Saturn Archives

August 11, 2009December 24, 2015 by Nancy Atkinson

More Equinox Oddities in Saturn’s Rings

Saturn's Rings with lit edge. Taken Aug. 7, 2009. Credit: NASA/JPL

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Today, August 11, 2009, is Saturn’s equinox. As we’ve been saying for a few months, the rings are going to disappear — at least from our vantage point on Earth. “Whenever equinox occurs on Saturn, sunlight will hit Saturn’s thin rings, the ring plane, edge-on,” said Linda Spilker, deputy project scientist for the Cassini spacecraft at Saturn. “The light reflecting off this extremely narrow band is so small that for all intents and purposes the rings simply vanish.” But even from the view at Saturn, weird things are happening in the rings, and the Cassini spacecraft has a front row seat watching it all take place. Above is one of the latest raw images sent back from Cassini, taken on August 7, 2009 showing how the sunlight is hitting the edge of the rings. It is a gorgeous shot. Below, see some of the strange sights Cassini has been seeing, including a buzzsaw, a blasted ring, and tell-tale moon shadows. For more about Saturn’s equinox, check out today’s 365 Days of Astronomy podcast with Emily Lakdawalla of the Planetary Society and she’ll tell you all about the weird and beautiful play of light and shadow going on at Saturn.

This image is really strange. Objects and waves in Saturn’s rings cast weird shadows back on the rings, and to me, this one looks like the jagged teeth of a saw. Interestingly, these shadows are faint, unlike the dark, black shadows we’ve seen in previous images. Check out our previous post about shadows on the rings.

Something blasts through Saturn's F Ring. Credit: NASA/JPL

In this image, it appears as though a small object on an inclined orbit has punched through Saturn’s narrow F ring, bursting out from underneath, and dragging behind it a wake of particles from the rings. Phil Plait discussed this image at length over at Bad Astronomy, so check out his take on this crazy picture.

Cassini B ring moonlet. Credit: NASA/JPL

The angle of the sun hitting the rings makes objects show up that we normally don’t see. Take, for example, this newly discovered moonlet in Saturn’s B-ring. The small moon has a diameter of approximately 400 meters, and it only becomes obvious when sunlight hits the rings edge-on, creating a 40 km (25 mile) -long shadow. If the Sun was above or below the rings, no shadow would be cast, and therefore no moonlet would be visible.

Enjoy the sights, because Saturn’s equinox only happens once ever 15 Earth years.

For more on the mission and the equinox, check out the Cassini website.

One other Cassini note, the spacecraft performed a flyby of Titan on August 9, the 61st time it has targeted Titan as a flyby object, passing at a distance of 970 kilometers (603 miles) above the surface at a speed of 6.0 kilometers per second (about 13,400 mph). Unfortunately, all the data it gathered during this flyby were lost due to a temporary outage at the Goldstone Deep Space Network Facility during Cassini’s transmission back to Earth. As unique, high-priority data from Saturn’s equinox was planned to be taken soon after flyby, there was no redundant playback scheduled.

Rats!

Sources: 365 Days of Astronomy, Bad Astronomy, Astroengine, Cassini website

Hat tip to Stu Atkinson on the buzzsaw image!

August 6, 2009December 24, 2015 by Anne Minard

Titan Shaping Up to Look a Lot Like Pre-Life Earth

An artist's imagination of hydrocarbon pools, icy and rocky terrain on the surface of Saturn's largest moon Titan. Image credit: Steven Hobbs (Brisbane, Queensland, Australia)

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It’s more than a billion kilometers (759 million miles) away, but the more astronomers learn about Titan, the more it looks like Earth.

That’s the theme of two talks happening this week at the International Astronomical Union meeting in Rio de Janeiro, Brazil. Two NASA researchers, Rosaly Lopes and Robert M. Nelson of the Jet Propulsion Laboratory in Pasadena, California, are reporting that weather and geology have very similar actions on Earth and Titan — even though Saturn’s moon is, on average, 100 degrees C (212 degrees F) colder than Antarctica (and certainly much more frigid than either California or Brazil; lucky astronomers).

The researchers are also reporting a tantalizing clue in the search for life: Titan hosts chemistry much like pre-biotic conditions on Earth.

Wind, rain, volcanoes, tectonics and other Earth-like processes all sculpt features on Titan’s complex and varied surface — except, according to additional research being presented at the meeting, scientists think the “cryovolcanoes” on Titan eject cold slurries of water-ice and ammonia instead of scorching hot magma.

“It is really surprising how closely Titan’s surface resembles Earth’s,” Lopes said. “In fact, Titan looks more like the Earth than any other body in the Solar System, despite the huge differences in temperature and other environmental conditions.”

The joint NASA/ESA/ASI Cassini-Huygens mission has revealed details of Titan’s geologically young surface, showing few impact craters, and featuring mountain chains, dunes and even “lakes.” The RADAR instrument on the Cassini orbiter has now allowed scientists to image a third of Titan’s surface using radar beams that pierce the giant moon’s thick, smoggy atmosphere. There is still much terrain to cover, as the aptly named Titan is one of the biggest moons in the Solar System, larger than the planet Mercury and approaching Mars in size.

New Cassini mosaic showing a dried-out lake at Titan's south pole.

Titan has long fascinated astronomers as the only moon known to possess a thick atmosphere, and as the only celestial body other than Earth to have stable pools of liquid on its surface. The many lakes that pepper the northern polar latitudes, with a scattering appearing in the south as well, are thought to be filled with liquid hydrocarbons, such as methane and ethane.

On Titan, methane takes water’s place in the hydrological cycle of evaporation and precipitation (rain or snow) and can appear as a gas, a liquid and a solid. Methane rain cuts channels and forms lakes on the surface and causes erosion, helping to erase the meteorite impact craters that pockmark most other rocky worlds, such as our own Moon and the planet Mercury.

Another Cassini instrument called the Visual and Infrared Mapping Spectrometer (VIMS) had previously detected an area, called Hotei Regio, with a varying infrared signature, suggesting the temporary presence of ammonia frosts that subsequently dissipated or were covered over. Although the ammonia does not stay exposed for long, models show that it exists in Titan’s interior, indicating that a process is at work delivering ammonia to the surface. RADAR imaging has indeed found structures that resemble terrestrial volcanoes near the site of suspected ammonia deposition.

Nelson said new infrared images of the region, also presented at the IAU, “provide further evidence suggesting that cryovolcanism has deposited ammonia onto Titan’s surface. It has not escaped our attention that ammonia, in association with methane and nitrogen, the principal species of Titan’s atmosphere, closely replicates the environment at the time that life first emerged on Earth. One exciting question is whether Titan’s chemical processes today support a prebiotic chemistry similar to that under which life evolved on Earth?”

Many Titan researchers hope to observe Titan with Cassini for long enough to follow a change in seasons. Lopes thinks that the hydrocarbons there likely evaporated because this hemisphere is experiencing summer. When the seasons change in several years and summer returns to the northern latitudes, the lakes so common there may evaporate and end up pooling in the south.

Lead image caption: Artist’s impression of hydrocarbon pools, icy and rocky terrain on the surface of Saturn’s largest moon Titan. Image credit: Steven Hobbs (Brisbane, Queensland, Australia)

Source: International Astronomical Union (IAU)

July 30, 2009December 24, 2015 by Anne Minard

Saturn Sees Days Shorter Than Thought, Winds Like Jupiter — Sort of

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A new estimate of Saturn’s rotation rate reveals days on the gas giant are five minutes shorter than previously believed — and that Saturn’s atmosphere has much in common with that of its planetary neighbor, Jupiter.

The new results appear today in the journal Nature.

(Image caption: Saturn as photographed by Cassini-Huygens. Credit: NASA)

An image of Saturn from NASA's Cassini spacecraft, clearly showing the 'geographic' South Pole of the planet (at the center of the circle of clouds, lower left). The bulk rotation of the planet is around an axis passing through the South Pole and Saturn's clouds (of ammonia ice) are organised into dark 'belts' and light 'zones' that are generally aligned with lines of latitude, indicating the influence of the planet's rotation on its meteorology.

For planets with solid surfaces, the spin rate can simply be determined by tracking the motion of landforms as they rotate across the surface.

Like the rocky planets, gas giant planets such as Jupiter and Saturn spin on their axes with well defined rotation periods. But, with no solid surface features to track, measuring the rotation period of a gas giant is a challenge. The approach that has worked for Jupiter, Uranus and Neptune — using the rotation of the planet’s magnetic field to infer its bulk rotation — gives results for Saturn that change with time, and implies a pattern of atmospheric winds that is very different from that seen on Jupiter.

Peter Read, of the University of Oxford in the UK, and his colleagues used atmospheric dynamics on Saturn to derive a rotation rate that is slightly faster than those inferred from magnetic measurements. When Saturn’s atmospheric winds are viewed relative to this new interior reference frame, they show a pattern of alternating eastward and westward jets similar to the pattern seen on Jupiter.

“This shifted reference frame is consistent with a pattern of alternating jets on Saturn that is more symmetrical between eastward and westward flow,”Read and his co-authors write. “This suggests that Saturn’s winds are much more like those of Jupiter than hitherto believed.”

The authors propose a new rotation rate of 10 hours and 34 minutes, as opposed to the previous estimate of 10 hours 39 minutes. The new rate also sheds light on Saturn’s interior structure, including its density and the mass of a possible rocky core. And it bears on the latitudinal gradient of temperatures below the clouds.

In a related editorial, Adam Showman of the University of Arizona in Tucson writes that there remain key differences between the atmospheres of Saturn and Jupiter: “Saturn’s winds are stronger than Jupiter’s, its banded cloud patterns and populations of hurricane-like vortices differ considerably, and its magnetic field, which is almost symmetrical about its axis — a puzzle in its own right — contrasts with Jupiter’s tilted dipole,” he notes. “These contrasts indicate that the planets are cousins rather than twins, whose intriguing mix of similarities as well as differences will keep planetary scientists engaged for years to come.”

Second image caption: An image of Saturn from NASA’s Cassini spacecraft, clearly showing the ‘geographic’ South Pole of the planet (at the center of the circle of clouds, lower left). The bulk rotation of the planet is around an axis passing through the South Pole and Saturn’s clouds (of ammonia ice) are organized into dark ‘belts’ and light ‘zones’ that are generally aligned with lines of latitude, indicating the influence of the planet’s rotation on its meteorology.

Source: Nature

June 16, 2009May 10, 2016 by Nancy Atkinson

Vertical Structures Tower Above Saturn’s Rings

Looming vertical structures, seen here for the first time and created by Saturn's moon Daphnis, rise above the planet's otherwise flat, thin disk of rings to cast long shadows in this Cassini image. Credit: CICLOPS.

Cassini has imaged towering vertical structures in the planet’s otherwise flat rings that come from the gravitational effects of a small nearby moon. This is the first time these structures have been seen. They reach up over one kilometer high, and are visible now as the sun nears “high noon” directly overhead at the planet’s equator, as Saturn approaches its equinox.

The search for ring material extending above and below Saturn’s ring plane has been a major goal of the imaging team during Cassini’s “Equinox Mission,” the two-year period containing the exact equinox. This novel illumination geometry, which occurs every half-Saturn-year, or about 15 Earth years, lowers the sun’s angle to the ring plane and causes out-of-plane structures to cast long shadows across the rings, making them easy to detect.

Images taken in recent weeks have demonstrated how small moons in very narrow gaps can have considerable and complex effects on the edges of their gaps, and that such moons can be smaller than previously believed.

The 8-kilometer-wide (5-mile) moon Daphnis orbits within the 42-kilometer-wide (26-mile) Keeler Gap in Saturn’s outer A ring, and its gravitational pull perturbs the orbits of the particles forming the gap’s edges. Earlier images have shown “waves” in the rings from Daphnis eccentric orbit.

But new images show the shadows of the vertical waves created by Daphnis cast onto the nearby ring. These characteristics match what was predicted by scientists.

Scientists have estimated, from the lengths of the shadows, wave heights that reach enormous distances above Saturn’s ring plane – as large as 1.5 kilometers (1 mile) — making these waves twice as high as previously known vertical ring structures, and as much as 150 times as high as the rings are thick. The main rings — named A, B and C — are only about 10 meters (30 feet) thick.

“We thought that this vertical structure was pretty neat when we first saw it in our simulations,” said John Weiss, lead author of a paper reporting on these images. “But it’s a million times cooler to have your theory supported by such gorgeous images. It makes you suspect you might be doing something right.”

Click here to watch a movie of the vertical structures and waves in motion.

Also presented in the paper is a refinement to a theory used since the Voyager missions of the 1980s to infer the mass of gap-embedded moons based on how much the moons affect the surrounding ring material. The authors conclude that an embedded moon in a very narrow gap can have a smaller mass than that inferred by earlier techniques. One of the prime future goals of the imaging team is to scour the remaining gaps and divisions within the rings to search for the moons expected to be there. “It is one of those questions that have been nagging us since getting into orbit: ‘Why haven’t we yet seen a moon in every gap?’” said Carolyn Porco, lead for the Cassini imaging team. “We now think they may actually be there, only a lot smaller than we expected.”

Source: CICLOPS

June 2, 2009December 24, 2015 by Nancy Atkinson

Book Your Tours of Titan and Enceladus Today!

Now's the time to book your Cassini tour!

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Looking to go somewhere far-flung and exotic? Now is the time to book your excursion, and the Cassini spacecraft has several flyby tours of the moons Titan and Enceladus scheduled for the next few months. Major tour operators say the cost of long-haul flights and summer holidays prices are at an all time low. But with Cassini, you can travel for FREE! just by following the along with Universe Today and the Cassini website. Thrill with some of the closest flybys ever of the mystery moon Titan, and delight in explorations of the geyser plumes of Enceladus. As a special bonus, if you book today, you can experience Saturn’s solar equinox, as in August the sun crosses from the southern hemisphere to the north. Wonderful worlds are beckoning – come away starting June 6 with Cassini! It’s a worry free vacation. See below for available tour dates! Destination fees do not apply.

Seriously now, here are the upcoming dates scheduled for Cassini flybys of Titan and Enceladus:

Dates listed in Spacecraft Event Time (SCET) — the time the event happens at the spacecraft based on Coordinated Universal Time (UTC).
Click here for details about time conversions.

June 6 — Titan flyby (965 kilometers) — T-56: This is the only dusk side observation at mid Southern latitudes, and the Ion and Neutral Mass Spectrometer (INMS) will be taking advantage of that, riding along with Cassini’s RADAR at closest approach. It’s the only time in the mission the spacecraft will get simultaneous coverage of the dusk side while in the wake magnetospheric interaction region. The Visual and Infrared Mapping Spectrometer (VIMS) and the Imaging Science Subsystem ISS will observe eastern Tsegihi, a bright region in Titan’s southern mid-latitudes.

June 22 –– Titan flyby (955.5) kilometers — T-57: RADAR and INMS again share prime opportunities near closest approach. The RADAR synthetic aperture radar (SAR) imager observation runs parallel to observations in the T-55 and T-56 flybys in the southern hemisphere mapping sequence. Earlier, RSS observes an occultation on the inbound leg. T-57 is another flank-out, post-dusk flyby, with a minimum altitude of about 1000 kilometers. As in T-55 and T-56 flybys, magnetometer measurements will provide a description of the draping and the pileup of the external magnetic field around Titan on the nightside hemisphere. The flyby will also be a good complement to previous flybys in order to characterize the background field for a similar local time with respect to Saturn and different SKR (Saturn kilometric radiation) longitudes.

July 8 — Titan flyby (965 kilometers) — T-58: The Ultraviolet Imaging Spectrograph UVIS observes a solar occultation while inbound towards Titan, and then a stellar occultation on the spacecraft’s outbound trajectory. RADAR’s SAR swath runs along the western edge of Xanadu to study its boundary with Shangri-La, a large equatorial dark region. The swath runs parallel to the T-55/56/57 mapping sequence and covers Ontario Lacus, a methane-ethane lake near the south pole of Titan.

July 24 — Titan flyby (955 kilometers) — T-59: The spacecraft’s instruments sample Titan’s southern mid-latitudes, with the Cassini Plasma Spectrometer (CAPS) controlling pointing at closest approach.

Aug. 9 –– Titan flyby (970) kilometers — T-60: RADAR takes a South pole pass. The resulting swath links up with the T-13 flyby swath at the edge of Xanadu, an Australia-sized, bright region on Titan. ISS will acquire high-resolution, low-phase-angle imaging of western Senkyo, a wide dark region near the equator.

Aug. 11 –– Saturn will go through the solar Equinox as the Sun crosses from the southern hemisphere to the north. For about two months on either side of that date rings scientists will be running an Equinox campaign to observe the rings in this season change.

The Cassini team will be watching for topographic features in the rings that can only be seen in this special geometry. Any features in the rings that are not exactly in the ‘ring plane’ will be seen to cast shadows.

Cassini scientists will also be looking at the thermal properties of the rings in this season change. Most rings have seen heating on the north side over the past 14.5 years, but the B ring’s densest portions have remained cold with no solar heat penetrating that ring for the past 14.5 years.

This Rings Equinox campaign is a unique opportunity provided by the long duration of the Cassini Mission.

Aug. 25 — Titan flyby (970 kilometers) — T-61: RADAR gathers a SAR swath over the Huygens landing site. The swath is near-equatorial, covering Dilmun, Adiri and Belet. As the SAR parallels and overlaps the T-8 flyby, this should provide a good stereo opportunity over the Belet sand dunes. T-61 is the only southern equatorial wake observation in the mission, so the Magnetosphere and Plasma Science (MAPS) instruments take advantage of the opportunity.

Oct. 12 — Titan flyby (1,300 kilometers) — T-62: This flyby offers excellent VIMS and UVIS observing opportunities, including a UVIS solar occultation that reaches down to Titan’s surface. CIRS takes observations while Titan is in eclipse, measuring the temperature, aerosol density and composition near 75 South. This is the only low altitude in nose side magnetospheric interaction pass in the extended mission.

Nov. 2 — Enceladus flyby (99 kilometers) — 120EN: This is the seventh targeted Enceladus flyby of the Cassini mission and will take the spacecraft to the lowest altitude above the active south pole region. This will also be the deepest plume passage of the tour, allowing for sensitive measurements of the geyser-like plume composition and density.

Nov. 21 — Enceladus flyby (1,603 kilometers) — 121EN: The eighth targeted Enceladus flyby, this is an approximate 1600 kilometer pass over the south pole enabling imaging of the warm, active tiger stripes.

Dec. 12 — Titan flyby (4,850 kilometers) — T-63: CAPS takes advantage of the T-63 flyby being the best wake passage in the extended mission to direct pointing at closest approach.

Dec. 28 — Titan flyby (955 kilometers) — T-64: RADAR captures HiSAR SAR over the North polar lakes to perform stereo and/or seasonal change detection. This is the only north polar SAR in the extended mission. Due to the location of the point of closest approach, this is a potentially important flyby in the effort to detect an intrinsic magnetic field within Titan. This is also an opportunity to sample the high northern atmosphere.

Source: Cassini website

April 28, 2009December 24, 2015 by Nancy Atkinson

Latest from Saturn: Pastel Rings and Moons by the Bunch

Saturn's rings in pastel. Credit: NASA/JPL/Space Science Institute

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The latest images from the Cassini spacecraft include this gorgeous natural color view of Saturn’s inner rings. Visible are the different degrees of transparency of the rings, which appear almost pastel in color. The dark shadows of the rings separate Saturn’s southern hemisphere in the bottom of the image from the north. The innermost D ring is invisible, laid over the planet’s northern hemisphere. The translucent C ring runs through the middle of the image. The denser B ring stretches across the top of the image. See below for more info on this image, as well as a challenge to “find the moons” on another Cassini image.

The view in the top image looks toward the sunlit side of the rings from about 48 degrees below the ringplane. Images taken using red, green and blue spectral filters were combined to create this natural color view. The images were acquired with the Cassini spacecraft wide-angle camera on Feb. 28, 2009 at a distance of approximately 1 million kilometers (620,000 miles) from Saturn. Image scale is 59 kilometers (37 miles) per pixel.

Three of Saturn's moons bunch together in this image by Cassini. Credit: NASA/JPL/Space Science Institute. Click for larger image.

Can you find the three moons in this image? Janus (179 kilometers, or 111 miles across) sits bright and overexposed outside the faint F ring. Prometheus (86 kilometers, 53 miles across) lies inside the F ring to the left of the center of the image.

Have you given up on finding the third moon? Well, don’t feel bad — Tiny Daphnis (8 kilometers, or 5 miles across) is in there somewhere, but it’s not visible in the thin Keeler Gap of the A ring just below Prometheus in this image.

This view looks toward the sunlit side of the rings from about 20 degrees below the ringplane. The image was taken in visible light with the Cassini spacecraft wide-angle camera on March 2, 2009. The view was acquired at a distance of approximately 1.1 million kilometers (684,000 miles) from Janus and at a Sun-Janus-spacecraft, or phase, angle of 47 degrees. Image scale is 63 kilometers (39 miles) per pixel.

Source: Cassini Equinox Mission homepage

April 21, 2009December 24, 2015 by Nancy Atkinson

Electrically Charged Particles Found in Enceladus’ Plumes

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A team of planetary scientists working on the Cassini-Huygens mission has discovered tiny, icy particles in the plume from Saturn’s moon Enceladus that offer a tantalizing glimpse of the interior of this enigmatic world. The spectrometer on Cassini, the Cassini Plasma Spectrometer (CAPS) discovered a surprise: the ice particles are electrically charged.

Cassini has been exploring Saturn and its moons since 2004. Enceladus is 500 kilometers (300 miles) wide and Cassini’s suite of instruments has found the moon to be active, with jets near its southern pole that spew gas and water thousands of kilometers out into space. During two particularly close flybys of the moon in 2008, skimming only 52 and 25 km from the surface at around 15 km per second (54,000 km per hour), the CAPS instrument on the spacecraft was pointed to scoop up gas as it zoomed through the plume.

Observations from the Cassini Plasma Spectrometer (CAPS) made during the Cassini flyby of Enceladus on 12th March 2008, superimposed on Cassini’s path. As the spacecraft passed the moon, CAPS detected streams of charged particles in individual jets within the plume; negative particles are shown in this view. Each ribbon in the image gives an indication of the measured particle energy per charge: high energy particle fluxes are shown nearest Enceladus, and lower energy particles are farthest. The red points marked on Enceladus show the locations of known jet sources found by other Cassini instruments. Credit: MSSL-UCL.

The CAPS instrument is designed to detect charged gas (plasma), but its measurements in the plume revealed tiny ice grains whose signatures could only be present if they were electrically charged. These grains, probably only measuring a few nanometres across (billionths of a meter – 50, 000 times thinner than a human hair), fall into a size range between gas atoms and much larger ice grains, both of which were sampled directly during previous Enceladus flybys. The particles have both positive and negative electrical charges, and the mix of the charges varied as the Cassini spacecraft crossed the plume.

Dr. Geraint Jones and Dr. Chris Arridge, both from University College London’s Mullard Space Science Laboratory, present the results for the CAPS team at the European Week of Astronomy and Space Science conference at the University of Hertfordshire.

Jones and Arridge suggest that the grains may be charged through so-called triboelectric processes, through bumping together in the vent below Enceladus’s surface before they emerge into the plume. This provides important hints to the conditions in the vents, and in turn may help with understanding conditions in the interior.

Jones and Arridge are intrigued by what their discovery reveals about Enceladus: “What are particularly fascinating are the bursts of dust that CAPS detects when Cassini passes through the individual jets in the plume” says Jones. “Each jet is split according to charge though”, adds Arridge, “Negative grains are on one side, and positive ones on the other”.

Arridge said that perhaps, as these charged grains travel away from Enceladus, their paths are bent by electric and magnetic fields in Saturn’s giant magnetosphere. In this way Saturn’s magnetosphere acts as an enormous mass spectrometer for the plume particles, allowing scientists to constrain their masses. Arridge has begun modelling the paths of these newly-discovered particles.

Ionised gas (plasma) in Saturn’s magnetosphere flows past Enceladus at over 80000 km per hour. Arridge’s results show that for this enormous mass spectrometer to work and for these dust particles to reach Cassini, this river of plasma must be significantly slowed down, in and near the plume, to speeds of less than 3200 km per hour. This slowing of the plasma is a result of the plume injecting particles into the plasma stream – making the whole flow slow down in a similar effect to when cars join a busy motorway. These new results provide further evidence that the material in the Enceladus plume has a huge influence on the moon’s surroundings.

Future Cassini flybys will help further understand the processes that occur at Enceladus and in its vicinity. William Herschel could not have suspected that the tiny point of light that he found in 1789 would turn out to be such an exotic place.

Source: RAS

April 12, 2009December 24, 2015 by Nancy Atkinson

Cassini’s Indirect Image of Boulders and Moonlets in Saturn’s Rings

Saturn ring shadows. Credit: NASA/Cassini

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Take a look at this — it is absolutely stunning. A couple of weeks ago, Anne wrote an article about moon shadows on Saturns rings. Because Saturn is approaching its equinox, in August the rings will “disappear” from our view from Earth, as the rings will be exactly edge-on. But as the rings ease into alignment with the sun, Saturn’s moons cast their shadows across the rings, growing longer as equinox approaches. See in the image above, a shadow is cast on the rings, likely by either the moon Mimas or Tethys. But the eagle-eyed folks over at UnmannedSpaceflight.com also noticed something else in this raw image from the Cassini spacecraft. Notice the area right near the middle of the image where the rings look kind of fuzzy? That’s not just camera blur; those are more shadows, created by thousands of boulders or moonlets in the ring! Amazing! We’ve never actually seen the small objects that make up the rings — and we still haven’t — but we’re seeing the shadows they are creating! Let’s zoom in for a closer look:

Shadows on the rings, closer. Credit: NASA/Cassini/UnmannedSpaceflight.com

Wow! As one of the members of UnmannedSpaceflight said, “Knowledge of the rings’ 3D structure is about to be revolutionized. And let’s not forget that these shadow will get much longer in the coming months.” Let’s zoom in a little more:

Saturn's rings closer yet. Credit: NASA/Cassini

The UnmannedSpaceflight crew has even created animations from combining several of the raw images sent back by Cassini. In the first movie, it doesn’t look like the moonlet shadows are moving at all, but in a more zoomed in version, it is obvious the shadows are moving as the objects orbit around Saturn. As Stuart Atkinson at Cumbrian Sky said, for a long time we’ve speculated that Saturn’s rings would look a bit like this, close up:

Artist concept of Saturn's rings. Image courtesy NASA/JPL/University of Colorado

And now we have the first image of the ring objects, or at least the shadows they create.

Head nod to Stuart Atkinson for the tip, and congrats to the sharp eyes (and image editing prowess) at UnmannedSpaceflight.com for this amazing find! Great work!

March 23, 2009December 24, 2015 by Anne Minard

Moonshadows on Saturn’s Rings Are Harbingers of Spring

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Moonshadows on Saturn’s rings are foretelling the planet’s equinox, when the sun will be exactly aligned with the planet’s equator and rings — and then will shift north from the southern hemisphere, kickstarting northern spring.

NASA’s Cassini spacecraft has captured, for the first time, the tell-tale moonshadows – sort of like groundhogs on Earth.

moonshadow2 — Click to play the short movie. Credit: NASA/JPL/Space Science Institute

The image above is a still from a movie, from Cassini’s hour-long observation of the shadow of the small moon Epimetheus.

Like Earth and most of the other planets, Saturn’s spin axis is tilted relative to its motion around the sun. So the sun, seen from Saturn, cycles from the southern hemisphere to the north and back again. A full sweep of seasonal changes on Saturn and its rings and moons takes a Saturnian year, equal to 29.5 Earth years. Thus, about every 15 Earth years, or half-Saturn-year, the sun passes through the plane containing the planet’s rings.

During these times, the shadows of the planet’s rings fall in the equatorial region on the planet. And the shadows of Saturn’s moons external to the rings, especially those whose orbits are inclined with respect to the equator, begin to intersect the planet’s rings. When this occurs, the equinox period has essentially begun, and any vertical protuberances within the rings, including small embedded moons and narrow vertical warps in the rings, will also cast shadows on the rings. At exactly the moment of equinox, the shadows of the rings on the planet will be confined to a thin line around Saturn’s equator and the rings themselves will go dark, being illuminated only on their edge. The next equinox on Saturn, when the sun will pass from south to north, is Aug. 11, 2009.

Because of these unique illumination circumstances, Cassini imaging scientists have been eager to observe the planet and its rings around the time of equinox. Cassini’s first extended mission, which began on July 1, 2008, was intended to gather observations during this time. Hence its name: Cassini Equinox Mission.

More than just pretty pictures, the observations could reveal any deviations across the rings from a perfectly flat wafer-like disk. Saturn’s ring system is wide, spanning hundreds of thousands of miles or kilometers. But the main inner rings (called A, B and C) are perhaps only 10 meters (30 feet) thick, and they are sometimes obscured from view inside thicker outer rings.

“We hope that such images will help us measure any vertical warping in the A and B rings,” said John Weiss, an imaging team associate from the Space Science Institute in Boulder, Colorado. “Because we know how big the moons are, and where they are in their orbits around Saturn when they cast these shadows, we have all the information we need to infer any substantial vertical structure that might be present.”

On Jan. 8, Epimetheus, a small moon 113 kilometers (70 miles) across, was the first moon observed casting a shadow onto the outer edge of the A ring. Next Pan, 30 kilometers (20 miles) across and orbiting within the rings, was caught casting a shadow on the A ring on Feb. 12. Eventually, more moons will cast shadows on the rings and all shadows will grow longer as exact equinox approaches.

Source: Cassini Imaging Central Laboratory for Operations (CICLOPS)

March 17, 2009December 24, 2015 by Anne Minard

Hubble Snaps Rare Moon Parade Across Saturn

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Once every 15 years, Saturn flashes its paper-thin rings in edge-on formation relative to Earth.

Because the orbits of Saturn’s major satellites are in the ring plane, too, this alignment gives astronomers a rare opportunity to capture a spectacular parade of celestial bodies crossing Saturn’s surface.

Leading this moon train is Titan – larger than the planet Mercury. The frigid moon’s thick nitrogen atmosphere is tinted orange with the smoggy byproducts of sunlight interacting with methane and nitrogen. Several of the much smaller icy moons that are closer in to the planet line up along the upper edge of the rings.

In the image, snapped by the Hubble Space Telescope on February 24, the giant orange moon Titan casts a large shadow onto Saturn’s north polar hood. Below Titan, near the ring plane and to the left is the moon Mimas, casting a much smaller shadow onto Saturn’s equatorial cloud tops. Farther to the left, and off Saturn’s disk, are the bright moon Dione and the fainter moon Enceladus.

Hubble’s exquisite sharpness also reveals Saturn’s banded cloud structure, which is similar to Jupiter’s.

hs-2009-12-b-print — The top frame captures the giant moon Titan and its shadow near Saturn's northern polar hood. Dione, the brightest of the icy moons in this view – which are closer in to Saturn – can easily be traced crossing the disk from far left to image center. In the center frame, the smaller moon Enceladus can be seen near the western limb of Saturn. Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

At the time, Saturn was at a distance of roughly 775 million miles (1.25 billion kilometers) from Earth. Hubble can see details as small as 190 miles (300 km) across on Saturn. The dark band running across the face of the planet slightly above the rings is the shadow of the rings cast on the planet.

Early 2009 was a favorable time for viewers with small telescopes to watch moon and shadow transits crossing the face of Saturn. Titan, Saturn’s largest moon, crossed Saturn on four separate occasions: January 24, February 9, February 24, and March 12, although not all events were visible from all locations on Earth.

This “ring plane crossing” occurs every 14-15 years. In 1995-96 Hubble witnessed the ring plane crossing event, as well as many moon transits, and even helped discover several new moons of Saturn.

Source (and more images!): HubbleSite