Saturday 25 May 2013

Stargazing Introduction: Four Basic Tours

I've been starting to do a little work with schools and planetariums related to navigating your way around the night sky.   I've often found that this sort of introductory information is hard to come by, so I've tried to collate a few sources and provide some simple diagrams (mostly because they'll help me give a few star tours!).  Here are a few stargazing tours for novices without telescopes or binoculars, biased towards mid-latitudes in the northern hemisphere.  The star charts were created by annotating the star images obtained from the Neave Interactive Planetarium, set for 10pm in July in the UK for the first three tours, and December for the final tour.

Tour One:  The Plough and the North Pole

[Ursa Major, Ursa Minor, Bootes, Leo]

The Plough is not a constellation by itself, but an asterism within the Great Bear, Ursa Major.  It's one of the easiest things to find, and always visible in our sky throughout the year, so we'll use it as our starting point.  The myth goes that Zeus was lusting after a young woman names Callisto, making is wife Hera so jealous that she turned Callisto into a bear.  Callisto's son, Arcas, mistakenly tried to kill the bear, so Zeus turned him into the Little Bear and cast them both skywards for their own protection, forming Ursa Major and Ursa Minor.  M51, also known as the Whirlpool Galaxy, is just to the south of the saucepan handle.

The 'saucepan' shape provides pointer stars to other objects.  Starting from the two stars defining the right hand edge of the saucepan (Merek to the lower right, Dubhe to the upper right), extend a line 'upwards' from the pan (5x the distance between Merek and Dubhe) to reach Polaris, the pole star, and the tip of Ursa Minor (the Little Bear).  Polaris is directly above the Earth's north pole, so all stars in the night sky appear to rotate around that one point as the Earth spins on its axis.

Returning to the Plough, we can now use the 'handle of the saucepan' as a pointer (the stars Mizar and Alkaid), directing us to Arcturus, the brightest star in the northern hemisphere in the constellation Bootes, the herdsman.  Bootes may have been responsible for driving the oxen that the Greeks thought to represent the Plough.  The constellation looks like a large kite extending up from the horizon, and Arcturus is an orange giant 37 light years from Earth.

Finally, if we drill a hole in the bottom of the saucepan, and let the liquid run out, it'll hit the head of the Leo the Lion, and the brightest star Regulus (a blue-white star 78 light years away marking the heart of the Lion), leading to the mnemonic: "A hole in the bowl will leak on Leo".  Leo was the Nemean Lion killed by Hercules during one of his twelve labours, and cast into the sky, but looks to me like a coathanger bent out of shape.    The Leonid meteor show November 14-15 comes from this direction.

Tour Two:  Cassiopeia's Court

[Cassiopeia, Cepheus, Andromeda, Perseus]

As before, we start from the Plough asterism in Ursa Major, finding our way to Polaris (the pole star) using the pointers Merek and Dubhe.  But now we continue on in the same direction that you took from the Plough to Polaris, for around the same distance again, and you'll come to the distinctive 'W' that makes up the constellation of Cassiopeia.   The vain Queen was cast into the sky by Poseidon as punishment for boasting that she was more beautiful than the Nereids.  Cassiopeia lies within the Milky Way, so contains many deep sky objects.

Cassiopeia can also be used as pointer stars, although the directions aren't quite as obvious as for tour one.  From the right hand side of the 'W', follow upwards towards the north to find Cepheus (Cassiopeia's husband and King of Ethiopia).  Cassiopeia's punishment also extended to Ethiopia too, as Poseidon commanded the sea monster Cetus to attack.

Now extend the same line down (southwards) to find the feet of Andromeda (Cassiopeia's princess daughter).  Cepheus was told that the only way to save his kingdom from attack by Cetus was to sacrifice his daughter, so Andromeda was chained to a rock to be eaten by Cetus.   Andromeda is intimately linked to the brightest star in the Great Square of Pegasus.  The Andromeda Galaxy (M31) is the closest spiral galaxy in the Milky Way and can be seen within the 'A' shape of Andromeda.

Finally, one of the central bars of the 'W' leads to the head of Perseus to the southeast.  Perseus is the hero of our story, saving Andromeda from the sea monster Cetus using the head of Medusa to turn the monster to stone.  Perseus became Andromeda's husband.  The variable star Algol within Perseus is said to be Medusa's eye, and lies 93 light years away.  The constellation is connected to Auriga to the east, and the Perseids' meteor shower (August 9-14) originates from this constellation each year.

Tour Three:  The Summer Triangle

[Cygnus, Lyra, Aquila, Hercules and Pegasus]

The summer triangle consists of three bright stars, each marking a constellation, that are readily visible in the summer skies.

Deneb is a bright star at the tail of Cygnus the Swan, a constellation lying in the plane of the Milky Way, and is easily recognisable due to the asterism known as the Northern Cross.  Deneb is a blue white supergiant 3200 light years away.  The beak is Albireo, a binary star of an orange giant and a blue-green star.  Transformations of Greek gods into swans seemed a common occurrence, with Zeus, Orpheus and Cycnus all having gone through the process!  The constellation contains the X-ray source Cygnus X-1, which is now thought to be caused by a black hole accreting matter in a binary star system.

Vega is in Lyra the Harp, a small constellation containing the 3rd brightest star in the northern hemisphere.  After the deal of the musician Orpheus, his lyre was thrown into a river but retrieved by an eagle from Zeus, to be placed into the sky.

The final star in the triangle is Altair, a bright star within Aquila the Eagle.  The eagle carried Zeus' thunderbolts.  Like the Swan, the Eagle lies within the plane of the Milky Way so is rich with deep sky objects.  Altair is one of the closest naked eye stars to Earth at a distance of only 17 light years. Interestingly, NASA's Pioneer 11 spacecraft (flew by Jupiter and Saturn in the late 1970s) is headed in that direction.

Both the Eagle and the Swan are flying in the direction of the Milky Way across the sky, and appear to be headed away from Cepheus (Cassiopeia's husband).  Extending a line from Altair through to Deneb shows you the way to Cepheus (see Tour Two).

A line at right angles to this, coming out of the centre of the Summer triangle, will lead you to the Great Square of Pegasus, the winged horse.  The square is made of four stars, alpha Peg, beta Peg, gamma Peg and alpha Andromedae, which it shares with Andromeda.  51 Peg features the first ever extrasolar planet to be discovered, and HD 2090458b provided the first evidence of water vapour from transit spectroscopy.  Pegasus carried Medusa's head to Polydectes, and was a bearer of thunder and lighting for Zeus.

A final line from Deneb to Vega will point in the direction of a trapezium of four stars making up the body of Hercules, known as the Keystone asterism.  Hercules is depicted as kneeling, praying to his father Zeus after winning a battle following his tenth labour.

Tour Four:  Orion's Hunting Ground

[Orion, Canis Major, Taurus, Gemini]

Orion is one of the brightest constellations in the winter sky, and its belt can be used as a starting point for some star-hopping.  The chart is depicted for 10pm in mid-December from northern mid-latitudes.  Orion lies on the celestial equator, and is depicted as a hunter with a bow and arrow.  Rigel, a blue white star; and Betelgeuse, a red supergiant, are the brightest stars in the constellation.  Orion was a supernaturally strong hunter in Greek mythology, son of Poseidon.  The three stars making up Orion's belt are Alnitak, Alnilam and Mintaka, and a sword hanging from the belt features the beautiful Orion Nebula (M42) 1344 light years away.

Follow the belt to the left (southeast), and we arrive at Sirius, the brightest star visible in our night sky and part of Canis Major.  Follow across the shoulders of Orion to the east and we find Procyon, part of Canis Minor.  Canis Major and Canis Minor were the two hunting dogs of Orion.  The Winter Triangle is made up of Sirius in Canis Major, Procyon in Canis Minor, and the red supergiant Betelgeuse, an asterism to rival the Summer Triangle.  Voyager 2 is slowly moving towards Canis Major.

Follow the belt to the right (northwest) and find the red giant Aldebaran, the brightest star in Taurus the bull.  Orion is typically depicted as fighting Taurus.  Several objects of interest lie in this constellation, including the Crab Nebula (M1, a supernova remnant), the Hyades and the Pleiades.  Keep on following the line through Taurus and you'll come to the Pleiades (M45), an open cluster of many stars, the seven most prominent giving the cluster its nickname.  Zeus took on the form of a white bull to abduct the Phoenician princess Europa, but the identification of Taurus goes back must further into our history due to its position in the zodiac.

Now follow the Hunter's right arm upwards through the red giant Betelgeuse towards Gemini, the twins, and the two stars Castor and Pollux.  Gemini is not in the plane of the Milky Way so features fewer deep sky objects, but is the origin of the Geminids meteor shower on December 13-14 each year. In Greek mythology, Pollux was the immortal son of Zeus and Leda, whereas Castor was the mortal son of Leda and the Spartan Kind, Tyndareus.  When Castor died, Pollux begged his father to give Castor immortality, and the two were united in the heavens.

Finally, a further asterism known as the Winter Hexagon can be found by connecting Sirius to Rigel, Aldebaran, Capella, Pollux and Procyon, with Betelgeuse roughly in the centre.

General Star Charts (Summer/Winter)

To give a sense of how all of these star hopping tours fit together, here's a star chart for the northern hemisphere constellations in mid-summer.
...and another for mid-winter.

Some Helpful Links

Superb Stargazing Live guide from the BBC:

Dave Snyder's 2003 guide to the constellations

Sky maps and planispheres from

A great beginners guide to constellations from

Visualisations of what the constellations really look like:

Friday 24 May 2013

Jupiter in the Infrared from VLT

Every now and then, you go back to look at some old data and discover something new.  I'm helping some colleagues with a proposal at the moment to study wave activity on Jupiter, and went back to some thermal infrared imaging of Jupiter obtained by the Very Large Telescope (VLT) down in Chile using the VISIR instrument.  Although we'd published it already as part of a study of the Great Red Spot (Fletcher et al., 2010, Thermal Structure and Composition of Jupiter’s Great Red Spot from High-Resolution Thermal Imaging, Icarus 208, p306-328), there's a lot more to this dataset than I first realised!  

I re-processed northern and southern hemisphere images at 8.6, 10.7 and 13.0 µm from August 15th 2007.  Jupiter was close to opposition at that time, and so large that it can't fit completely onto the detector array.  Furthermore, we have to use a technique called nodding in the infrared, switching between two slightly different views of the target and then subtracting them, meaning that we detect Jupiter differentially on top of a very bright background (our own warm atmosphere).  Three images for the southern hemisphere are shown below.

Jupiter at 8.6 µm, where bright emission means a gap in the clouds, dark tones represent cloudier regions.

Jupiter at 13 µm, sensitive to the temperatures of the hydrogen and helium gas, so bright colours indicate warmer regions.  Imperfect correction for the nodded image can be seen as a bright arc at the top of the frame.
Jupiter at 10.7 µm, sensitive to a combination of tropospheric temperatures and ammonia gas.  So bright colours can either mean an absence of ammonia, or warmer temperatures.
The real magic happens when we combine these three filters together to make a false colour image.  Now I'm a novice when it comes to this sort of thing, but I used Adobe Photoshop, shifting the three Jupiter images to register them properly (they were taken a few minutes apart, and Jupiter rotates rather quickly!) and adding them to the R, G and B layers.  I then tweaked the colour balance slightly, and voila.

A crude false colour image of Jupiter, using red=8.6 µm, green=13 µm and blue=10.7 µm.
Credit:  L.N. Fletcher/University of Oxford/ESO
I'm pretty happy with these results, and certainly make nice images for a proposal, even if the corrections for the nodded images aren't perfect (particularly for the southern hemisphere, where auroral emission at 13 µm makes a well-defined south pole that's hard to correct for).  Where you see red, it means an absence of tropospheric cloud/haze.  Green means warmer temperatures.  Blue means an absence of ammonia or warmer temperatures.  The warm/cloud-free/ammonia-free southern edge of the Great Red Spot shows as white, as does the northern edge of the SEB.  The turbulent wake to the northeast of the GRS is cloud-free and warm.  You can see cloud-free rifts near the southern edge of the NEB, and a whole host of vortices in both hemispheres.  In the southern hemisphere image, you can even see the glow of Jupiter's auroral hotspot (this is acetylene emission from high over the southern pole).  If I were more of an expert at photoshop, I might be able to clean these even more, but I suppose I should get back to doing some real work!!

Why Does Uranus Emit Very Little Heat?

As we've been preparing white papers on science themes for ESA's future L class missions, I've learned  a lot from the conversations flying back and forth via emails in the community.  One was about the Voyager-era result that the self-luminosity of the ice giants, Uranus and Neptune, was so different (Neptune's internal energy emission was a factor of ten larger than the of Uranus, such that the ratio between emitted and absorbed energy is 1.06±0.08 for Uranus but 2.61±0.28 for Neptune) that some strange scenarios had to be invoked to explain the vast difference.  Planets are warm inside and cool down as they age.  Voyager measurements suggest that Uranus’ evolution produced a planet with negligible self-luminosity, smaller than any other planet in our Solar System (Pearl et al., 1990). Combined with the sluggish appearance of the atmosphere as viewed by Voyager, this suggested that the interior of Uranus was either (a) not fully convective or that (b) it suffered an early loss of internal heat. For Case (b), a catastrophic event such as a collision early in Uranus' history could have shocked the matter in the interior and led to a catastrophic loss of internal energy.  
The interiors of the giant planets from evolution modelling.
Credit: Lunar and Planetary Institute

However, two scenarios were described for case (a) in the Neptune and Triton book (Hubbard, Podolak and Stevenson, 1995), which implies that the interior of Neptune is colder than the adiabatic case.  One idea suggested that Uranus might be stable to convection in the inner 40-60% of its radius, so that we essentially only measured the cooling history of the outer part of the planet.  The interior would be warmer, but that immiscibility of certain consitiuents (e.g., helium and carbon) prevents the efficient transport of heat from the interior outwards.  The other suggestion was that Uranus started cold, so cold that the ices in its deep interior could have been of a crystalline form.  

Since that time there have been improved thermal evolution models and data on the equation of state of the interior composition, leading to different solutions for the internal convection permissible within Uranus.  Nadine Nettleman described new Uranus evolution models using an assumption of an adiabatic interior that can explain the low luminosity of Uranus.  Essentially, with the new models, Uranus can have a convective interior and still cool enough over the age of the solar system to be consistent with the upper limits on the measured luminosity from Voyager. The inferred size of a non-convective, stable internal region is extremely sensitive to the measured intrinsic heat flux value: a mostly stable interior is predicted if the heat flux is close to zero, but a fully convective interior remains possible, as for Neptune, should the upper limit of the Voyager heat flux value prove true.  

These inferences come from a single measurement from the Voyager flyby in 1986, at a single point in Uranus’ seasonal cycle, and with error bars so large that many different scenarios are consistent with the data.  The heat balance arguments put forward by Barney Conrath, John Pearl and colleagues come from integrating the spectral irradiance measured by Voyager/IRIS over as broad a wavelength range as possible to estimate a total emitted power in Watts ((5.60±0.11)e15 W for Uranus, (5.34±0.29)e15 W for Neptune), and then relating this to the temperature of the equivalent blackbody producing the same spectrally-integrated irradiance (59.1±0.3 K for Uranus and 59.3±0.8 K for Neptune).  The fact that they are so similar, despite Uranus absorbing more than twice as much solar energy as Neptune, is the source of this enduring mystery.  IRIS' spectral coverage was narrow for the ice giants (25-50 µm or so), but included the peak of the black body emission of these planets, so it remains one of the best estimates we have.  The energy balance measured by Voyager is simply the ratio of this integrated power to the absorbed power.  The latter depends on the albedo of the planet, which is also rather hard to estimate (you need to know precisely how the planet - i.e. the clouds - reflect light over a full range of phase angles) and is a source of uncertainty on the ratio.  If we could obtain more accurate measurements of the intrinsic heat flux, via a mission seeking to determine the balance between Uranus' emitted energy and that absorbed from the Sun, then we might be able to distinguish between these competing scenarios for the evolutionary history of one of the strangest planets in our solar system.

Wednesday 22 May 2013

Planetary Science and ESA's L Class

The last few weeks have been crazy, as the European planetary science community has been organising itself to submit responses to ESA's call for science themes for its L2 and L3 missions, slated for 2028 and 2034, respectively.  Despite the selection of JUICE (the Jupiter Icy Moons Explorer) as the first L-class mission, for launch in 2022, the planetary science community was urged to actively participate in this call.  After all, there's no shortage of exciting, ambitious and ground-breaking ideas for the exploration of our solar system, addressing themes and questions right at the heart of ESA's Cosmic Vision.

The L class missions are the cornerstones of ESA's scientific programme.  SOHO, Cluster, Rosetta, XMM-Newton and Herschel are good examples of the scope and ambition required; along with Gaia and BepiColombo which are currently being implemented.  The L missions are interspersed with the smaller 'M' missions, in which planetary science missions like EChO (the exoplanet characterisation observatory) and Marco-Polo-R (asteroid sample return) are currently competing for M3.    ESA wanted an open community consultation for the science themes to be addressed by the next L missions, and given the long decadal timescales, it's not surprising that it's been the younger members of the planetary community (postdocs and research fellows in search of permanent positions, hint hint) who have jumped at this opportunity.  Every now and then, it's nice to look at the bigger picture, about why we believe it's so important to continue the exploration of planetary environments in our solar system and beyond.

Here's a round up of some of the planetary science white papers that have been flying around over these past few weeks, and I sincerely hope that some of them will inform ESA's planning, technology development and international collaborations in the next few decades!  Eventually I'll include links to the finished papers, but here's an incomplete list of topics, in no particular order:

[UPDATE!  All the L2/L3 White Papers are now available as an e-book available from ESA's website here, and the majority of the topics described below have been selected for presentation in Paris in September 2013].
Cover for the planetary science
observatory and the giant planet 

entry probes, themes obviously close to
my heart!

Joint Exploration of Titan and Enceladus (Gabriel Tobie and Nick Teanby)

The Exploration of Titan with an Orbiter and a Lake-Probe (Guiseppe Mitri)

Exploring Planetary Origins and Environments in the Infrared:  A Planetary Science Infrared Observatory (PSIO)  (Leigh Fletcher)

In Situ Exploration of the Giant Planets and a Saturn Entry Probe Concept (Olivier Mousis and Leigh Fletcher)

The Science Case for an Orbital Mission to Uranus (Chris Arridge)

Neptune and Triton:  Essential Pieces of the Solar System Puzzle (Adam Masters)

Venus Exploration (Colin Wilson)

Venus: A Natural Planetary Laboratory (Sanjay Limaye)

Main Belt Comet Mission (Geraint Jones)

In Situ Exploration of the Diversity of the Asteroid Belt  (Pierre Vernazza)

Exploring Habitable Worlds Beyond our Solar System (Andreas Quirrenbach,

Lunar Science as a Window into the Early History of the Solar System  (Ian Crawford)

In-Situ Investigations of the Local Interstellar Medium (likely featuring giant planet flyby) (Robert F. Wimmer-Schweingruber)

European Ultraviolet-Visible Observatory (EUVO) (Ana Inés Gómez de Castro)

The ODINUS Mission Concept:  The Scientific Case for a Mission to the Ice Giant Planets with Twin Spacecrafts to Unveil the History of our Solar System (Diego Turrini)

Master: A Mission to Return a Sample from Mars to Earth (Monica Grady)

Thursday 9 May 2013

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)

Perhaps more useful is this artists concept of the structure of the rings, and where the main shepherding moons are located.  Credit: NASA/JPL,

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)

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.

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.

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.

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: (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.

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.

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  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.

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.

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.

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.

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.

Tuesday 7 May 2013

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)

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)

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)

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)  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)

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)

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)

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)

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)

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)

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) and 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)

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)

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)

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)

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)

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)

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 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)

Cassini repeated a shadowed view of Saturn in October 2012, this time from below the ring plane rather than above it (  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 ( (Credit: NASA/JPL/Space Science Institute).