Notes and Ne Notes and News

Notes and News Mercury & Venus Section Messenger at Mercury, 2008 January In the last issue of the Journal [(118(1), 6−9 (2008)] we re- paratively young, and may be one of the largest and youngest viewed recent BAA observations of Mercury. NASA’s Messen- basins in the Solar System. The basin is an area of smooth plains ger spacecraft encountered the planet on Jan 14, approaching to but is also pockmarked by a number of young craters; it also has just 200km at the closest point. As can be seen from Figure 2, it a large number of radial and tangential lineaments and some unu- imaged part of the hemisphere that Mariner 10 did not view sual dark-rimmed craters (Figure 3A). during any of its three encounters of 1974−’75. The Messenger website adds the following: ‘Near the centre of It is easy to calculate what part of the planet was sunlit. Extrapo- the basin, an area unseen by Mariner 10, this remarkable feature – lation from the BAA Handbook shows that Mercury (during its E. nicknamed ‘the spider’ by the science team – was revealed. A set of elongation) on January 14 at 0h UT had a CM longitude of 224° troughs radiates outward in a geometry unlike anything seen by and a phase of 0.82. The phase angle i is given by: Mariner 10. The radial troughs are interpreted to be the result of phase = 0.5 (1 + cos i) extension (breaking apart) of the floor materials that filled the Caloris therefore i=50°. This implies that longitudes from 94° to 274° basin after its formation. Other troughs near the centre form a (at the observable terminator) were sunlit and therefore imaged. polygonal pattern. This type of polygonal pattern of troughs is (Messenger arrived later than 0h UT, but the calculation will also seen along the interior margin of the Caloris basin. An impact serve our purposes.) crater about 40km (~25 miles) in diameter appears to be centred on Messenger returned a number of excellent images, and in this ‘the spider.’ (Figure 3B).’ short note we illustrate some examples (Figure 1 (on the cover), Elsewhere on the planet, Messenger observed further examples and Figures 2−4). One of the highlights of the new mission was of scarps (Figure 4), and therefore provided more evidence of its the imaging of the entire Caloris basin (Figures 1 & 3), leading to past crustal shrinkage. It re-observed some features discovered by a larger estimate of its diameter from 1300km to close to 1550km. Mariner 10, under different lighting conditions (Figure 5). It also This basin had appeared bisected at the terminator on the Mari- ner images. Under the near-vertical lighting prevailing this time, Caloris appears relatively bright and therefore geologically com- Figure 3A (top). A close-up view of Caloris. On the left, Messenger’s view; stitched onto the right is a collage from Mariner 10, under more oblique lighting. Note how the modern estimate of the basin diameter (in blue) has been revised upwards Figure 2. The standard albedo map of Mercury, compiled from from that of the 1970s (in yellow). North is uppermost. ground-based photographs and drawings, after Camichel & Dollfus Figure 3B (bottom). The striking pattern of fractures in the (1968), upon which are superimposed the longitudes imaged by centre of Caloris basin, centred upon a 40km wide impact crater. Mariner 10 and Messenger. South is uppermost. North is uppermost. J. Br. Astron. Assoc. 118, 2, 2008 67 Notes and News carried out spectroscopic analyses to deter- cord is very good, and the resolution of the mine the surface mineralogical composition ground-based work surprisingly high. An- as well as investigating the planet’s sodium other puzzling feature of the older telescopic ‘tail’, magnetic field and magnetosphere. work was the fact that Mercury’s S. cusp It is important to compare Messenger’s always appeared darker than the N. one. new images of the previously unmapped These latest images show that this is simply hemisphere with Earth-based images and because the S. polar region is more densely drawings: see Figures 6−7. In Figure 7 we cratered and rougher than the northern. compare the classic work of G. V. Schiaparelli Messenger will encounter Mercury and E. M. Antoniadi with a blurred and de- again in October before it finally enters graded version of Figure 1. Both observers into orbit around the innermost planet. had clearly mapped the Caloris basin with Watch this space! its darker surroundings. In Figure 6 we compare more recent CCD work. Again the ac- Richard McKim, Director Figure 4. Mercurian cliffs: a new long scarp on the previously unknown hemisphere. This Messenger image is 200km wide. North is uppermost. Figure 7. Reduced and blurred copies of Figure 1 compared with the classic work of Schiaparelli and Antoniadi. South is uppermost. All illustrations show Caloris as a light oval with dusky borders. Figure 5. Vivaldi, a double-ringed crater whose outer diameter is about 200km. From the President: Meetings! Craterlets down to 1km diameter can be Meetings? How many of our members ac- dinary’ meeting, is again held in BH but resolved. This crater had previously been imaged by Mariner 10 at lower resolution tually attend the very many excellent meet- with a special keynote speaker. The Exhi- and under different illumination. North is ings that our Meetings Secretary organises bition Meeting, one of our premier meet- uppermost. each year? Some are certainly well attended ings, is currently being held in the National but others... less so. I realise that for some, Space Centre; and finally there is the Out it is very difficult due to many factors, but of London Meeting, which as the name sug- for others it must be because they’ve never gests, may be held anywhere other than ever attended a BAA Meeting anyway or London! This year it will be in Cambridge perhaps that they did so many years ago as part of the AAVSO/VSS extravaganza. and found it, well, less than stimulating. We then have a number of extended meet- Let me assure everyone that All BAA Meet- ings. Firstly, there is the infamous ‘Win- ings are Excellent nowadays! chester Weekend’, this year moving away So, what types of meetings are organised from King Alfred’s College to nearby each year? This year (from October 2007 to Sparsholt College; the first time in its 40- September 2008) there are four ‘Ordinary year-plus history that ‘Winchester’ will not Meetings’ which are now held in the splen- be at King Alfred’s. In addition, there is didly refurbished Burlington House (BH), also the BAA Out of London Weekend. (I’m Piccadilly. Sometimes these take place on a not sure why it is so termed as we have Saturday afternoon but more often on a never had a weekend meeting in London to Wednesday evening. These meetings have my knowledge.) These are starting to get a been among the least well supported recently reputation for excellence, as not only are with attendances sometimes dropping be- the speakers as good as at any meeting but low 70, but hopefully this will improve now the venues are great and the visits to local we are back in our permanent home. attractions extremely interesting. Last Figure 6. Recent CCD images by Ed Lomeli All other ‘general’ meetings are ‘one- year’s in Glasgow (see the February Jour- (Sacramento, CA, USA, 235mm SCT) and offs’, although they take place annually. nal) was a case in point and this year’s in John Boudreau (Saugus, MA, USA, 279mm The AGM in October must be held in Lon- Dublin promises to be just as successful. SCT) compare very well with Messenger’s don according to the by-laws, and also takes The BAA is of course principally an as- image of the Caloris hemisphere. (Boudreau’s place at BH on a Wednesday. The Christ- sociation of observers. Therefore, to en- image kindly supplied by Frank Melillo.) mas Meeting, which is really a special ‘Or- courage more people to become active in 68 J. Br. Astron. Assoc. 118, 2, 2008 Notes and News this area we have the Observers’ Work- shops, and in February we had the ninth in the series, which concentrated on observ- ing the planets. Then there are the ‘Back to Basics’ meetings, which are intended for newer members or even those who have not yet joined the Association and wish to know more. They are usually organised in conjunction with a local society and are proving very popular. Finally there are the Section Meetings. Not all Sections hold an annual meeting, for some may not be suited to such a regular event. But given that delegates of such meetings are a subset of the whole BAA mem- bership, they are usually very well attended, often with a similar attendance to a full BAA Ordinary meeting. Why are our ‘Ordinary’ meetings sometimes rather poorly supported? The venues are usually excellent (like BH) and the speakers can be the same as those who will speak at a Sec- tion meeting which will attract a relatively larger audience. What is more, you usually have to An almost-capacity audience for the talks at a recent Exhibition Meeting. Photo: pay, only a modest sum admittedly, for a Sec- Hazel McGee tion Meeting, whereas Ordinary meetings are free! And you do get refreshments. deed, if you are a society member, why not writing I’ve not had a reply to my request for We need our meetings as one way of at- involve several others as well and make a somebody (or bodies) to help Hazel Collett tracting and retaining new members as well day, or at least an afternoon or evening, of organise the Back to Basics Workshops (see as informing existing members about many it? And don’t forget, the President will be also page 113).

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