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Jupiter

    MILLION-YEAR PICNIC

    The Gas Giant That Rules the January Sky

    Observing the storms stirred by Jupiter’s strange chemistry

    By

    Step outside after sunset this month and you’ll see a brilliant gold-white star high, far brighter than any other, high in the east. It’s not the legendary star in the east that the three magi supposedly followed to a manger outside Bethlehem. It’s not even a star at all. It’s the planet Jupiter.

    Jupiter is the largest planet in our solar system, and descriptions of it can involve lots of big numbers. It has over 90 moons. It’s 11 times the size of Earth with a diameter of 88,000 miles. Thirteen hundred Earths could fit inside it. In fact, all the other planets could fit inside Jupiter.

    Jupiter’s Great Red Spot and turbulent southern hemisphere captured in three composite images from Juno on Feb. 12, 2019. (Courtesy NASA/JPL-Caltech/SwRI/MSSS, processing by citizen scientist Kevin M. Gill)

    Like Saturn, the other planet to watch in the winter night sky, Jupiter is what astronomers call a gas giant. This means it has a thick, dense atmosphere of hydrogen mixed with some helium and trace amounts of ammonia and methane. Our own atmosphere has a depth of about 60 miles; Jupiter’s is 1,900 miles.

    The pressure and heat of such a deep atmosphere create strange chemistry. At the lowest altitudes, the pressure compresses the gaseous hydrogen so much that it transitions into an immense sea of liquid hydrogen. Farther in, as pressure and heat soar, electrons are stripped from nuclei, transforming the liquid hydrogen into a bizarre metallic superfluid where electricity flows freely. Theoretically, Jupiter has a core made of nickel and iron like our own and larger than the Earth.

    If we zoom out to a comfortable distance of 10,000 miles or so, the planet presents a visual spectacle. The topmost layer of its atmosphere contains clouds of ammonia crystals. Jupiter’s fast rotational speed (it rotates once every 10 hours as opposed to our 24 hours) causes powerful jet streams and turbulence. Sheared by these forces, the clouds separate into many distinct bands. Other material, like hydrocarbons, sulphur, and phosphorus, rises from the depths and reacts with ultraviolet light from the Sun, taking on different colors. This colorful material then gets caught up with the motion of the cloud bands, creating Jupiter’s whirling, striped appearance.

    Juno was about one Earth diameter from Jupiter when it captured this view of the planet’s atmosphere on Dec. 16, 2017 from 8,292 miles above its clouds. (Courtesy NASA/JPL-Caltech/SwRI/MSSS, processing by citizen scientists Gerald Eichstädt and Seán Doran)

    The frenzied activity in Jupiter’s uppermost layer reaches its apex in the Great Red Spot — a most underwhelming name for such an astonishing phenomenon. This mighty storm, larger than two Earths, has been raging at least since 1831 and possibly since 1665, when the Italian astronomer Giovanni Cassini first recorded observations of a “permanent spot” in Jupiter’s south equatorial belt. The Great Red Spot is an anti-cyclone, a storm rotating counterclockwise, with wind speeds of over 250 miles per hour. You can see it with any backyard telescope. And I recommend seeing it while you can. The storm appears to be at last diminishing in size and intensity. It may go on for only another century or so.

    While looking for the Great Red Spot in a telescope, you can’t miss the four largest of Jupiter’s moons. These four — Io, Europa, Ganymede, and Callisto — appear as pinprick stars in line on one or both sides of the planet.

    In 1610, Galileo Galilei first pointed one of the earliest telescopes at Jupiter. Besides Jupiter’s cloud bands, he observed these four “stars.” He also observed their motion.

    Stay at the telescope for an hour or two and you’ll witness the moons changing position as they orbit Jupiter. One might disappear behind the planet, while another appears on the other side. This clockwork procession around Jupiter provided the first empirical proof that Earth was not the center of the universe.

    Two storms merging in Jupiter’s atmosphere. Taken from NASA’s Juno spacecraft on Dec. 26, 2019 at about 44,900 miles from the tops of Jupiter’s clouds. (Courtesy NASA/JPL Caltech/SwRI/MSSS, image processing by citizen scientist Tanya Oleksuik)

    Galileo got into serious trouble with the Catholic Church for promoting his observations and deductions. In honor of his dedication to science and evidence-based thinking, these four satellites are known now as the Galilean Moons.

    Jupiter’s name, like that of Saturn, also comes to us from the Romans. They called it the Star of Jupiter, a god of the Roman pantheon. Other cultures have associated this bright star with important mythological figures. Ancient Hindu astrologers called it Brihaspati, who was the teacher and counselor of their gods. The Babylonians connected the star with Marduk, who, like Jupiter (and the Greeks’ Zeus before him), was the king of their pantheon.

    Jupiter will shine after sunset until early spring. You can also still see Saturn (details on how can be found in last month’s column), but it is already dipping toward the horizon at sunset and setting earlier every night. By the end of January, Saturn will vanish for a time as it moves behind the Sun from our point of view, leaving Jupiter alone to rule the early evening sky. For a time.

    Come spring, Jupiter will vanish behind the Sun, too, giving another planet a chance to claim the title of brightest evening star. Venus perhaps? Clear skies!

    MILLION-YEAR PICNIC

    The Golden Glow of a ‘Wandering Star’

    December is for watching Saturn in the southwestern sky

    By

    It’s Jupiter and Saturn season, the time of year when both planets are favorably positioned in the sky after sunset. Over the next few weeks, you can find and appreciate them easily with the unaided eye as pretty and colorful stars. And if you have binoculars, you may be astonished at the detail you can see with them.

    Saturn is probably the planet most casual skywatchers know best, after our own: its rings, whether seen in NASA photos or through a telescope, are an unforgettable sight.

    The planet’s name comes to us from the religion and mythology of the Romans. Saturn was a Roman god associated with agriculture, renewal, wealth, and freedom. Roman mythology refers to an ancient Golden Age when Saturn ruled over all creation, and humanity enjoyed peace and prosperity — without having to do any work.

    An image of Saturn and its rings taken by NASA’s Cassini spacecraft in 2016 at the relatively close distance of 1.9 million miles. The Cassini Division, a gap in the rings caused by the moon Mimas, is visible in sharp detail, as are the many distinct cloud bands in Saturn’s atmosphere. (Photo courtesy NASA/JPL-Caltech/Space Science Institute)

    Much later, having learned the value of hard work, the Romans conquered Greece. The conquerors of the first century B.C.E. admired and envied Greek culture and ideas. Among many acts of cultural appropriation, which included stealing art, they adopted Greek deities and, in one of the great retcons of history, identified them with their own Roman deities. Zeus, Hera, and Aphrodite, with a few modifications, became Jupiter, Juno, and Venus. And Kronos, leader of the Titans who ruled before the more familiar Greek gods, became Saturn.

    There are five stars that follow strange, meandering paths in the sky. Their motion is markedly different from the two thousand or so other well-behaved stars. The ancient Greeks called them planetes — wandering stars. We know them today as the five planets in our solar system that are visible to the unaided eye.

    The Greeks called one of these wandering stars Kronos in honor of the Titan. The Romans called it Saturn, and that name has stuck for the cultural descendants of ancient Rome.

    Saturn is about 800 million miles from Earth and is a huge planet, second only to Jupiter in size. Our planet is about 7,900 miles in diameter; Saturn is about 75,000 miles across. You could pack 764 Earths inside it. Astronomers categorize Earth as a terrestrial planet, meaning it’s mostly composed of iron, nickel, and rock, with a relatively thin atmosphere. In our case, it’s about 62 miles thick and, at lower altitudes, very pleasant. Saturn is a gas giant, made almost entirely of hydrogen and helium, with an immensely thick atmosphere. And it’s not a pleasant one at any altitude.

    If you could descend into Saturn’s atmosphere in a spaceship, you’d first pass through a layer with ferocious winds blowing over 1,000 miles per hour and clouds made of ammonia ice. Temperature and pressure would increase rapidly as you descended, crushing or baking your spaceship long before you could reach a deep sea of liquid hydrogen. Below that sea the hydrogen, under ever-increasing heat and pressure, transitions to a bizarre metallic hydrogen, which has properties of both liquid and solid. Finally, there is a dense rocky core, similar to Earth’s in composition, but much larger and under extreme heat and pressure.

    Prophesy had it that Kronos, known to the Romans as Saturn, would be overthrown by his children. He tried to prevent this by eating them. The painting on this terracotta jar from around 450 B.C.E. shows the god with his wife, Rhea, giving him a stone wrapped in a blanket instead of their newborn son Zeus. (Photo courtesy Metropolitan Museum of Art)

    Saturn’s magnificent rings are made of countless small pieces of ice, which range from the size of pebbles up to about 30 feet. The rings are about 170,000 miles in diameter (with an enormous gap in the center for Saturn). But this vast structure is, on average, only 60 feet thick. Ice reflects sunlight very well, which is why the rings are visible.

    Our best evidence to date suggests that the rings are the remains of an icy moon whose orbit decayed, causing it to lose altitude and venture closer and closer to Saturn and its powerful gravity. Once it got too close, tidal forces (like those between Earth and our moon that cause ocean tides) tore the moon apart. The debris then spread out into the rings we see today. This probably happened about 400 million years ago. (The Sun and planets, including Earth, formed about 4.5 billion years ago.)

    About 300 million years from now the rings will be gone, as piece by tiny piece the ice succumbs to Saturn’s gravity and falls into its atmosphere. It’s happening even now as you read this.

    To find Saturn in the sky, head outside on any clear evening before the end of the year. Give your eyes time to adjust to the darkness, then look to the southwest. Saturn is not a bright star, but its color is a distinct yellow-gold. Some people see it more as red-gold — individual color perception in dim conditions can vary. And in contrast to true stars, planets don’t twinkle (usually). If you see a star in the southwest that’s any kind of gold hue and doesn’t twinkle, you’ve found Saturn.

    If you have binoculars, rest them on a railing or tree branch and point them toward Saturn. With a steady view Saturn will appear as an oblong shape like a football. Children and adults with very sharp eyesight may be able to resolve the rings. And anyone will be able to see at least one pinprick star very close to Saturn. That’s Titan, its largest and brightest moon. (Saturn has over 100 moons, though most of them are just large rocks and chunks of ice.)

    Saturn’s Golden Age is over. But Roman mythology doesn’t say that it was necessarily the one and only time that Saturn might rule over humanity. I’m looking forward to peace and prosperity without the hard work in some future Golden Age Part Two. Clear skies!

    million-year picnic

    Two Planets Prepare to Leave the Night Stage

    A last fall look at Jupiter, Saturn, and their moons

    By

    Jupiter shows its Great Red Spot
    An image of Jupiter showing the Great Red Spot. (Photo courtesy NASA)

    The constellations of summer are passing from the night sky, while our old wintertime friends, like Orion the Hunter, are making their way onto the stage. The planets Jupiter and Saturn are still visible in the early evening, but not for much longer. They’re setting earlier each night, and will soon be lost in the Sun’s glare. When they reappear in January and February 2020, they’ll be in the early morning sky before sunrise.

    Why is that? The motion of the planets in the sky is complex, and the explanation for it eluded astronomers from ancient times until about 400 years ago, when Galileo and a few other luminaries settled the matter through careful observation. A full description of celestial mechanics is beyond the scope of this column (and, sadly, perhaps the columnist) but let’s try to tackle the basics.

    The planets of the solar system orbit the Sun at different speeds. The farther a planet is from the Sun, the slower it orbits, and the longer it takes to complete that orbit. Earth is much closer to the Sun than Jupiter and Saturn, and completes one orbit for every 12 of Jupiter’s and every 29 of Saturn’s. One orbit equals one year, so a year on Jupiter spans 12 Earth-years, and on Saturn, 29 Earth-years.

    Thus, over the course of our year, we approach Jupiter and Saturn in our collective race around the Sun like a runner on the inside lane of a racetrack. Opposition is the astronomical term for when we catch up with them. Then we pass them and pull ahead in our faster inner lane. The race continues as we swing around the Sun opposite them (called conjunction), and they vanish completely from our view for several weeks. Then we emerge from conjunction, and the pattern repeats. As these movements occur, the time of Jupiter’s and Saturn’s rising and setting in our sky shifts.

    For now, Jupiter and Saturn are still easy to view at night, so let’s see what we can see. You’ll find them low in the southwest after sunset (see my last column, “Celestial Navigation,” published Aug. 23, though they’ll now be lower in the sky than described there).

    To the naked eye, Jupiter looks like a bright gold-white star. Through binoculars, you’ll see Jupiter as a bright disk, with up to four little stars very close to it, in a line together or on either side of the giant planet. These are the Galilean moons (named after Galileo, who first observed them); they are the four largest of Jupiter’s 79 moons. If you watch for long enough, as Galileo did, you’ll see them move over the course of a couple of hours. Some might disappear behind Jupiter, while others may appear over its edge.

    Through a telescope you’ll see distinctive bands of dark clouds across Jupiter. If you’re lucky, you might see the Great Red Spot, a giant storm larger than the Earth that has been raging for centuries. It will appear as a dark swirl extending from one of the cloud bands.

    On to Saturn. To the eye, it looks like a moderately bright gold star. Binoculars enhance the color, but to see its rings you’ll need a telescope. Yes, you really can see the rings. It’s a spectacular and unforgettable sight. You might also see Titan, its largest moon, as a small dim star close to Saturn.

    I mentioned summer and winter constellations in the opening of this column. Next time, we’ll talk about this phenomenon of seasonal stars and constellations, which is similar to the progression of the planets in the sky that you just learned about. Clear skies!

    million-year picnic

    Celestial Navigation

    Finding Jupiter and Saturn at the end of your arm

    By

    Stargazing on Wellfleet pier
    Passers-by on the Wellfleet pier get a look through the telescope at Jupiter and Saturn. (Photo Eliza Miller)

    The planets Jupiter and Saturn returned to the evening sky last month and will be with us through autumn. Jupiter looks like a brilliant white-gold star, and Saturn is a dimmer, rich gold color. They’re a beautiful sight against the night sky, but bright objects like these can be seen earlier, just after sunset, when the sky is still blue.

    Here is a clever celestial navigation trick to find them.

    By a convenient quirk of human anatomy, your fist held out at arm’s length covers about 10 degrees of the sky. You can use this to direct your gaze to specific positions. Try this: extend your arm straight out in front of you and make a fist. Turn your fist vertical (thumb-side up). Now make sure your thumb is on top of your fist, not tucked in. With the heel of your fist resting on the horizon, the bump of your thumb knuckle now marks a spot in the sky about 10 degrees above it. Placing your other fist on top of the first moves you up to 20 degrees relative altitude. Leapfrog your fist again, and you’re at 30 degrees above the horizon.

    Now that you have the technique down, on to the fun part. First, find a place where you have a clear view to the south (a beach, big parking lot, or open field). Then stand facing south after sunset; currently 7:30 p.m. Using your new navigation technique, raise your gaze to 25 degrees altitude (two and a half fists, stacked vertically).

    Justin Samaha showing young stargazers the planets
    Young stargazers get a good look at the planets with help from street astronomer Justin Samaha. (Photo Thea Samaha)

    Slowly scan back and forth, until you see a star shining against the dark blue sky. Only it’s not a really a star. It’s Jupiter.

    Now move your eyes left of Jupiter, a little east of south, and drop half a fist to 20 degrees altitude. Look carefully for a dimmer, gold star — that’s Saturn. It’s harder to find than Jupiter, since it’s not as bright, and you might not see it until closer to 8 o’clock.

    Seeing how early you can catch these planets and watching them as they move across the night sky is a good way to train your astronomical eye. Next time: more about the detail you can see of both planets with your eyes, binoculars, and small telescopes. Clear skies!

     

     

    Note to readers who are not on the Outer Cape: Jupiter and Saturn are visible everywhere right now, and the altitude figures given will work from roughly the latitudes of Washington DC to Montreal. The further south you are, the higher the altitude of Jupiter and Saturn will be. The further north, the lower the altitude. If you want to learn more, look for astronomical almanacs online. They will tell you the altitude in degrees of objects in the sky at your location for a given time and date.

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