Thursday 12 December 2013

The Plumes of Europa

2013 has been a rather exciting year for Europa scientists, even without a spacecraft anywhere near the jovian system.  We've seen evidence of the salty composition of Europa's oceans; models explaining the deep ocean flow and influence on surface features; evidence of surface phyllosilicates from a cometary or asteroidal impact, and today's exciting news:  the Hubble Space Telescope (HST) discovery of water vapour plumes from the south pole of this icy moon.
Illustration of icy Europan plumes
(Credit: NASA/Caltech)

Hubble's Plumes

The ultraviolet observations by HST are reported in Science (Roth L., J. Saur, K. D. Retherford, D. F. Strobel, P. D. Feldman, M. A. McGrath, F. Nimmo, "Transient Water Vapor at Europa's South Pole," Science, 12 Dec 2013), and suggest water vapour plumes being dissociated by electron bombardment into their constituent atoms, revealing themselves to Hubble as ultraviolet hydrogen emission (Lyman alpha at 121.6 nm) and oxygen emission (130.4 nm and 135.6 nm).  The excess emission rises 200 km from Europa's south pole, reminiscent of the icy geysers of Enceladus in the Saturn system, with incredible implications for our ability to probe the potentially-habitable conditions on this small satellite.  But it's important to note that we've only seen this once, in December 2012, when Europa was at apocentre (its furthest point from Jupiter), so it'll be extremely important to follow this up with future observations.  Thankfully, the team is led by researchers at the Southwest Research Institute (SwRI) in San Antonio, Texas, who happen to be the brains behind the Ultraviolet Spectrograph (UVS, on which I'm very lucky to be a science co-investigator) on ESA's Jupiter Icy Moons Explorer (JUICE).  The results are being presented at the AGU meeting in San Francisco on Thursday (Roth et al.) and Friday (Retherford et al.), and were subject of a press conference earlier today.

Artist impression of the Europa south polar plume.
Credit: NASA, ESA, and L. Roth (Southwest Research
Institute and University of Cologne, Germany)
P42A-01. HST Observations of Europa's UV Aurora Morphology 
Lorenz Roth; Joachim Saur; Kurt D. Retherford; Darrell F. Strobel; Paul D. Feldman; Melissa A. McGrath; Francis Nimmo

P53A-1838. Discovery of Europa's Water Vapor Plumes:  Europa's Atmosphere and Aurora: Recent Advances from HST-STIS and Plans for Plume Searches with JUICE-UVS
Kurt D. Retherford; Randy Gladstone; Lorenz Roth; Melissa A. McGrath; Joachim Saur; Paul D. Feldman; Andrew J. Steffl; Darrell F. Strobel; Thomas K. Greathouse; John R. Spencer; Fran Bagenal; Leigh N. Fletcher; John S. Eterno

Europa's Sub-Surface Ocean

But first back to enigmatic Europa, the smallest and smoothest of the four Galilean satellites (Io, Europa, Ganymede and Callisto) at 1940 miles across, so roughly a quarter of the size of the Earth.  What makes Europa so intriguing is the suggestion of a global sub-surface ocean, beneath an icy crust somewhere between 10-100 km thick depending on the model you consider.  The ocean is kept as a liquid by the energy released by powerful tidal forces raised by Europa's 3.5-day orbit around Jupiter.  What's more, that ocean is thought to be in direct contact with the rocky silicate mantle, and with the surface ices, meaning that all the necessary ingredients for habitability (a source of energy, water as a chemical solvent, as well as a source of elements and minerals) come together in this fascinating environment.  Europa's icy surface is fractured, cracked and in some places 'geologically-young', meaning that there are few craters because of the resurfacing processes at work.  As Europa is tidally-locked, with one side continually facing Jupiter, it exhibits stark differences between the leading (forward-orbit-facing) and trailing hemispheres, with the latter being bombarded by the materials being swept around by Jupiter's powerful magnetic field.  And the Galileo spacecraft discovered a weak 'induced' magnetic field, caused by the interaction of Jupiter's magnetosphere with a highly-conductive layer beneath the crust, most likely the liquid ocean.  For all these reasons and more, Europa has long been the top destination for a future mission to the outer solar system.

Dark striations across Europa's cracked surface
(NASA/JPL/University of Arizona/University of Colorado)
But how might we probe this potentially-habitable sub-surface ocean?  Cryobots that melt their way down to the dark oceans remain in the realms of science fiction for now, so we're left with whatever observations can be done remotely.  2013 has seen plenty of new evidence come to light that we can probe the interior by careful observation of the surface and external environment.  The first was a paper in the Astronomical Journal in April by Mike Brown and Kevin Hand that used the Keck Observatory to detect magnesium sulphates (potentially epsomite) on Europa's trailing side.  They hypothesised that salty ocean brines containing sodium, potassium and magnesium chlorides are somehow delivered to the icy surface (by some geologically-active process), where they are bombarded from behind by sulphur being emitted by its neighbour Io to form sulphates [the sulphur being whacked into the trailing hemisphere of Europa by the rotation of the magnetosphere].  Most of the sodium and potassium are sputtered (i.e., knocked off the surface) to create a thin atmosphere (e.g., Brown 2001), leaving behind the magnesium sulphates as the product of radiolysis occurring on the ocean brines.  So that means we can get a good idea of what the interior ocean is like by looking at the chemistry of the surface materials.

The second result came from theoretical modelling of Krista Soderlund and colleagues in early December ("Ocean-driven heating of Europa’s icy shell at low latitudes") in Nature Geoscience.  These authors used ocean dynamics simulations to try to understand the chaotic terrain that covers approximately 40% of Europa's surface and is more common at the equator than at the poles.  The jumbled, criss-cross patterns could be caused by thinner regions melting and refreezing, or by solid-state convection within the ice shell.  The new models suggest that turbulent convective motions within the global ocean serve to focus Europa's internal heat at lower latitudes, making the ice thinner there.  The oceanic model suggests three zonal jets and two Hadley-like circulation cells.  Once again, the properties of the sub-surface ocean can be inferred by 'reading' the surface features, and I particularly like how this paper bridged the gap between oceanic circulation models and the features of the ice shell.

Artist impression of Europa's plumes
(Image: NASA/ESA/K. Retherford, SwRI)
But both of these results rely on indirect remote observations - either 'reading' the geology, or conducting infrared spectroscopy to measure the composition.  They still don't allow us to directly probe that deep ocean.  Until now.  The Europa plumes revealed by Lorenz Roth's paper ("Transient Water Vapor at Europa's South Pole") in Science offer the tantalising prospect of directly sampling the chemical composition of material spewed out of the global ocean by some future mass spectrometer.  Now, we don't know for sure that these plumes have a water source in the sub-surface ocean, and HST only has one plume detection so far, in December 2012.  The plumes appear highly variable, and were not spotted when Europa was closer to pericentre in November 2012 (i.e., at the closest point to Jupiter), which suggests that changing stresses along the cracks can open fissures when the moon is 'unsquished' at apocentre, as it was in December 2012 for the full seven hours of Roth's Hubble observations.  This variability mirrors the processes governing Enceladus' plumes.  Galileo didn't really cover the poles during its 11 passes of Europa, so it doesn't really help us here.  But it looks like we have two great examples of icy moon plumes in our solar system (Europa and Enceladus), but we're basing all this on one observation, and we really must go back for more!

Ice rafts in Conemara Chaos, a region targeted by
JUICE in 2031 (Credit:  NASA/JPL/Univ. Arizona)

A Tantalising Prospect for JUICE

The plume discovery makes the UV observations from ESA's Jupiter Icy Moons Explorer (JUICE) even more tantalising.  The JUICE mission is currently in the planning and definition phase, but it is envisaged that it will make two close flybys over Europa's chaos terrains in February 2031, reaching within 1000 km of the surface.  These chaos terrains will be targeted as the potentially-active and thinner crust offers our best opportunities to map the ice-ocean interface.  JUICE will be using radar to sound through the ice, laser altimetry to map the topography, magnetic field measurements to measure the ocean conduction, and a range of remote sensing to understand the composition, chemistry and physical properties of the icy surface.  The UVS observations envisaged by the SwRI team responsible for the plume discovery now take on a great deal more importance:  UVS will conduct detailed plume searches via stellar occultations and far-UV imaging scans of auroral emission (driven by interactions of Europa's plasma with the magnetosphere).  Limb imaging will be performed within the several hours of the closest approach to Europa (less than 1000 km above the icy surface), supplemented by stellar occultations at relatively large distances from the moon.  A movie of the proposed flyby is shown below, and although it's still 18 years away, these data will be worth the wait!  [PS.  That also means that the graduates who'll be working on these data are probably in nursery today...].

Video courtesy of C. Arridge, UCL, created for the Royal Society Ice Worlds Exhibit 2013

Beneath the icy crust, these global oceans will be perpetual darkness.  Any life that exists there would presumably be as simple as it comes, but that doesn't matter - if we can one day reveal that life developed somewhere other than Earth, no matter its complexity, or whether it's on Mars, Europa or Titan, it'll be the most profound discovery we'll ever make.  

More Reading:

Flow of an alien ocean, Jason Goodman, Nature Geoscience (2013)
Ocean-driven heating of Europa’s icy shell at low latitudes, K. M. Soderlund et al., Nature Geoscience (2013)
Tilting at Europa, Emily Lakdawalla, Nature Geoscience 6, 899 (2013)
Jupiter's Icy Moon: Window Into Europa's Ocean Lies Right at the Surface, Science Daily
Salts and Radiation Products on the Surface of Europa, Brown and Hand, 2013.
Europa’s Underground Ocean Surfaces, Phil Plait (Bad Astronomy)
Jupiter Icy Moons Explorer (JUICE): An ESA Mission to Orbit Ganymede and to Characterise the Jupiter System, Grasset et al., Planetary and Space Science
Transient Water Vapor at Europa's South Pole, Science, (2013)

Monday 2 December 2013

PhDs in Planetary Science at Oxford (2014)

With apologies for using this blog as a way of advertising, but there are plenty of opportunities here at the University of Oxford for planetary science studies.  The University of Oxford has several Planetary Physics DPhil positions (i.e., PhDs) available in the Atmospheric, Oceanic and Planetary Physics (AOPP) department from October 2014, some working on projects with me (see below).

The available projects cover a broad range of planetary atmospheric science: studies of brown dwarfs spectroscopy; Rosetta investigations of cometary atmospheres; ground-based observations and Spitzer studies of planetary processes at work on ice giants Uranus and Neptune; Cassini studies of Saturn's seasons; ground-based observations of Jupiter's weather; and experimental studies of planetary heat transport.  Please visit the following website for specifics of each project:

Specific Planetary Science Projects:
  • Modelling the spectra of Brown Dwarfs
  • Rosetta-VIRTIS cometary studies
  • Infrared remote sensing of the Ice Giants: atmospheric temperature, composition and clouds
  • Seasons of Saturn: evolution of Saturn’s temperature, clouds, chemistry and dynamics from Cassini
  • The weather of Jupiter: observation of dynamical tracers from ground-based spectroscopy
  • Testing parameterisations of large-scale planetary heat transport in the laboratory
What You Need to Know:

Deadlines:  Candidates should apply by 24 January 2014 in order to be considered for Departmental Studentships and any funding the University administers. Candidates able to secure external scholarships should apply by 14 March 2014.  Both research council and scholarship funding is available, please see our admissions website for further details.

We would be very grateful if you could distribute this information to potentially-interested students.  If you have any queries or require further information that is not available on these pages please email:

For some great student insights into what it's like to study here in AOPP, watch the following video on Youtube:

Thursday 28 November 2013

Hubble: Galaxies Across Space and Time

I spent the summer of 2003 working on the Great Observatories Origins Deep Survey (GOODS), supervised by Frank Summers, at the STScI in Baltimore, Maryland. I took an image of the GOODS Deep Field and converted it to a three dimensional fly-through of the galaxies. The movie was converted to an IMAX film ‘Hubble: Galaxies Across Space and Time’, which subsequently won Best Short Film at the LFCA (Large Format Cinema Association) annual film festival in Los Angeles, and can be seen at the Baltimore Science Centre.  I was happy to find that it had found it's way onto Youtube earlier this year (2013), and you can watch it below.   Below you'll also find some links to some of the news releases about the movie, but a comprehensive description can be found here: Making a Short, but Very Large, Movie

The IMAX visualisation was reported at the American Astronomical Society Meeting in 2003:

Summers, F. J., Stoke, J. M., Albert, L. J., Bacon, G. T., Barranger, C. L., Feild, A. R., Frattare, L. M., Godfrey, J. P., Levay, Z. G., Preston, B. S., L. N. Fletcher, GOODS Team. 2003. Hubble Goes IMAX: 3D Visualization of the GOODS Southern Field for a Large Format Short Film.  Bulletin of the American Astronomical Society, Vol. 35, p.1345

Hubble IMAX Film Takes Viewers on Ride Through Space and Time

Hubble Space Telescope Science Institute, June 24, 2004

Take a virtual ride to the outer reaches of the universe and explore 10 billion years of galactic history, from fully formed and majestic spiral galaxies to disheveled collections of stars just beginning to form.

This unforgettable cosmic journey is presented in the award-winning IMAX short film, "Hubble: Galaxies Across Space and Time," which transforms images and data from NASA's Hubble Space Telescope into a voyage that sweeps viewers across the cosmos. Using the 650-megapixel-mosaic image created by the Great Observatories Origins Deep Survey (GOODS), more than 11,000 galaxy images were extracted and assembled into an accurate 3-D model for the three-minute movie. The large-format film was created by a team of Hubble image and visualization experts in the Office of Public Outreach at the Space Telescope Science Institute (STScI) in Baltimore, Md. The film was directed by Frank Summers, an astrophysicist and science visualization specialist.

Galaxies are vast assemblages of stars, gas, and dust. And viewers experience these majestic cities of stars on a movie screen as tall as a five-story building. The film opens with looming images of two mature galaxies that are relatively nearby Earth, and then pans through the vibrant and diverse panorama of thousands of galaxies in the GOODS mosaic.

The ensuing 3-D journey through these galaxies provides more than just a new perspective in space, it also takes the audience back in time. Because light takes time to cross space, the galaxies farther away from Earth are seen further back in cosmic history. The virtual voyage reveals galaxies as they appeared billions of years ago, when they were still in the process of forming.

The movie has been so well received that it recently won the "Best Short Feature" award at the Large Format Cinema Association's 2004 Film Festival in Los Angeles, CA. The Hubble movie premiered in April at the Maryland Science Center in Baltimore, and is currently also playing at the Rueben H. Fleet Science Center in San Diego, Calif., and the New Detroit Science Center in Detroit, Mich. Distribution to several dozen other large-format theaters will occur over the coming months and years.

The film is based on data from the GOODS project, a collaboration between Hubble, NASA's Chandra X-ray Observatory and Spitzer Space Telescope, and several ground-based observatories. The observations with the Advanced Camera for Surveys, one of the largest Hubble projects ever, provided deep images of a small patch of sky covering about one-third of the projected area of the full moon. That patch contains nearly 30,000 galaxies, which were cross-matched against a ground-based redshift survey to get distances for the 3-D model.

Actress Barbara Feldon is the film's narrator, and space music composer Jonn Serrie wrote the surround-sound score. The STScI film team consists of John Stoke, Zoltan Levay, Lisa Frattare, Greg Bacon, John Godfrey, Bryan Preston, and summer intern Leigh Fletcher.

The Space Telescope Science Institute (STScI) is operated by the Association of Universities for Research in Astronomy, Inc. (AURA), for NASA, under contract with the Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency (ESA). DKP/70MM Productions, Inc., a subsidiary of IMAX Corporation donated their services and created a negative and first print from the Hubble digital frames.

An Award-Winning IMAX® Super Short from the home of the Hubble Space Telescope

A wonderful confluence of events has given the team that operates NASA’s Hubble Space Telescope a unique opportunity to display Hubble’s universe on the biggest of screens.

In March of 2002, during the final completed flight of the Space Shuttle Columbia, astronauts installed the Advanced Camera for Surveys aboard Hubble. This new instrument is now providing images of such resolution and clarity that large-format film screens are an ideal medium for displaying them.

With the generous support of David Keighley Productions 70MM, Inc., an IMAX company, we have created Hubble: Galaxies Across Space & Time, a journey across 9 billion years of cosmic history that takes a mere 2 minutes 51 seconds, short enough to be spliced into the “trailer space” before a main feature.

Awarded “Best Short Feature” in the Large Format Cinema Association’s 2004 Film Festival, the film is available for showing at institutional IMAX theaters in the USA and Canada at no cost. At the moment we have 5 prints available. We have the soundtrack on DTAC disc; other soundtrack formats can be procured at cost directly from the IMAX Soundtrack Mastering Facility.

The highlight of the film is a fantastic computer-generated flight through a field of over 10,000 galaxies that takes audiences on a journey back through time to an era when galaxies were newly formed. Viewers will see the universe as it appeared when it was young.

These galaxies were photographed by Hubble as part of the Great Observatory Origins Deep Survey (GOODS) project. The original source image contains over 600 million pixels. Hubble scientists and imaging specialists worked for months to extract individual galaxy images, placing them in a 3D model according to their approximate true distances as determined by ground-based photometric redshift data.

Wednesday 30 October 2013

Saturn Science in 2013

Completing a trio of posts on the state of giant planet science in 2013, based on notes, tweets and abstracts submitted to two large conferences, EPSC in London and the DPS in Denver.  Late 2013 finds Saturn's northern springtime hemisphere (four years after the equinox, and four years until summer solstice) continuing to recover from the major 2010-11 storm, with the continued presence of the stratospheric 'beacon' (see 'Saturn's Stratospheric Vortex') and tropospheric anticyclone (see 'Saturn's Storm Vortex Survives!') spawned by the storm, and observations of the north polar hexagon and polar cyclone as they come into view from Earth (see 'Saturn's Hexagon Viewed from the Ground').

Cassini Returns to Saturn's Poles
Cassini ISS stares down into the heart of Saturn's north polar
cyclonic vortex.

My own research (presented at EPSC) concerned new observations of Saturn's poles by Cassini, now that the spacecraft has returned to an inclined orbit around the gas giant.  I used thermal spectroscopy from Cassini/CIRS to show the continued presence of hot spots at both poles of Saturn, which correspond to cyclonic vortices at both poles irrespective of season.  We also showed the cooling of the south polar 'autumn' stratosphere and hints of a warming north polar stratospheric vortex, as atmospheric temperatures, composition and clouds change in response to the increasing sunlight in the northern hemisphere.  Saturn's south pole is now in the darkness of approaching winter, and Momary et al. (DPS) show that the hot cyclone still corresponds to a hurricane-like vortex with a cloudless 'eye' lying deeper than the surrounding cloud decks.  

The north polar hexagon is now readily visible in spring sunlight, with new results from Cassini/VIMS (Momary et al., DPS), Cassini/ISS (Sayanagi et al., DPS) and ground-based observations (Sanchez-Lavega et al. EPSC) suggesting that the hexagon is slowly rotating westward in longitude, rather than being stationary as we previously thought.  On the other hand, ground-based amateur observations presented by Delcroix et al., (EPSC) were able to resolve the hexagon vertices throughout 2013, but were not accurate enough to see any slow drifts.  Sanchez-Lavega et al. suggest that the hexagon has the most stable rotational period of any feature on Saturn, making it an excellent candidate for constraining the deep internal rotation rate of the planet.  Finally, Momary et al. showed discrete clouds racing around the edges of the hexagon, and a massive storm residing just poleward of the hexagon system, which seems to have become increasingly cloudy since 2008 and could be a 'shepherding storm' for the hexagon.

Saturn's northern hemisphere as seen from above
on October 10th 2013.
Seasonal Saturn

Scientists studying Saturn are interested in far more than the polar latitudes, and Cassini continues to provide a unique opportunity to study the seasonal evolution of a gas giant.  Sinclair et al. (EPSC) and Li et al. (DPS) find intriguing differences in Saturn's temperatures, winds and composition between 2009/10 from Cassini and 1980/81 from Voyager, exactly one Saturnian year earlier.  Edgington et al. (DPS) showed that Saturn's moving ring-shadow influences photochemistry and haze content during a year; Guerlet et al. (EPSC) and Sylvestre et al. (EPSC) used limb spectroscopy from Cassini to look at Saturn's evolving stratosphere, which will nicely feed into the new global climate model being developed by Spiga et al. (EPSC) for Saturn's middle atmosphere.  Orton et al. (DPS) focussed on a type of wave activity, namely slowly moving thermal waves observed on Saturn by Cassini and ground-based infrared observations since 2003.  These are very large-scale waves, and the strongest wave activity was discovered between 30-45S and 0-30N during southern autumn, with a strong correlation between tropospheric and stratospheric wave activity. They also found that that Saturn's waves occurred in trains over a limited longitude range but with little apparent correlation with known atmospheric storms (one possible cause of the waves), and with peak activity just before Saturn's northern spring equinox in 2009.

Over a duet of talks, Barth and Rages (DPS) describe Cassini observations of Saturn's limb at high phase angles and spatial resolutions of around 10 km, showing the presence and structure of particulates in Saturn's stratosphere.  The hazes are likely a mix of material, including solid organics formed as a result of methane photolysis and electron deposition, as well as water condensation and hydrocarbon ices (e.g., butane, diacetylene).  At even higher altitudes, Koskinen et al. (DPS) use Cassini/UVIS solar and stellar occultations to probe the temperatures of the thermosphere, finding exospheric temperatures (an exobase 2700-3000 km above Saturn's 1-bar level) ranging from 370-540 K and increasing from equator to pole by 100-150 K, consistent with auroral heating being redistributed to lower latitudes by some as-yet uncertain circulation.
Aftermath of the Great Storm

Finally, researchers are still using the wealth of Cassini and ground-based data from the springtime storm of 2010-11 to understand the processes governing Saturn's meteorology.  Sromovsky et al. (DPS) summarise their recent Icarus paper on Cassini/VIMS reflected sunlight imaging of the storm system, finding an aerosol population of ammonia and water ice, with some ammonium hydrosulphide as a third component.  This is the first identification of water ice on Saturn, required to improve the spectral fits around the 3-µm ammonia ice features, and supports the idea that the storm was powered by strong convection from the 10-20 bar depths of the water cloud.  Fouchet et al. (EPSC, DPS) followed up on our Cassini study of Saturn's stratospheric vortex (the beacon), using the TEXES instrument on the IRTF in July 2011 to determine the vertical structure of temperatures near the vortex, probing higher altitudes than Cassini could sense.  Finally, Li et al. (DPS) suggested that the 20-30 year periodicity of these enormous storms is caused by a prohibition of strong convection when the troposphere is warm, and the presence of water makes the column 'heavy'.  As the troposphere cools below some critical point, convection can begin and produce a warm column that overshoots into the stratosphere.  The resulting large-scale atmospheric adjustment causes ammonia vapour to condense and precipitate out as snow, causing the high brightnesses observed by the Cassini/RADAR teams at microwave wavelengths (indicating a depletion of ammonia).  However, it remains unclear why these eruptions should occur only at certain latitudes, but it certainly sounds like progress.

Finally, my own use of the IRTF/TEXES instrument back in February 2013 yielded the first results on the origins of nitrogen on both Jupiter and Saturn, in my second presentation at EPSC.  Although it's still a work in progress, the initial results seem to suggest that Saturn's primordial nitrogen came from the same place as Jupiter's..... so watch this space as I get the article written!

Key 2013 Saturn Papers

Monday 28 October 2013

Jupiter Science in 2013

Continuing the theme of my previous post on ice giant science in 2013, this time I'll take a look at the latest Jupiter studies presented at the European Planetary Science Congress (EPSC, September) in London and the Division of Planetary Sciences Meeting (DPS, October) in Denver.  I've cobbled these results together from tweets, abstracts, papers and conversations with colleagues, and any mistakes or omissions are my own!  And my apologies if your favourite result isn't here, I never meant to be totally comprehensive...

Damian Peach's image of a double shadow transit of Io
and Europa, October 5th 2013.
2013 found Jupiter in a more 'normal' state than in 2012, with the planet in the late stages of a 'global upheaval' in its banded structure, characterised by the episodic fading (whitening) of the typically red-brown belts, followed weeks or months later by dramatic storm-like revivals of their intense colours.  The current life cycle started with the fade and revival cycle of the South Equatorial Belt (SEB) between 2009 and 2011 (the chaotic rifting activity northwest of the Great Red Spot is now present and active), and most recently we've seen a revival of the North Equatorial and Temperate Belts (NEB and NTB) in 2012.   Amateurs are tracking a strange light patch to the east of the Great Red Spot, possibly a cyclonic oval within the SEB.  As usual, head to the Jupiter Section of the British Astronomical Society for apparition reports and more details.

The Colourful Clouds of Jupiter

The episodic changes in Jupiter's global appearance offer a unique glimpse into the processes responsible for generating the cloud colours and albedo patterns, both for entire red-brown belts and also for discrete red features.  The chromophore responsible for the Great Red Spot's distinctive colour remains a mystery because of difficulties in identifying unique signatures of any particular chemical, but several authors are making headway.  Simon-Miller et al. (DPS) presented Hubble Space Telescope images of an intense red cyclone visible in 1994-95, suggesting that the Great Red Spot, intense red ovals and North Equatorial Belt might indicate the presence of the same chromophore but under different conditions (e.g., different amounts of mixing with white clouds, or longer UV irradiation at high altitudes), with multiple components involving NH4SH (one of Jupiter's condensate clouds) and hydrocarbons (produced photochemically above Jupiter's clouds and then sedimenting downward) required to reproduce the spectra. Chanover et al. (DPS) presented work in a similar vein, exploring 300-1000 nm spectra of Jupiter acquired from Apache Point Observatory, whereas Bjoraker et al. (DPS) presented high-resolution 5-µm spectra of CH3D lines to probe structural differences between cloud-free 'hot spots' and cloudy vortices like the Great Red Spot, indicating that a water cloud near 5 bar is responsible for the main opacity of the Great Red Spot at 5 µm, and not the ammonia (0.7 bar) or NH4SH (2-3 bar) clouds we might have otherwise expected.  Conversely, their hot spot observations were consistent with a complete absence of the water cloud.

Giles et al. (EPSC) focussed on the processes responsible for the revival of Jupiter's reddish SEB colours, using 7-25 µm imaging of Jupiter's thermal emission from the VLT to determine the deep warming taking place during the revival and subsequent sublimation of the white 'faded' aerosols.  Furthermore, Tejfel et al. (DPS) used visible-light spectroscopy to show the higher density of scattering aerosols when the SEB was faded, likely due to enhanced ammonia ice condensation as the SEB cooled during the fade.  Pulling this infrared thermal emission (the long-wave) together with the changes in visible albedo (the short wave) will be a crucial element for understanding the cloud decks of Jupiter and the origins of the colours, but there's still a long way to go.

Making Waves

For atmospheric changes on Jupiter over even shorter timescales, Hueso et al. (EPSC) presented estimates of the impact flux into Jupiter's atmosphere based on the three recent bolide events (i.e., impact flashes) seen between 2010 and 2012.  These impactors were in the 5-20m diameter category and released similar amounts of energy to the Chelyabinsk meteor earlier this year.  Hueso et al. estimate 18-160 impacts of this nature per year on Jupiter.  Pond et al. (DPS) used numerical models of jovian impacts (the ephemeral flashes and observations of Shoemaker Levy 9 and the 2009 'Wesley' impactor) to study the propagation of shock waves through the atmosphere.

Impact flashes on Jupiter.
Rogers et al. (EPSC) report on the possible influence of a planetary-scale wave on Jupiter's south temperate belt (STB, bounded by jets at 26-29S and 36S and home to Oval BA), where there are always 2-3 structure sectors of small-scale turbulence, one of them headed by Oval BA at its eastern end.  Oval BA is typically followed by a dark segment that sometimes contracts to form a cyclonic oval (a dark barge).  From Rogers:  "The other structured segments begin as  small dark spots or streaks remote from oval BA, then expand, and eventually catch up and merge with  the dark segment at BA, inducing intense disturbance in and around it. This cycle has been completed three times in 15 years, maintaining at least 2 structured sectors at all times. The major changes in drift rate of oval BA appear to be due to the impacts and subsequent shrinkage of the structured segments." Interestingly, Rogers predicts that Oval BA will shrink in the coming years so that it no longer controls the dynamics of the STB, so the STB cyclonic segments will develop a long-lived pattern, with the spaces between developing into anticyclonic circulations that may herald the next generation of larger, anticyclonic white vortices (i.e., the state of the STB before 2000).

Comparing the Giant Jets

Liu et al. (DPS) presented numerical models seeking to explain why Jupiter has narrower and weaker atmospheric jets than Saturn's broad and strong jets.  Although the radii, rotation rates and atmospheres of the two worlds are rather similar, Jupiter has 15-20 off-equatorial jets with speeds of around 20 m/s at the cloud tops, whereas Saturn has only 5-10 wider off-equatorial jets, with speeds of around 100 m/s.  Both planets have strong super-rotating equatorial jets, and vortices fill the spaces between the jets.  Liu suggests that Jupiter's jets experience stronger magnetohydrodynamic drag in the planetary interior than on Saturn.  Heavens et al. (DPS) looked at why Jupiter's jet structure disappears poleward of 65 degrees (and vortices come to dominate the flow, referred to as 'polar turbulence'), whereas on Saturn the organised jet-like structure extends all the way to the pole.  They suggest that the stability of the jets is the main criterion for the transition to polar turbulence rather than jet-dominated flow.

A Selection of 2013 Jupiter Papers

Wednesday 23 October 2013

Ice Giant Science in 2013

September and October saw three major planetary science conferences taking place - the European Planetary Science Congress (EPSC) at UCL in London; the Division for Planetary Sciences (DPS) meeting in Denver, Colorado; and a Uranus after Voyager conference in Paris.  Sadly I couldn't make it to the latter two, but the increasingly large number of 'space tweeps' meant I could follow along in a virtual sense via Twitter, even asking the odd question from afar when necessary!  Although this means missing all those discussions over coffee, the poster sessions and the lively Q&A sessions with the speakers, it meant I could still get the gist of the new giant planet system research being presented by my friends and colleagues.  With Cassini presently executing high-inclination orbits around Saturn; a strong showing for outer solar system exploration for ESA's L-class science proposals; Juno en route to Jupiter in 2016 (it swung by Earth for a final gravitational assist in October) and the ESA JUICE mission gathering speed to its 2022 launch, there's plenty to be excited about.  In this first post, I'll focus on the latest news from the Ice Giants, Uranus and Neptune.  Uranus' northern hemisphere is emerging into spring sunlight after the northern spring equinox of 2007, with the atmospheric dynamics, brightness and polar banding responding to the increased sunlight.  Neptune's Southern Hemisphere remains in summer sunshine after the 2005 equinox, with the northern winter hemisphere hidden from view.  2013 saw the emergence of an extremely bright storm feature at 45S being tracked by amateurs and professionals alike.

Uranus from the Great Observatories
Uranus from Damian Peach (2013)

In 2013 Uranus and Neptune are truly within reach of the most talented amateur observers, with observers able to see the polar collars of Uranus in strong methane absorption (e.g., this image from Damian Peach) and track distinct bright storm-like features on Neptune (e.g., the excellent blog from Christophe Pellier).  However, in the absence of any dedicated ice giant missions, today or in the near term, we rely on the resources of the 'great observatories' to reveal new insights into the composition of Uranus and Neptune.  Cavalie et al. (DPS & EPSC) used the Herschel Space Observatory to detect CO in the Uranian atmosphere, using the sub-mm spectra to understand the potential origins of this stratospheric species - material from icy rings/satellites, interplanetary dust or potentially large cometary impacts (such as CO in Jupiter's atmosphere after the Shoemaker Levy 9 collision, Bezard et al., 2002). Fry & Sromovsky (DPS) report Hubble observations of Uranus from September 28th 2012 using STIS to measure spectra from 0.3-1.0 µm now that the north pole is coming into view (we passed Uranus' spring equinox in 2007).  These observations indicate that methane depletion at mid-to-high latitudes in Uranus' troposphere is symmetric about the equator - i.e., it's not seasonally-forced like the atmospheric brightness or discrete cloud features.  This high-latitude depletion of methane has implications for how Uranus' atmosphere redistributes material from equator to poles.  Finally, Moses et al. (DPS) interpret the equinoctial Spitzer observations of Uranus by Orton et al., using a photochemical model to interpret the relative abundances of the soup of hydrocarbon and oxygenated species in Uranus' stratosphere.

Uranus in July 2012 from Keck II telescope.
...and from the Ground...

Improvements of adaptive optics (i.e., giving better spatial resolution) and spectral resolution in diagnostic wavelength bands means that ground-based observatories also plug the remote sensing gap for the ice giants.  Irwin et al. (EPSC) presented near-IR images and spectroscopy of both ice giants from Gemini and VLT in 2009, finding similarities in the scattering properties (and hence composition) of their main 2-3 bar cloud decks, and near-identical deuterium-to-hydrogen ratios suggesting that these two worlds shared a rather similar origin.  Tice et al. (EPSC) used an integral field unit (SWIFT) with Palomar's AO system to produce high-resolution 0.65-1.0 µm spectra of Neptune, using them to investigate the latitudinal distributions of clouds and methane on the most distant ice giant.  Roman et al. (DPS) presented H- and K-band images and spectra of both planets from Palomar between 2001 and 2007, using them to determine the distributions of clouds, aerosols and para-hydrogen (a tracer of atmospheric motion and chemical equilibriation).  de Pater et al. (DPS) report a multi-wavelength campaign of near-infrared, thermal-infrared and microwave observations of Neptune from Keck and the VLA in 2003, particularly focussed on the warm temperatures and volatile depletion near the southern summer pole.  In a similar vein, Norwood et al. (DPS) use microwave observations to determine the chemical abundance of Neptune's troposphere (e.g., the volatiles H2S and NH3).  Iino et al. (EPSC) used 2010 observations from the 10-m NOAJ Atacama Sub-Millimetre Telescope Experiment (ASTE) to measure CO, HCN and CS (not detected) in Neptune's atmosphere, and show that the ratio of CS to CO is 300 times smaller on Neptune than on Jupiter, casting doubt on ideas of cometary origins for Neptune's atmospheric composition.

Beyond the observations, numerical models are attempting to understand how an ice giant atmosphere circulates, and how this differs from the gas giants.  Sussman et al. (DPS) explored the dynamics of planets with axial tilts exceeding 54 degrees (i.e., the poles receive a greater insolation than the equator when averaged over a year), and showed how the fine balance between thermal gradients and Rossby wave generation and propagation governs whether you'll get eastward or westward jets at mid-latitudes. The remarkably uniform temperature gradient on Uranus suggests that the mechanisms transporting heat latitudinally are rather efficient.  Kaspi et al. (DPS) report on their recent Nature article, using the gravitational fields of the ice giants (particularly the fourth gravity harmonic determined from Voyager and HST observations) to constrain the atmospheric 'weather' to the outermost 0.15% of the mass on Uranus and 0.2% on Neptune (i.e., a thin layer no more than 1000 km thick).  Finally, Friedson (DPS) discusses reasons why the heat flux from an ice giant might be small - water is sufficiently abundant in its condensation zone that both 'ordinary' and 'diffusive' convective transport are inhibited (i.e., the layer is stable), limiting the transport of heat to nothing more than a weak, oscillatory convection which does a poor job at moving energy upward through these atmospheres.

Looking to the Future

With ESA's next round of large-class cosmic vision proposals just around the corner, interest in an ice giant mission was strong throughout 2013, with three white papers (Uranus Pathfinder by Arridge et al., a Neptune mission from Masters et al., and the dual ODINUS concept from Turrini et al.) submitted for the call for science themes, not to mention my own efforts with Olivier Mousis for an ice giant entry probe.  Along with several others, I helped organise a 3-day workshop in Paris in September called "Uranus Beyond Voyager 2" - the rich discussions from this meeting were summarised in a recent blog post by Geraint Jones.   Hopefully the community will build on these new scientific results and collaborations as we prepare to respond to new ice giant mission calls in 2014 and beyond!

Saturday 27 July 2013

Back to Civilisation

Saturday July 27th, New York City
40.7N, 74.0W

The last 24 hours were probably the most challenging of the entire voyage from Southampton to New York.  As we came onto the continental shelf a few hundred miles off Boston, and travelled on a rhumb line straight into New York harbour, a force 7 gale was blowing Queen Mary 2 from side to side, creating the roughest seas that we'd encountered on the whole journey.  My final lecture, on the Cassini exploration of the Saturn system, was one of the most challenging I've ever given.  I had to adopt a wide stance, clutching the podium as the lecture theatre pitched from side to side.  This was followed by two live planetarium shows, giving a much smaller audience a tour of the night sky from the comfort of the auditorium, and making the most of the superb system onboard QM2. I received lots of warm feedback at an informal 'meet the speakers' event in the bar that afternoon, and I hope that people learned at least one thing from each of the lectures I gave!

From noon on Friday, when we heard our last navigational announcement from the Commodore at 40.3N, 67W, we travelled the final 280 miles to New York.  The ship was relatively quiet that evening as people packed and prepared for the early morning.  At 04:30 am we passed under the Verrazano-Narrows bridge with our narrow clearance, and by 5 am we were staring out at Manhattan at dawn, with the Statue of Liberty out to the port side and our final destination, Brooklyn cruise terminal, on the starboard.  It was our first glimpse of land and civilisation in almost 7 days, and it was with a mix of emotions that we realised the relaxed and peaceful days at sea were over.  Tugs moved us into place, and we were alongside shortly after 7am.  I bade farewell to the Queen Mary 2, and saw her again later in the day from Battery Park, Lower Manhattan, as she departed for her 202nd crossing of the Atlantic Ocean.

Friday 26 July 2013

Cassini-Huygens Exploration of the Ringed Planet

Friday July 26th, 11am

It’s our final day at sea, and the vast bulk of the Atlantic Ocean is now behind us, with anticipation for our arrival in New York City tomorrow morning.  The fourth and final lecture in this Royal Astronomical Society series concerns an enormously successful robotic explorer in the distant solar system, the Cassini-Huygens mission to Saturn.  Many of the topics I’ll describe have been the subject of previous posts on this blog, so I’ll refer the interested reader to those for more details.  For example, I begin with a brief history of observation of the Saturn system, from the strange appendages observed by Galileo, to the explanation of the rings and the discovery of enigmatic Titan by Christiaan Huygens; Jean Dominique Cassini’s discoveries of a division in the rings and a multitude of icy satellites; and William Herschel’s in depth observations of Saturn’s changing appearance.  Saturn’s first human-made satellite was named for Cassini; and Titan’s first entry probe was named for Huygens, honouring their pioneering studies of the ringed planet.

The Mission

The Cassini mission was born of the early 1980s, as one of the first fully international robotic space missions, a collaboration between NASA and the European Space Agency.  Indeed, when budgets were being slashed it was the international aspect of this mission that probably saved it.  It launched in 1997; spent seven years in the frigid depths of space and finally arrived in orbit around Saturn in 2004 just after the northern winter solstice.  It has been working out there on its lonely orbit for almost a decade now – following completion of its four-year primary mission; it’s lifetime was extended and extended and we now hope it will survive through until 2017, northern summer solstice.  That is, the spacecraft will have observed all four seasons at Saturn, from winter to spring, summer and autumn, providing arguably our most comprehensive picture of a giant planet ever obtained.  Ignoring the wonderful science from this mission for just one moment, the Cassini mission is a triumph of engineering – a spacecraft almost 7-m long and 4-m wide, weighing 2150 kg and powered by the radioactive decay of Plutonium-238.   Hydrazine rocket thrusters allow the spacecraft to be agile and turn to point its suite of instruments at the variety of targets, but it is this which limits the lifetime of the mission – once the hydrazine supply is exhausted, the mission will be over.

Saturn and its Rings

This lecture gives a whistle-stop tour of some of the discoveries of the Cassini-Huygens mission.  Starting from the gas giant itself, we’ll talk about the different atmospheric layers, what we think is present deep down inside the planet, and what forces drive the weather we see.  When you observe Saturn through a telescope, you’re seeing light being reflected from the clouds and hazes in the upper atmosphere; possibly fluffy clouds of ammonia ice rather than the water clouds we observe in our own atmosphere.  Those clouds are blown around the planet by winds racing east and west.  We’ll talk about small storms and lightning (including the southern storm alley), and the seasonal eruptions of globe-encircling storm systems that endure for many months.  One such storm erupted in 2010, and its aftermath is still being felt today.  Cassini is currently in a high inclination orbit, allowing it to gaze down at the poles of the giant planet to explore the mysterious hexagonal wave around the North Pole, and the twin hurricane-like cyclones churning like giant plugholes at both poles.  It has also captured movies of the aurora dancing in Saturn’s high atmosphere in response to pressures from the solar wind and plasmas being injected within the planets magnetic field environment.

Saturn’s delicate and beautiful rings serve as a wonderful laboratory for gravitational interactions.  We’ll describe the ring structure, their potential origin and the process of continual renewal and recycling.  Discrete features like elusive spokes in the B ring; structures towering above the ring plane and casting shadows back across the main rings; shepherding moons generating wakes in the rings and beautiful gravitationally sculpted structures seen by Cassini’s high-resolution cameras.  Beyond the rings, a vast array of unique satellites orbit the giant planet, including Phoebe (a remnant of solar system formation captured by Saturn’s immense gravity); Hyperion (with the appearance of a sponge); Mimas (with the huge Herschel crater making it look suspiciously like a science fiction icon); and Iapetus (with it’s asymmetric brightness and enormous equatorial ridge).  Tiny Enceladus, with its four south polar fissures actively venting ice and gas into space, is intricately connected with Saturn’s diffuse E-ring and a truly remarkable discovery.

Enigmatic Titan

The jewel in Saturn’s crown is this enormous satellite, shrouded in a thick smoggy orange atmosphere and the second largest moon in our Solar System.  Although Voyager had captured images of Titan thirty years ago, the thick hazes were impenetrable, preventing any glimpses of the surface of the moon.  What would we discover there, and what might this unexplored terrain look like? The mystery of Titan was one of the driving goals of this mission to the Saturn system.  Imaging systems on the Cassini orbiter provided access to wavelengths where the hazes are transparent, providing glimpses through the smoggy atmosphere.  Radar swaths have shown the undulating terrain, from dunes to impact scars, mountain ranges and river valleys.  And the Huygens probe, designed by the European Space Agency, became the first human-made object to touch down on Titan in January 2005.  At that time no one knew what we might be landing on, from solid surfaces, to hydrocarbon sludge, or even into a vast ocean of liquid methane and ethane.  The probe descended beneath a parachute, being buffeted by the winds until coming to rest in a dried up river bed; pebbles of water ice appearing rounded by the flow of fluids across the surface, and hints of moisture in the upper layers of the soil.  It’s too cold on Titan for that fluid to have been water – instead, methane is a fluid at these temperatures, and a ‘methane-cycle’ on Titan mimics the ‘water-cycle’ on Earth, with methane clouds, methane rain, methane rivers and lakes.  Evidence for this methane cycle can now be seen everywhere on Titan, and the river networks and lakes reveal a very Earth-like world.

One of Cassini’s most enticing discoveries is that of lakes and seas of hydrocarbons at the northern pole of the planet.  For the first time in the history of our solar system exploration, we have seen standing bodies of liquid on another planet, and can envisage what it would be like to sail those Titanian seas.  We have the technology to do so, and could one day see a long-live vessel bobbing on the hydrocarbon sea, floating in and out of different drainage deltas and looking back at the hills, valleys and mountains on the Titanian shore.  It’s a lovely idea, and one I hope we’ll one day see with our robotic explorers.

The Future

By 2017, the Cassini mission will have completed an in-depth orbital reconnaissance of the whole Saturn system, and it will be time to draw this hugely successful mission to a close.  It cannot simply be left in the Saturn system to potentially collide with, and contaminate, any of the pristine environments to be found on the satellites.  So it must be disposed of by burning up in Saturn’s atmosphere, a dramatic fireball at the end of the mission.  But before that happens, mission planners have designed a dramatic series of final orbits, taking more risks with this grand old spacecraft.  It’ll be flying closer to Saturn than ever before, within the rings and skimming just above the cloud tops.  It’ll be measuring the strength of the close-in magnetic field, and also the gravitational parameters of the planet itself, using these to probe down to the centre of the planet at depths we’ve never seen before.  It’s been described as a new mission, a new lease of life for this ageing spacecraft, and contributing even more to our knowledge of the Saturn system.

As we were preparing to embark on our passage to New York, on Friday July 19th at around 21:30, the Cassini spacecraft trained its powerful cameras back onto the inner solar system, to capture another stunning image of the Earth and moon system, as seen by a lonely robotic explorer a billion miles away in orbit around Saturn.  While we’ve been sailing, that image was beamed to Earth, assembled by computers in California and processed to show the results to the world.  All of humanity, once again sharing just a few pixels, and showing how small, how fragile, and how precious our home world truly is.  It seems like the perfect place to end this series of lectures on board the Queen Mary 2. 

Thursday 25 July 2013

Gulf Stream and Star Watching

Thursday July 25th, 40.8N, 57.7W

The ocean around us today has changed from the green and rather forbidding cold, foggy ocean of the Labrador Current to the warmer, indigo blue colours of the Gulf Stream.   A fresh wind is blowing, but we have clear blue skies and a beautiful blue sea.  The depth under our keel is 5000 m, deeper than at any other point in this journey, as we pass within 200 nautical miles of Sable Island, Canada, a small, narrow crescent-shaped sand island around 13 square miles in size and home to only a handful of people.  We’re now 2408 miles from Southampton, with only 745 miles to run to New York on a straight line.  The Commodore describes this area as the region of strongest hydrographic contrast in the world, as the ocean temperature has changed by 10 degrees from the cold arctic currents of yesterday to the warm Gulf Stream today.  But the Gulf Stream itself is complex and inconsistent, with whorls, eddies, gyres all changing the speed and temperatures as we make our passage, and showers and squalls are expected for this evening.  We can also spot brown clumps of weed floating in the open ocean, Sargasso weed from the Bahamas being pushed north by the Gulf Stream and appearing like brown clumps of grapes in the seawater.

It is this changeable weather that has prevented us from hosting star parties from the upper deck on this particular crossing – the entertainment folks need time to advertise these events, and rely on weather forecasts.  The full moon and long summer days also mean that conditions are best late at night, when they want to minimise disturbance to guests.  That said, I’ve been doing my best to point out to interested guests Venus on the western horizon after sunset; the summer triangle (Deneb, Vega and Altair; in Cygnus, Lyra and Aquila); star hopping from the Plough to Polaris, Arcturus and Leo; as well as Cassiopeia and her entourage east of North at 10-11 pm (Cepheus, Andromeda, Perseus, etc.).  The great square of Pegasus should be on the horizon to the northeast.  Saturn is a good object for viewing in the early evening too towards the South West in the Constellation Virgo; and Jupiter is rising just before dawn.  We’ll talk about all of these in Fridays’ live planetarium shows, but please do stop me to ask me if you’re out on the upper deck this evening!