Formation of the Giants

The giant planets Jupiter and Saturn were known to the ancients, and became the first targets of telescopic observations during the renaissance, with Cassini observing a giant red spot and banded structure on Jupiter, and Huygens interpreting the 'ears' of Saturn as a large, flat, inclined ring.  Today, Jupiter has been visited by Pioneer 10 & 11, Voyager 1 & 2, Galileo, Cassini and New Horizons, with Juno expected to arrive in 2016, followed by ESA's JUICE spacecraft in the 2030s.  The Cassini Huygens mission is in its ninth successful year in orbit around Saturn.  

Uranus and Neptune came later.  William Herschel discovered Uranus in 1781 with a 15 cm telescope, Neptune was first observed by J. Galle in 1846 after Le Verrier and J. Adams both predicted the position of the eighth planet.  These ice giant worlds have been visited only once by Voyager 2, and much of what we know about these distant worlds has been revealed by Hubble and ground based observations over the past two decades.

Planet Formation

One of the driving goals behind the exploration of the giant planets is he quest to understand the formation of our solar system.  We know that planets orbit in the same plane (almost), on nearly circular orbits, and that  they rotate anticlockwise just like the Sun.  This suggests that they all formed from a disk of material, an idea supported by the observations of protoplanetary disks around young nearby stars.  Furthermore, there are two types of planets in our solar system: the high density terrestrial planets (3-5 g/cm3) with very few satellites, and the low density giants (0.7-1.7 g/cm3) with extensive satellite systems.

This hierarchy of planets is due to the conditions in the nebula as the planets formed. Embryos of solid particles were embedded in the hydrogen-rich disk, and as more refractory elements (I.e., metals and silicates) were found in the high temperature inner solar system, versus the more icy materials at the cold outer reaches, this explains why the inner and outer solar system look so different from one another.  The available solid mass was lower in the hot inner solar system, whereas abundant ices in the outer solar system produced big embryos of ten earth masses or more.  Once a critical mass for these embryos was reached, the extensive gassy atmospheres of the giants was accreted directly from the gas-rich nebula.  

This core accretion scenario was developed by Mizuno (1980) and Pollack et al. (1996).  More recently, the idea of planetary migration has been added, known as the "Nice model" (A. Morbidelli et al.), where the 2:1 resonance crossing of the Jupiter-Saturn system led to intense perturbations of other bodies in the system and generated the Late Heavy Bombardment, whose impact scars dominate the appearance of rocky bodies in our solar system to this day.  In an earlier phase, Jupiter may have approached the orbit of Mars (stopping its growth) and then receded due to Saturn's interaction (Walsh e al. 2011).

The giant planets can be further subdivided into the gas giants (Jupiter (5 AU) & Saturn (10 AU); 318 and 95 Earth masses, respectively) comprised mostly of hydrogen and helium, and the ice giants enriched in heavier materials (potentially delivered as ices, hence their name).  Uranus and Neptune are smaller (14 and 17 earth masses), and their different atmosphere composition is usually attributed to their formation timescales - they formed after Jupiter and Saturn at a time when there was less gas available in the disk (it was being dissipated). The migration hypothesis suggests that the ice giants formed closer in (10-15 AU) and migrated outward to their present positions (19 and 30 AU). 

Atmospheric Composition

The bulk atmospheric compositions support the idea that the planets formed in a disc with temperatures decreasing with distance from the young sun.  At low temperatures, thermochemical equilibrium implies that the key elements (C, H, O, N) were in hydrogenated, reduced forms of methane, water and ammonia - the key ices thought to have formed the giant planets and still present in their atmospheres.  At the higher temperatures of the inner solar system, those species exist as CO, CO2 and N2, precisely the species found in the terrestrial atmospheres (although these came from later out gassing of secondary atmospheres, rather than accretion of primary atmospheres like the giants).  

Remote sensing of giant planet atmospheres, in addition to the Galileo probe, have provided supporting evidence of the core accretion hypothesis of planetary formation.  Both the carbon and deuterium abundances increase from Jupiter to Neptune, as expected if the ratio of gassy envelope to planetary core decreases with distance from the Sun.  The Galileo in situ entry probe discovered that almost all of the elements making up Jupiter were enriched over solar composition, supporting the existence of planetary embryos enriched in abundance compared to the rest of the gassy nebula.  This direct measurement is something scientists are keen to repeat on the other giant planets in our solar system, particularly an ice giant, to understand the differences in how these bodies formed.  Remote sensing cannot measure the abundances of the noble gases, which are key indicators of the way in which materials like ice and gas were accreted into the giants.

One big mystery about Jupiter's composition remains unanswered - the abundance of oxygen.  If all the other materials were encased in cages of water ice, then there should be lots of it.  Unfortunately Galileo was unable to measure the deep water abundance to confirm this.  The Juno probe, due to arrive in 2016, carried with it a microwave instrument capable of peering beneath the Jovian clouds to sample the deep water abundance.  Scientists wait with baited breath for the answer to this crucial question of how Jupiter formed, with far reaching implications for the evolution of our whole planetary system.

The Galileo Mission to Jupiter

Artist's impression of the Galileo mission to Jupiter.

In a previous post, I summarised some of the stories of Voyager's Grand Tour of the outer solar system, as discussed by John Casani at the Alpbach summer school in 2012.  After the Pioneer missions to Jupiter in the 1970s, NASA began to think about its next voyage to the outer solar system, and conceived the Pioneer-class Jupiter Orbiter with Probe mission, a spin stabilised platform with a despun component to allow remote sensing.  At an early stage, the Galileo project was forced to move away from a Titan launch vehicle like the Voyagers, and were instructed to use the Space Shuttle as the launch system, in a move away from the expendable launch systems of the past to reusable technologies.  From the first planned launch in 1982 to the final launch in 1989, Galileo was continuously delayed by problems with the shuttles launch schedule and deteriorating performance.  When it finally arrived in 1995 to drop its probe into the churning atmosphere of Jupiter, it was ten years later than planned, a lesson for all future planners of outer solar system missions!  To accommodate the space shuttle launch, Galileo went through several costly redesigns, including the introduction of a Probe Carrier, a Mars Flyby Module and a Centaur upper stage which never saw the light of day.  Ultimately, it may have been less costly to stick with the original expendable launch vehicle!

Galileo's main hurdle to overcome was a broken antenna, which failed to unfurl in the early days of the mission, considerably limiting the amount of data returned. For example, remote sensing coverage in the infrared was restricted to small postage stamps of discrete regions if the atmosphere, rather than the global coverage that is still desired by we Jupiter scientists.  In the thermal infrared, the PPR instrument featured a rotating wheel used to change the filter wavelengths, but this got stuck and meant that most of our Jupiter observations are restricted to just two wavelengths.  Indeed, with the exception of the Cassini/CIRS experiment that flew by in 2000, we've never obtained thermal infrared observations of Jupiter sufficient to properly get to grips with Jovian meteorology.  That's something we hope to correct with future missions.

Nevertheless, Galileo provided excellent reconnaissance of Jupiter's churning atmosphere and its diverse satellite system, whetting our appetite for a future return to the gas giant.  Chief among its achievements was the in situ probe, which penetrated deep into the Jovian cloud layers to measure the composition of the planet, providing us with clues to how our solar system formed and evolved from the original protosolar nebula.  Ground based observations by my colleague Glenn Orton (JPL) using the NASA Infrared Telescope Facility showed that the probe had entered a region of unique meteorology known as a hotspot, with powerful down-draughts clearing the air of its main volatile species, ammonia and water.  

The Galileo orbiter was eventually plunged into the atmosphere in 2003 after an 8-year mission.

NASA's Juno Mission at EPSC 2012

The European planetary science convention (EPSC) took place in September 2012 in Madrid, and for the second year running I was asked to convene the session on the giant planets.  With European activities on JUICE, ESA's proposed mission to Jupiter in the 2030s, in full swing, I hoped to persuade the Juno team to 'cross the pond' to update us on the status of he Juno mission.  Scott Bolton, the mission PI, provided an overview, while Mike Janssen explained the microwave remote sounding of Jupiter's deep interior and Glenn Orton explained the need for coordinated ground based observations in support of these activities.  From the hastily-written notes as the rain fell in Madrid, here's a record of the Juno sessions at EPSC.

Bolton - The Juno Mission

With three solar arrays 8.5 m long, Juno is a big spacecraft, cartwheeling along towards the gas giant.  Juno's science goals at Jupiter will fall into four categories, studying the origin, interior, atmosphere and magnetosphere of the giant planet. The mechanisms by which Jupiter formed are a key question for the origins of our solar system and our habitable planet, and Juno will provide answers to two mysteries - how much oxygen is present, and does Jupiter have a rocky core at its centre? 

The motivation for this is the impasse reached in our understanding of Jupiter's formation since the Galileo probe 17 years ago.  We know that Jupiter is enriched in materials compared to the solar composition, and that all the elements are enriched by roughly the same amount.  Juno will help answer how those materials were incorporated into Jupiter by measuring the abundance of water (molecules were probably delivered trapped in water ice cages), something that escaped Galileo in 1995 when it descended into the Sahara desert of Jupiter (a dry hot spot). 

Probing the core of Jupiter, if one exists at all, will require both precise gravitational measurements as Juno orbits the gas giant, in addition to magnetic field measurements to understand how the interior rotates, where convection occurs, where the magnetic field is generated and why it appears to be asymmetric.   It will help answer how Jupiter's interior rotates, whether as a single solid body, or as a series of concentric cylinders with deep winds.   The polar orbits of Juno will be ideal for such measurements, as well as revealing the unique dynamics and chemistry of the polar atmosphere and auroras for the first time.  The trajectory will take the spacecraft through the regions where particles are accelerated along magnetic field lines.

To achieve its science aims, Juno must get closer to Jupiter than any other spacecraft, within the intense radiation belts.  Those belts, rather like the earth's van Allen belts, contain high energy particles that pour through sensitive detectors and electronics, meaning that Juno will be short-lived, and the prospects for an extended mission slim. There will be 32 primary science orbits roughly 11 days long, starting with the spacecraft close to the equator (perijove at 5000 km above the cloud tops) and distant over the poles, but eventually moving so that Juno enters more and more of the radiation belts as the planet's gravity tweaks the orbit.

Juno features a suite of instruments required to address its four science themes, including a gravity science instrument from JPL, a magnetometer from Goddard space flight center, a microwave radiometer from JPL, energetic particle detectors from APL in Baltimore, a UV spectrometer from SWRI, and a near infrared spectrometer from Italy.  Finally, there's a colour camera on board (JunoCAM, provided by Malin) intended for public outreach activities.  Bolton described Juno as a antenna farm, with more antennae than any other mission.

Juno was launched on schedule (!!) on August 5th 2011, and will return for a swing by of the earth in September 2013.  It will arrive at Jupiter on July 4th 2016 for a year-long mission, at approximately the same time as Cassini will be completing the proximal orbit phase at Saturn (i.e., close inside of the rings).  Juno's main engine was ignited in September for the first time, a correction manoeuvre in deep space, and came as a huge relief to the team.  That engine is needed to insert successfully into Jupiter orbit, without it they'd go sailing right past!

After orbital insertion, Juno will be in a polar orbit, with Jupiter rotating by 192 degrees longitude between each perijove.  For the first 15 orbits, we'll have ground tracks, north to south, spaced every 24 degrees longitude.  After 15 orbits, Juno will shift slightly and fill in the gaps, so the final product will be scans spaced by 12 degrees of longitude around the planet at very close perijove.  When complete, Juno will hopefully have revealed the existence of a core, the deep water abundance, and investigated the structure of the interior and polar regions for the first time.  Finally, Juno carries with it three Lego figurines, of the gods Jupiter and Juno, and Galileo himself, as a fitting tribute to our exploration of the giant planet.

Janssen - Microwave Observations below the Clouds

Microwave radiometry will be used to peer beneath the clouds of Jupiter to measure its internal water abundance and structure.  The MWR on Juno features six antennae from 1.3 to 50 cm in wavelength, with longer wavelengths probing deeper into the planet.  The antennae are huge, with one taking up a whole side of the spacecraft, and the other five mounted together on another side.  The antennae will provide footprints 12 degrees in size from 1.37-11.55 cm, and 20 degrees in size from 20-50 cm.  The longest wavelengths are most likely to probe the water cloud.  This type of science is impossible to do from the earth, as the synchrotron emission from Jupiter's powerful radiation belts hides the atmosphere from view.

The MWR uses a clever technique to calibrate, and get accuracy on the measurements below 0.1%.  Firstly, the spinning of Juno means that the antennae alternately see the synchrotron emission from the radiation belts and the radiance from the planet, meaning that the difference can be used to calibrate.  But the dependence of the radiance on emission angle is independent of the radiometric calibration, allowing the team to achieve high accuracy relative measurements.  This accuracy will allow Juno to peer down below Jupiter's churning clouds for the first time.

Orton - Ground-Based Support during Juno

EPSC had a session devoted to amateur contributions to astronomy, and this year Glenn Orton presented plans for earth based support of Juno in the years preceding the mission.  Juno's suite of eight instruments misses spectral ranges that could provide crucial contextual information. For example, although the near infrared instrument will cover the 2-5 micron range, and the microwave instrument covers beyond 1.3 cm, there's nothing in between (the mid infrared). The visible camera eill have no science-grade calibration.  Furthermore, Juno will be so close to Jupiter that data will come in narrow north-south strips, missing the global spatial context that telescopes on earth could provide. Finally, the Juno mission is short, and given that we know Jupiter's atmosphere varies over long timescales, we may miss the temporal context of our observations.

The Juno team will address these issues via a coordinated campaign of ground-based imaging, from both professional and amateur observatories.  Whole-disk images, at the same time as the microwave observations during orbits 3 through 8 (November 2016 to January 2017), will provide this context for the connection between the lower troposphere and the upper clouds.  Furthermore, ground based images may allow them to predict the location of features, allowing orbits to be tweaked to redirect remote sensing efforts. 

Real time support will be challenges, as Jupiter will be only 35 degrees from the sun in November 2016, so the team emphasises the need for contextual studies during the preceding apparition, before solar conjunction.  Spatially resolved visible and near infrared spectroscopy are desirable, filling in the gaps and calibration of JunoCAM and JIRAM. But the team are aware that all this will be done on a best-effort basis, with no formal contracts for image reduction and calibration, relying on the passion and expertise of amateur observers worldwide to support the Juno mission.

Personally, I don't think they'll have a problem!