A Digital Supplement to Astronomy Insights Astronomy Magazine © 2018 Kalmbach Media Saturn Looms Large

June 2019 • Astronomy.com Visible to the naked eye Visible with binoculars SKY THIS MONTH Visible with a telescope THE SOLAR SYSTEM’S CHANGING LANDSCAPE AS IT APPEARS IN EARTH’S SKY. BY MARTIN RATCLIFFE AND ALISTER LING

SATURN LOOMS LARGE when it reaches opposition this month. With the rings tipped wide open, observers should look for the dark Cassini Division and other fine details. NASA/ ESA/A. SIMON (GSFC) AND THE OPAL TEAM/

J. DePASQUALE (STScI)

JULY 2019 The ringed planet at its peak These warm summer In between the brilliant Mercury and Mars are in planets continue to drop lower nights bring magnifi- flashes and explosions of your conjunction with each other with each passing day and will cent views of Saturn. The ringed local July 4 fireworks, look to July 7. That evening, magnitude disappear within a week. planet remains visible all night, the west for a noticeably fatter 2.0 Mercury lies 4° south (lower You won’t have to search a feat nearly matched by its crescent Moon, now some 15° left) of magnitude 1.8 Mars, as hard to find Jupiter. The equally stunning sister world, high a half-hour after sunset. though both prove difficult to giant planet stands clear of the Jupiter. Alas, the solar system’s The two planets lie about 10° see in the bright twilight even southeastern horizon after sun- smaller planets don’t fare as to our satellite’s lower right. through binoculars. The two set and climbs nearly 30° high well. Mercury and Mars hang in the south well before mid- low in evening twilight, while night. It shines at magnitude Saturn takes center stage Venus appears just before dawn. –2.5 against the faint backdrop Let’s start our celestial tour of southern Ophiuchus the with Mercury and Mars, which OPHIUCHUS Serpent-bearer. we can see only in early July. Any telescope shows Altair The two planets glow faintly in Jupiter’s disk. In mid-July, the the west-northwest shortly after AQUILA world spans 44.4" across its sundown. Your best opportu- equator and 41.6" through its nity to find them comes July 3, Jupiter poles. The 6 percent discrep- when a waxing crescent Moon Antares ancy, caused by a combination points the way. Saturn SCORPIUS of Jupiter’s gaseous nature and The one-day-old Moon then its less than 10-hour rotation stands 5° above the horizon period, becomes obvious once 30 minutes after sunset. Once SAGITTARIUS you know to look for it. you locate it, use binoculars to 10° Even more striking are the July 9, 11 P.M. spy magnitude 1.8 Mars 3° to Looking south-southeast details you can see in the jovian the upper left. Mercury glows a atmosphere. Look for two dark bit brighter, at magnitude 1.4, The gas giant shines at magnitude 0.1 at opposition July 9, when it remains cloud belts straddling a brighter some 4° to the Red Planet’s left. visible all night amongDiagram the stars ASY-SM0719_10: of Sagittarius. ALL ILLUSTRATIONS: ASTRONOMY: ROEN KELLY zone that coincides with the

2 ASTRONOMY INSIGHTS • JUNE 2019 SKY THIS MONTH RISING MOON I A crater worthy of an eminent astronomer TERRACED WALLS, complex central peaks, Copernicus a rough ejecta blanket, and an expansive ray system at Full Moon — it’s no surprise that lunar observers visit the crater Copernicus again and OBSERVING again. The Sun first rises over this crater’s tall, Copernicus modestly sharp rim July 11, two nights after HIGHLIGHT First Quarter phase. SATURN peaks July 9, when The low Sun angle on the 11th casts long it shines at magnitude 0.1 and spans 18.4" with rings shadows, accentuating the visibility of peaks extending 41.8" when seen and bumps as well as holes and mild depres- through a telescope. sions. Notice the jagged black teeth the eastern rim projects onto Copernicus’ floor. A textured apron surrounds the entire crater. It origi- N nated when surface material got splat- tered outward in all directions in the E equator. The southern belt car- immediate aftermath of the impact that created Copernicus. Lunar geologists This spectacular crater first appears ries the Great Red Spot, which July 11, but it remains a fine sight until appears obvious whenever the call it an ejecta blanket. after Full Moon on July 16. CONSOLIDATED LUNAR planet’s spin brings it onto the The terraced walls are a delight to see ATLAS/UA/LPL; INSET: NASA/GSFC/ASU Earth-facing hemisphere. at high power. To imagine how they formed, Jupiter’s four bright moons dig a hole in a wet sandy beach. Within seconds, on the 60-mile-wide crater. Thread in a filter to — Io, Europa, Ganymede, and the steep walls collapse inward to form giant reduce the glare, pump up the power, and take Callisto, in order of increasing staircases down to the floor. Sharp-eyed observ- in the scenery. The rough terrain and differences distance — circle the gas giant ers may notice that Copernicus’ terraces tilt in elevation and texture have completely disap- peared, replaced by a land of light rays and in periods ranging from a cou- down to the outside because the higher inner edge casts a shadow toward the rim. The eastern darker mare material showing through from ple of days to a couple of weeks. terraces show up best on the 11th. In case you underneath. These brightness variations help The three inner satellites cross get clouded out, the sequence almost repeats astronomers piece together the history of the in front of and pass behind the the evening of August 9, but with an even lower lunar surface. The impacting asteroid may have planet once each orbit. Many Sun angle than the image shown here. landed in a dark lava field, but the blast exca- observers consider these events For a few days on either side of the July 16 vated down to lighter-hued rocks and hurled the highlight of a Jupiter view- Full Moon, the nearly overhead Sun beats down them outward to create the superb ray system. ing session. Let’s take a look at a few of July’s best. Io transits Jupiter’s disk during the evening hours METEOR WATCH I The year’s longest-running show July 4, 11, 18, and 27. On the 4th, the transit begins at 10:02 p.m. EDT while the Southern Delta Aquariid meteors THE SOUTHERN STRAND of moon’s shadow follows 34 min- the Delta Aquariid meteor stream utes later. Both appear against encounters Earth during much of Enif July and August, but reaches a broad the planet’s North Equatorial PISCES peak from July 28–30. And with New Belt. The July 11 transit starts at DELPHINUS Moon arriving on the 31st, viewing 11:48 p.m. EDT with the shadow AQUARIUS conditions this year should be close trailing 42 minutes behind. The to perfect. Under a dark sky with the longer gap arises from a shift in CETUS radiant — the point in Aquarius from the relative positions of Jupiter, Radiant which the meteors appear to ema- the Sun, and Earth. nate — overhead, observers could Io circles Jupiter quickly, Fomalhaut CAPRICORNUS see up to 25 meteors per hour. and the following evening you Unfortunately for Northern can see the moon as it comes Hemisphere skygazers, the radiant around the planet’s far never passes overhead. From mid- side. At 11:51 p.m. EDT on SOUTHERN DELTA GRUS northern latitudes, the radiant peaks 10° the 12th, Io emerges from AQUARIID METEORS at an altitude of 35° between 3 and July 30, 4 A.M. Jupiter’s shadow some 14" Active dates: July 12–August 23 4 a.m. local daylight time. This cuts Looking south off the eastern limb and Peak: July 30 the observed number of meteors Moon at peak: New Moon adjacent to the South roughly in half, with observers in the Maximum rate at peak: ASY-SM0719_11With New Moon arriving just one day after Equatorial Belt. 25 meteors/hour this shower peaks, viewing conditions southern United States seeing a few should be ideal. more and those in Canada a few less.

WWW.ASTRONOMY.COM 3 WHEN TO SKY THIS MONTH— Continued from page 49 VIEW THE PLANETS See Saturn’s top six satellites remains within a few hun- dredths of this magnitude EVENING SKY S all month. Mercury (west) For viewers at mid-northern Mars (northwest) Saturn Jupiter (south) W Titan Enceladus latitudes, Saturn reaches a Tethys Saturn (southeast) Iapetus Dione peak altitude of nearly 30° at Rhea 1:30 a.m. local daylight time MIDNIGHT July 9, 11:30 P.M. EDT 1' July 1 and about 30 minutes Jupiter (southwest) earlier each week thereafter. Saturn (south) With outer Iapetus in line with Titan and inner Enceladus near greatest Neptune (east) elongation, the Diagramringed world’s ASY-SM0719_12: moons show up well at opposition July 9. Skywatchers farther north will find Saturn lower in the sky, MORNING SKY Europa transits Jupiter’s southeast of magnitude 3.8 and those farther south will see Venus (northeast) disk starting at 9:54 p.m. EDT Omicron (ο) Sgr. it climb higher. Saturn (southwest) on July 23. The satellite tra- The ringed planet reaches Observers should take every Uranus (east) verses three-quarters of the opposition and peak visibility opportunity to view the ringed Neptune (south) disk before its shadow drops July 9, when it lies opposite the world through a telescope. The onto the jovian cloud tops at Sun in our sky and thus stays best times are within an hour 11:45 p.m. The icy white moon out all night. Opposition also or two of its peak altitude, is Saturn’s beautiful ring sys- crosses the North Temperate brings the world closest to when the planet’s light passes tem, which spans 41.8" and tilts Zone and practically disap- Earth, so it shines at its bright- through less of Earth’s turbu- 24° to our line of sight. Look pears against the bright cloud est for the year, magnitude 0.1. lent atmosphere. At opposition, for details, most noticeably the band, while its black shadow As an outer planet, however, Saturn’s equator measures 18.4" dark Cassini Division that sep- stands out against the same Saturn changes slowly, and it across and the polar diameter is arates the outer A ring from the 16.9". (This 7 percent flattening brighter B ring. The average number of days between is more pronounced than Saturn provides observers Jupiter’s.) But the star attraction with more moons than Jupiter. 378 successive oppositions of Saturn. zone. Europa tracks farther north than Io because the sat- COMET SEARCH I Take a dip in the celestial sea ellites’ orbits currently tip 3° to our line of sight. AFTER A FEW MONTHS’ LULL, Comet 168P/Hergenrother Jupiter’s darker and largest things are looking up for comet moon, Ganymede, crosses the observers. It appears we’ll see gas giant’s north polar region at least one comet glowing 11 N at 10th magnitude or brighter the evening of July 24. The ARIES every month starting this transit begins before sundown, 9 but viewers in the eastern half autumn and running through of North America can watch it the end of next year. wrap up at 9:58 p.m. EDT. The You can get a head start 7 Path of Comet 168P ο moon’s shadow first touches before dawn breaks these short summer nights. Comet 168P/ ξ1 the jovian disk at 11:26 p.m. E Hergenrother swims the waters 5 PISCES EDT and takes 2 hours 27 64 near the border between Pisces minutes to finish its trek. the Fish and Cetus the Whale CETUS 3 1° Ganymede offers an encore before emerging onto the dry performance on the 31st. land of Aries the Ram during Lovely Saturn lies among July’s second week. July 1 the background stars of Hergenrother loops through Sagittarius the Archer, just the inner solar system every The stars near the intersection of the constellations Pisces, Cetus, and Aries south of that constellation’s 6.8 years. On its last time serve as steppingstones to the dim glow of this periodic visitor. Teaspoon asterism. It moves through in 2012, the dirty snow- Diagram ASY-SM0719_13: westward relative to this back- ball experienced an outburst and brightened past 10th magnitude. If we aren’t lucky again, the drop throughout July, passing comet may reach only 12th magnitude in July and be a difficult fuzzball. The best observing win- 1.1° south of magnitude 2.9 dow comes during the month’s first two weeks when the Moon stays out of the morning sky. Pi (π) Sagittarii on the 20th As its high number attests, 168P/Hergenrother is a relatively recent find. American astronomer and ending the month 0.7° Carl Hergenrother first spotted it in 1998 during a survey to discover asteroids.

4 ASTRONOMY INSIGHTS • JUNE 2019 LOCATING ASTEROIDS I SKY THIS MONTH A dwarf planet swings through Libra THE DWARF PLANET CERES climbs highest in the south as darkness falls, making a tempting target during the early evening

Totality comes to Chile and Argentina hours. Glowing at 8th magnitude, it’s easy to find through binoc- ulars or a small telescope among the bright stars near the border between Scorpius the Scorpion and Libra the Scales. Ceres lies about 15° west of Jupiter all month, but it’s sim- pler to locate by hopping from the lovely 2nd-magnitude dou- ble star Beta (β) Scorpii. The dwarf planet lies within 3° of Beta all month. It ventures even closer to magnitude 5.0 Lambda (λ) Librae during July’s final week. Only 8' separate the two at their closest the evening of the 27th. To identify Ceres from the background stars, make a sketch at the eyepiece that includes three or four stars in addition to the dot you think is the dwarf planet. Return to the same field a night or two later and see which object shifted position. The “star” that moved is Ceres. Fortunately, the Milky Way dust lanes that thread through this region help suppress the num- ber of faint background stars that could confuse matters. Ceres is the undisputed king of the asteroid belt. Its sub- Observers along a narrow track can see the Sun’s corona blossom into view stantial size, nearly 600 miles across, and nearly spherical during the July 2 total eclipse. PATRICK MARTINEZ shape helped persuade astronomers to raise its status from asteroid to dwarf planet. Any telescope reveals 8th- time July 1 and by 10 p.m. on magnitude Titan, which circles the 31st. It lies in Aquarius, just the planet once every 16 days. east of 4th-magnitude Phi (ϕ) Ceres barely evades the Scorpion’s reach When the giant satellite lies Aquarii. The planet’s westward N north or south of Saturn, it motion against this backdrop July 1 appears 1.3' away. At greatest reduces the gap between it and 6 eastern or western elongation, Phi from 1.3° to 0.9° during 11 LIBRA ν Titan lies 3.1' from the planet. July. Magnitude 7.8 Neptune 16 Three 10th-magnitude shows up easily through bin- Path of Ceres 21 moons — Tethys, Dione, and oculars; a telescope at high β 26 λ Rhea — lie closer to Saturn magnification shows the plan- ω1 31 than Titan and appear through et’s 2.3"-diameter disk and E 4-inch and larger scopes. You’ll blue-gray color. ω2 need a bigger instrument to Uranus stands 20° above spot 12th-magnitude Enceladus. the eastern horizon as twilight It orbits in 1.4 days and shows begins July 1 and more than up only when it is farthest east doubles that altitude by SCORPIUS or west of the planet. It happens month’s end. It resides in to be at greatest eastern elonga- southern Aries, 10° south and δ 0.5° tion on opposition night; hunt slightly east of 2nd-magnitude The invisible border between Scorpius and Libra seemingly blocks for it 16" from the A ring’s edge. Hamal (Alpha [α] Arietis), the the Scorpion’s claws from snatching this 8th-magnitude object. Outer Iapetus glows at 10th Ram’s brightest star. The mag- Diagram ASY-SM0719_14: magnitude in the days around nitude 5.8 planet stands out its July 15 greatest western surprisingly well through bin- PLEASEas a guide. PROOF: It appears 5° high in and Argentina. The Moon’s Title Astronomy Illustrator Roen Kelly elongation. Your best chance oculars in a sparse region 2.3° Individualthe east-northeast illustrators, 30 minutes umbral shadow first touches designers, art directors, Issue July 2019 Designer to see it comes on opposition south of the magnitude 5.7 star andbefore editors sunrise, must proof and VenusJob # liesMAG-ASY-JUL19 South AmericaArt Dir. on the Chilean night when it lines up with 19 Ari. When viewed through a andone sign binocular this form. field toCode the lowerSM coast nearStory La SerenaEd. and com- Titan. The outer moon then lies telescope, Uranus shows a dis- left. Despite gleamingProof at magni1 - pletes its trekCopy Ed.as the Sun sets 8.4' west of Saturn, some 2.7 tinctive blue-green color on a tude –3.9, the planet Datebarely4-1-19 just southMan. of Buenos Ed. Aires. 4-12-19 times farther away than Titan. disk that spans 3.5". shows up against theReturn brighten- Editor The two outer planets July 1 provides your best ing sky. You might be able to Martin Ratcliffe provides appear best during the morn- chance to see Venus before it track Venus for a few more planetarium development for ing hours. Neptune rises disappears in the Sun’s glare. days, but it drops out of view Sky-Skan, Inc., from his home around midnight local daylight Use the waning crescent Moon by the end of July’s first week. in Wichita, Kansas. Alister A total solar eclipse occurs Ling, who lives in Edmonton, Alberta, has watched the skies GET DAILY UPDATES ON YOUR NIGHT SKY AT July 2 along a narrow track that www.Astronomy.com/skythisweek. crosses the South Pacific, Chile, since 1975.

WWW.ASTRONOMY.COM 5 This intrepid spacecraft spent 13 years studying the ringed planet, transforming our view of this captivating world. by Liz Kruesi

Saturn’s globe blocked the Sun while the evening of September 11, The onlookers could see the beautiful Cassini captured this panoramic view 2017, Griffith Observatory rings circling Saturn, the planet’s yellowish showing the planet’s ring system in exquisite detail. The imaging team hosted an enthusiastic group cloud bands, and the orange-tinged dot of created this mosaic from 165 separate of observers. The assembled the big moon near the planet; what they images taken over a three-hour period. crowd looked through the couldn’t make out was a much smaller, NASA/JPL-CALTECH/SSI 12-inch Zeiss refracting tele- human-made target. On that late summer scope, the centerpiece of evening, the Cassini spacecraft was just the venerable public astron- 75,000 miles (120,000 kilometers) from omy venue in Los Angeles. Titan on its final path toward Saturn. The They watched as light from Saturn and its spacecraft and Titan had enjoyed their largest moon, Titan, passed through the “goodbye kiss,” as the astronomers and telescope’s optics, where lenses bent and engineers on the mission called the last focused it onto their eyes. gravitational yank that would send the

6 ASTRONOMY INSIGHTS • JUNE 2019 CASSINI 2004-2017

CASSINI 2004-2017 2004-2017 CASSINI

CASSINI 2004 2017 2004 -2017 -

CASSINI -2017 2004 CASSINI 2004-2017

spacecraft into the planet it had been Cape Canaveral, Florida, nearly 20 years At 4:55 a.m. PDT, they saw the last signal studying for 13 years. earlier. But their thoughts were not all on from Cassini fade away on the screen. The These observers at Griffith were no the past: Cassini was still collecting data room erupted in applause — not for the end ordinary members of the public. They and sending it back to Earth. of the mission, but for what the spacecraft were members of Cassini’s Project Science On September 15, at 3:31 a.m. PDT, and those hundreds of people had achieved. Group, watching their beloved spacecraft Cassini entered Saturn’s upper atmosphere Cassini revealed surprise after surprise on its final journey around the giant world. at a shallow angle. It would travel through at Saturn: an incredibly complex system of “It was a magical evening,” says Cassini’s the gas for nearly 11/2 hours. The team moons and moonlets, rings that change Project Scientist Linda Spilker. members were gathered at NASA’s Jet structure on hourly timescales, and a beau- Over the next few days, hundreds of Propulsion Laboratory in Pasadena, tiful atmosphere wracked by huge storms. scientists and engineers on the Cassini California, where they watched and waited. The 13 years of images and measurements mission team would reminisce about the “The room got quieter and quieter as we got changed humanity’s view of the ringed spacecraft, which had launched from down to those final minutes,” says Spilker. world. But there’s still more to learn from

WWW.ASTRONOMY.COM 7 vinyl record circling the yellow gas giant. But bring a camera close to Saturn, and the smooth disk resolves into belt after belt after belt, with spaces separating them. That was the view revealed by the Voyager flybys in 1980 and 1981, which led scien- tists to think the rings were probably made of tiny ice particles that slowly bump into one another as they orbit the planet. Use Cassini’s instruments to watch as the rings filter light from a background star, however, and all of a sudden those belts become far more complex. The particles clump together and form bigger bodies. The gravity of those objects — boulders and minimoons — controls the rings, herding smaller particles and building structures and patterns. And they change quickly, says Larry Esposito, principal investigator on Above: Cassini could probe Saturn’s ring Cassini’s Ultraviolet Imaging Spectrograph, structure by sending radio signals through the rings. In this simulated view of the A ring who has studied Saturn’s rings for more and the Cassini Division (at left center), red than four decades. “Structures develop denotes particles 2 inches (5 cm) or more in within hours in the rings.” diameter; green indicates particles less than Planetary scientists have identified sev- 2 inches across; and blue signifies particles less than 0.4 inch (1 cm) across. NASA/JPL-CALTECH eral different types of structures. Some, which come and go and come back again, Right: Vertical structures at the B ring’s are called kittens — “because they seem to outer edge cast long shadows onto the rings have multiple lives,” says Esposito. Others, two weeks before Saturn’s August 2009 equinox. The structures rise some 1.6 miles (2.5 km) called propellers, migrate slightly inward above the rest of the rings, which average or outward. They are consequences of about 33 feet (10 m) thick. NASA/JPL-CALTECH/SSI gravitational interactions between a small moon embedded within the rings and the ring particles themselves. The moon tries, unsuccessfully, to clear away the particles and create a gap. the last several months of data that Cassini Solstice extension, which ultimately bled Bigger moons tend to have more notice- collected. Scientists hope those final obser- into the Grand Finale. This final stage able effects. Prometheus, for example, vations will tell them about Saturn’s interior commenced in April 2017 and featured whose diameter averages 53 miles (86 km), — in particular, how it generates a mag- 22 close-in orbits that skimmed just above “dips in almost to the edge of the F ring and netic field and how its mass is distributed. Saturn’s cloud tops. pulls out streamers,” says Spilker. Several Because Cassini’s 12 instruments were other moons also leave their gravitational An extended stay attached directly to the spacecraft, the imprints. Scientists had long known that Spacecraft already had visited Saturn three entire contraption had to rotate for an Mimas creates the Cassini Division — like times before Cassini arrived in mid-2004, instrument to point toward a specific tar- the spacecraft, named for the 17th-century so scientists had some inkling of what they get. That meant multiple instruments Italian-French astronomer Giovanni might find. But as with any new mission couldn’t observe the same spot at the same Cassini — the broadest gap in the rings. But — especially one involving a machine with time. Instead, while one looked at a moon, it took data from the Cassini probe to reveal 12 sophisticated instruments that would another might observe Saturn’s rings. that seven midsized moons combine to remain in orbit instead of flying past as And that made the last five months of the keep the outer A ring from dispersing. its predecessors had — Cassini revealed a mission — the Grand Finale — a work The rings also contain density waves complex planet full of surprises. And that’s of impressive coordination. Although that show up as variations in brightness a good thing. “If Saturn had been exactly 22 orbits might sound like a lot, they and thickness. After studying these pat- as expected, it would have been a lot more aren’t much to work with when you have to terns, scientists, including the University of boring,” says Spilker. divide the limited time during those close Idaho’s Matt Hedman, showed that these Cassini arrived at Saturn for a primary flybys among the full instrument lineup. brightness changes are tied to Saturn’s inte- mission set to last four years. But when rior. The researchers used fine-scale den- mid-2008 came, the spacecraft continued Rings, rings, and more rings sity variations in the rings as a seismometer with its Equinox Mission extension. And Saturn’s rings are the planet’s defining of sorts to learn about how the planet’s in September 2010, the mission began its characteristic. From afar, they look like a interior oscillates, in much the same way

8 ASTRONOMY INSIGHTS • JUNE 2019 BEGINNER INTERMEDIATE ADVANCED

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B&H Photo – 800.947.9970 – bhphotovideo.com Astronomics – 800.422.7876 – astronomics.com Adorama – 800.223.2500 – adorama.com High Point Scientific – 800.266.9590 – highpointscientific.com OPT Telescopes – 800.483.6287 – optcorp.com Focus Camera – 800.221.0828 – focuscamera.com Optics Planet – 800.504.5897 – opticsplanet.com Woodland Hills – 888.427.8766 – telescopes.net Agena AstroProducts - 562.215.4473 - agenaastro.com Above left: A disturbance in Saturn’s narrow F ring appeared April 8, 2016. The disorder likely arose when a small body embedded in the ring interacted with material at the ring’s core. The small moon Pandora (lower right) was a mere bystander. NASA/JPL-CALTECH/SSI

Above right: Potato-shaped Prometheus (lower left) dips into the F ring’s inner edge once each 15-hour orbit, pulling particles into a streamer. This image captures the moon as it creates a new streamer; the dark streamers at upper right formed during the moon’s previous two incursions. NASA/JPL-CALTECH/SSI

Left: Tiny Daphnis orbits in the Keeler Gap near the A ring’s outer edge. Here, the 5-mile-wide (8 km) moon makes waves from the fine particles at the gap’s edge. The waves dissipate quickly, however, as the moon travels toward the image’s right side. NASA/JPL-CALTECH/SSI that solar astronomers have studied how rings, we could be missing things about, says Luciano Iess, who is leading Cassini’s brightness variations at the Sun’s surface say, how the solar system formed.” The pro- gravity data analysis. Disentangling the correspond to its inner pulsations. cesses going on in the rings could give two will not be easy, however. The prelimi- Despite all the incredible ring structure astronomers valuable insights into how nary analysis, he says, “seems to indicate that Cassini’s cameras and spectrometers planetary systems develop. that the rings did not form with Saturn.” resolved, scientists still have questions. The Grand Finale data are getting scien- It will take more research to firm up this The biggest one concerns the ring system’s tists closer than ever to figuring out the result, and to find out when and how the mass. They don’t want to know this mass rings’ mass. During those final months, rings formed. just for knowledge’s sake. Instead, the mass Cassini flew between the inner rings and is linked to the age of the rings and how Saturn’s upper atmosphere 22 times. Cloudy weather they formed. Throughout the previous 121/2 years of Beneath Saturn’s majestic rings lies the This is important because Saturn’s rings Saturn exploration, the spacecraft stayed planet’s equally magnificent cloud tops. are the closest example astronomers have of outside the rings, and thus it felt the com- Cassini unveiled churning and swirling astrophysical disks — such as the flattened bined pull from Saturn and the rings. clouds in the upper atmosphere, and places disks of gas and dust out of which solar sys- “When you are between the rings and where warm gases rise up through cooler tems form. “It’s not the same, but it’s analo- Saturn, the rings are pulling in one direc- layers and erupt into long-lasting thunder- gous,” says Hedman. And this means, “if we tion, and Saturn is pulling in the other, storms. Cassini resolved these thunder- don’t understand what’s going on in the so you can disentangle the two effects,” storm clusters into minute detail, watching

10 ASTRONOMY INSIGHTS • JUNE 2019 Tiny embedded moonlets create “propellers” as they unsuccessfully try to open gaps in the rings. In one of its final images, Cassini captured one such feature just above the Keeler Gap in the outer A ring. NASA/JPL-CALTECH/SSI

The informally named Earhart propeller resides in the A ring just inside the Keeler Gap (right). Earhart is the attempt of an unseen moonlet to create a ring gap, but the large mass of the surrounding material quickly fills the nascent breach. Dozens of small propellers occupy the so-called propeller belts in the NASA/JPL-CALTECH/SSI middle of the A ring. The propellers look like double dashes and appear on both sides of the density wave that cuts diagonally across this scene. NASA/JPL-CALTECH/SSI them evolve and listening to the radio Cassini’s imaging camera first saw high-frequency radio emissions created in static from lightning flashes. the storm December 5, at the same time lightning strokes. While normal photos painted pretty another instrument heard it — or at least, The jet streams in Saturn’s atmosphere pictures of the whirling atmosphere, infra- the radio bursts created by its lightning. A carried the northern hemisphere storm red images let scientists see below the cool similar phenomenon happens on Earth. If along its cloud band. By late January 2011, cloud tops to warmer regions beneath. “And you have ever been in a car listening to an it wrapped around the planet and stretched that’s our secret weapon for how to analyze AM radio station during a thunderstorm, 9,000 miles (15,000 km) north-south. As the depths of Saturn,” says Cassini scientist you probably heard what sounded like the storm progressed, scientists used the Kevin Baines. “[It’s] how Saturn was static. “That static is not actually static,” imaging instruments and RPWS to view revealed to be not this nice demure place, says William Kurth. “It’s actually radio it. In summer 2011, after some 200 days of but this roiling dynamic place.” He and his emissions from the lightning strokes and roiling, swirling, and spreading, the storm colleagues watched as clouds in the upper the thunderstorm, and they propagate at died out and the atmosphere cleared. The atmosphere blocked heat from below. They the speed of light.” Kurth is the principal region, says Baines, “has been very boring also identified vortices and a giant cyclone investigator of Cassini’s Radio and Plasma ever since.” at each of Saturn’s poles, though only the Wave Science (RPWS) instrument, which Because scientists could watch the great north pole features a hexagonal jet stream. listened in on the Great White Spot’s storm evolve with Cassini’s broad array of But one storm stood out from all the others. The Great White Spot erupted unexpectedly December 5, 2010. Earth- based observations of Saturn over the past 140 years had shown that a giant, long- lasting storm pops up every 30 years or so, alternating between cloud bands in the northern hemisphere and near the equator. In 1876, one appeared at the equator; in 1903, another developed at mid-northern latitudes; and in 1933, a storm emerged back at the equator. The pattern continued over the decades, and scientists expected the next storm would arrive around 2020 — after Cassini’s reign. But it fortuitously arrived 10 years early, and gifted Cassini Saturn’s north polar hexagon is a meandering A giant vortex resides at Saturn’s north pole. jet stream near 77° north latitude. Each side The storm, which appears red in this false-color scientists with an up-close look at how of the hexagon measures slightly longer than image, spans 1,250 miles (2,000 km) and has these giant storms evolve. Earth’s equatorial diameter. NASA/JPL-CALTECH/SSI winds up to 330 mph (540 km/h). NASA/JPL-CALTECH/SSI

WWW.ASTRONOMY.COM 11 The Great White Spot appears as a multihued snake in this false-color mosaic from February 2011. Yellow and white reveal high, thick clouds associated with thunderstorms; red shows deep clouds with no towering tops; and blue areas are cold spots. NASA/JPL-CALTECH/SSI instruments, they could piece together a coherent picture of what causes these long- lived events. Caltech’s Andrew Ingersoll and his then-graduate student Cheng Li put forth the most likely theory. They say it’s due to a convective competition between water-rich clouds and the lighter- weight atmosphere of mostly hydrogen and helium. The heavier, wet clouds can’t rise until the lightweight upper clouds become denser and sink. But this competition is a marathon. “The air above has to cool off, radiating its heat to space, before its density is greater than that of the hot, wet air below,” said Li in a press release. “This cooling process takes about 30 years, and then come the storms.” Once the storm rains out its water The Great White Spot erupted in December 2010 and quickly evolved into a massive storm. By the content, convection shuts down, and the time Cassini captured this image 12 weeks later, Saturn’s jet streams had carried the storm completely storm stops. around the planet. NASA/JPL-CALTECH/SSI

Magnetic makeup the region the field controls, called the generate, in much the same way it heard When you think of Saturn, the ornate magnetosphere. radio flashes associated with lightning. rings and cloudy atmosphere likely come Previous observations of Saturn had “We’ve been able to use the intensity of to mind first, but no object exists in isola- shown aurorae at the planet’s poles, similar these radio emissions as a proxy,” says tion. So, how does the giant planet affect its to the northern and southern lights seen in Kurth, to address questions of “how surroundings? That’s where Saturn’s mag- Earth’s polar regions. Cassini’s RPWS intense are the auroras and is there a lot of netic field factors in, and it’s why Cassini instrument monitored auroral activity activity going on.” RPWS also monitored brought along instruments to study it and by detecting the radio waves that aurorae how Saturn’s magnetosphere and aurorae changed when the Sun delivered a burst of high-energy particles and radiation. But how does Saturn produce its mag- netic field? To find out, scientists used Cassini’s magnetometer. This instrument measures the strength and location of the planet’s magnetic field lines, which trace how charged particles travel. Electrons, for example, have a negative charge, and they always move toward a magnet’s positive pole. Both Saturn and Earth are essentially giant dipole magnets: They have a positive pole and a negative one. Each planet gener- ates its magnetic field deep in its interior. For Earth, researchers have a pretty good Although Saturn’s north polar hexagon has lasted for at least 35 years (the Voyager spacecraft idea of how it happens. “You have heat, you first imaged it in the early 1980s), it does change. These natural-color views show the hexagon in June 2013 (left) and April 2017. Scientists think an increase in solar radiation during those four years have convection taking place in the inte- caused yellowish smog to form. NASA/JPL-CALTECH/SSI/HAMPTON UNIVERSITY rior, you have rotation in the interior, and

12 ASTRONOMY INSIGHTS • JUNE 2019 Enceladus prepares to set behind Saturn’s limb September 13, 2017. This was one of the last images Cassini took of the geologically active moon before the probe crashed into the gas giant September 15. NASA/JPL-CALTECH/SSI

magnetic field should be decaying — and scientists have seen no evidence of a diminishing magnetic field at Saturn. Above: Saturn’s aurora glows blue while the underlying When Cassini flew close to Saturn during atmosphere appears deep red in this infrared composite image. As on Earth, the aurora arises as Saturn’s magnetic the Grand Finale, the magnetometer col- field funnels energetic solar particles to the polar regions. lected data about the magnetic field. “We NASA/JPL-CALTECH/UNIVERSITY OF ARIZONA really expected these Grand Finale orbits to clearly measure the tilt, and all we’ve been Left: Cassini captured the ultraviolet glow from Saturn’s aurora one day before the spacecraft crashed into the planet. The able to do so far is put a limit on it,” says north pole lies at the center of this image, while the bottom Dougherty. The angle between the two faces the Sun. NASA/JPL-CALTECH/UNIVERSITY OF COLORADO/UNIVERSITY OF LIEGE-LPAP axes must be less than 0.06°. The team has had the data for only a you have flowing electrical currents,” says thick atmosphere hides the planet’s solid couple of months, however, and Dougherty Michele Dougherty, principal investigator core — assuming it has one. is confident that after she and her col- of Cassini’s magnetometer. “All of those To measure Jupiter’s day, for example, leagues complete their careful and thor- combine to give you the magnetic field that scientists track the magnetic axis and find ough analysis, they’ll know what Saturn’s you measure outside the planet.” it wobbles with respect to the planet’s rota- internal magnetic field is like. The biggest A key component in understanding tion. The magnetic field’s axis and the rota- hurdle is accurately calibrating the instru- Saturn’s magnetic field is the length of a tion axis tilt relative to each other, and that ment. The analysis requires absolute preci- saturnian day, and this was a major ques- wobble relates directly to how fast the plan- sion — the exact location and timing of the tion scientists hoped Cassini would resolve. et’s core is spinning. The problem with spacecraft’s trajectory, and knowledge of This shouldn’t be a difficult question, Saturn, though, is that the two axes are where Cassini was when the instrument right? It’s just the rotation period. But that’s nearly perfectly aligned. This makes it collected each bit of data. Researchers have a much harder problem to solve for gas awfully hard to find that wobble. predicted orbits, positions, and times, but giant planets than it is for Earth. The cloud The precise alignment also perplexes they have to know whether Cassini’s actual tops rotate at different speeds, and the researchers because it implies that the orbit followed them precisely. For example,

On May 28, 2017, Cassini flew between Saturn’s rings and its cloud tops, capturing the images for this mosaic. Saturn appears in the left foreground, adorned with shadows cast by the rings. The rings themselves emerge from behind the planet’s limb and extend to the right. NASA/JPL-CALTECH/SSI

WWW.ASTRONOMY.COM 13 Saturn posed for Cassini one last time September 13, 2017. The imaging team assembled this natural-color mosaic from 42 wide-angle images taken through three color filters from about 15° north of the ring plane. NASA/JPL-CALTECH/SSI

During its Grand Finale mission, Cassini captured subtle atmospheric details. In this view, the Sun shines at a low angle near Saturn’s terminator, Cassini crashed into Saturn’s atmosphere September 15, 2017, at the spot marked by the oval. where day turns to night, and some high clouds This nighttime infrared view shows heat coming from the planet’s interior in red; the dark regions cast shadows on lower regions. NASA/JPL-CALTECH/SSI are silhouetted clouds. NASA/JPL-CALTECH/UNIVERSITY OF ARIZONA might there have been a half-second delay and Cassini and measured the slight chang- at different speeds than others. The because the craft felt more drag from the es in radio frequency. Those changes arose researchers still have more orbit trajectories atmosphere than expected? “It’s a really from gravitational tugs of mass pulling on to calibrate, and thus are still months away complicated process,” says Dougherty of the spacecraft — the more mass, the bigger from a major announcement. the analysis. “It’s like trying to find three the tug. So Iess and his colleagues can use Revealing that the interior doesn’t align or four needles in a haystack that’s chang- those tiny frequency changes to map the with models would be a fitting discovery ing shape and size at the time.” distribution of mass within Saturn. Because from a mission that already has found so Cassini skimmed the planet’s cloud tops many surprises at the Saturn system. Mapping gravity’s pull during its final months, it felt a stronger Cassini’s suite of instruments offered the The magnetic field analysis isn’t the only gravitational pull from those mass distribu- flexibility that allowed scientists to make one proving to be extremely complex and tions, and was able to sense finer details. those discoveries. The mission’s scientists requiring precise calibration. Scientists also Precisely understanding those Grand and engineers worked in sync for decades want to know about Saturn’s interior, and Finale orbits is crucial to the gravity analy- to perform what Spilker calls Cassini’s in particular, how the planet’s mass is dis- sis of Saturn. So far, the team has learned “intricate ballet.” tributed. To do that, they need to measure that theoretical models of Saturn’s gravity “It’s for the unknown, the unexpected,” the planet’s gravity. That’s not as simple as do not match the data. “The gravity field of she says. “That’s why you do science.” it might sound. “There is no instrument Saturn is surprising,” says Iess. “We found aboard a satellite which can reveal the grav- Saturn has features that can be explained Contributing Editor Liz Kruesi writes about ity field by itself,” says Iess. Instead, scien- only by differential rotation,” meaning distant objects from her Earthbound home tists passed radio signals between Earth some portions or layers of the planet move in Austin, Texas.

14 ASTRONOMY INSIGHTS • JUNE 2019