Tuesday, 5 January 2016

Planetary Science PhDs at Leicester

Interested in joining us here at the University of Leicester?  Applications are now open for PhD projects within our group to explore the extreme climates of the giant planets.  You'd be joining a friendly research environment with broad-ranging expertise from the complex churning atmospheres, to the rarefied upper atmosphere and ionosphere, to the powerful magnetospheres and beyond.  Leicester is intricately involved in the Cassini mission to Saturn, the Juno mission to Jupiter, and future plans for Europe's first mission to Jupiter and its moons (JUICE), providing plenty of future opportunities in the planetary science community!  Plus you'd get to spend 3-4 years in the heart of the UK, well-connected to both the North and South from the Midlands powerhouse (Leicester, Birmingham, Warwick, Nottingham, etc.).  Please get in touch if you're interested.

This year's projects include:
Exploring Jupiter’s Climate Variability during the Juno Mission (with myself and Emma Bunce):

Revealing the Chemistry and Circulation of the Ice Giants (with myself and Tom Stallard):

...among other planetary, exoplanetary and planet formation project ideas.  Please drop me a line if you'd like any further information, deadline is February 3rd 2016.

Monday, 19 October 2015

Jupiter Weather Report: 2015/16 Apparition

[Work in Progress]

Jupiter will be intensely scrutinised over the next six or seven months to understand the state of the atmosphere immediately prior to the arrival of the Juno spacecraft in July 2016.  The spacecraft team hopes to use guidance from the citizen science record to target specific features of interest, from storms and plumes to large-scale changes in Jupiter's banded structure.  At the end of the last apparition, we were awaiting both an outbreak on the North Temperate Belt jetstream and an expansion event in the North Equatorial Belt.

Some of the first images of the apparition started to arrive in October 2015, and have once again been assembled into a glorious map by Marco Vedovato of the Italian Amateur Astronomers Planet Section.  He uses the WinJUPOS software tool to create global maps of Jupiter regularly during the apparition - his index of maps can be found here.

JUPOS map of Jupiter at the start of the 2015/16 apparition (October 15-18 2015).  Credit:  M. Vedovato.
South Tropical Domain:
The GRS remains extremely orange in colour, with chaotic activity in its northwestern wake region.

North Tropical Domain:
White Spot Z (WSZ) is still apparent on the ragged northern edge of the NEB near 19N, but a conspicuous new Red Spot can also be seen sat between the NTropZ and the NEB.  There are no signs yet of the NEB expansion event starting.

South Temperate Domain:
The chain of Anticyclonic White Ovals (AWOs) still persists in the South South Temperate Belt near 40S.

North Temperate Domain:
The northern barges on the North Temperate Belt (NTB) that were so prominent for much of the previous apparition are no longer quite so visible.

Tuesday, 29 September 2015

Jupiter Weather Report: 2014/15 Apparition

In an earlier post, I described how the army of citizen scientists provide near-continuous updates on Jupiter's atmospheric phenomena, allowing the British Astronomical Association and others to provide regular digests on jovian weather and reasonable forecasts of what might be coming.  At the time of writing (September 2015) Jupiter is a few weeks past solar conjunction and ready to make it's reappearance in our dawn skies, so the 2014/15 apparition that was centred on the February 6th opposition (when Jupiter was at its biggest and brightest) is now over.  Scientists all over the world are preparing observing proposals for the next 2015/16 apparition on Earth-based observatories, which will be the final opportunity to characterise Jupiter's churning weather before the arrival of the Juno spacecraft on July 4th 2016.  For details of the nomenclature of Jupiter's belts and zones, please refer to my previous post.

From the amateur community, the first images of the apparition were uploaded to PVOL on August 31st 2014, and the last ones on July 31st 2015.  I'm indebted to Marco Vedovato of the Italian Amateur Astronomers Planet Section for using the WinJUPOS software tool to create global maps of Jupiter regularly during the apparition - his index of maps can be found here.   Further insights into Jupiter's complex goings-on have been provided by the wonderful Hubble Space Telescope's OPAL (Outer Planet Atmospheres Legacy) project, led by Amy Simon, Mike Wong and Glenn Orton. Hubble observations of Jupiter on January 19th 2015 are available here, and a new '4K movie' released by HST can be seen below.

Jupiter as observed on January 19th 2015 by the Hubble Space Telescope WFC3 instrument.  The findings of the new dataset are described by Simon et al., 2015, ApJ, 812, 55

Below I've provided links to some of Vedovato's JUPOS maps for various points during this apparition - they prefer having north to the bottom of the images as this is how Jupiter would really be seen through the eyepiece.  The following four images were taken between October 2014 and April 2015, and you can see the evolution of features by toggling between them (all should be credited to Marco Vedovato and the JUPOS team).

South Tropical Domain:  GRS Shrinkage has Slowed?

Jupiter's Great Red Spot, sat between the SEB and STropZ, has been steadily shrinking in east-west extent for many years (we've known this for a while, despite recent news reporting!).  A couple of years ago the community discovered that this shrinkage had gone through an unprecedented acceleration, with measurements by citizen scientists suggesting a longitudinal width of 13.6±0.7 degrees in 2013/14, down from 15.3±0.8 degrees in 2011/12.  In 2014/15 this longitudinal shrinkage seems to have stopped or slowed down, with the size being the same as last year - approximately 13.8±0.9 degrees longitude according to measurements by the JUPOS team (Vedovato et al. - see his charts here).  M. Jacquesson was able to use November 2014 images to measure the internal rotation of the GRS at approximately 3.8 days, consistent with values measured in the 2013/14 apparition.

Observations from the Hubble Space Telescope (Simon et al., 2015) captured some wonderful spiralling filamentary structure within the GRS core, and indicated that the core of the GRS remains just as orangey-red as it did in the previous 2013/14 apparition, consistent with the general appearance in the amateur imaging.  It has narrowed in latitudinal (north-south) extent too, implying a less severe interaction with the SEBs retrograde jet that flows around its northern periphery.

As for the rest of the South Equatorial Belt (SEB), it appears to be business as usual - lots of complex rifting in the turbulent wake to the northwest of the Great Red Spot, and no sign of any 'fading activity' such as that last witnessed in 2009/10, when the entire SEB whitened over.  It's really hard to predict these SEB whitening cycles, so who knows when the next might begin!

North Tropical Domain:  Waiting for an NEB Expansion:

North Equatorial Belt (NEB):  The width of this dark-red belt changes over time, having shrunk during 2013 with the disappearance of the dark, distinct brown barges on its northern edge.  At the end of the 2014/15 apparition it may be on the verge of expanding again (the last such event took place in 2012), meaning that those brown barges could reappear during the next apparition.  NEB broadening events have been occurring at 3-5 year intervals since 1988, so we might expect another in the next couple of years - observers will be waiting to see if such a dramatic change in the belt/zone structure happens next year.  One of the most conspicuous features is 'White Spot Z', a white oval right on the ragged northern edge of the NEB (near 17N) that had a more reddish tinge last apparition.  This white spot can also be seen in the Hubble imaging.

The OPAL images in January 2015 captured some fine-scale waves within the NEB near 16N, a type of structure known as a baroclinic wave, not seen in Jupiter imaging since the Voyager days.  This fine wave was superimposed on top of other NEB cloud features, including a chain of cyclones at the same latitude and whiter anticyclones on the northern edge of the NEB (the retrograde jet called the NEBn).

North Temperate Domain:  Brown Barges:

The Northern Temperate Domain last underwent an upheaval back in 2012, and the cloud structures in this region still appear complex 3 years on.  In particular, there are several cyclonic 'Brown Barges' on the North Temperate Belt (NTB) near 30N:  4-5 brown barges are now evident within the NTB and are extremely elongated in east-west extent.  John Rogers suggests that rifting during the previous apparition generated an extremely dark spot in the Northern Temperate Zone (NTZ), which lengthened to become a dark streak that somehow evolved into the extremely long barge that we see today.

In general though, there were no signs of large NTB outbreaks (plumes on the jets), which occur at approximately 5-year intervals - the next one is likely to occur in 2016 or 2017, right smack bang in the middle of the Juno mission.  The NTB might have been showing signs of fading, as it does just before an outbreak, but when Jupiter disappeared from view the NTB was still unchanged...

The amateur community caught the birth of a new Little Red Spot in December 2014, just to the north (32N) of the brown barges.  It can be seen in the Hubble imaging, below.  It started as a white spot interacting with other ovals (that didn't survive), becoming larger and fawn-coloured, before appearing brighter and redder in January 2015.

Annotated version of Hubble imaging in January 2015, showing structures at all latitudes, including the brown barges of the North Temperate Domain and the newly-emerged Little Red Spot just to the north.  Credit:  NASA/ESA/Simon/Wong/Orton
South Temperate Domain:  Spots!

Oval BA continues it's slow eastward traversal of the temperate region, with a slightly weaker red colour than the previous apparitions (could the chemistry responsible for its reddening somehow be weakening, or the materials aging?).  BA passed the GRS again in the October 2014 (BA moving east, the GRS moving west), an event which happens every couple of years.  This close passage generated lots of chaotic structure in the STropZ that's sandwiched between BA and the GRS, providing insights into how these two giant anticyclones interact with one another. Intriguingly, a faint bluish region currently exists in the South Temperate Belt (STB) known as the STB Ghost - this cyclonic structure interacts with any spots that impinge upon them, but it's importance for the dynamics of the STB is still being explored.  The structured segments of the South Temperate Domain were subject of a detailed report by Rogers et al. that can be found here.

Even further south, the South South Temperature Belt (SSTB) is home to lots of anticyclonic white ovals (AWOs) - they can be clearly seen in the amateur imaging against the darker background, with more than ten of them moving towards the east over time.


The 2014/15 apparition was largely business as usual for Jupiter, with the birth and death of new small red spots; chaotic activity in the equatorial belts and numerous white ovals in the south.   The GRS shrinkage may have slowed for now, and the brown barges in the North Temperate Domain are longer than we've seen in a long time, but the really exciting changes might still be to come in the next apparition.  In his three-year forecast for Jupiter, John Rogers suggests that we're waiting for both an outbreak of plumes on the North Temperate Belt jetstream and an expansion event for the North Equatorial Belt for the first time since 2012 - who knows what we'll find when Jupiter reappears for the first 'Juno apparition' of 2015/16.

Tuesday, 15 September 2015

Towards a Jupiter Weather Forecast

Trying to keep track of the ever-changing face of Jupiter is a pretty big challenge, given that it is prone to unexpected outbursts of spots, plumes and weird meteorological activity, in addition to large-scale variations between the ever-present belts and zones.  Far from having a static and unchanging appearance, Jupiter is a dynamic world that can fascinate and surprise every time we turn our telescopes towards it.

Researchers here on planet 3 have only just begun to investigate the enormous forces and energy shaping the colourful bands that we see, and for some of us (cough cough) it's a life-long process of trying to understand what's going on deep within planet number 5.  For that ambitious goal, we'll need to throw our whole arsenal of atmospheric science at the problem - cloud microphysics and haze formation; thermochemistry, photochemistry and ion chemistry; meteorology, dynamics and circulation; and many other strands of natural science.  These are all diverse pieces of a puzzle that, when assembled into a whole, will allow us to understand the changing face of Jupiter, with implications for how atmospheres 'work' throughout our solar system.

But the starting point is an account of the phenomenology, looking for patterns and trying to explain how what we observe (changing colours and discrete spots, waves and bands) is related to the shifting environmental conditions and deep atmospheric flows.  Amateur observers, or citizen scientists, have amassed a truly incredible amount of data, an observational record that now spans many decades.  As astronomy opens up further, and the use of WebCam technology to capture 'lucky images' through our turbulent atmosphere becomes more mature, we're faced with a mountain of observational data to parse through.

Thankfully, there are dedicated teams out there doing just that, and keeping we "professionals" updated with what's changing on Jupiter (I use the term "professionals" lightly, meaning the few of us getting paid to do our hobby).  The problem is that the data is scattered far and wide, and it's not very easy to stay 'current' on what's going on.  I'm going to try to keep track of Jupiter's changing weather on this blog, and I'm basically summarising the enormous efforts of the British Astronomical Association's Jupiter Section, headed by my friend and colleague Dr. John Rogers.  John works closely with the JUPOS Project, a great team of software developers and astronomers who keep track of jovian features at regular intervals, measuring winds and identifying new phenomena within Jupiter's atmosphere.

Nomenclature for discussing jovian weather, from the BAA Jupiter Section (redrawn from John Roger's excellent book: Rogers JH, 'The Giant Planet Jupiter', Cambridge University Press, 1995).
The diagram above provides the necessary starting point for discussing Jupiter's ever changing weather.  Jupiter's powerful jet streams whip east and west in the troposphere - prograding jets (i.e., going with the direction of planetary rotation) go from west to east (westerlies), retrograding jets (i.e., going against the planet's rotation) go from east to west (easterlies).  These jets separate the coloured bands, and instabilities on the jets can excite waves, storms and vortices.  The forces powering these jets is still a subject of debate, but they have been shown to be reasonably constant over time.  Indeed, it's excursions from the norm that get people excited (amateurs and professionals alike).

Zones are typically brighter than belts, which have a red-brown appearance.  The colour differences are possibly (but not definitely) related to upwelling in zones and subsidence in belts).  The names of the most prominent features are shown in the diagram on the left, and moving from equator to pole they become more and more obscure.  But they form the framework in which Jupiter's climate can be discussed.   In future blog posts, I'll try to subdivide these as follows:

1.  Tropical Domain:  Comprises the equatorial zone (EZ) between two fast-moving prograde jets at 7N (NEBs jet) and 7S (SEBn jet); the North Equatorial Belt (NEB) from 7N to the retrograding jet at 18N (NEBn jet); and the South Equatorial Belt (SEB) from 7S to the fastest retrograding jet on the planet, the SEBs jet at 20S.  Poleward of the SEB and the NEB are two further zones, the South Tropical Zone (STropZ) and North Tropical Zone (NTropZ) that go up to prograde jets at 25N and 27S.  These mid-20s jets define the edges of the tropical domain.  Jupiter's Great Red Spot (GRS) sits within this domain, impinging on both the SEB and the STrZ and disrupting the flow of the retrograding jet at 20S (the SEBs jet).  Note that there are some more ephemeral features here too, like a reddish equatorial belt (EB) and a whitish SEB zone (SEBZ) that form every once in a while (right hand side of the left figure in the diagram).

2.  Temperate Domain:  Zones and belts become more closely packed as we move to higher and higher latitudes beyond the mid-20s.  The darker belts are characterised by prograde jets at the equatorward edge and retrograding jets at their poleward edge.  In the southern hemisphere we have the South Temperate Belt (STB), Southern Temperate Zone (STZ), then a series of further belts known as SSTB, SSTB, S3TB, etc.  The north follows suit with the NTB, NTZ, NNTB, NNTZ, etc.  Things get more and more complex, and in practise we find notable spots (e.g., newly forming red spots or white ovals) or outbreaks within these narrow bands.  For example, Oval BA sits within the South Temperate Belt.

3.  Polar Domain:  The organised patterns of the belts and zones finally give way to turbulent structures in the northern and southern polar regions (NPR and SPR), where high hazes and small-scale chaotic structures appear to dominate, bounded by prograde jets that exhibit waves.  The polar regions are the hardest to view from Earth so you won't hear much about their meteorology (until Juno provides us with a better view in 2016-17).

With this organisation in place, we can begin to discuss what's going on in each region, summarising the Herculean efforts of the amateur community to record these details.  It's then up to the atmospheric scientists to try to explain what's being recorded - and we're by no means there yet.  But in the next few years, with these expanding climatological databases, a Jupiter weather forecast might just be within our grasp.

Zonal windspeeds measured by both Voyager and Cassini, showing the relationship between the banded structure and the prograde and retrograde jet streams.  

Friday, 28 August 2015

Observing the Giant Planets in 2016

The VLT semester goes from April to October.

Jupiter is at opposition on March 8th 2016, and available above airmass 1.8 for 6 hours on April 1st, 4 hours on June 7th, 2 hours by July 11th, 1 hour by July 26th.  Sadly Jupiter is a daytime object for much of the period when Juno is first arriving at Jupiter, but we'll hope to pick up again in the latter part of the year.

Saturn is available for 6 hours on April 1st, opposition on June 2nd 2016, and is still around for 2 hours on October 1st.

Uranus is at opposition on October 15th 2016, and is up for an hour on June 1st, 2 hours by June 14th, 4 hours by July 14th.

Neptune is at opposition on September 2nd 2016, and is up for an hour by April 17th, 4 hours by May 29th.

Tuesday, 25 August 2015

ESA’s Europa Targets for JUICE

In a previous post I discussed the preliminary plans for two flybys of Europa in February 2031 by ESA's Jupiter Icy Moons Explorer (JUICE).  The flybys are constrained by a multitude of complex factors, including the illumination conditions, the geometry to allow the radar to work (we must be on the anti-jovian far side), the available data volume, the accumulated radiation dose and the need the power all the instruments simultaneously during the dense flybys.

First the basics:  Europa is a highly evolved world, with a low crater frequency implying a young age of the surface material (as low as 60 million years).  That youth is inherently linked to the ocean and gravitational tides, which trigger resurfacing, cracking and release of fresh materials from the interior. We expect a metallic core, a silicate rock mantle, and then an outer layer of water ice some 100-200 km thick, some of which is a salty liquid (evidenced from the induced magnetic response to Jupiter’s magnetic field).  The ice shell above the liquid might be 10-30 km thick (from morphological studies of landforms), although it might be as thin as 3 km in some regions.  The ice penetrating radar should be able to help us resolve that mystery.

Dark mottled terrain dominates the trailing hemisphere, with ridges and double ridges hundreds of kilometres long being the most ubiquitous landform.  The darker terrains are associated with materials like salts, sulphates, carbonates and/or sulphuric acid on the surface, whereas brighter areas are richer in water ice.  The trailing hemisphere appears brighter, possibly as a result of the more modest particle bombardment compared to the trailing hemisphere.  The main process shaping the surface appears to have been tectonism, with tidal stress (from Europa's 85-hour period for orbiting Jupiter) generating linear ridged plains with dark bands, which subsequently evolved through faulting to create the chaotic terrains at lower latitudes.  However, the mechanisms creating specific features remain highly uncertain, and surface features could be linked to the sub-surface ocean, tidal effects and possible exchange processes.

Only 10% of Europa’s surface was imaged by Galileo at a resolution of better than 100 m, and Europa remains poorly imaged at regional resolutions of 200-500 m.  The highest resolution image obtained by Galileo was at 6m/pixel, revealing the surface to be extremely rough at small scales, but this was only done once.  The JUICE team selected eight regions of interest for their geological, chemical and astrobiological significance, and then designed the flybys to give close views of these locations.  I’ll try to summarise each of those regions below - the Regio, the disrupted terrains, linear features and impact craters.
Regions of interest for JUICE, with the darker
mottled areas known as Regio.  Ground-tracks for the two
flybys are shown. Credit:  ESA Red Book.

1.  Regio:  Darker Terrains:

At low spatial resolution, Europa’s Regio are locations of darker terrain and are given names from Celtic mythology.  On the trailiing side during the approach phases JUICE will be able to observe Annwn (20N, 40E), Argadnel (15S, 151E), Dyfed (10N, 110E), Falga (30N, 150E) and Moytura (50S, 65E) Regio.  On the leading side, JUICE will observe Balgatan (50S, 330E), Powys (0N, 215E) and Tara (10S, 285E) Regio during the departure phases.

2.  Disrupted Terrain:  Chaos and Lenticulae

Conomara Chaos from Galileo.
Conamara Chaos (A1, 8N, 85E)
Fractured plates of ice that have shifted with respect to one another, possibly due to localised melting of the salty ice shell due to ice convection or oceanic plumes, form the jigsaw of the chaos terrains.  The chaos rafts show pre-existing ridged plains that have been broken up.  Some chaos units appear to stand higher than the surrounding terrain, whereas other appear to have foundered into a finer-grained material.   Reddish material is associated with the chaotic terrain, possibly hinting at a relationship with the deeper ocean.  JUICE will be able to investigate the Conamara Chaos region (A3, 8N, 85E) that has previously been explored by Galileo, but will additionally study chaos terrains in the central and northern parts of the Dyfed Regio region (B1e, B1b, B1c) associated to Conomara, where the activity has disrupted the Asterius, Glaukos, Agave and Belus Linea.

Thera Macula, from Paul Schenk's Atlas of the
Galilean satellites. 
Thera (47S, 179E) and Thrace (46S, 188E) Macula (A3)
These two features, enriched in dark materials potentially emplaced in a liquid state, were considered the highest priority for the flyby.  Thrace is the largest of the Maculae at 180 km diameter, Thera is only 95 km wide.  Here we find chaos material with a matrix of pre-existing structure associated with dark plains, possibly with emplacement of liquid via some sort of cryovolcanism.  Three other Maculae (Boeotia, Castalia and Cyclades) are found in the southern hemisphere (54, 2 and 64S, respectively) around the antijovian point. The inset region of Thrace was imaged by Galileo at 40m per pixel, the rest of the image has a scale of 250m.

Lenticulae (A5, 45N, 145E):
Pits, spots and domes all suggest ice shell convection and are found throughout Europa’s mottled terrain.  These relatively recent circular/elliptical domes and pits (10-15 km across) are associated with prominent intersecting ridges Minos, Udeaus and Cadmus Linea.  Within the domes there is a texture referred to as ‘microchaos’.  Their possible origin is from upwelling of warm, ductile ice, with evidence of melting and exchange with the subsurface.
Lenticulae (Latin for 'freckles') on Europa.  NASA / JPL / University of Arizona / University of Colorado

3.  Linear Ridges, Bands and Fractures    

Linear features cross Europa’s surface for hundreds of kilometres, possibly due to fracturing of the icy crust.  Ridges have widths as wide as 2 km and can be several hundred metres high.  Some are double structures separated by a central trough; some are cycloidal and form chains of arcs.

 Three common morphologies of linear features on Europa (a) trough (b) ridge (c) band.  NASA / JPL / Marshall and Kattenhorn

Double ridge on Europa, Feb 1997, NASA / JPL / ASU

Ridged Plains (8S, 140E)
Complex network of ridges, bands and chaos.  Double ridges could form from extrusion or intrusion of water or warm ice, with frictional shear heating from motions (strike-slip faults) along fractures causing warming and melting, creating mobile ice to squeeze through fractures to form the ridge.  Bands could be formed by the pulling apart of the crust by separation and spreading.  

Band Wedges (A6, 5S, 160E):  
These appear to be lineaments that were opened, separated and then filled by a darker (low albedo) non-ice material, much like sea-floor spreading on the Earth.   These wedge-shaped pull-apart bands provide evidence for the original configuration of the ice before the surface began to move.  The youngest bands tend to be the darkest, whereas older bands are bright.

Fractures are narrower than the ridges and bands, and are seen down to the 10-m resolution limit of the best images to date.  They can exceed 1000 km in length, cutting across nearly all other features to suggest that deformation of the ice shell occurs over short timescales.  The youngest fractures could even be active today, in response to tidal flexing.

Pwyll crater with bright rays, NASA/JPL/Arizona

4.  Impact features:

Only 24 impact craters larger than 10km in diameter have been identified on Europa, providing strong evidence for a youthful surface.  Taliesin (22S, 222E) is the largest at 50 km diameter, followed by bright Pwyll at 45km diameter.  Multi-ring structures from an impact, can provide information about the physical properties of the sub-surface.  There are very few large craters on the surface, hinting at an age of around 60 million years. Pwyll on the trailing hemisphere (25S, 89E), named for the Celtic god of the underworld, is the most striking of the impact features, and the bright rays suggests it formed less than 5 million years ago.  Pwyll is shallow and relaxed. Multi-ring structures like Tyre (34N, 214E) suggest that the impactor punched through 20-km thick ice, with a central peak from a rebound and surrounded by faulting.

More details of these features can be found in the Gazetteer of Planetary Nomenclature maintained by the USGS:

ESA's Europa Flyby Plans

While excitement builds for NASA’s flyby mission for Europa, ESA’s plans for the Jupiter Icy Moons Explorer (JUICE) are now in the implementation phase (known as B2), following successful mission adoption in November 2014.  JUICE will conduct two close flybys of Europa over a couple of weeks in October 2030, before moving onto a wider jovian orbit to complete a reconnaissance of the rest of the jovian system, ending up in orbit around Ganymede in 2032-2033.  JUICE’s sophisticated instrument suite of cameras, spectrometers (UV, near-IR and sub-mm), field and plasma instruments, laser altimeter and sub-surface radar will study Europa’s surface and sub-surface with a clarity, resolution and sensitivity far in excess of any previous exploration, including the ill-fated Galileo mission.  JUICE aims to understand the composition of non-ice material on the surface (particularly those related to habitability of the sub-surface ocean); search for liquid water below the surface; and to study any active processes.

However, the harsh radiation environment of Europa, coupled with the desire to study Jupiter and Ganymede over a 3-year mission, means that JUICE’s brief foray close to Europa will only occur once, with two quick flybys to limit the radiation dose.  Indeed, particle fluxes are 20 times higher at Europa than they are at Ganymede.  We chose to focus these two flybys on regions of high interest for geology, chemistry and astrobiology, including those where Galileo image suggested potential recent activity - chaotic terrains where exchange of materials between the surface and subsurface might have been possible, providing potential insights into the nature of Europa’s subsurface ocean.  Two flybys will no-doubt leave us wanting many more, but it should be remembered that Ganymede is the primary target of this mission.

Regions of interest and trajectory for the two JUICE
flybys of Europa, from the JUICE Definition Study Report.
JUICE will fly on the anti-jovian side of Europa (the far side, from Jupiter’s perspective), with a closest approach of 400 km.  We need to be on the far side for the radar sounding to work.  The far side must also be sunlit for the remote sensing experiments.  This will provide regional (500-1000 m resolution) and local (50 m resolution) imaging for the study of geological processes; the potential for radar sounding down to a maximum penetration depth of 9 km (depending on the surface properties) with a vertical resolution of 50 m or larger; and laser altimetry with a vertical resolution of less than 5 m.

Plans for the Flybys

The spacecraft will approach Europa from the trailing hemisphere (longitude of 90 degrees) that receives the largest radiation dose from the co-rotating jovian magnetosphere; closest approach will be over the anti-jovian point (longitude 180 degrees); and departure is then over the leading hemisphere (longitude 270 degrees).  One flyby will occur over the northern hemisphere (up to latitudes of 45 degrees), the second will be over the southern hemisphere.    JUICE will zip past at 3.6-3.9 km/s.  

The Europa flybys represent the most highly packed observational sequence of the JUICE mission.  Observations will start many hours before the closest approach, using regional imaging and spectroscopic mapping to compare the geology and composition of the leading and trailing hemispheres. JANUS will lead 3 spacecraft slews during the inbound and outbound phases, MAJIS will lead 2, with other instruments riding along to view both the nadir and limb of the moon.   All instruments will be operating simultaneously when the spacecraft is within a distance of 150,000 km of Europa, placing one of the toughest requirements on the design of the JUICE spacecraft.  

During the 2 hours surrounding the closest approach, JUICE will switch from the power-optimised yaw-steering mode to the inertial pointing mode.  After more spacecraft slews, the instruments become purely nadir-pointing for the ±30 minutes around closest approach, and then active instruments (laser altimetry and sub-surface radar) operate for the ±7 minutes surrounding closest approach.  Depending on data volume constraints, distant Europa observations for plume searches will be performed two days either side of the closest approach, and distant views of Europa will continue to be used to study materials ejected from Europa’s surface, such as plume activity from the southern pole.  

Eight regions of interest have been identified, seven of which are on the trailing hemisphere to be viewed during approach.  I’ll try to describe each of these regions in a future blog post, and why we think they’ll be exciting to explore.  The plans continue to be formulated as JUICE moves through the implementation phase, under the guidance of the working groups for the SWT (science working team), but a comprehensive summary of the plans can be found in the JUICE Definition Study Report (Red Book) here: