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Cassini-2

Seven hundred million miles away the Cassini spacecraft is preparing for death this coming September (2017). Launched in 1997 (when I moved to Brooklyn) the joint Italian/American space exploration mission to Saturn has seen and done things beyond comprehension. Lifted out of Earth’s gravity well by means of a Titan IVB/Centaur It flew through the nothingness and slingshotted around Venus (twice), the Earth, and Jupiter. It discovered new oceans on Enceladus and launched a lander onto the supermoon Titan (the first ever landing in the outer solar system). Cassini was used to tested general relativity: the craft broadcast radio past the sun to the Earth so that scientists could measure how the star’s gravity distorted the electromagnetic waves. Powered only by pluck (and, uh, 33 kilograms of plutonium-238) the little probe visited 20 moons.
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But all good things come to an end, and this final phase may be the most dramatic. On April 26th the craft began weaving between Saturn’s rings and the top of the planet’s atmosphere. The image at the top is an artist’s conception of how this might look for Cassini. The second image is a picture of the enormous hexagonal storm at the north pole taken April 30th. The image below is an infrared picture of Saturn. Cassini is scheduled to make 20 more of these passes before its final fiery plunge into Saturn itself, so prepare for more mind-boggling images of the gas giant.
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One of the smaller moons in the Saturn system is Daphnis, a little 8 km (5 mile) irregular satellite which orbits the gas giant within the outer rings of the planet (although I guess really the famous rings themselves are composed of innumerable “moonlets”).  Daphnis, which has the irregular shape of a potato, orbits Saturn in a 42-kilometer (26 mile) wide belt in the rings—the Keeler Gap.  The moon is responsible for clearing this narrow track, and it is felt that by studying this interaction we may learn about accretion and the enigmatic happenings of the early solar system (when more things looked like Saturn). Here is a picture from NASA’s Cassini probe which was released yesterday which shows little Daphnis producing waves in the Keeler belt.  What a remarkable image!   I need to post more Cassini pictures here. They fill the heart with wonder and give us a chance to get off-planet for a little breather.

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The North Pole of Enceladus during the October 30th, 2015 Cassini Flyby (NASA/ESA/ASI)

The North Pole of Enceladus during the October 30th, 2015 Cassini Flyby (NASA/ESA/ASI)

Since 2004, the Cassini spacecraft has been in orbit around Saturn. The robot probe (a joint effort of NASA, ESA, and the Italian space agency) received the most press when it launched a flying saucer lander onto Saturn’s planet-like moon Titan, but it is still out there doing amazing work. Last week, while I was busy writing about Halloween themes, the probe made its closest pass yet to Saturn’s ice moon, Enceladus. Enceladus is only 500 kilometers in diameter and it is coated in ice, but it is of great interest to scientists because ice plumes venting from the moon’s south pole seem to indicate a large polar subsurface ocean of liquid water. Warmed above freezing by tidal flux, this ocean beneath the ice probably has a thickness of around 10 km.

View of Enceladus’ south pole geyser, backlit by Saturn

View of Enceladus’ south pole geyser, backlit by Saturn

On October 30th, Cassini flew by the icy moon at the dangerously close distance of 30 kilometers (18.6 miles). The probe was directly above the south pole of Enceladus and it collected a little flake of ice to analyze (which strikes me as incredibly amazing and beautiful). It will take some time for the ship’s devices to assay the drop of water from an alien ocean, but Cassini also snapped some photos which we already have. These are taken from point blank range above the south pole. The ocean is down there beneath the scratches and scars. What is the nature of this icy ocean? How long has it been there? Could it possibly harbor life?

The Great Basin on Saturn's Tethys  (Credit: Cassini Imaging Team)

The Great Basin on Saturn’s Tethys (Credit: Cassini Imaging Team)

It’s been too long since we headed out to space.  This is true of humankind, but it is also true of this blog…so today we are going to cast our eyes across the solar system to Tethys a mid-sized moon of Saturn. In 1684 the Italian astronomer Giovanni Domenico Cassini discovered Tethys. He initially named the moon in honor of Louis XIV, but his choice was later changed so that the moon is named for a first generation Greek titan-goddess.  The moon has been approached by several human spacecraft, most notably…Cassini, which has dropped by several times (the robot space probe is named after the astronomer—the poor fellow has not been drifting in space since the 17th century).

Giovanni Domenico Cassini

Giovanni Domenico Cassini

Of all the major moons in the solar system, Tethys has the lowest density: 0.98 g/cm3 ! This means that almost the entire moon is made of frozen water—it is essentially a huge round ice cube floating around Saturn. Tethys has two extremely prominent features—a giant crater 450 kilometers (280 miles) across (named Odysseus) and a huge ice canyon 2000 kilometers (1200 miles) long, 100 km (62 miles) wide, and 3 km (1.8 miles) deep, which stretches most of the way across the moon.  Unsurprisingly astronomers speculate that the two features are related and the massive impact which created Odysseus melted a chasm along the entire side of the planetoid.

Tethys

Although you might be inclined not to expect much activity from a ball of ice in the depths of space, Tethys seems like it may be geologically active, or, at least, it may have been once.  The area around the hemisphere is comparatively flat and free of craters—which suggests that tidal flux from Saturn causes some melting—and possibly cryovolcanoes.

Ithaca Chasma: The Great Rift on Saturn's Tethys  (Credit: Cassini Imaging Team)

Ithaca Chasma: The Great Rift on Saturn’s Tethys
(Credit: Cassini Imaging Team)

Paleontologists and sharp-eyed readers already know the name Tethys.  During the age of Pangaea (when all of the world’s continents joined to form a single land mass), the great ocean in the midst was named the Tethys Ocean. In Greek mythology, Tethys was the daughter of Gaea (the mother earth) and Uranus (the heavens).  She was regarded as the mother of all waters and was married to her brother Oceanus, the first lord of the seas.  The astronomers of the age of enlightenment who renamed the moon, could not have known it was composed mostly of water, but they chose well.

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Voyager I

Voyager I

Today (September 12, 2013) NASA announced that Voyager I has officially left the solar system.  The probe is the first human-made object to enter interstellar space: it is farther away from Earth than anything else people have ever made.  Launched on September 5, 1977, Voyager’s primary mission was to fly by Jupiter and Saturn and take pictures (and electromagnetic radiation readings) of the two worlds and their systems.  The probe reached Jupiter in 1979 and Saturn in 1980.  After a close fly-by of Titan, the moon with an atmosphere, the spacecraft was flung out of the plane of the solar system.  Only this summer has it reached the heliopause, where the sun’s electromagnetic energy is matched by the ambient energy of the cosmos (although since only minimal instruments are running on Voyager, astrophysicists may be a long time arguing about when exactly the craft slipped out of the solar system).

The Great Red Spot of Jupiter as imaged by Voyager I

The Great Red Spot of Jupiter as imaged by Voyager I

A Volcanic Eruption on Io (imaged by Voyager I)

A Volcanic Eruption on Io (imaged by Voyager I)

 

The Atmosphere of Titan (imaged by Voyager I)

The Atmosphere of Titan (imaged by Voyager I)

Voyager is not moving as quickly as the solar probes mentioned in yesterday’s post, but neither is it moving slowly (its current velocity is 38,000 miles per hour).  However such speed is minimal in the face of interstellar vastness (although Voyager is due to pass within 1.6 light years of the red dwarf star Gliese 445 in 40,000 years).

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janus_1

In the Roman pantheon, Janus is the two-faced god of beginnings, limits, doors, gateways, and departure.  Unlike the other Greco-Roman deities, Janus was not imported from Greece to Rome.  How he arrived in the Roman pantheon is unclear: some scholars believe that he was originally a gatekeeping deity of the near East while others argue he was an original Latin deity who was worshipped in Italy before Rome rose to power.  Similarly there are different myths concerning his origin.  The most dramatic tale of his creation asserts that he was made by Uranus, god of the primal heavens as a love present for dark Hecate.  Janus despised being in the underworld so he escaped from Hecate by diving into the river Styx and swimming to the world above.

Janus

After fleeing the underworld, Janus acted as one of the earliest kings of Rome in the golden era when the titans ruled the world, however at the end of the titanomachy—the epic war between titans and Olympians—he made the poor decision to give shelter to Saturn, hated father of Jupiter.  Jove was furious at Janus because of this betrayal and he cursed him with immobility and with a second face.  Thereafter Janus stood at the threshold of heaven to open and close the gate as Jupiter came and went.

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Janus was a popular god for the Romans and they worshipped him whenever they started a new venture or embarked on a trip.  January is named after the god and the first day of every month is dedicated to him. The ancient temple of Janus stood in the center of Rome was open during war and closed during times of peace.  Since the Romans were a warlike people the temple was rarely closed and sometimes stood open for hundreds of years at a time.

The surface area of Earth is about 510 million square kilometers.  That number adds some perspective to the giant storm which has been raging on Saturn since December and now covers approximately 4 billion square kilometers of the gas giant planet.

The Storm Raging on Saturn (photo from the Cassini probe, NASA)

Saturn’s atmosphere is usually calm and tranquil–although powerful storms have been observed by telescope in the past. Now however Saturn is being closely observed by NASA’s Cassini space probe which is in orbit around the planet and we have some precise details.  At the storm’s height, Cassini detected over 10 lightning strikes per second.  Additionally, these lightning bursts can emit 10,000 times the amount of electrical energy as a typical lightning burst on Earth.  Saturnian meteorologists (or whatever weather scientists for the great ringed planet are called) speculate that this super lightning is so powerful because of the juxtaposition of layers of water ice with layers of crystallized ammonia.

A Detailed False Color Picture of the Storm (NASA)

Saturn’s weather is known to fluctuate with the change of the season on the frigid planet and the huge rings are presumed to affect the weather in unknown and unpredictable ways. The current giant storm is taking place in the northern hemisphere of Saturn, which is entering spring.

Although Saturn’s storms are not as well-known as the Great Red Spot on Jupiter, the planet’s north pole does feature a hexagonal storm which has persisted for at least 25 years.  Named for Jupiter/Zeus’ father (who was known as Cronus to the Greeks), the planet Saturn is the second largest in the solar system with a surface area of 4.27 x 1010 square km.  The planet is orbited not only by its famous rings but also by at least 62 known moons including Titan, the only known satellite with a dense atmosphere, and Mimas, which features the largest known impact crater.

Les Saturnales (Antoine-François Callet)

Today is the Feast of Saturn!  In Ancient Rome this holiday was officially celebrated on December 17 (XVI Kal. Jan.) and it initiated the multiple day festival of Saturnalia—the biggest holiday of the Roman Year. The Roman god Saturn was based on the Greek deity Cronus.  Although the Romans recognized that Saturn was a deposed ruler, a murderer, and a cannibal, Saturn was worshiped in Rome as an agricultural deity whose reign had been a golden age of abundance and innocence.  Saturn’s time had been one of gold–an age when people were naked, free, and kind. Jupiter’s age was one of iron when all men struggled greedily against one another–an age of wars, lawyers, oppression, and struggle.

Saturnalia was therefore a time to return to the imagined happiness of the past.  The cult statue of Saturn was freed from the shackles with which he was bound during the rest of the year and filled with olive oil (for the figure was hollow).  Schools and offices were closed so that special sacrifices could be made.  Great feasts were held and small presents were exchanged–particularly earthenware figurines called sigillaria and candles (which were a sort of symbol of the holiday and represented the return of light after the short dark days of the solstice).  There was a special seasonal market, the sigillaria. People decorated their houses and themselves with greenery and garlands.  Best of all, Rome’s famously rigid discipline was set aside during Saturnalia.  To quote the online Encyclopedia Romana:

During the holiday, restrictions were relaxed and the social order inverted. Gambling was allowed in public. Slaves were permitted to use dice and did not have to work. Instead of the toga, less formal dinner clothes (synthesis) were permitted, as was the pileus, a felt cap normally worn by the manumitted slave that symbolized the freedom of the season. Within the family, a Lord of Misrule was chosen. Slaves were treated as equals, allowed to wear their masters’ clothing, and be waited on at meal time in remembrance of an earlier golden age thought to have been ushered in by the god. In the Saturnalia, Lucian relates that “During My week the serious is barred; no business allowed. Drinking, noise and games and dice, appointing of kings and feasting of slaves, singing naked, clapping of frenzied hands, an occasional ducking of corked faces in icy water—such are the functions over which I preside.”

Various cults celebrated their mysteries during this time of year.  People from all walks of life lost themselves in uninhibited drinking, merrymaking, and fertility rituals.  Many Romans were born 9 months after Saturnalia (which would be approximately August 22nd on our calendar).

Roman painting (Unknown Artist, Villa dei Misteri in Pompeii, Italy)

Saturnalia had started in Rome in 217 BCE after Rome had suffered a series of crushing defeats at the hands of the Carthaginians (and the citizens needed a morale booster), but the deep roots of the holiday stretch back to prehistory.  Additionally the various people whom Rome had conquered all had solstice rituals of their own–which became incorporated into Saturnalia.  The year-end ceremonies of the Gauls and Celts focused on evergreen trees particularly the yew.  In Roman Egypt, the ancient deities were still worshiped (indeed, worship of Isis spread through the Roman world).  During the solstice time Egyptians celebrated how their greatest god, Osiris, had returned to life after being murdered by Set. Strangely the Egyptians too focused their resurrection rituals around a tree–albeit the palm tree. Rome’s mightiest neighbor, the Persian Empire, burnt great fires for Mithras, a deathless god born in a cave on December 25th.   The Mithraic mysteries were particularly popular in the Roman military (although many of the details about the cult are unknown to us).  Across the complicated cosmopolitan Roman world, people of all classes and faiths dedicated themselves to pleasure and to getting through the cold darkness to a new year. Catullus called the time of Saturnalia, “optimo dierum” (the best of days) and that was definitely true in an empire which was otherwise beset by political unrest, war, agricultural failure, greed, injustice, and decline.

On an unrelated note, I will be away for a week to celebrate Christmas.  I might post some things here or I might be too busy eating, relaxing, and exchanging small presents with loved ones.  In the mean time I wish the very happiest of holidays to all of my family, friends, readers, and, in fact, everyone.

Mimas (Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA)

Mimas is a moon of Saturn.  Discovered late in the 18th century by the astonishing Sir William Herschel, Mimas is the smallest (known) astronomical body which is spherical from self-gravitation (here is an explanation of what that means).  The moon’s most noteworthy feature is an enormous impact crater named Herschel which is 130 kilometres (81 mi) across–about the same as the distance between New York and Philadelphia. Wikipedia gives some additional dimensions of the crater:

Herschel’s diameter is almost a third of [Mimas’] diameter; its walls are approximately 5 kilometres (3.1 mi) high, parts of its floor measure 10 kilometres (6.2 mi) deep, and its central peak rises 6 kilometres (3.7 mi) above the crater floor.

If you were standing in the crater (which you should not do!) it would be a great broken plain surrounded by cliffs thirteen times taller than the Empire State Building.  In the middle you could see a huge mountain slightly shorter than the tallest mountain in North America.  Jagged craters and valleys as deep as Lake Baikal would lie around you.

Mimas orbits above Saturn. The dark lines are shadows cast by the rings (Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA)

A great series of impact cracks on the opposite side of the moon would seem to indicate that the collision which created Herschel nearly shattered Mimas (which is composed principally of ice).

The moon’s name might be of passing interest to followers of my Deities of the Underworld category. In Greek mythology, Mimas was one of the monstrous sons of Gaia.  He was born with snakes for legs and he was clad in full armor.  In the Aeneid, Virgil tells the story of how Hephaestus imprisoned Mimas under Vesuvius during Gaia’s great rebellion against the Olympian gods.  As the imprisoned giant shakes so to does the area around the Bay of Naples.

Mimas--Yikes! (Also, what happened to his armor?)

As I was researching this article, I was struck by how many moons Saturn has! As a special bonus feature, here is an alphabetic list of Saturn’s named moons (several more remain anonymous): Aegaeon, Aegir, Albiorix, Anthe, Atlas, Bebhionn, Bergelmir, Bestla, Calypso, Daphnis, Dione, Enceladus, Epimetheus, Erriapus, Farbauti, Fenrir, Fornjot, Greip, Hati, Helene, Hyperion, Hyrrokkin, Iapetus, Ijiraq, Janus, Jarnsaxa, Kari, Kiviuq, Loge, Methone, Mimas, Mundilfari, Narvi, Paaliaq, Pallene, Pan, Pandora, Phoebe, Polydeuces, Prometheus, Rhea, Siarnaq, Skadi, Skoll, Surtur, Suttung, Tarqeq, Tarvos, Telesto, Tethys, Thrym, Titan and Ymir.

An immense hexagonal storm twice the diameter of earth is locked around the north pole of Saturn.  Humankind discovered the feature by means of the Voyager 1 space probe in 1980 and we continue to study it with our Cassini space probe.  So far, aside from hurricanes on Earth, this is the only eyewall atmospheric feature scientists have found in the solar system (an eyewall is a cloud formation where towering clouds swirl around an empty still center).  Each of the sides of this feature is 8,575 miles long and the eyewall towers 20 to 45 miles tall.  The eyewall clouds do not shift in longitude like the other striations in Saturn’s visible atmosphere.  The huge honeycomb shape rotates every 10 hours 39 minutes and 24 seconds–the same period of rotation as that of the planet’s radio emissions (which is therefore assumed to be equal to the period of rotation of Saturn’s icy interior).  Saturn’s south pole has no comparable feature–although there is a prominent hot spot there.

Nightime movie of the storm taken from the Cassini probe (Image credit: NASA/JPL)

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