Jupiter’s Internal Powerhouse and Ultracool Dwarfs

Jupiter, the solar system’s largest planet, is turning out to be as majestic as its ancient name implies.  High definition images taken of its poles, transmitted back to Earth by the space probe Juno, show a vibrant, churning cloudscape which appear to have been artistically generated in oils (1).  The gnarly appearance of the storms and tempests which are woven into this mind-blowingly immense vista seem peaceful enough from space, but the ferocity of their winds can only be imagined.  Although the colours have been enhanced to a certain extent artificially (2), Juno’s imaging equipment has captured the incredibly beautiful blue colours of the polar zones and the immense set of storms swirling within.

(Image credit: NASA/JPL-Caltech/SwRI/MSSS/Gerald Eichstädt/Seán Doran)

These dramatic regions contrast strongly with the generally dull series of beige bands wrapping around the more familiar equatorial region (although Juno has also allowed us to better appreciate the intricate patterns of these banded zones, too) .  These blues are more reminiscent of the ice giants Neptune and Uranus, and perhaps even of Earth – although the constituent gases of the atmospheres of these worlds can differ significantly from Jupiter’s.  which give the clouds.  The different colours and properties of Jupiter’s clouds can be attributed to their constituent gases – mostly hydrogen and helium, but also water, ammonia, methane and sulphur.

It seems to me that the solar system is starting to come to life – not in the way of biological life, although that may yet come to be, but instead in terms of our appreciation of its rich complexity and visuality.  The Pioneer and Voyager space probes provided what were incredible images of the outer solar system planets back in their day.  But limitations in the image-capturing technology also created a sense in those images of dull uniformity.

In the decades before the space-probe images had been sent back from the outer solar system, sci-fi writers, film-makers and scientists had created an amazing array of ideas about what these worlds might be like.  This potential had become ingrained within the public collective consciousness, and to some extent helped drive NASA’s ambitious space programme forward.  This was enhanced by a sense of mystery – and a hope of alien life.  However, the images returning to our television screens in the latter part of the 20th century clearly did not do these worlds justice.  So, although obtaining the planetary images were astonishing achievements in themselves, the disappointing lack of features within them dashed many hopes, and provided the public with a new view of the outer solar system.  Like lifeless Mars and overheated Venus, the outer solar system consisted of a rather mundane set of giant planets distinctly lacking in the vibrant complexity of our own Earth. 

But the 21st century has brought renewed vigour to this visual record of our solar system.  Cassini’s long mission to Saturn and its moons provided stunning images of great beauty, as well as the realisation of the existence of liquid water bursting out of the moon Enceladus.  Saturn’s weird hexagonal patterning over its north pole broke the established mould of beige uniformity for the gas giants.  Then New Horizons’ flyby of Pluto presented a multiplicity of surface features which few had dared to dream of.  Now, Juno has sent us these incredible images of the incredibly complex cloudscapes of our solar system’s largest known planet.  It feels as though the solar system is beginning to reveal itself in a new way – showing a complex and unpredictable side to itself which is both tantalizing, and also rather wonderful.

Jupiter’s Northern Lights

It is not just the new visual imagery which is challenging us to look anew at these immense worlds.  Cassini and Juno have sent back valuable scientific data from their on-board instruments, gathered from the highly charged and radioactive regions of space surrounding Saturn and Jupiter.  Juno’s data about Jupiter’s polar aurorae indicates that the acceleration of particles through the Jovian atmosphere due to its strong magnetic and electrical fields is being driven by a different, more powerul process than the one we observe here on Earth:

(Image Credit: NASA/JPL-Caltech/Bertrand Bonfond)

“Jupiter has the most powerful auroras in the solar system, so the team was not surprised that electric potentials play a role in their generation. What’s puzzling the researchers, [Barry] Mauk [of the Johns Hopkins University Applied Physics Laboratory (APL), Laurel, Maryland] is that despite the magnitudes of these potentials at Jupiter, they are observed only sometimes and are not the source of the most intense auroras, as they are at Earth. 

“”There are hints in our latest data indicating that as the power density of the auroral generation becomes stronger and stronger, the process becomes unstable and a new acceleration process takes over, “ [said Mauk, who leads the investigation team for the APL-built Jupiter Energetic Particle Detector Instrument (JEDI)]…. Scientists consider Jupiter to be a physics lab of sorts for worlds beyond our solar system, saying the ability of Jupiter to accelerate charged particles to immense energies has implications for how more distant astrophysical systems accelerate particles.” (3)

Juno appears to be observing two processes at play: A distinct, high-energy, downward, discrete electron acceleration in Jupiter’s auroral polar regions; as well as upward magnetic-field-aligned electric potentials of up to 400,00 electronvolts. This latter electrical potential is an order of magnitude greater than the largest potentials observed on Earth (4).  Jupiter’s own ability to accelerate high-energy electrons may indicate the process which, taken up to a larger scale, becomes capable of converting gas giant planets like Jupiter into dwarf stars.

Brown Dwarf Magnetic Fields

If this is what is happening on Jupiter, then perhaps we can extrapolate further upwards for the kinds of particle acceleration processes that must be occurring within its more massive cousins, the sub-brown dwarfs, and onwards to the broad category of brown dwarf ‘stars’.  These failed stars would seem quite capable, then, of generating even more potent aurora displays from their immense magnetic and electrical fields.  Brown dwarfs, like their less massive gas giant cousins, are thought to generate strong fields by a convection-driven dynamo process within their electrically conducting interiors (5).  Dynamos occur when the interior of the body rotates faster than its surface layer.  This accelerates electrons to create magnetic forces and thus magnetic fields within and around the star or planet.

(Image Credit: NASA’s Goddard Space Flight Center/S. Wiessinger)

A study published last year, examining radio flares emitted by ultracool dwarfs, indicated that brown dwarfs and sub-brown dwarfs might be capable of experiencing magnetic field reversals, similar to stars, on decade-long timescales (6).  Up until the publication of this research, it had been thought that the complex internal structure of stars was very different to that of smaller ultracool dwarfs, meaning that the latter should be incapable of strong disturbances, like magnetic field reversals:

“There’s a major difference between ultracool dwarfs and Sun-like stars: their internal structures. Sun-like stars have a convective envelope that surrounds a radiative core. The interiors of cool, low-mass objects, on the other hand, are fully convective. Based on theoretical studies of how magnetism is generated in stars, it’s thought that the fully convective interiors of ultracool dwarfs can’t support large-scale magnetic field formation. This should prevent these stars from exhibiting activity cycles like the Sun.” (7)

Brown dwarfs sometimes seem to be a lot more like stars than planets, even though the processes by which they emit their dim light are very different from the nuclear fusion driving stars.  For instance, brown dwarfs can exhibit surprisingly strong surface magnetic fields and, more importantly,  these fields can show unexpectedly high levels of disturbance (8).  In a way, all of this tells us something we already know – brown dwarfs sometimes behave like stars, sometimes more like gas giant planets like Jupiter.  What we’re perhaps now appreciating is that Jupiter itself is more complex than had previously been imagined.

Written by Andy Lloyd,  18th November 2017

References:

1)  The Space Academy “NASA’s $1 Billion Jupiter Probe Just Sent Back Stunning New Photos Of Jupiter” 7th November 2017 http://www.thespaceacademy.org/2017/11/nasas-1-billion-jupiter-probe-just-sent.html

2)  NASA’s Juno Mission 16th November 2017 https://twitter.com/NASAJuno

3)  D. Agle & D. Brown “Jupiter’s Auroras Present a Powerful Mystery” NASA/JPL, 6th September 2017 https://www.missionjuno.swri.edu/news/jupiters-aurora-presents-powerful-mystery

4)  B. Mauk et al “Discrete and broadband electron acceleration in Jupiter’s powerful aurora” Nature 549, 66–69, 6th September 2017, https://www.nature.com/articles/nature23648

5)  Open PhD Project “Dynamos in giant planets and brown dwarfs” International Max Planck Research School, https://www.mps.mpg.de/phd/astrophysics-dynamo-giantplanets-browndwarfs

6)  Matthew Route “The Discovery of Solar-like Activity Cycles Beyond the End of the Main Sequence?” 25th September 2016, Astrophysical Journal Letters, 830, L27, https://arxiv.org/abs/1609.07761

7)  Susanna Kohler “Could Ultracool Dwarfs Have Sun-Like Activity?” 9th November 2016 http://aasnova.org/2016/11/09/could-ultracool-dwarfs-have-sun-like-activity/

8)  Jesse Emspak “Brown dwarfs have strong magnetic fields just like real stars” New Scientist, 15th September 2017 https://www.newscientist.com/article/2147636-brown-dwarfs-have-strong-magnetic-fields-just-like-real-stars/

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