captured by it. All but one of the irregular satellites orbit the planet backward, opposite the direction it spins. Retrograde irregular moons outnumber prograde ones because the Sun’s gravity TRITON can’t as easily yank a retrograde moon away from its planet.

Far-Out Moons At Neptune, the standout moon is Triton. It is nearly twice as large as Uranus’s largest moon and slightly larger than . Yet Triton revolves backward, a sign that it, too, is a captured world, one that once roamed through space on its own as E H T A M A q TRITON This color mosaic of Voyager 2 images shows the nitrogen- S P ice surface of Neptune’s largest moon. The dark streaks overlying the south polar cap’s pinkish ice may be from geyser plumes.

LA OM ED E SO IA NE

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20 DECEMBER 2019 • SKY & TELESCOPE Pluto still does. Triton is the largest Return to the Ice Giants? retrograde moon in the . All these moons and the planets they Its arrival doomed most of Nep- NEPTUNE STATS orbit provide rich targets for future tune’s other moons. Triton probably spacecraft. An orbiter like the ones smacked into some, tossed others into AVERAGE MEAN DENSITY Jupiter and Saturn have received would the planet, and ejected still others DISTANCE FROM (WATER = 1.00) scrutinize these systems for years. To THE SUN 1.64 altogether. One moon that managed 4.5 billion km reach the planets quickly, a spacecraft to hang on was Nereid, but its orbit (30.1 a.u.) KNOWN MOONS must swing by Jupiter, whose gravity 14 became stretched out due to the new- YEAR would fl ing the craft outward. The comer’s gravity. Nereid now has the 165 Earth years RINGS next launch opportunities to Uranus most elliptical orbit of any moon in the 6 and Neptune occur around 2030, lead- ESTIMATED DAY solar system, with an orbital eccentric- 16 hours 7 minutes DISCOVERED ing to encounters around 2040. ity of a whopping 75%. September 23, No such missions are yet funded, AXIAL TILT 1846 Triton suffered, too. Its initial orbit 28.3° but planetary scientists in both the around Neptune was elongated, but United States and Europe are draw- MASS Neptune’s tides forced its path to 17.15 Earths ing up plans. After all, every become circular, internally scorch- planet from to Saturn RADIUS ing the moon and melting all its ice 3.9 Earth radii has received at least one orbiting and maybe even its rock. Volcanic spacecraft, and we now know eruptions must have spewed out lots that planets the size of Uranus of gas, wrapping the moon in a thick and Neptune abound throughout atmosphere. Today, Triton’s atmosphere resembles Pluto’s, the galaxy (S&T: Sept. 2019, p. 16). tenuous and full of nitrogen. Geysers send additional gas Orbiters to the two nearest examples would divine clues to into the air. their interiors, compositions, and atmospheres. Moreover, Triton is about as close to Neptune as the Moon is to such spacecraft would fl y past moon after fascinating moon Earth. Inside Triton’s orbit lie seven other satellites, all on cir- and could discover additional satellites as well. cular paths, all but one found by Voyager. The largest, and the Uranus in particular presents a timely opportunity. By bad second largest orbiting Neptune, is Proteus. Voyager images luck, Voyager fl ew past when one of the planet’s poles pointed revealed a battered world with a huge crater named Pharos. nearly sunward, which meant that, even as Uranus and its The crater might be a scar from an impact that created the regular satellites turned, one side of each world stayed hidden small moon Hippocamp, which Showalter spotted in 2013 in in darkness. In contrast, sunlight will illuminate them in an orbit that lies near Proteus. full at the next equinox, in 2050, so a spacecraft still in orbit Beyond the orbits of Triton and Nereid, fi ve other irregular then could see all of Uranus and its regular moons. satellites pursue elliptical and inclined orbits around Nep- The challenges of reaching these distant worlds are great, tune. The most distant, , ventures farther from its planet but the scientifi c rewards are likely to be far greater. than does any other moon in the solar system. At its extreme, Neso skirts 72 million kilometers from its master — nearly ¢ Ever since childhood KEN CROSWELL has been especially half the distance between the Sun and Earth. So distant is intrigued by distant and mysterious Uranus, Neptune, and Neso that it takes 26 years to orbit Neptune once, nearly as Pluto. He is the author of eight books, including Planet Quest long as Saturn takes to circle the Sun. Yet Neptune can retain and Ten Worlds. this remote retrograde moon because the planet itself is so far

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T In 2003 Scott Sheppard (Carnegie Institution for Science) D A E N M S I ( U

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I are four other moons and a couple of rings. The black bar prevents the T S P L S A E L N N A N I planet’s light from swamping the image.

skyandtelescope.com • DECEMBER 2019 21