Friday, 17 March 2017

JUICE Moves into Phase C

Some excellent news to round the week off - the JUICE mission has passed its PDR (Preliminary Design Review), which means that the mission can officially move from Phase B2 (the preliminary definition phase, where we've been ever since mission adoption in November 2014) into Phase C (the detailed definition phase).  This is a pretty important milestone in the life cycle of a mission, which proceeds throughout this whole implementation phase (B2/C/D/E1).  Phase D is the qualification and production phase, and Phase E1 is the start of the utilisation phase.  The most exciting thing is that the main contractor, Airbus DS, can begin building the prototypes.

From the ESA website:

JUICE will be equipped with 10 state-of-the-art instruments, including cameras, an ice-penetrating radar, an altimeter, radio-science experiments, and sensors to monitor the magnetic fields and charged particles in the Jovian system.

In order to ensure it can address these goals in the challenging Jovian environment, the spacecraft's design has to meet stringent requirements.

An important milestone was reached earlier this month, when the preliminary design of JUICE and its interfaces with the scientific instruments and the ground stations were fixed, which will now allow a prototype spacecraft to be built for rigorous testing.

The review also confirmed that the 5.3 tonne spacecraft will be compatible with its Ariane 5 launcher. Operating in the outer Solar System, far from the Sun, means that JUICE needs a large solar array: two wings of five panels each are foreseen, which will cover a total surface area of nearly 100 m², capable of providing 820 W at Jupiter by the end of the mission.

After launch, JUICE will make five gravity-assist flybys in total: one each at Mars and Venus, and three at Earth, to set it on course for Jupiter. Its solar panels will have to cope with a range of temperatures such that when it is flying closer to the Sun during the Venus flyby, the solar wings will be tilted to avoid excessive temperatures damaging the solar cells.

The spacecraft's main engine will be used to enter orbit around the giant planet, and later around Jupiter's largest moon, Ganymede. As such, the engine design has also been critically reviewed at this stage.

Special measures will allow JUICE to cope with the extremely harsh radiation that it must endure for several years around Jupiter. This means careful selection of components and materials, as well as radiation shielding.

One particularly important topic is JUICE's electromagnetic 'cleanliness'. Because a key goal is to monitor the magnetic fields and charged particles at Jupiter, it is imperative that any electromagnetic fields generated by the spacecraft itself do not interfere with the sensitive scientific measurements. This will be achieved by the careful design of the solar array electrical architecture, the power distribution unit, and the reaction wheels – a type of flywheel that stabilises the attitude.

The review also ensured that JUICE will meet strict planetary protection guidelines, because it is imperative to minimise the risk that the potentially habitable ocean moons, particularly Europa, might be contaminated by viruses, bacteria or spores carried by the spacecraft from Earth. Therefore, mission plans ensure that JUICE will not crash into Europa, on a timescale of hundreds of years.

"The spacecraft design has been extensively and positively reviewed, and confirmed to address the many critical mission requirements," says Giuseppe Sarri, JUICE Project Manager. "So far we are on schedule, and are delighted to begin the development stage of this ambitious large-class mission."
ESA's industrial partners, led by Airbus, now have the go-ahead to start building the prototype spacecraft units that will subjected to tough tests to simulate the conditions expected during launch, as well as the extreme range of environmental conditions.

Once the design is proved beyond doubt, the flight model – the one that will actually go into space – will be built.

TEXES on Gemini North: Blazing Jupiter!

All of this week the TEXES team has been out on Mauna Kea running a programme of observations that included ten hours of time scanning Jupiter's tropics.  I proposed this to solve a key issue that we have - TEXES has provided fantastic spectral maps from the IRTF but with a limited spatial resolution from the 3-m primary mirror, whereas VISIR on the VLT (among others) provides superb imaging at high spatial resolution, but without decent spectroscopy.  By moving TEXES to Gemini-North for this special run, we were able to get the best of both worlds.

Sadly neither I nor the Leicester team could join them this time, but Tommy Greathouse, Glenn Orton, James Sinclair and Rohini Giles were sending me nearly continuous updates, and provided the data in a raw form on Tuesday morning.  I processed the spectral data into a map at just one wavelength (1165 cm-1, which senses deep temperatures and jovian aerosols, and always contains a lot of structure) to share in the Gemini e-cast.  There's also a nifty 3-colour image, generated from three wavelengths in the same spectral setting, which we'll be using in a future GeminiFocus magazine.  Needless to say, we're all pretty delighted with these data - the highest spatial-resolution spectral map of Jupiter ever acquired, period.  This is going to keep us going for years.

TEXES Gemini and Jupiter:

To truly understand the atmospheric phenomena at work in Jupiter, we must investigate three different domains - spatial, temporal, and spectral.  Past investigations have allowed us to target one of these domains, but today we are able to explore all three by combining the Gemini observatory, the TEXES spectrograph and the worldwide campaign of Earth-based support for NASA’s Juno mission.  This three-colour map reveals Jupiter’s weather layer near 8.6 microns, where Jupiter’s spectrum is governed by temperatures, cloud opacity, and gaseous species like deuterated methane and phosphine. The map was constructed from spectral scans over two nights (March 12th-13th 2017), and represents the highest spatial resolution ever achieved by the TEXES instrument.  Every pixel in this map represents a spectrum of Jupiter.  Red colours use a wavelength that senses deep, warm temperatures at the cloud tops; blue colours sense cooler temperatures at higher altitudes near the tropopause, and green colours sense an intermediate altitude.  The equatorial zone and the Great Red Spot in the bottom right are cold and dark at all three wavelengths.  The turbulent wake to the west of the Great Red Spot is darker (cooler) and distinct from the rest of Jupiter’s South Equatorial Belt.  An outbreak of dark, cold and cloudy plumes can be seen in the southern belt near 270W.  Finally, the pattern of cold, cloudy plumes (dark) and warm, bright hotspots (white) can be seen encircling the planet near latitude 7N, on the edge of Jupiter’s Northern Equatorial Belt.

Credit:  TEXES team & L.N. Fletcher/University of Leicester, UK.

From the Gemini e-cast #93 (March 16th 2017)

TEXES, the visiting high-resolution mid-IR spectrograph, is back for another visit on Gemini North. This time the instrument is supporting a wide-ranging set of science programs, including summer-solstice observations of Saturn’s polar vortex, three programs studying Jupiter’s atmosphere, stratosphere and aurora, and (beyond the solar system) studies of the chemistry of the gaps in protoplanetary disks, organics in hot star-forming cores and the motions of gas in embedded super star clusters. At mid-IR wavelengths most of the seeing is due to image motion, which is removed by the rapid tip-tilt secondary mirror on Gemini, producing diffraction-limited images as small as 0.3 arcseconds without the use of adaptive optics.

The TEXES team has been sharing part of each night with GMOS CCD commissioning activities, reported in the previous story in this newscast, and the team is grateful for their flexibility in accommodating this TEXES visitor instrument run.

The TEXES team and Gemini staff preparing the instrument to mount on the up-looking port of Gemini North in March 2017. The beachballs are part of the instrument’s helium overflow system.

Jupiter in the 8-micron region, in a spectral scan taken by TEXES on Gemini North, March 2017. Note the cool wake of the Great Red Spot (lower right). For more details and a full color mid-IR image of the Jupiter weather layer, see the upcoming April issue of GeminiFocus. Image credit: TEXES team & L.N. Fletcher/University of Leicester, UK.

Wednesday, 15 March 2017

Ten Years of the European Research Council

I was honoured to be listed among Leicester's ERC grant holders in a recent press release coinciding with the tenth anniversary of the European Research Council.  A copy of the text can be found below, or via Leicester's website:

More details of this anniversary can be found here:

Researchers based at UK institutions won the largest share of mid-career and proof-of-concept grants handed out by the European Research Council in the latest awards rounds.

The news comes in the week that the European Research Council – a success story of the EU’s Horizon 2020 programme – marks its tenth anniversary with ‘ERC week’ (13-17 March) and celebrates its impact on strengthening Europe as a global centre of excellence in research.

The University of Leicester is a part of that success story having secured almost €10 million of ERC funding since 2011 – highly prestigious awards given only to ‘frontier’ research projects. ERC grant holders are in good company with some previous grant holders going on to win a Nobel Prize or to be awarded the Fields Medal.

UK-based researchers received a total of 58 grants in the latest Consolidator Grant round, equivalent to 18% of the awards handed out. This was followed by 48 for researchers located in Germany, 43 in France and 29 in the Netherlands.

Ten of the 44 Proof-of-Concept grants awarded by the ERC on 31 January went to researchers who will work at UK universities. Germany and Spain will host the second and third most grantees with six and five recipients respectively. This is the third time that the UK has topped the Proof-of-Concept awards recipient list since it voted to leave the EU in June 2016.

Leicester’s ERC grant holders include: Leigh Fletcher- Physics Consolidator Grant (2016) c. E €2 million.; Richard Alexander - Physics Consolidator Grant (2015) c. €2 million; Clare Anderson - History Starting Grant  (2013) – c. €1.5 million; Laura Morales - Politics Starting Grant  (2011) – c. €1.5 million; and David Mattingly - Archaeology Advanced Grant (2011) – c. €2.5 million. You can find out a bit more about their groundbreaking research on the Research and Enterprise funding pages.

Professor Iain Gillespie, Pro Vice Chancellor for Research and Enterprise commented: “We are very pleased to celebrate the achievements of our European Research Council (ERC) grant holders on the ten-year anniversary of the European Research Council scheme.

“These researchers epitomise leadership in world-class research, and we are proud that they also represent Leicester’s continuing, strong engagement with the European research community.”

Academic and research staff are reminded that the Treasury is continuing to financially underwrite UK participation in EU projects submitted before any official Brexit takes place. Funding will be guaranteed for UK organisations submitting projects before an official exit, even if the project will continue beyond the UK's membership of the European Union.

Wednesday, 1 February 2017

Twenty Years of the Sutton Trust

Twenty years ago, Sir Peter Lampl founded the Sutton Trust, an organisation dedicated to improving social mobility through education, firstly in the UK and more recently in the USA.  As an 17-year old in Leicestershire, I benefited from winning a place on one of their summer schools in 1999, only the third year that the programme had been running.  My Nan had spotted a story about the new summer school in the Daily Mirror and encouraged me to have a go.  Now, 18 years later, I’ve been extremely fortunate to  work in some of the world’s top universities, and I look back extremely fondly on my experiences with the Sutton Trust.

What did your parents do when you were growing up, and what sort of school did you attend?

I was a student at the John Cleveland College in Hinckley, a state school catering for 14-18 year olds in Leicestershire’s peculiar system of primary, middle and secondary schools.  It was a big place, with more than 1500 students - easy to fall through the cracks if you didn’t want to succeed.  I was one of the hard-working ones though - I enjoyed being at the school, and knew I wanted to continue my education post-18.  I’d be the first person in my immediate family to do so - my parents left school at 16, my Dad was owner and director of a successful timber merchant in Market Harborough, my Mum did secretarial work and accounts.  My family were enormously supportive of my ambitions to go to university, and had been saving for many years to help me to afford the tuition fees and living costs.

Did you have any preconceptions about going to a top university?

Certainly - my entire knowledge of top UK universities came from the media and television:  elite institutions, home to those with extreme talents and sufficient funds to see them comfortably through life without struggles, and destined to go into high-paid and high-power jobs.  Naturally, I was worried about fitting in, about what sort of people I’d be living and studying with, and whether I’d be able to compete with those that might have been given a leg-up in their education.  For me, the most important result of the summer school was the deconstruction of those preconceptions, showing me that many students at Oxford and Cambridge were just like me - with a little talent but insecure, willing to take on a challenge, and wondering about fitting in so far from home.

What was your experience like on the summer school? What sort of things did you do?

I got a place on the Physics summer school at Cambridge in the summer of 1999, while I was between Year 12 and Year 13 and heading for my 18th birthday.  We stayed in Newnham College (I later discovered that it was still a single-sex college), engaging in lectures, tutorials, team exercises and social activities.  Our two incredible lecturers, Julia Riley and Dave Green, took us through a range of university-level lectures, followed by mathematical and physical problems that we’d solve together, then discuss in small groups.  This, I came to realise, was about giving us a taste of Oxbridge education:  the bombardment of information in lectures, followed by careful thought, practise and questions during smaller tutorial groups.  It was really eye-opening, showing me how different the learning experience would be at a university compared to school.  We also had group discussions about interview techniques, providing me with suggestions for what interviewers were looking for in those short, make-or-break meetings.  I’m sure this is the sort of thing that privately-schooled students would get all the time, but for me it was extremely helpful.  

But more than any of that, the most important consequence of the summer school was that I’d engaged and interacted with fellow summer students, current undergraduates, and university lecturers.  I could see that I shared a huge amount of common ground with them, and that I need not be worried about ‘fitting in’ if I were successful in gaining a place at one of these top UK universities.

What did you go on to study at university?

After returning from the 1999 summer school, it was time to put in my UCAS applications - I recall applying to UCL, Leicester, Sheffield - and probably others.  But my heart was set on Emmanuel College, Cambridge.  My interviews were in December 1999, and shortly after Christmas I had an offer of a place to study Natural Sciences.  A-levels, and the required STEP papers for entry to Emmanuel, followed in the summer of 2000, and I started my first year at Cambridge in October. Although I’d always thought that I wanted to do physics at uni, the Natural Sciences course was absolutely the right thing for me:  I fondly look back on my first year studying Maths, Physics, Chemistry and Cellular Biology, and my later specialisms in physics.  Everything I’d learned on the summer school stood me in good stead to be part of the Emmanuel community - from the lectures to the small supervision groups - and the people I was living and studying with have become lifelong friends.  

What have you done since leaving university?  

I never really left!  I moved from Cambridge to Oxford in 2004, studying for a PhD in planetary science in Oxford’s Atmospheric, Oceanic and Planetary Physics department.  My PhD took me to the USA as a NASA Postdoctoral Fellow, working in the Jet Propulsion Laboratory (JPL) in California.  I returned to Oxford for five more years as a research fellow, first with a Glasstone Science Fellowship and later a Royal Society fellowship.  That fellowship allowed me to move to the University of Leicester in 2015 to set up my own group in planetary atmospheres, so that I’m now lecturing and leading world-class research.  I’m directly involved in robotic spacecraft missions exploring the outer solar system, and in the use of giant ground-based observatories to study the atmospheric patterns on other worlds.  I’m extremely lucky to be paid to do a job that I love!

What would your advice be to young people who might be in a similar position to you as a teen?

The Sutton Trust’s central message of social mobility through education is more important now than ever before, as liberal politics and fundamental equalities appear to be on the backfoot.  Every child, no matter their social background or economic status, deserves the opportunity to shine, to develop their talents, and to access the best possible education.  Have confidence in your talents, and don’t be satisfied if people tell you you haven’t got what it takes to make it at the best UK universities.  The only way you’ll ever know is by giving it a try. You can put yourself in the right position by being pro-active:  ask your school for help with interview techniques, attend open days at a range of universities, and have a go at getting onto a Sutton Trust summer school.  They were so helpful to me in demystifying Oxbridge for someone who had never had prior exposure to them.  Find a subject that you enjoy, that you’re genuinely passionate about.  Lastly, there’ll always be rejections and let-downs, but don’t ever lose faith in your own abilities.  

Articles on Sutton Trust:

Wednesday, 3 August 2016

The Jupiter Time Capsule

Originally posted by @LeighFletcher on the Leicester to Jupiter blog.

Given that we don’t yet know whether a planetary core exists within Jupiter, much of our understanding of giant planet formation comes from a different line of investigation:  the bulk composition of the planet.

The composition of the atmosphere that we observe today results from a combination of many processes – chemistry initiated by the Sun’s UV rays, condensation of gases to form cloud decks, and dynamic weather causing the mixing and transport of materials from place to place.  Nevertheless, Jupiter’s immense gravity means that all this chemistry is determined by the original balance of chemical elements and isotopes.  The materials that the young Jupiter captured from the protosolar nebula were never allowed to escape.  Jupiter can then be thought of as an ideal time capsule, revealing the processes that formed this gargantuan world billions of years ago.

Juno will explore the bulk composition of Jupiter’s atmosphere to constrain its origins. Credit: NASA

Why is this important?  Well, different theories of giant planet formation predict a different balance of elements and isotopes.  By creating a precise inventory of Jupiter’s composition, it might help us distinguish between those competing ideas.  For example, if Jupiter’s formation didn’t require a core, and simply started from a collapse of the nebula gas that surrounded our young Sun, then we might expect Jupiter to have the same sort of inventory as the Sun itself.  Conversely, if Jupiter did require a core for the formation to begin (core accretion theory), then that core material would have helped to enrich the planet in elements, over and above what we measure in the Sun.

A Job Left Undone

There are a couple of ways to determine this compositional inventory.  Remote sensing, using spectroscopic measurements of the soup of gaseous species from ground-based telescopes or visiting spacecraft, is one potential method.  Each gas has a different spectral signature, so provided the gas is present above the cloud tops, we can determine the composition by modelling the spectra.  This has been done for all four giant planets (although it’s really hard to do on the ice giants), and the general picture that has emerged is that the elements and isotopes are enriched with respect to conditions in the Sun.  Place a tick in the box for core accretion theory.

However, the upper atmosphere represents only a tiny fraction of Jupiter’s enormous bulk and might not be representative of the rest of the planet.  Indeed, some species like ammonia and water condense out at the cold temperatures of Jupiter’s upper atmosphere, forming the primary cloud decks.  The main repositories for these condensable species are therefore hidden from view, down below the cloud decks.  So lots of the species we’re interested in are inaccessible to us.

A better way of measuring the composition below the clouds is by firing a probe into the planet itself, with sophisticated instruments designed to sniff out the gases and aerosols present at levels hidden from view.  That’s precisely what we did in 1995 with the Galileo probe, which descended under a parachute for 60 minutes, reaching down to the 20 bar level, deep below the expected cloud layers.

But there was a problem. The cloud decks weren’t where they were supposed to be, and, crucially, the amount of water measured was much lower than expected (about 490 ppm, or 30% of the amount found in the Sun).  With only a single probe measurement, we fell into the trap of a Jovian hotspot, a dry, desiccated region of air just north of the equator where all the gases appeared to be depleted by powerful downwelling motions.  It was like trying to infer the abundance of water on Earth if we only had one measurement from the Sahara desert.  The Galileo probe did, however, measure enrichments in most gasesous species that were above the solar values.  So, another tick for core accretion theory.  However, the abundance of water remained a mystery.

The enrichment of elements in Jupiter is consistent with the formation of a protoplanetary core, except for water…

Juno and the Microwaves

Step in Juno, with its microwave radiometer on board, the first every flown to a giant planet.  At microwave wavelengths, we sense light emitted from extremely high pressures down to 100 bar or so, well below the levels sampled by the Galileo probe, and well below the expected cloud decks.  Here, water should be well mixed, allowing Juno to definitively constrain the water abundance in Jupiter and hence the amount of oxygen mixed into the giant planet as it was forming.

Oxygen is key, as core accretion theory would expect it to be similarly enhanced over the solar abundances, maybe by 400-1000%, depending on the details of the formation model. Indeed, one theory involves water ice cages trapping gases (like methane, ammonia, nitrogen, etc.) in the early solar system and delivering them to Jupiter.  If that’s the case, then we should find more oxygen (from those original water ice cages) than the other gaseous species.  The precise water abundance has the potential to rewrite the textbooks on how Jupiter first formed, and to complete the task that was left over from Galileo.

Wednesday, 20 July 2016

Birth of Giants

Originally posted by @LeighFletcher on the Leicester to Jupiter blog.

The presence of Jupiter has had a profound influence on the architecture of our solar system, shaping the conditions that have led to the stable, habitable environment that exist here on Earth.  But it didn’t have to be this way.  Maybe the young Jupiter could have wandered in too far to the inner solar system, scattering all the young terrestrial worlds so that Earth never formed properly.  Or maybe Jupiter’s interaction with other giants could have ejected it completely, to be a free floating planet.  This might have meant that Jupiter never shaped the population of icy comets and watery asteroids that delivered key species – water, and maybe even the ingredients for life – to our forming home planet.  And maybe, without Jupiter’s stabilising presence, cataclysmic impacts could be going on to this day, never allowing intelligent life to develop on Earth?

Probing the unknown interior of Jupiter with the Juno spacecraft, and the possible presence of a core. Credit: NASA

It’s likely that all of these scenarios have played out somewhere in our universe.  But Jupiter’s formation and influence on the other planets is key to the question of how our Solar System came to be.  And yet, there remains a surprisingly large gap in our understanding of how Jupiter first formed and how it evolved.  Three key questions remain unanswered – did Jupiter form with a massive protoplanetary core, is that core still present today (or are the heavy materials mixed throughout the planetary interior), and did accretion of icy materials massively enrich the water content of Jupiter?  Indeed, Jupiter can be thought of as a time capsule, its massive gravity being so powerful that the proto-planetary material from the birth of our solar system never managed to escape.  Resolve these mysteries and we’ll have a much better idea of how giant planets form, both in our Solar System and around other stars.

Two prevailing theories exist for the formation of Jupiter.  The favoured model is known as the core accretion theory, where a massive rocky core (a protoplanet, or planetary embryo) is formed first in the young solar system, and when it attains a high enough mass (somewhere around 10 Earth masses) it begins to suck in all the surrounding hydrogen and helium gas from the forming solar system, growing to become the giant we see today.  If that’s true, then the presence of the original core material will serve to enrich Jupiter’s chemical abundances over and above that found in the nebula.  The composition of our own Sun is a good measure of the composition of the original solar nebula, so we have a good point of comparison.  On the other hand, if no planetary core was required, then the planet could begin to form by gravitational collapse (just like a star). In this case, there’d be no significant enrichment in the planetary chemical abundances.  So, was a core required to form Jupiter, and is the planet’s chemical inventory enriched relative to the Sun?

Gravity Mapping

NASA’s Juno mission is tantalisingly close to providing those answers via two techniques – precision mapping of the gravitational field, and microwave mapping to determine the composition of this time capsule far below the clouds.  We’ll look at the second point in a later blog post, but for now let’s look at Juno’s gravity mapping.

Measuring the internal density structure of Jupiter via gravitational mapping, using slight perturbations in Juno’s orbit. Credit: NASA

Juno’s close-in orbit is specifically designed to map the gravity field, as small changes in the interior distribution of densities will pull and tweak Juno’s orbit around Jupiter over its 37 orbits.    By monitoring the Doppler shift in Juno’s radio signal, Juno will be able to map those perturbations to assemble a 3D map of the insides of Jupiter.  If there are discrete layers, then the gravity mapping should be able to reveal their depth and the density changes.  For example, if a rock/metal core exists – the remnant of the protoplanet that initiated Jupiter’s formation – then maybe Juno will be able to detect its gravitational signal.  At the very least, Juno will probe the exotic transitions in Jupiter’s hydrogen-helium mixture as it is compressed by crushing pressures (up to 30-50 million bar) and temperatures (tens of thousands of degrees) at the planet’s centre.

The main expected phase transition is to a bizarre state called metallic hydrogen – at high pressures, hydrogen’s single electron can be detached from its proton and allows it to become electrically conducting.  Droplets of helium and neon might rain out in this strange fluid.  The properties of metallic hydrogen are poorly understood, given that it might only be produced for fleeting instances between diamond anvils in labs on Earth (and this is highly contested), so this could all add a lot of complexities to interpreting Jupiter’s gravity field.  We think the metallic hydrogen layer might start about 25% of the way down, where temperatures exceed 10000 degrees and pressures exceed 2 million bar, but this is highly uncertain.  To put that in perspective, the pressure of the Earth’s core is around 3-4 million bar.  Furthermore, convective motions in this fluid might have served to erode any original protoplanetary core away, redistributing its materials over the aeons since the planet first formed.  Nevertheless, if Juno detects the presence of a core it would be a smoking gun for the core accretion theory of planetary formation.

Tuesday, 12 July 2016

When it Rains....

Originally posted by @LeighFletcher on the Leicester to Jupiter blog.

Water, water, everywhere.  

Dr. Leigh Fletcher appeared on this month’s episode of BBC Sky at Night to discuss Juno’s goals at Jupiter, and describes the importance of Jupiter’s water in this new post.

If our ideas about the formation of giant planets stand up to the observational tests of the Juno spacecraft, then Jupiter’s extensive atmosphere should be moist, humid and drenched in water. Moist air contains energy, released as gaseous water condenses to liquid droplets.   That energy may be powering the fascinating meteorology that shapes the face of the giant planet, and driving lighting storms that flicker and flash in Jupiter’s belts.  In turn, the distribution of water may help to explain the contrasts in storm activity and colouration between the white zones and brown belts that criss-cross the face of Jupiter.  Understanding Jupiter’s water might be the key to understanding its churning weather.

But if the distribution and availability of water is key to explaining the meteorology of Jupiter, then why don’t we already have a better handle on this question?  The answer lies in Jupiter’s cold atmospheric temperatures.  The topmost clouds that we can see through our telescopes are composed of crystals of ammonia ice, mixed with various chemical contaminants that cause the different cloud colours.  These condense at a lower temperature (roughly -100 degree Celsius) than water vapour, so the clouds of ammonia ice sit higher up than the water clouds.  In fact, this cloud layer almost completely hides the deeper layers from view, and we have only glimpsed water ice in very limited locales under very special circumstances, when a powerful storm dredges the water ices upwards to the levels where we can see it, before precipitation (as rain or snow) causes it to sink back down again.

Jupiter in infrared light – spectra of the brightest regions show some signs of the presence of water, but cannot map the deep, drenched interior. Credit: ESO/L.N. Fletcher 

Certain wavelengths of light can start to probe down through these topmost cloud layers, like removing the skin of an onion to see what lies beneath.  For example, Earth-based telescopes can use observations of Jupiter’s infrared glow at 5 microns to peer through gaps in the ammonia ice clouds, such as those recently released by the Very Large Telescope in Chile.  Previous spacecraft, such as Galileo and Cassini, have had instruments that can observe Jupiter in this spectral `window’, and allowed us to place lower limits on the amount of water present (around 0.04% by volume).  However, they never sense down deep enough to see the amount of water in the deeper atmosphere, which is expected to be in the 0.1-1.0% range, depending on the different models used.  Juno carries a similar infrared instrument called JIRAM, provided by the Italian Space Agency, which will perform similar measurements.  But the deep water abundance remains out of reach for these infrared mappers.

A much better solution, then, is to actually send a probe into Jupiter itself.  And that’s precisely what we did in 1995 with the Galileo probe, which carried all the sensors required to sniff out the gases and clouds in Jupiter’s cloudy layers.  However, with only one probe there’s always the risk that you’ll find a region that isn’t representative of the full planet.  Imagine trying to understand the amount of water on Earth if you only sampled the Sahara desert.  The probe descended into a location known as a ‘hotspot’ just north of the equator, where powerful downwelling regions dried the atmosphere out and removed almost all traces of water.  The probe transmission ended at 20 bar of pressure, below the expected altitude of the water cloud (around 5-7 bar), but still without finding the deep abundance.  The maximum was 490 ppm near 20 bar (0.05% by volume), around 30% of the solar abundance.  Based on Galileo’s measurements of other species, like methane and ammonia, we’d expect that value to be more like 400-1000% of the solar abundance.

Mapping of Jupiter’s deep water and ammonia, key condensible species, far below the obscuring clouds. Credit: NASA 

It’s this mystery that’s driving Juno, and its microwave radiometer experiment.  Microwave light is another way of peering below the topmost clouds, and this should give us access down to 100 bar of pressure to see the distribution of water down at great depths.  We’ll be going deeper into the churning, convective weather layer than ever before to understand not only the bulk abundance of water, but also its distribution.  Maybe there’s more water available beneath the belts to power the moist convection and lightning storms that we see there?  But it’s a balancing act, and too much water actually stabilises the atmosphere and prevents convection – maybe that’s what’s happening beneath the white zones, where we don’t see as much convection?  Or is water really depleted throughout the cloud forming region, as the Galileo probe results suggested?  Juno is about to provide the answer.

At the same time, we’ll be amassing data from Earth that reveals the temperature, cloud and compositional structure above the clouds to see how this relates back to the distribution of water far below.  Will the deep atmospheric dynamics be different to what we’re used to above Jupiter’s clouds?  We hope to finally have a handle on the ‘Jovian water cycle’ that powers the weather on this gas giant world.