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.

Thursday 7 July 2016

Juno Tour of the BBC

The last 48 hours have been incredible, emotional, exhausting, and inspirational!  I was really lucky to be involved in some of the BBC's coverage of the NASA Juno mission's arrival at Jupiter.  I'll try to write a post on all the excitement at some point soon, but here are a collection of video clips from my day!

An interview with myself and Glenn Orton on BBC Breakfast at 07.40am, July 5th:

An interview on BBC Breakfast at 06:15am, July 5th:

An interview on the BBC News Channel at 5:20pm on July 4th, before arrival: