Future Exploration of the Outer Planets

On Friday December 14th the Royal Astronomical Society hosted a one-day discussion meeting on the future exploration of the outer solar system, organised by myself and Chris Arridge.  The details of the meeting can be found below, along with the list of speakers from the UK and overseas.  More details will appear in this blog after Christmas, and the outcome of the meeting will be described in an article for Astronomy and Geophysics in 2013. The meeting was a great success, with a broad range of topics covered and plenty of discussions over coffee about how we might push for future exploration of the outer solar system!

RATIONALE:

Exploration of the giant planets of our solar system over the past several decades has revealed four unique, complex and dynamic worlds.  They serve as natural planetary-scale laboratories for the fundamental physics and chemistry at work throughout our solar system, and can be viewed as miniature solar systems in their own right.  Our understanding of these planets remains in its infancy, but the four giants serve as templates for the interpretation of exoplanetary systems being discovered throughout our galaxy.  The purpose of this Royal Astronomical Society Specialist Discussion Meeting on December 14th 2012 is to bring together experts in giant planet systems to identify the key science questions for future exploration of the outer solar system.  The themes of this meeting include:

1. •Drivers for exploration of the Jupiter system by Juno and JUICE – open questions and the scientific potential.
2. •Orbital exploration of an Ice Giant planetary system – science questions and technological feasibility.
3. •In situ exploration of giant planet atmospheres (probes) and satellite surfaces (landers) to provide a window onto the formational history of our solar system.
4. •Future space-telescopic observations of our solar system in the post-Hubble/Spitzer/Herschel era (e.g., capabilities of JWST and the need for planetary spectroscopy).

Abstracts concerning recent research and open questions on the present state of these planets (dynamics, chemistry and vertical structure); their temporal evolution and coupling between their atmosphere, interior, magnetosphere, satellites and ring systems are also welcome.   This meeting will explore the rationale and drivers for outer solar system exploration in the coming decades, and discuss the possibilities for future telescopic and spacecraft missions.   Further information on the meeting (plus details of how to get to Burlington House) can be found at http://www.ras.org.uk/events-and-meetings/ras-meetings

Keynote Speakers

1. •Mark Hofstadter (JPL) - Missions to the Ice Giants
2. •Michele Dougherty (Imperial) - Exploration of the Jovian System and JUICE
3. •Olivier Mousis (Toulouse) - Formation Processes in the Outer Solar System

Programme:  Friday December 14th 2012

Keynote talks will last 30 minutes (including 5 min discussion), all other talks will last 20 minutes (including 4 min discussion).

10:00 - 10:30 Arrival and Coffee

10:30 - 13:00 Morning Session

10:30 - 10:40 Intro & Aims (10 mins) - Leigh Fletcher, Oxford

10:40 - 11:10 Prospects for a NASA-led Ice Giant Mission (Keynote) - Mark Hofstadter, NASA/JPL

11:10 - 11:30 Uranus Pathfinder - Chris Arridge, MSSL

11:30 - 11:50 Gas Giant Ionospheres and Aurora - Tom Stallard, Leicester

11:50 - 12:20 Origin of Uranus and its Satellite System (Keynote) - Olivier Mousis

12:20 - 12:40 Radioactive Power Sources and Technology Development - Richard Ambrosi, Leicester

12:40 - 13:00 Outer Solar System Technology Development - Matthew Stuttard, Astrium

13:00 - 14:00 Lunch

14:00 - 15:30 Afternoon Session

14:00 - 14:30 Exploration of the Jovian System and JUICE - Michele Dougherty, Imperial

14:30 - 14:50 Future Exploration of Titan - Mark Leese, Open University

14:50 - 15:10 Instrumentation for in-situ measurements of the gas giants and their satellites - Andrew Morse, Open University

15:10 - 15:30 Exploration of Planetary Rings - Carl Murray, QMUL

15:30 - 16:00 Tea at the Geological Society

16:00 - 18:00 RAS Monthly A&G (Ordinary) Meeting

18:00 - 19:00 Drinks Reception (in the RAS' Burlington House Apartments)

Further Details:

Recent observations of the gas and ice giants have revealed complex evolving systems, from (i) short-term variability (comet/asteroidal impacts on Jupiter, giant planetary-scale storms on Saturn, discrete features on Uranus and Neptune, dynamic moon activity at Io and Enceladus), (ii) medium-term changes (the life cycle of Jupiter's South Equatorial Belt, seasonal storms on Saturn, seasonal effects on natural satellites, seasonal changes in ring systems and their atmospheres/ionosphere) and solar cycle variability in the magnetosphere.  The seasonally-induced hemispheric asymmetries, polar vortices and equator-to-pole contrasts on Saturn, Uranus and Neptune allows us to study generalised seasonal and polar phenomena without the complicating terrestrial influence of topography.  Temporal variability within the weather layer may provide key diagnostics of processes occurring in regions inaccessible to remote sensing, within the deep troposphere and planetary interior.   The diverse satellite and ring systems host a plethora of unique environments that have yet to be fully explored, from Io’s volcanoes, the sub-surface oceans of Europa, Ganymede and Callisto, the plumes of Enceladus and Triton and the thick atmosphere of Titan.  And finally, the magnetospheres of the giant planets act as giant particle accelerators (from plasma to dust), exhibit pulsar-like behaviour (Jupiter) and interact with the rest of their planetary environments in complex ways via a range of mass, energy and momentum exchange processes.  This meeting aims to capture each of these diverse planetary exploration fields to discuss the rationale for future missions to the outer solar system.

Seasons in the Saturn System

Cassini gazing down into Saturn’s spectacular north polar vortex, revealed in reflected sunlight as the north pole enters spring time conditions after years of winter darkness, November 2012. Credit:  NASA/JPL

The EPSC in Madrid in September 2012 featured sessions on giant planets, satellites and seasonal processes in our solar system, all of which provided insights into some of the latest discoveries by Cassini in the Saturn system.  Cassini has been orbiting Saturn since mid winter in the northern hemisphere (when Saturn's northern clouds had a distinctly blue, Neptune like appearance).  The spring equinox was in 2009, when the sun passed from the south into the northern hemisphere for the first time in fifteen years (half of a Saturn year) and the rings were seen edge-on from earth.  Cassini will hopefully remain operating all the way to northern summer solstice in 2017, fully characterising half of a Saturnian year.

Linda Spilker, Cassini project scientist, gave a keynote talk describing some of the latest findings.  During equinox, Saturn's thin F ring remained glowing, revealing that it is slightly inclined compared to the other rings, which were only partially illuminated by Saturn shine (light reflected from the clouds below). Elevated features in the rings cast long shadows, and dusty streaks above the ring plane were evidence of recent impacts, their debris sheared out by keplerian motion and catching the sunlight.  Edge waves on the Keeler Gap, created by the tiny moon Daphnis, cast shadows due to a 4 km lifting out of the ring plane, whereas similar edge waves in the Encke Division, created by tiny Pan, had no similar vertical extension.  Bizarre propeller objects in the rings were also projected above the ring plane.  Thermal mapping showed that the rings were at their coldest at the equinox at 45K, in equilibrium with e thermal radiation from the planet itself.  Intriguingly, the corrugated nature of the rings (ripples observed at equinox) could be modelled and traced all the way back to an impact event in the rings in 1983.  A similar corrugated wave in the Jovian system could be traced back to the collision of comet Shoemaker-Levy 9 in 1994. 

Beyond the rings, the tidally-locked satellite Mimas has been shown to have an unusual interaction between the surface and the magnetospheric environment.  A dark lens shape on the leading face of Mimas coincides with a cold region, bounded by sharp warm edges that give the moon a "pacman" appearance in certain geometries.  Cassini scientists suggest that energetic electrons, travelling retrograde around Saturn, slam into the surface to cate material differences in colour and thermal inertia.  These electrons are mainly funnelled into the corner ansae, creating the lens-shaped appearance.  On the trailing hemisphere, cold plasma and E ring grains cause erosion.  Amanda Hendrix also talked about the UV darkening of water ice due to photolytic creation of hydrogen peroxide, produced in the summer but destroyed in winter by electron bombardment, leading to seasonal asymmetries in UV brightness of the moons.  They predict that the northern hemisphere will darken during southern summer as more and more of this H2O2 is produced.

Alice le Gall reported evidence of seasonal variations on Iapetus, the solar systems most distant tidally-locked moon.  Its inclined orbit makes Iapetus difficult to reach, but also means that the moon passed equinox back in 2007 before the rest of the Saturn system.  Passive radar scans of the Cassini Regio showed that the north was cooler than the south, despite more illumination in the north, and suggestive of heat buried in the upper few centimetres of the moon during the long summer season.  Beneath about 1 m, however, the temperature is just symmetric about the equator.

On Saturn itself, we have been gripped by observations of the gigantic springtime storm system since 2010.  The churning tropospheric storm persisted from December 2010 to July 2011, but produced after effects in the stratosphere that persist to this day, and will be the topic of future posts when the papers are all published.  But Spilker described one interesting development - it was thought that we'd never see lightning on Saturn because of light reflected from the rings.  But the northern storm was so powerful that Cassini could observe the flashes in blue light.  Away from the storm, the slow seasons march on, with Saturn's northern blue hues now completely replaced by the familiar yellow-ochre colours we are all familiar with.  My own work is showing that northern temperatures are warming, as expected, and that the hot south polar stratosphere we reported a few years ago is showing signs of diminishing.

Saturn's enigmatic moon, Titan, continues to undergo severe changes in its atmospheric circulation, as reported by Ralph Lorenz.  When Cassini arrived, the North Pole featured a dramatic vortex, enriched in chemicals due the downward branch of a global circulation system.  That dark north polar hood has now become a distinct dark lane with a detached haze layer.  The height of the detached layer of haze itself seems to change, rising and falling depending on the strength of the meridional circulation. A similar polar hood is now seen to be developing at the South Pole, with recent images showing a dramatic vortex structure reminiscent of Venus' dipole vortex discovered by Venus Express. The whole atmospheric circulation may be shifting direction as spring progresses.

One of the most spectacular Cassini results are the recent observations of specular reflections from Titan's northern lakes, observed by the VIMS instrument at 5 microns.  Glinting sunlight from a titanian sea is a wonderful thought.  

The Cassini spacecraft is still in excellent health, 8 years into the mission and 15 years since launch.  If all goes to plan, the proximal orbits in 2016 will see Cassini closer to Saturn than ever before, flying within the rings themselves as we approach northern summer solstice.  Then, around September 2017 Cassini will plunge to a fiery death, a fitting end to an incredible mission.