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Jupiter’s polar polygons: Clusters of cyclones around the poles


The Juno spacecraft orbiting Jupiter has discovered that cyclones around the planet’s poles are tightly packed into remarkable polygonal patterns.  At the south pole, five cyclones form a pentagon around a central cyclone; at the north pole, eight cyclones form a modified octagon or ‘double square’ around a central one.  These patterns, discovered by the JunoCam and JIRAM instruments, are disclosed in a paper published today in ‘Nature’*, and we present more details and animations from the JunoCam observations in a report posted today on the BAA Jupiter Section web pages:, & animations at

The spacecraft’s camera, JunoCam, has been viewing the circumpolar cyclones (CPCs) since the first close orbital passage (perijove), but as it can only view the side that is illuminated as it flies over each pole, the polygonal patterns only became clear over the first few perijoves.  In contrast, JIRAM (the Jovian Infrared Auroral Mapper) views not reflected sunlight, but thermal infrared emission at 5 microns wavelength, and so it recorded the entire polar regions at perijove-4, and large parts at perijove-5.  Together, the two instruments have produced complete views of the polar polygons and their circulations.

The north polar CPCs have diameters of 5200-7000 km, and the eight contiguous ones in the octagon are separated from the central cyclone by a narrow chaotic zone with westward flow and some vortices.  The central cyclone, which can only be viewed by JIRAM as it is currently in darkness, is centred only ~0.5º latitude off the true pole. The octagon of CPCs around it has not changed substantially in over a year, with only limited swings to and fro around the pole.

The south polar CPCs have diameters ranging from ~6400-8200 km, and the five forming the pentagon are mostly in contact with each other and with the central one.  But the pentagon is not entirely symmetrical. It is approximately centred on the central cyclone, whose centre is offset from the pole by 1.0º to 2.5º latitude, and the JunoCam images show that this offset has varied cyclically.  On the side furthest from the pole, there is a gap between two cyclones in the pentagon, and this gap grew larger over the first eight perijoves; it has now shrunk again.

The wind speeds within the CPCs, measured independently from JunoCam and JIRAM images, are mostly about 200-350 km/hr at a radius of 1500 km.  Closer to the centres, JunoCam measurements show that some of them have even faster speeds, exceeding 450 km/hr near the centre of the south polar cyclone.   So these cyclones are similar to terrestrial hurricanes in wind speeds, or even faster, and they are several times larger.

These patterns are different in nature from the famous north polar hexagon on Saturn, which is a kinked jet-stream surrounding a single giant cyclone. In contrast, each pole on Jupiter has a polygonal cluster of cyclones, which are are remarkably stable.  They have persisted with little change over the first eleven perijoves, despite the furious winds which blow within them, and the buffetting from the chaotic, rapidly changing storm systems around them.  It is not yet clear theoretically how these polygons can be stable, but they seem to be real-world examples of patterns called ‘vortex crystals’ which were previously known only from laboratory modelling.

* Adriani A, Mura A, Orton G, Hansen C, Altieri F, Moriconi ML, Rogers J, Eichstädt G, et al. ‘Clusters of Cyclones Encircling Jupiter’s Poles.’  Nature (8 March 2018)


Links to the full-size images:   News-Fig0_Polar-polygons.jpg


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Mercury and Venus well displayed in the evening sky

Mercury has a reputation for being difficult to see and it is often reported that Copernicus failed to see it. I think the main problem is not that it is difficult to see, but rather that it is difficult to find. Once found, however, it can be an easy naked eye object. You just have to know when and where to look and this March, in the western evening sky, there is just such an opportunity. 

 Jupiter, with an orbital period of slightly under 12 Earth years and an average distance from the Sun of 5.2AU, is frequently visible in the sky for months on end (it is now rising before midnight and will dominate the southern sky for the next few months – albeit at a very low altitude). Mercury, however, with an orbital period of 88 days and an average distance from the Sun of only 0.39AU is too close to the Sun to be seen in a truly dark sky from the UK and needs to be caught when at its greatest angular distance from the Sun relative to the Earth – known as greatest elongation. With an orbital period of only 88 days it might be thought that these occasions would occur frequently enough to make Mercury a regular visitor to the evening or morning sky. The problem though is that not all elongations are created equal. In addition to being at greatest angular distance from the Sun it also needs to be at its greatest height above the horizon. 

Mercury, along with the other planets, sticks closely to the path in the sky known as the ecliptic and the angle that the ecliptic makes with the horizon varies throughout the year. In spring the ecliptic in the evening sky rises steeply from the horizon meaning that Mercury, if it reaches greatest elongation around that time, will be at its greatest height above the horizon. Conversely in autumn the ecliptic in the morning sky is at its most vertical, so morning apparitions of Mercury are best seen then. An additional complication with Mercury is that its orbit around the Sun is highly elliptical and this affects how far above the horizon the planet appears to an observer. 

This March, in addition to Mercury being well displayed, Venus also joins in the fun and the two planets get within 1 degree of each other on March 4. Unfortunately this will not be an easy observation as with the Sun setting at 17:46 UTC from London (17:53 from Edinburgh) the planetary pair will be only around 6 degrees above the western horizon at the end of civil twilight. A clear tree, building and murk free horizon will therefore be essential. Mercury will be shining at magnitude -1.2 and Venus considerably brighter at -3.9.  If the sky is at all murky binoculars may help in the hunt, but never attempt this before the Sun has fully set. One glance at the Sun through any optical aid can lead to instant and permanent blindness. 

The planets remain close together for the next few days, so if poor weather prevents observation on March 4 there will be other opportunities, although the angular distance between them will have increased. On March 5 the distance will be just under 1.5 degree and on March 10 just under 3.5 degree (see chart below). Mercury itself will be best seen on March 15, the date of greatest eastern elongation (18 degree), when at sunset (18:05 UTC from London) it will be 16 degrees above the western horizon. Mercury reaches perihelion (its closest point to the Sun) on March 10 so, for the reasons outlined above, this restricts its height above the horizon. On the day of Mercury’s greatest elongation Venus will be around 12 degrees up. After this, hopefully, splendid evening display Mercury moves ever closer to the Sun, reaching inferior conjunction – directly between the Sun and the Earth – on April 1. Before that though a young very slender Moon also gets in on the act (New Moon is on March 17) creating another interesting observing opportunity. And all at a reasonable time in the evening sky. 

If you fail to see Mercury this time around there will be a further opportunity for northern hemisphere observers in August, but that will be in the morning sky.

[Thumbnail image on homepage taken from Stellarium 0.17.0]

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Digital membership

On 1 February 2018 we launched a new way to subscribe to the BAA - our Digital Membership.

Digital subscribers will be able to download electronic versions of the BAA Journal and Handbook and enjoy all the other benefits of a traditional BAA subscription at a greatly reduced cost. For further information about the BAA and what a membership could mean for you, check out the About Us pages. To advertise the launch of our digital subscription you may well see copies of our flyer (right) on the BAA stands at the various astronomy events throughout the year, including Astrofest in under two weeks time.

We acknowledge that many members still prefer to receive paper copies of the Journal and Handbook and the traditional subscription will continue to be offered.

For existing members who are interested in a digital subscription, once your current year’s membership expires and you come to renew, you have the option to choose either a traditional or a digital membership. If you have any questions about this when your renewal dates approaches, ask on the forum, or contact the office. Membership types cannot be altered part way through a member’s year.

We are excited about the launch of digital membership, and about broadening the options for our members.

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