Saturn's Rings: A Guide in Pictures

Continuing the theme of the last post, which used Cassini images as an introduction to Saturn's weather, I thought I'd bring together a collection of ring science images in an attempt to learn something about these beautiful phenomena.  The water ice rings are coloured by impurities (dust, silicates, tholins) and have particles with a wide range of sizes, from a few microns across to maybe 10 metres wide.  The age and origins of the rings remain hard to assess, as the system is in a continuous state of flux - particles clump together to form larger bodies due to gravity, but are then disrupted by interactions and collisions to reform the rings and dust.  Because of this continuous recycling there's still no consensus on how the rings came to be in the first place.

Distribution of Rings

Enlarging this composite image (45 images from Cassini's Narrow Angle Camera in total, obtained in May 2007) will give you a good first guide to the ring system.  Moving from the inner edge outwards, we have the faint and innermost D ring, Colombo Gap, C ring, Maxwell Gap, the main B ring, Cassini Division, A ring with its Encke and Keeler Gaps, the Roche division and then the narrow F ring (140,220 km from Saturn).  The G ring and the diffuse E ring, due to active venting from Enceladus, are both further out and not seen in this image.  (Credit:  NASA/JPL/Space Science Institute)  http://photojournal.jpl.nasa.gov/catalog/PIA08389

Perhaps more useful is this artists concept of the structure of the rings, and where the main shepherding moons are located.  Credit: NASA/JPL, http://photojournal.jpl.nasa.gov/catalog/PIA03550

Removing the planet entirely from this 2007 image gives us a great view of the rings in all their glory, from the outer narrow F ring, through the main A and B rings (separated by the Cassini division), and then the inner C ring.  (Credit:  NASA/JPL/Space Science Institute)  http://photojournal.jpl.nasa.gov/catalog/PIA08361

This 2008 image shows sunlight scattering off of the B ring, the brightest and most massive of all of Saturn's rings.  It was in this ring that a mysterious phenomenon known as ring spokes was observed, producing dark radial striations on the rings sunlit side, which appear to come and go with time and may be a seasonal phenomenon.  http://photojournal.jpl.nasa.gov/catalog/PIA09860

This image of the B ring in 2010 shows spoke phenomena, appearing bright when viewed at a high phase angle.  They appear dark in images taken at lower phase angles, telling us something about the nature of the particles making up the spokes. http://photojournal.jpl.nasa.gov/catalog/PIA12605

A colour image of the Cassini division from 2005, separating the main A and B rings, possibly consisting of more contaminated ices than the fresher material comprising the two rings.  http://photojournal.jpl.nasa.gov/catalog/PIA07631

A large ring of dust was discovered in 2009 by the Spitzer Space Telescope in infrared light, possibly originating from impact events on Phoebe (a retrograde satellite with an inclined orbit).  The full story can be found here.

Probing the Ring Properties

Just as for Saturn, astronomers use images of the rings in different wavelengths to deduce the composition, sizes and structure of the various ices.  This comparison image from the Visual and Infrared Mapping Spectrometer (VIMS) in 2004 shows scattered light coming through the rings on the left (so thicker rings appear darker); then the strength of a signature of pure water ice that seems to grow strong in the A ring; and finally a signature of some unidentified 'dirty' material causing darkening of the rings.  For more details see:  http://photojournal.jpl.nasa.gov/catalog/PIA06350 (Credit:  NASA/JPL/University of Arizona)

There are other ways to deduce the properties of the rings - this is a comparison of a natural-colour image from 2005 with a simulated image based on a radio occultation.  Using radio signals in the Ka, X and S bands (0.94, 3.6, and 13 cm wavelengths), the modulation of the signal strength by the rings can be used to deduce ring optical depths and particle sizes.  The colours correspond to the presence or absence of ring particles of different sizes.  http://photojournal.jpl.nasa.gov/catalog/PIA07874

Furthermore, the Cassini Ultraviolet and Imaging Spectrograph (UVIS) can measure the strength of water ice signatures.  In this 2004 image, we can see the Cassini Division in red on the left (thinner, dirtier with less of an ice signature) compared to the A ring in turquoise on the right (with a stronger water ice signature).  The redder Encke gap is also visible.  Credit:  NASA/JPL/University of Colorado.  http://photojournal.jpl.nasa.gov/catalog/PIA05075

Finally, Cassini's Composite Infrared Spectrometer (CIRS) is able to measure the thermal emission from the rings at a variety of phase angles and illuminations.  The thermal characteristics vary notably with phase angle, over a range of temperatures from 65-110 K.  The comparison between lit and unlit sides tells us how effective sunlight is at penetrating the optically thicker rings to cause heating.  For an explanation of the figure, see http://photojournal.jpl.nasa.gov/catalog/PIA03561  Credit:  NASA/JPL/GSFC

Dynamical Phenomena

The 2009 equinox was an ideal opportunity to observe vertical structures in Saturn's rings, as they would cast long shadows across the narrow ring plane.  These structures at the edge of the main B ring (the Cassini Division is the dark expanse at the top of the image) tower 2.5 km above the plane, which is enormous compared to the expected thicknesses (tens to possibly hundreds of metres) of the rings themselves.  This pileup of material might be being caused by the gravitational effects of moonlets at the edge of the B ring. http://photojournal.jpl.nasa.gov/catalog/PIA11668

Another example of vertical structures observed in May 2009 when tiny Daphnis, sat within the Keeler gap within Saturn's A ring, interacts with the surrounding material.  The shadows indicate structures some 1.5 km tall, compared to the expected 10-m thickness of the main rings. The continuous interaction creates an edge wave which propagates around the circumference of the Keeler gap. http://photojournal.jpl.nasa.gov/catalog/PIA11653

The rings continuously interact with the tiny satellites such as Prometheus, seen here creating a streamer from the F ring.  This dynamic ring appears to evolve over hourly timescales, being shepherded by both Prometheus and Pandora. Here, the satellite has reached its apoapsis (furthest point from Saturn) and may be pulling material away from the ring, creating kinks, gaps and other discontinuities in the rings in a continually evolving dance. http://photojournal.jpl.nasa.gov/catalog/PIA06143

This dance between Prometheus and the F-ring carves channels into the ring every time it pulls out a streamer of material.  As it rotates slightly faster than the F ring around Saturn, each apoapsis interaction is in front of the last, creating this wonderful sequence of striations in the F ring.  http://photojournal.jpl.nasa.gov/catalog/PIA12684

Looking even more closely at Saturn's active F ring in September 2006, it appeared that additional tiny moonlets were interacting with the ring and drawing out tiny streamers of material, a miniature version of the interaction with Prometheus observed above.  http://photojournal.jpl.nasa.gov/catalog/PIA08290

Saturn's Weather: A Guide in Pictures

The May 2013 episode of the Sky at Night is all about Saturn, and I was asked to collect together some of Cassini's nicest images to discuss our latest discoveries in Saturn's dynamic and evolving atmosphere. I thought it'd be nice to collect all these sources into one place, as I'm constantly having to track them down for presentations, so here they are!  The vast majority are available via NASA's Photojournal, and I've tried to tell a story with some of the images, including all the links and credits.  Enjoy!

Seasons on Saturn

Saturn during southern summer just before Cassini arrived in May 2004, showing the familiar yellow-ochre appearance of its cloud tops, and a faintly banded structure less prominent than that of Jupiter.  The rings cast long shadows on the northern winter hemisphere, where a hint of blue colours can be observed.  Saturn is 95 times the mass of Earth, 9 times the diameter, only 12.5% of the density and receives around 1% of the solar illumination compared to the Earth. (Credit: NASA/JPL/Space Science Institute) http://photojournal.jpl.nasa.gov/catalog/PIA06077

A better view of the northern blue hues from November 2004, showing tiny Mimas against the ring shadows.  Saturn's atmosphere responds to the different levels of sunlight, with aerosols growing larger and more opaque in the spring and summer, but vanishing over the winter, explaining this asymmetry between the hemispheres.  Where there are fewer scattering hazes in the north, light has to travel through longer paths of atmospheric methane before it reflects from the cloud tops.  As methane absorbs red light very strongly, the remaining light is mostly blue, just like on Uranus and Neptune. (Credit: NASA/JPL/Space Science Institute) http://photojournal.jpl.nasa.gov/catalog/PIA06142

Saturn orbits the Sun once every 30 Earth years, so the seasons are around 7.5 years long.  Saturn's obliquity of 26 degrees is slightly larger than that of the Earth (23 degrees).  As northern winter (2002) marched on to northern spring (2009), the north pole emerged from the shroud of winter darkness, and aerosols grew to give Saturn its typical yellow-ochre appearance, as in this image captured at the equinox.  Here, the Sun equally illuminates the northern spring and the southern autumn hemispheres, and the rings would have vanished to a thin line as viewed from Earth.  (Credit: NASA/JPL/Space Science Institute) http://photojournal.jpl.nasa.gov/catalog/PIA11667

At the equinox, the shadow of the rings drops to a tiny line at Saturn's equator.  This shift in illumination from south to north seems to have coincided with a number of changes in Saturn's weather, generating more convective, turbulent activity in the north where the most dramatic changes have taken place. Note Rhea on the far right of this image. (Credit: NASA/JPL/Space Science Institute) http://photojournal.jpl.nasa.gov/catalog/PIA12513  Emily Lakdawalla has a great blog post explaining some of these changes in more detail.

One method Cassini uses to diagnose these seasonal changes are images taken across lots of different wavelengths, from the ultraviolet to the far-infrared.  This image from Cassini's Visual and Infrared Mapping Spectrometer brings together a blue 2.3 µm image (water ice in the rings is very reflective, atmospheric methane very absorbing), a green 3.0 µm image (water ice rings absorbing, but lots of reflection from the sunlit portion of Saturn) and a red 5.1 µm image (showing thermal emission from the planet itself).  Note that you can see the thermal emission from the non-illuminated side of Saturn, and all the fine cloud structures are seen in silhouette against the deep internal red glow.  (Credit: NASA/JPL/University of Arizona) http://photojournal.jpl.nasa.gov/catalog/PIA09212

Saturn's Seasonal Storms

Saturn's atmosphere is dominated by hazy material, either formed from cloud particles mixed from the deeper atmosphere, or from photochemically-produced materials raining down from above.  For that reason, a lot of the really interesting dynamics is hidden from view.  But we'd be mistaken for thinking that Saturn is a lot less active than Jupiter.  Small-scale storms do occur, and for much of Cassini's mission they were confined to a band at 35S known as storm alley.  This particular storm was imaged in March 2008, after it had been detected via the radio emission of its cracking lightning a few months earlier.  (Credit: NASA/JPL/Space Science Institute)  http://photojournal.jpl.nasa.gov/catalog/PIA08411

However, once every Saturnian year an enormous eruption of billowing white cloud material occurs on Saturn, generating structures that enthral amateur and professional astronomers alike.  This eruption was the sixth on record since 1876, and occurred in Saturn's northern hemisphere near the peak of a westward jet, which helped spread cloud material around the planet.  This image was obtained around 12 weeks after the eruption was first discovered in December 2010.  (Credit: NASA/JPL/Space Science Institute)   http://photojournal.jpl.nasa.gov/catalog/PIA12826

A close-up of the western storm head in February 2011, and details in the tail to the east.  Yellow-white clouds are thick and high; the blue colours represent the highest semi-transparent clouds lofted by the storm; and the reds are those that are deeper, so these false colour images give us an impression of the three-dimensional structure of the eruption.  Billowing material downstream also created a large anticyclonic vortex (blue oval, bottom right) which has persisted to this day (April 2013).  (Credit: NASA/JPL/Space Science Institute) http://photojournal.jpl.nasa.gov/catalog/PIA12825

This natural colour view of the storm band was obtained in March 2011, showing the snake-like appearance of the westward moving storm head, and the chaotic activity in the tails moving to the east.  The storm wrapped its way around the whole planet, the head encountering the tail which signalled the end of the convective activity and lightning. (Credit: NASA/JPL/Space Science Institute)   http://photojournal.jpl.nasa.gov/catalog/PIA14904

The long term evolution of the storm is captured in this montage of Cassini images between December 2010 and August 2011.  By the end of the sequence, the original storm head was no longer visible, lost in the chaotic jumble of the storm band.  The storm has had long-term repercussions for this region of the atmosphere, leaving a distinct cloud-free band in the northern hemisphere still visible today (2013).  (Credit:  NASA/JPL-Caltech/Space Science Institute)  http://photojournal.jpl.nasa.gov/catalog/PIA14905

Saturn's storm didn't just affect the visible atmosphere, it also had repercussions in the high atmosphere, sending waves of energy into the stratosphere to form an enormous, hot, circulating anticyclone.  This image was captured in July 2011 with the VISIR instrument on the Very Large Telescope in Chile, sensitive to stratospheric emission.  The stratospheric vortex persists to the present day, and is continuing to move west around the planet like an enormous glowing beacon.  See more details and a movie of the forming beacon in my blog post here.  (Credit:  University of Oxford/L.N. Fletcher/ESO) http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=50997 and http://photojournal.jpl.nasa.gov/catalog/PIA16190 for the movie.

Saturn's Polar Atmosphere

Cassini has spent much of its mission exploring the equatorial region of Saturn, but every so often it ramps up to higher-inclination orbits to provide an unprecedented glimpse of the polar atmosphere.  The poles are unlike any other region on Saturn, being the apex of a planet-wide circulation, and a site where the charged particle environment of the magnetosphere can actually interact with the atmosphere itself, via aurorae.  This movie from VIMS covers six hours in 2008, when the pole was still in winter darkness.  You're seeing a flipped image, so that clouds appear white against a dark background, whereas the real measurements at 5 µm saw clouds silhouetted against Saturn's internal glow.  You can see cloud motions within the polar vortex, and the bizarre hexagonal wave.  (Credit: NASA/JPL/University of Arizona) http://photojournal.jpl.nasa.gov/catalog/PIA11215

Staying with the VIMS instrument, this image from 2008 compares the northern and southern poles in infrared light, showing striking similarities between the small polar cyclones.  Both are located right at the pole, and may be long lived features permanently present irrespective of season.  The Cassini Composite Infrared Spectrometer (CIRS) had previously shown that these cyclones were glowing hot in infrared emission, having temperatures higher than their surroundings.  (credit: NASA/JPL/University of Arizona) http://photojournal.jpl.nasa.gov/catalog/PIA11216

Another view of the north polar hexagon, this time without inverting the 5-µm brightness, so that you're seeing dark clouds against a red glow.  This image is in fact a combination of an atmospheric image from 2008 and an auroral image from 2006 (auroral emission at 4 µm) (Credit: NASA/JPL/University of Arizona)  http://photojournal.jpl.nasa.gov/catalog/PIA11396

Shifting to even longer wavelengths, the Cassini Composite Infrared Spectrometer discovered that the hexagon was also visible in the thermal field, and that a compact hot polar cyclone was present at both north and south poles of Saturn, surrounded by rapid peripheral jets.  This image is a map of the atmospheric temperatures in the troposphere at a time when the north pole was shrouded in winter darkness.  The mean temperatures at this altitude are around -190 degrees Celsius. (Credit: NASA/JPL/GSFC/Oxford University) http://photojournal.jpl.nasa.gov/catalog/PIA10217

In 2008, the Cassini Imaging Sub System gazed right down into the heart of the south polar vortex, showing convective clouds within the swirling cyclonic vortex.  Other views of this vortex showed the outer edge to be a hurricane-like eyewall, casting shadows across the saturnian cloud tops.  (Credit: NASA/JPL/Space Science Institute) http://photojournal.jpl.nasa.gov/catalog/PIA11104

This more oblique view, also from 2008, shows the shadows cast by these concentric eyewalls around Saturn's south polar vortex.  These images of the southern pole were obtained while it was still in sunlight, before it disappeared into darkness in August 2009, not to be seen again in reflected sunlight for the remainder of the Cassini mission.  With sunlight now returning to the northern hemisphere, Cassini has begun to capture images of the northern pole.  (Credit: NASA/JPL/Space Science Institute) http://photojournal.jpl.nasa.gov/catalog/PIA11103

The combination of spring sunlight and a high orbital inclination in 2012 finally allowed Cassini to view the hexagon in reflected light, rather than in the infrared. Here it is in all its glory, as well as the compact north polar cyclone mirroring the one at the south pole. This colour composite is based on http://photojournal.jpl.nasa.gov/catalog/PIA14646 from November 2012, with a colour composition by Jason Major.  (Credit: NASA/JPL/SSI/Jason Major) and you can read more about it at Universe Today.

Also from November 2012, this is a raw Cassini/ISS image processed very lightly to remove some bad pixels (again by Jason Major, read more at Universe Today), and the result is stunning - swirling clouds in the heart of Saturn's north polar vortex.  Even more amazing is the 7-frame animation compiled by Bjorn Jonsson and seen here.  (Credit: NASA/JPL/Space Science Institute)

Saturn's Shadow

Without a doubt Cassini's most stunning image of Saturn, obtained back in September 2006 as Cassini moved into Saturn's shadow.  The sun can be seen refracted through Saturn's upper atmosphere, and the pale dot of Earth can be seen just interior to the G ring, from a robotic vantage point over a billion kilometres from home.  The diffuse E ring, being actively vented from icy Enceladus, encircles the planet; the narrowly-confined G ring is easily seen just beyond the main rings; and these images even allowed astronomers to discover two faint new rings around the planet associated with satellites Janus, Epimetheus, and Pallene. (Credit: NASA/JPL/Space Science Institute)  http://photojournal.jpl.nasa.gov/catalog/PIA08329

Cassini repeated a shadowed view of Saturn in October 2012, this time from below the ring plane rather than above it (http://photojournal.jpl.nasa.gov/catalog/PIA14934).  No Earth this time (although you can just make out Tethys and Enceladus on the left of the planet), but another image in November 2012 was able to spy bright Venus between the planet and the innermost rings (http://photojournal.jpl.nasa.gov/catalog/PIA14935) (Credit: NASA/JPL/Space Science Institute).

Saturn's Storm Vortex Survives!

It seems to be the norm these days that amateur observers spot interesting features on the giant planets before us so-called professionals!  Their overwhelming advantage is long-baseline observations, working as a team to observe night after night to assemble a near-continuous record of meteorological activity at visible wavelengths.  I can't wait to see where this professional-amateur collaboration is going to take us in the next few years, but here's a very current example for Saturn, which is about to reach opposition (April 28th) to give us our best view of the ringed world.

False colour image of the storm on January 11th 2011,
splitting the longitude circle into three panels and
showing the anticyclone at the top.  (Credit:  NASA/JPL/SSI)

Back in 2011, shortly after Saturn's Great Northern Storm had erupted in December 2010, I was looking to compare thermal-infrared imagery of Saturn's storm temperatures with visible albedo patterns.  Colleagues put me in touch with Trevor Barry, a talented observer from down-under, who had captured some images at the same time as VLT, and we managed to publish them side by side in a paper on the Saturnian storm.  We spotted a new dark vortex downstream (i.e., to the east) of the main storm head, having formed among the churning, roiling turbulence of the storm system itself.  That vortex was 4000x5500 km in size in January 2011 (although it had apparently shrunk since it had first formed), colder than the surroundings (implying an anticyclone), and seemed to mark a transition between the western storm head and the eastern storm tail.  And there was something special about this vortex, the heart of the storm, as it seemed to coincide (at least to begin with) with the enormous glowing beacon of stratospheric air that we've been tracking ever since.  The Cassini cameras produced some stunning views of this anticyclone, seen at the top of the inset figure, and a paper was recently published by Kunio Sayanagi that traced its evolution from December 2010 until October 2011.  Kunio and the Cassini team watched as the vortex, originally at 39N, shifted northward and shrank in size, changing from westward motion to eastward motion as it moved into the region of an eastward jet near 47N.

Saturn on February 28th 2013,
Credit: NASA/JPL/Space Science Institute

Anthony Wesley's image of Saturn's dark oval,
March 18th 2013.

That could have been the end of it, with the anticyclone slowly disappearing from view as the storm came to its conclusion in the middle of 2011.  Through 2012 it was noted that the storm had left behind a new band in Saturn's northern hemisphere, free of cloud material and with slightly warmer temperatures, but in early 2013 we started to hear of a dark spot being tracked by the amateur community, Trevor Barry and Anthony Wesley among them.  Anthony had spotted it in images on March 18th and asked the Cassini folks what they made of it, and we pointed him to a raw Cassini image obtained on February 28th capturing the same feature.  This seemed to get everyone excited, and by combining all their various observations of the dark spot's longitude, they used the freely-available WinJUPOS software to measure the location.  From this, we come up with an eastward drift rate of 3.37 degrees per day at the present time, allowing us to predict when it'll be on the central meridian for all to see.  Trevor Barry's dataset went back even further to the last apparition in early 2012 (see chart below), and by co-plotting his dark spot longitudes with the Cassini/ISS observations from Kunio's paper, it looks likely that we're seeing the same anticyclone, two years after it was first born.  It may still be slowly drifting northward into the eastward jet, and accelerating slightly from 2012 to 2013. Note that this eastward-moving vortex is now completely disconnected from the westward moving stratospheric anticyclone.

Charting the drift of the dark oval from combining Cassini/ISS data from 2011 (Sayanagi et al., 2013) with observations from  Trevor Barry in 2012 and 2013.  Note that this is a work in progress, we'll continue to update this tracking as we go along.  To get the longitude on a particular date, just add multiples of 360 until you get a sensible number between 0 and 360!  You can see the original westward motion and the transition to eastward motion in July-August 2011.

What will be its fate in the coming months?  Have we witnessed the stormy birth of a long-lived oval in Saturn's northern hemisphere?  It's an exciting thought, and I'm sure that both Cassini and professional observers will be trying to figure out the properties of this vortex for some time to come...  Want to see this dark spot for yourself?  It should be approaching 93W longitude on April 28th, meaning that it'll transit the central meridian of Saturn at 23:00UT on April 27th, and 09:30 and 20:10 UT on April 28th.    All times are universal times.  Happy storm chasing!

William Herschel's Exploration of Saturn

I've been continuing to read through Alexander's excellent historical overview of Saturn observations before what we think of as the modern era of planetary exploration, and I've been astounded at how quickly our knowledge expanded in those early years.  My first post covered the work of Galileo and Huygens to explain the observations of Saturn's rings, and the second post looked at Cassini's discoveries of the major icy satellites.  But now I'm interested in observations of activity and structure on the planet itself, which seemed to be largely ignored for nearly a century.

Cassini had first noted vague observation of contrasts associated with a South Equatorial Belt (SEB) in 1676 (northern winter solstice in Saturn's third year).  Ninety years later, Charles Messier's observation in March 1766 (approaching northern winter solstice in Saturn's sixth year) noted 'two darkish belts, they were indeed extremely faint, and difficult to be discerned, directed, however, in a right line parallel to the longest diameter of Saturn's ring.' (Phil. Trans., 1769, vol. 59, p459).  Those bands must have both been in the southern equatorial and temperate regions, given the ring opening angle at the time.  Ten years later in May 1776 (northern spring), Messier observed 'a belt of a fainter light on the body of Saturn.... pretty broad and almost as distinct as those of Jupiter', which was presumably the north equatorial belt (Phil. Trans., 1776, vol. 66, p543).  But a comprehensive study of features on Saturn had to wait for a new hero of planetary astronomy to come along.  

William Herschel

William Herschel

William Herschel (1738-1822) had been born in Germany, but migrated to Britain at the age of 19 to seek refuge from war with France.   He constructed over four hundred telescopes during his lifetime, and his discovery of Uranus in 1781 came as he was cataloguing multiple star systems using his 160-mm aperture, two meter long reflecting telescope from the back garden of his home in Bath, Somerset.   Herschel had first taken up astronomy as a hobby in 1773, and observed a ring plane crossing in April 1774 (spring equinox at the start of Saturn's seventh year):  "I found the planet as it were stripped of its noble ornament, and dressed in the plain simplicity of Mars...".

In 1789 at the autumnal equinox he used a 40-foot reflector to discover two additional satellites, Enceladus and Mimas, bringing the total count to seven (the names were later provided by William's son, Sir John Herschel).  Saturn had been moving across the sky rather rapidly, so Herschel commented that over 2.5 hours "I had the pleasure of finding, that the planet had visibly carried them all away from their places," confirming that these points of light were indeed associated with Saturn.  Herschel crater on Mimas is named in his honour. He measured their orbital periods and comparable brightnesses; observed a shadow of Titan transiting Saturn's disc; and made detailed assessments of the leading/trailing brightness asymmetry of Iapetus, suggesting that the regularity of the observations implied the absence of any discernable atmosphere. He confirmed that the Cassini division was a true gap in two concentric rings, and measured the angular dimensions of the ring system.  These satellite and ring observations were all reported in Phil. Trans. 1790, vol. 80, p427 and Phil. Trans., 1791, vol. 82, p1.

Saturn's Atmosphere

Herschel's observation of Saturn, and a dusky
spot in June 1780.

And now we come to my original reason for reading Alexander's book:  the first study of features on Saturn's disc.  During Saturn's northern spring and summer between 1775-1780, Herschel observed the bright equatorial zone and one or more dusty dark belts, of variable width and brightness, in the northern hemisphere.  A large dusky spot observed in 1780 seemed to move from the centre to the limb, showing for the first time that Saturn rotated on an axis, just like Jupiter.  He surmised that he was seeing atmospheric features, and that "very probably Saturn has an atmosphere of a considerable density."  

As the rings opened and closed, the belts appeared to curve and then straighten, suggesting bands around a spherical object.  At the autumnal equinox and ring plane crossing in 1789, he observed a symmetry in the belts between the two hemispheres, and made precise measurements of the equatorial and polar radii of the planet, noting the polar flattening of Saturn due to it's rapid rotation.  Despite such apparent differences, Herschel was beginning to compare Jupiter and Saturn, stating "that every conclusion on the atmosphere and rotation of the one, drawn from the appearance of its belts, will apply equally to the other."  That theme of comparative planetology of the two gas giants continues to this day, two centuries later.

Herschel's discussion of quintuple rings in the southern
hemisphere, November 1793.

In November 1793, Herschel recorded more and more features in the southern summer hemisphere - a 'quintuple belt', probably consisting of the dark SEB in Saturn's tropics and two dusky temperate belts at higher southern latitudes.  He also noted that the south polar regions were of "a pale, whitish colour; less bright than the white equatorial [zone]."  (Phil. Trans. 1793, vol. 84, p28).  He used a series of observations of any heterogeneous features in Saturn's belts between November 1793 and January 1794, selecting pairs of similar features at two different times to estimate the rotational period of Saturn.  He was guided by his earlier deduction of a ring rotation period of 10.5 hours, and over many trial solutions arrived at an estimate of 10 hours, 16 minutes and 4 seconds (Phil. Trans., 1794, vol. 84, p48), around 15-30 minutes off from the current (variable!) rotation rate being measured by the Cassini spacecraft.  Alexander's book notes how incredible this result is, given that Herschel was using very slight differences in intensity in 'ill-defined stretches of belts.'  

More than a decade later, Herschel was compelled to return to his Saturn observations, comparing new observations of Saturn's north pole in 1806 (northern spring), to south polar views from 1793-1795 (southern spring).  At both poles, he noted that they had changed from their former whiteness to a duller tone during continued exposure to the summer sunshine.  Drawing analogies from the seasonal changes to the polar caps of Mars, Herschel suggested seasonal changes in Saturn's atmosphere, creating "large, dusky looking spaces of a cloudy atmospheric appearance" at the poles (Phil. Trans., 1806, vol.96, p455).

I had never truly appreciated Herschel's contribution to the exploration of Saturn before.  Since Cassini's time the planet had been rather neglected, but a century later Herschel provided dramatic insights into the satellite system, rings and the atmosphere that have stood the test of time. His enthusiasm for Saturn exploration was real:

(Phil. Trans., 1805, vol. 95, p272).  Sounds like the start of hundreds of subsequent research proposals!

Summary of Saturn Years, Measured from Spring Equinox (Heliocentric Longitude of Zero)

Saturn Year One:  1597-1627:  Galileo discovers Saturn's 'strange appendages'.
Saturn Year Two:  1627-1656:  Several theories proposed to explain Saturn's servants.
Saturn Year Three:  1656-1685:  Huygen's solves the Saturn problem and discovers Titan, Hooke and Flamsteed observe Saturn's ring progression; Cassini discovers Iapetus, Rhea, Dione and Tethys.
Saturn Year Four:  1685-1715:  Repeated observations of the new satellites, early evidence of C ring.
Saturn Year Five:  1715-1744:  Rev. James Bradley accurately measures ring diameters and satellite orbits.
Saturn Year Six:  1744-1774:  Charles Messier's observations of equatorial belts.
Saturn Year Seven:  1774-1803:  Herschel's discoveries of Enceladus and Mimas, discoveries of atmospheric features, measurement of rotation rate.
Saturn Year Eight:  1803-1833:  Herschel compared polar colouration on Saturn.

Astronomy & Geophysics Article: Future Exploration of the Outer Solar System

Cover of the April 2013 issue of A&G,
Credit:  Oxford Journals

The Oxford Journals magazine Astronomy and Geophysics has just published my report on a meeting hosted by Britain’s Royal Astronomical Society back in December  (Fletcher, 2013, Future Exploration of the Outer Solar System, Astronomy & Geophysics 2013 54: 2.14-2.20).   The article goes into much more detail, but here I've posted a similar report that I delivered to NASA's Outer Planets Assessment Group back in January 2013.  The Royal Astronomical Society has been around since 1831, and is chartered to promote astronomy, geophysics and solar system science here in the UK, with over 3000 members and premises in Burlington House, Piccadilly.  Each month they sponsor one-day discussion meetings on topics of interest to the community.  These are mainly aimed at raising awareness of issues within the UK community, but every so often we have meetings that encourage more international participation, such as this one. 

Chris Arridge and I proposed and ran a meeting on future exploration of the outer planets with the following aims:  (1) to continue the momentum of our studies for an ESA-led mission to an ice giant; (2) to review and discuss the high priority objectives of future missions and telescopic observations of the outer solar system; and (3) to bring together scientists and engineers involved in preparation for ESA’s JUICE mission.  After a busy couple of years with proposals for both ESA’s M3 mission (Marco-Polo-R and EChO currently the planetary missions in competition) and L mission (JUICE selected last May), it was hoped that this would be a rather optimistic and hopeful end to 2012.  What emerged from the meeting, from my perspective, was that there’s no shortage of great ideas, achievable concepts and tantalising destinations for the outer planets community. 

We were fortunate to have the funding to invite two overseas speakers to provide keynote talks, and asked Mark Hofstadter to provide us with an update on the prospects of a US-led mission to an ice giant, and Olivier Mousis from Toulouse to discuss giant planet origins and the French intentions for future ESA proposals.  Both of these presentations were heavily biased towards Uranus, with good reason.  As many of you will be aware, in 2010 Chris Arridge led a team of European and American scientists in a bid for ESA’s M3-class mission slot, with a concept called Uranus Pathfinder. The aim was to show ESA that an ice giant mission captured all of the essential elements of the Cosmic Vision, and that Europe could come up with viable strategies to achieve these aims.

Mark Hofstadter reviewed the outcome of the US decadal and budgetary cuts, starting with a Dickens quote – the best of times and the worst of times, the best because a Uranus flagship was recognised as high priority, and the worst because it doesn’t look likely any time soon.  He discussed the prospects of Discovery and New Frontiers Uranus missions, particularly with European involvement possibly in the form of bilateral agreements and instrument builds.  Chris Arridge reviewed the Uranus Pathfinder concept, and our intentions to rework the design for the next L class round. Olivier Mousis reviewed theories for the origins of Uranus and its satellite system, providing observational tests to be addressed by in situ exploration (both entry probes and INMS sampling from an orbiter).  In particular, we talked about a revival of a Saturn entry probe mission for the next ESA M-class call with a single probe.  Although this would target Saturn, this would be equally beneficial to the Uranus Pathfinder community, who saw an entry probe as a vital addition to address our questions about the origins of the ice giants.   Underpinning all of this, we had a presentation by Richard Ambrosi from the University of Leicester, part of a European consortium working towards efficient power conversion technologies for the decay of Americium-241.  Although the power density is only a quarter that of plutonium-238, its being seen as a viable alternative, and the consortium hopes to have a full-scale flight system in the near future.

The rest of the day was filled with review talks of key science drivers for future exploration, including presentations on auroral measurements from Juno and JWST from Tom Stallard of the University of Leicester; a review of comparative ring science from Carl Murray of QMUL; descriptions of future Titan exploration, such as TANDEM, TSSM, TiME and other ideas from Mark Leese of the Open University; and mass spectrometry concepts for future Titan landers, gas giant entry probes and penetrators for Europa, Ganymede and Enceladus from Andrew Morse of the Open University. 

Finally, we had two presentations reviewing the Jupiter Icy Moons Explorer, which has been top of the European planetary science community’s agenda for the past twelve months.  Michele Dougherty described the science case, with which I’m sure you’re all familiar, and Matthew Stuttard from Astrium described a high level overview of the company’s studies for the definition phase. Instrument proposals were all submitted in mid-October during the DPS, and then selected back in February by ESA.  The dust should hopefully settle over all of this in the next couple of months, and the selected PI teams can begin to move forward with the JUICE mission development. 

In summary, we had a successful meeting reviewing the scientific questions and key destinations for future exploration, and discussions about collaborations on future ESA M and L-class calls for ice giant missions and in situ exploration.  Here's a link to the full text of the A&G article:

http://astrogeo.oxfordjournals.org/cgi/content/full/att032?ijkey=tCO9H18kEN76Ogr&keytype=ref

Disclaimer:  Oxford Journals allow the author to include these links on their personal websites, or those of their host institutions, providing free access to the offprint to interested readers irrespective of whether you're a subscriber to A&G.  The idea is that Oxford Journals can then track the readership of these articles via their own website.

Future Exploration of Neptune's Atmosphere: Three Big Questions

Neptune stands apart from the other giant planets, possessing the most meteorologically active atmosphere in our solar system despite its great distance from the Sun.   Unlike sluggish Uranus, which was radically altered by collisional processes, Neptune may be typical of a whole class ice giant planets being discovered beyond our solar system.   And yet some of the basic dynamical, chemical and cloud-forming processes at work within this churning atmosphere, along with the competing influences of seasonally changing insolation and internal heat flux on atmospheric structure, remain an unresolved mystery. The European astronomy community is currently assembling a series of white papers to inform ESA's large cornerstone missions in the coming decades, and I've been asked to contribute ideas for Neptune orbital exploration.  In my opinion, future exploration of Neptune’s atmosphere must focus on what makes this world unique, so here's my list of top questions.

Neptune through the wavelengths, from visible-light imaging from Hubble, to near-infrared imaging from Keck (Credit I. de Pater) and thermal-infrared imaging from VLT (Credit:  G. Orton)

What powers the circulation and meteorology of an ice giant? Neptune provides an excellent test for models balancing seasonally dependent insolation (due to the 26^o axial tilt and the 165-year orbit) and excess internal heat flux (emission exceeding solar inputs by a factor of 2.6, the largest in the solar system).  The source of this intrinsic luminosity is uncertain, but it likely drives the complex meteorology of the troposphere and is the key factor distinguishing Neptune from Uranus, which has a negligible internal heat.  Neptune has a different relation between the banded cloud structures, atmospheric temperatures and the zonal wind structure when compared to Jupiter or Saturn.  Rapidly evolving convective cloud activity prevails at cooler mid-latitudes, with retrograde flow at the warmer equator and a high-latitude prograde jet confining a seasonally variable polar vortex of unusually high temperatures and unique chemical composition.  Dark ovals (such as the Great Dark Spot observed by Voyager 2) are sometimes associated with bright white orographic clouds at higher altitudes.  Neptune’s zonal winds are among the strongest in the solar system, possibly as a result of less atmospheric turbulence dissipating the energy when compared to Jupiter.  A future mission must correlate visible changes to cloud albedo, winds, eddies and vortices with environmental changes (e.g., latent heat release from cloud condensation, conversion between different spin states of molecular hydrogen, long term seasonal variability in temperature and composition) to understand the processes controlling the changing face of Neptune.

What is the origin and distribution of the zoo of chemical species in Neptune’s atmosphere?  Neptune’s atmospheric composition is determined by condensation chemistry (removing volatiles such as CH₄, NH₃, H₂S, and H₂O to the condensate phase), vertical mixing (dredging CO and possibly other species from the warmer interior), external influx of oxygenated species from infalling comets and dust, and a rich hydrocarbon photochemistry due to the UV destruction of methane.  Measurements of elemental enrichments (C/H, N/H, O/H), isotopic ratios (D/H, ¹³C/¹²C, ¹⁵N/¹⁴N) and noble gas abundances (via an entry probe) would provide constraints on the delivery of these materials to the forming proto-Neptune and conditions in the early solar system.  Furthermore, mapping the spatial distributions of cloud-forming volatiles, disequilibrium species and photochemical products teaches us about the chemical processes and cloud formation at work within the ice giant, and their variability from equator to pole.  The latitudinal distribution of methane will reveal whether it is enhanced by tropical uplift or by warming of the cloud trap at the seasonally heated poles. Indeed, the polar vortices are the sites of unique conditions due to a close connection with the planet’s magnetosphere, and require exploration via a high-inclination orbital phase. 

 

The highly variable atmpsphere imaged by Hubble in 2011 (left); and
high-altitude clouds of methane ice observed by Voyager in 1989 (right).

What are the atmospheric structure and cloud properties from the troposphere to the thermosphere?  Determinations of the three dimensional profiles of temperature, density, gaseous composition and aerosols provides the key to understanding the balance between internal heating, convective mixing, latent heat release and radiative heating and cooling throughout an ice giant atmosphere.  Vertical sounding should reveal the circulation regimes, zonal winds and turbulence characteristics at a variety of depths both within and above the tropospheric clouds; the nature and spatiotemporal variability of ice giant clouds and hazes; meridional motions in the stratosphere; and the importance of wave activity in redistributing energy and material with altitude.  The importance of wave breaking and ionosphere-magnetosphere drag processes should be determined in studying the abnormally high temperatures of the ionosphere and thermosphere.  This three-dimensional planetary-scale characterization of an ice giant atmosphere will provide a bridge between the deep circulation and the external magnetospheric environment, to be directly compared with Galileo, Cassini and JUICE reconnaissance of the gas giants.