April and May 2016
This has been an extremely busy period for the section with Jupiter very well placed for evening viewing and with Mars reaching opposition late in the period, then making its closest approach to the earth since 2005 on the 30th of May. In addition Saturn is approaching its own opposition event in early June and there was a transit of the planet mercury across the face of the Sun on the 9th of May! This means there are a large number of observations and contributions to get through in a limited space. With this in mind I have placed all the ‘Transit of Mercury’ observations as a news item on the Society website rather than covering them here. I must also apologise that some late observations from the last period went unreported as I was selfish enough to take a holiday abroad at the end of March, so those observations are incorporated in this report.
I will start with Jupiter and, because there have been a very large number of individual observations, I am forced to cover the general features observed in the period rather than dwelling on individual images. Starting with the South Polar Region; the area has been generally inactive, appearing as a dark blue-grey with occasional faint darker texturing. In this regard the area looks very similar to the North Polar Region but both areas appear very bright in reflected sunlight when methane-band filters are used for imaging ‘though the SPR’s bright cap is smaller than that of the NPR. These filters show bright areas where there is little or no methane in the atmosphere to absorb light close to a frequency of 890 Nm, and dark patches where methane is plentiful. With this in mind the bright polar caps represent high reflective ammonia-ice based cloud with little methane contents.
In visible light the edge of the SPR is dark and quite distinct and just above this area there is a chain of obvious white oval storms rotating in a band around the circumference of the planet. These are officially designated A0 to A8 and all nine of them can be discerned in images contributed by the section. These features are clearly seen in visible light observations as well as near infra-red (IR) and methane-band images. The white ovals are rotating in a distinct South-South Temperate Belt (SSTB) but north of this area there is an off- white zone with very little in the way of obvious features. Oval BA, otherwise known as the little red spot, usually sits in a dark South Temperate Belt (STB) but this belt is almost completely absent with only a faint ghost of it appearing in small patches around the circumference. Trailing Oval BA there is a dark diamond shaped patch or area of small patches that has persisted throughout the period and looks like it may be growing. Perhaps there will be an outbreak of the STB growing from this patch but without it the South Tropical Zone and South Temperate Zones are merged into one large area devoid of obvious features.
Oval BA itself is very hard to find visually. It started the period at a longitude (L2) of 340 degrees and slowly drifted to 324 degrees, making steady progress towards the longitude of the Great Red Spot (GRS). It is a very pale pink colour, hard to discern against the near white of the temperate zone, but is more obvious in near IR images and very prominent in the methane band. There was the suspicion of a darker annulus around the heart of Oval BA at the start of April but this was not evident by late May.
The GRS is strikingly pink visually with a definite darker annulus surrounding it and a further dark concentration at its very heart. There is no obvious sign of this massive storm having shrunken any further since the last reporting period but the spot is still reported to be shrinking and it will be interesting to make year-on-year comparisons to look for changes. A thin light band separates it from the South Equatorial belt making a clear socket for the ‘eye’ of the GRS to sit in and north of this a darker area of the SEB forms an obvious ‘eyebrow’. This eyebrow has thickened and extended in length through the period covering just half of the length of the socket in early April and extending all the way around it and beyond by late May; giving the GRS the appearance of a stylised Egyptian eye hieroglyphic as seen on the walls of an ancient tomb. The eyebrow is the extension of the southern-most of two dark bands that bifurcate the SEB prograde of the GRS. These bands are obvious over nearly 200 degrees of longitude with the northern band eventually becoming dominant and completing the circumference all the way around to the extremely disturbed area within the SEB that sits just retrograde of the GRS. This disturbed area is very distinct with folds and whirls of cloud forming intricate patterns, some of which are very bright in the methane band and show that cloud is being thrown very high by this disturbance. The GRS itself is a pure-white spotlight in methane and this probably represents the highest point achieved by cloud anywhere in the Jovian atmosphere.
The broad Equatorial Zone is bland visually giving only vague hints of detail, photographically it shows more features with perhaps ten large dark barges tied to the northern ‘shore’ of the zone and thin festoons extending out into the zone itself. In IR frequencies the zone becomes very chaotic with the festoons tangling with each other and with the arches they form, near to the dark barges, enclosing regions which are white visually, pale pink in IR and bright in the methane band. Last period there was a hint of a dark coherent equatorial band within this zone but no trace of it remains within the current chaos.
The North Equatorial Belt (NEB) is very distinct and appears as a brown belt with a generally uniform southern boundary and with obvious waves distorting the northern edge as it is cut into by storm features running around the North Tropical Zone (NTrZ) next to it. Last period there was a noticeable thickening of the NTB that steadily extended around its circumference making both the NEB and the SEB identical in width over much of the planet but this thickening seems to have stopped, and perhaps even reversed, towards the end of the period. This is quite unusual, even unprecedented, in that the ‘normal’ cycle of activity seen during a thickening event sees the thickening extend around the full circumference and then persist for some time before fading away and leaving dark barges and ovals which are eventually destroyed by white-oval storm activity in the NTrZ. Quite why the belt expansion has stopped this time is unknown and, no doubt, will be one area of study for the Juno probe shortly to arrive in orbit at Jupiter. The belt has been very dynamic with, for example, a short-lived collection of three white spots appearing between the 12th and the 14th of May, near the same longitude as the GRS, and then disappearing completely by the 16th.
The North Tropical Zone is, like its southern partner, generally broad and bland though the large storm White Spot Zulu (WSZ) is still obvious cutting into the northern edge of the NEB and a similar storm becoming clear some 70 degrees retrograde of it, if not quite as large or as well developed. There is little evidence of either a North Temperate Belt (NTB) or of a North-North Temperate Belt (NNTB) as separate coherent entities but faint strips of both systems persist around the northern circumference of Jupiter perhaps hinting at an outbreak later in the year. A distinct white oval storm was seen in high northern latitudes on the edge of the North Polar Region at the end of the period close to the same longitude as WSZ and the GRS.
A number of excellent images were received showing moon or shadow transit events at Jupiter and it is good news that an increasing number of people are managing to get such detail that faint albedo features on the Galilean moons is discernible in their work.
Moving on to the other planets; Mercury is rarely observed in detail or photographed but given the rare event of a Mercury Solar Transit on the 9th of May this tiny planet received more attention than normal and both Martin Lewis and Simon Kidd managed to image it in mid-April using IR filters and fast modern cameras. Occasionally faint features can be caught in this way and successfully correlated with existing maps of the planet. Saturn also received a little attention in the period despite being very low from UK skies. Both Steve Norrie and Robin Scagell took pictures of the planet with Robin’s results coming from a small 130mm reflector from the less-expensive end of the market. I mention this because his images are good as well as scientifically interesting and they go to show that with low-altitude targets, or when our atmospheric conditions obscure the smaller details, all telescopes can make a valid contribution not just the massive back-yard canons available to some observers.
Robin also imaged Mars as did a number of other contributors with interesting results. From early in the period the angle at which we can view the planet should have made the norther polar ice-cap visible but it was impossible to see clearly, nor was there any frost visible in the Hellas basin but large and dark surface features were on show along with morning and evening cloud near the limbs and some orographic cloud over the high points of the Tharsis shield volcanoes and the Elysium plateau. The southern hemisphere of Mars is moving from late winter into spring and extensive cloud occasionally appeared around the South Pole. Then, at the end of the period, the western end of the Elysium area suddenly darkened from its usually bright appearance and it seems likely that a large dust storm has broken out in the area or moved in from Amazonis. Such storms are uncommon near Elysium and the area will bear watching during the next reporting period to see if there is any development or spreading of the dust.
I must now list individual contributors from the section during the period. Alan heath sent in extensive notes on his Jupiter observations giving comparisons of brightness, colour and intensity for features seen. Matthew Barrett also sent in sketches using the SPA reporting ‘blanks’ allowing direct comparisons with both Alan’s notes and other images. Then, cast in order of appearance, we have images sent in from Simon Kidd, Martin Lewis, Robin Scagell, Steve Norrie, Dave Finnigan, Drew Sullivan, Paul Crossland, Alexei Pace, Carl Bowron, Graham Taylor and myself. It may be of interest to other section members that Martin Lewis has started using an Astronomik Pro-Planet 642 BP (band pass) filter for his IR imagery of Jupiter and found it very successful with his monochrome ASI cameras. At the same time the deep-sky section director, Dave Finnigan, experimented with both 685Nm and 807Nm filters for IR work and found that while they were both successful in helping to pull detail from a turbulent atmosphere the lower resolution of the longer wavelength filter meant it tended to get used less often than the 685Nm filter. Dave also started using an atmospheric Dispersion Corrector with his images and found that, while fiddly to set up correctly, they do help illuminate colour fringing effects in images and therefore sharpen detail and are well worth the money in his opinion; for what it’s worth I definitely agree.
With Saturn at opposition on the second of June I hope many people will take the opportunity to observe and, as always, I look forward to seeing your results.