White spot Z: its history and characteristics, 1997-2013
White spot Z: its history
and characteristics, 1997-2013
John Rogers (British Astronomical Association)
White spot Z (WSZ) is a prominent, long-lived anticyclonic white oval (AWO) at ~19ºN, in the North Tropical domain. It first appeared in 1997
and is the only feature in the domain to have lasted more than a few years. It has claimed attention in autumn 2013 because it has become
unusually methane-bright and now is developing slight reddish colour. Here we summarise the previous history of WSZ, from our previous reports
and from JUPOS data, and provide illustrations of its recent behaviour.
To a first approximation, WSZ is a great AWO at 19ºN, and it appears thus when the N. Equatorial Belt (NEB) is broadened to encircle it with
dark material. However, when the NEB is narrowed so that WSZ is in the N. Tropical Zone, its oval form is often obscured by a faint grey
streak, whereas a tiny, bright white spot appears on its N edge at 20.2ºN. During the great NTBs jetstream outbreaks in 2007 and 2012, WSZ
became dark grey.
WSZ almost always has a faster drift than other stable spots in the domain (AWOs and cyclonic barges), and lies slightly further north than
other AWOs. In its early years, it eliminated other spots preceding it; later it became more likely to accelerate them to its own speed,
‘pushing’ them ahead of it and sometimes causing them to merge with other spots further ahead. In exceptional circumstances in 2007 and
2009, it failed to accelerate barges preceding it and almost halted its drift, but only temporarily. Conversely, after the great northern
upheaval of 2012, it accelerated to a rapid speed which was unprecedented and anomalous for its latitude, and collided with an AWO
preceding it in early 2013. Collisions between AWOs in this domain are uncommon and lead to only partial merger; a remnant of the leading oval
always persists for a short time.
These ovals are not usually
methane-bright, but WSZ became modestly methane-bright after the 2013 merger,
and by 2013 Sep. the whole oval
was strongly methane-bright. In visible light, the aspect of a grey streak and a northerly bright spot was notable. The grey area acquired
slight brown tint during Oct., then in late Nov. more distinct reddish colour appeared around the northerly bright spot. We wait to see
whether WSZ will now transform itself into a Little Red Spot.
Download the full report in PDF
Download the report Appendix in PDF
the full resolution figures (ZIP file, 10MB.)
John H. Rogers, Ph.D.
Jupiter Section Director,
British Astronomical Association