Monday, 28 October 2013

Jupiter Science in 2013

Continuing the theme of my previous post on ice giant science in 2013, this time I'll take a look at the latest Jupiter studies presented at the European Planetary Science Congress (EPSC, September) in London and the Division of Planetary Sciences Meeting (DPS, October) in Denver.  I've cobbled these results together from tweets, abstracts, papers and conversations with colleagues, and any mistakes or omissions are my own!  And my apologies if your favourite result isn't here, I never meant to be totally comprehensive...

Damian Peach's image of a double shadow transit of Io
and Europa, October 5th 2013.
2013 found Jupiter in a more 'normal' state than in 2012, with the planet in the late stages of a 'global upheaval' in its banded structure, characterised by the episodic fading (whitening) of the typically red-brown belts, followed weeks or months later by dramatic storm-like revivals of their intense colours.  The current life cycle started with the fade and revival cycle of the South Equatorial Belt (SEB) between 2009 and 2011 (the chaotic rifting activity northwest of the Great Red Spot is now present and active), and most recently we've seen a revival of the North Equatorial and Temperate Belts (NEB and NTB) in 2012.   Amateurs are tracking a strange light patch to the east of the Great Red Spot, possibly a cyclonic oval within the SEB.  As usual, head to the Jupiter Section of the British Astronomical Society for apparition reports and more details.

The Colourful Clouds of Jupiter

The episodic changes in Jupiter's global appearance offer a unique glimpse into the processes responsible for generating the cloud colours and albedo patterns, both for entire red-brown belts and also for discrete red features.  The chromophore responsible for the Great Red Spot's distinctive colour remains a mystery because of difficulties in identifying unique signatures of any particular chemical, but several authors are making headway.  Simon-Miller et al. (DPS) presented Hubble Space Telescope images of an intense red cyclone visible in 1994-95, suggesting that the Great Red Spot, intense red ovals and North Equatorial Belt might indicate the presence of the same chromophore but under different conditions (e.g., different amounts of mixing with white clouds, or longer UV irradiation at high altitudes), with multiple components involving NH4SH (one of Jupiter's condensate clouds) and hydrocarbons (produced photochemically above Jupiter's clouds and then sedimenting downward) required to reproduce the spectra. Chanover et al. (DPS) presented work in a similar vein, exploring 300-1000 nm spectra of Jupiter acquired from Apache Point Observatory, whereas Bjoraker et al. (DPS) presented high-resolution 5-µm spectra of CH3D lines to probe structural differences between cloud-free 'hot spots' and cloudy vortices like the Great Red Spot, indicating that a water cloud near 5 bar is responsible for the main opacity of the Great Red Spot at 5 µm, and not the ammonia (0.7 bar) or NH4SH (2-3 bar) clouds we might have otherwise expected.  Conversely, their hot spot observations were consistent with a complete absence of the water cloud.

Giles et al. (EPSC) focussed on the processes responsible for the revival of Jupiter's reddish SEB colours, using 7-25 µm imaging of Jupiter's thermal emission from the VLT to determine the deep warming taking place during the revival and subsequent sublimation of the white 'faded' aerosols.  Furthermore, Tejfel et al. (DPS) used visible-light spectroscopy to show the higher density of scattering aerosols when the SEB was faded, likely due to enhanced ammonia ice condensation as the SEB cooled during the fade.  Pulling this infrared thermal emission (the long-wave) together with the changes in visible albedo (the short wave) will be a crucial element for understanding the cloud decks of Jupiter and the origins of the colours, but there's still a long way to go.

Making Waves

For atmospheric changes on Jupiter over even shorter timescales, Hueso et al. (EPSC) presented estimates of the impact flux into Jupiter's atmosphere based on the three recent bolide events (i.e., impact flashes) seen between 2010 and 2012.  These impactors were in the 5-20m diameter category and released similar amounts of energy to the Chelyabinsk meteor earlier this year.  Hueso et al. estimate 18-160 impacts of this nature per year on Jupiter.  Pond et al. (DPS) used numerical models of jovian impacts (the ephemeral flashes and observations of Shoemaker Levy 9 and the 2009 'Wesley' impactor) to study the propagation of shock waves through the atmosphere.

Impact flashes on Jupiter.
Rogers et al. (EPSC) report on the possible influence of a planetary-scale wave on Jupiter's south temperate belt (STB, bounded by jets at 26-29S and 36S and home to Oval BA), where there are always 2-3 structure sectors of small-scale turbulence, one of them headed by Oval BA at its eastern end.  Oval BA is typically followed by a dark segment that sometimes contracts to form a cyclonic oval (a dark barge).  From Rogers:  "The other structured segments begin as  small dark spots or streaks remote from oval BA, then expand, and eventually catch up and merge with  the dark segment at BA, inducing intense disturbance in and around it. This cycle has been completed three times in 15 years, maintaining at least 2 structured sectors at all times. The major changes in drift rate of oval BA appear to be due to the impacts and subsequent shrinkage of the structured segments." Interestingly, Rogers predicts that Oval BA will shrink in the coming years so that it no longer controls the dynamics of the STB, so the STB cyclonic segments will develop a long-lived pattern, with the spaces between developing into anticyclonic circulations that may herald the next generation of larger, anticyclonic white vortices (i.e., the state of the STB before 2000).

Comparing the Giant Jets

Liu et al. (DPS) presented numerical models seeking to explain why Jupiter has narrower and weaker atmospheric jets than Saturn's broad and strong jets.  Although the radii, rotation rates and atmospheres of the two worlds are rather similar, Jupiter has 15-20 off-equatorial jets with speeds of around 20 m/s at the cloud tops, whereas Saturn has only 5-10 wider off-equatorial jets, with speeds of around 100 m/s.  Both planets have strong super-rotating equatorial jets, and vortices fill the spaces between the jets.  Liu suggests that Jupiter's jets experience stronger magnetohydrodynamic drag in the planetary interior than on Saturn.  Heavens et al. (DPS) looked at why Jupiter's jet structure disappears poleward of 65 degrees (and vortices come to dominate the flow, referred to as 'polar turbulence'), whereas on Saturn the organised jet-like structure extends all the way to the pole.  They suggest that the stability of the jets is the main criterion for the transition to polar turbulence rather than jet-dominated flow.

A Selection of 2013 Jupiter Papers

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