The opposition of Mars, 1997 Richard McKim A report of the Mars Section (Director: R. J. McKim) The opposition marked the successful return to Mars of lander-type spacecraft, the first since Project Viking two decades earlier. In 1997 the BAA Mars Section helped to provide ground- based coverage for the Mars Pathfinder and Mars Global Surveyor (MGS) missions. Only small- scale surface changes were evident since 1995, of which the continuing pallor of Cerberus− Trivium Charontis was especially notable. Several dust storms were detected by the BAA, the Hubble Space Telescope (HST) and the spacecraft in martian orbit: these included a large regional event in 1997 November which began in Noachis. The Equatorial Cloud Band (ECB) effect was rather evident around opposition, and its gradual decline was traced. The seasonal behaviour of the NPC was typical, excepting small differences in the timing of the disappearance and reappearance of the polar hood. The recession of the cap was followed in detail. Introduction planet was very well observed, and UK observers enjoyed periods of excellent seeing in 1997 March−April. Due to the very high volume of CCD images the overall number of ob- NASA’s Mars Pathfinder landed on 1997 July 4 at the con- servations exceeded that for 1995. fluence of the Ares and Tiu Valles outflow channels (lat. +19°, In addition to publishing observational hints,2 the Direc- long. 33°), and MGS underwent orbital insertion on Sep- tor produced a large number of Section Circulars,3 achiev- tember 11. A timetable of BAA and spacecraft imaging of ing a real-time meteorological summary as the Association’s Mars between 1993 and 2003 was given in the Section Re- contribution to the ground-based Marswatch effort2,4 or- port for 1995.1 ganised by Dr J. F. Bell (Cornell University) to monitor the Mars came to opposition on 1997 March 17, right at the planet during the Pathfinder mission. These circulars were start of northern summer. The planet’s N. pole was tilted distributed as paper copies and by email (the first use of the towards the Earth throughout the apparition, increasing from latter medium within the Section), whilst a new website of- +6° in early 1996 August to + 24° by mid-November, decreas- fered an online archive.5 Six Interim Reports were published ing a few degrees in early 1997, then rising again to +26° by in the Journal,6−11 as well as one BAA Circular.12 Other June, decreasing steadily to 0° by November and subse- amateur organisations have produced both preliminary quently reaching −23° by the close of the apparition. Other (OAA,13 ALPO14) and final (ALPO15) reports on this appa- physical data are listed below: rition, and others (UAI16) were active. Accounts of personal work by Nakajima & Minami17 and by Schmude18 have also Physical data for the 1997 opposition been published. Dr S. Ebisawa made several reports of his − Solar conjunction 1996 Mar 4 observations from Tokyo.19 20 MGS data continued beyond the period reviewed, and HST Spring equinox N. hemisphere/ − Autumn equinox S. hemisphere 1996 Aug 26 Ls= 0° data spanned 1996 September 18 1997 October 9. BAA ob- servations covered the extreme range 1996 August 4 (Ls= 349°, Aphelion 1997 Jan 27 Ls= 70° Warell) to 1998 January 21 (Ls= 259°, Parker), and therefore Summer solstice N. hemisphere/ spanned late winter, all of spring, summer, and most of au- Winter solstice S. hemisphere 1997 Mar 13 Ls= 90° tumn, in the martian N. hemisphere. 84 observers (Table 1) in Opposition 1997 Mar 17 Ls= 92° 14 countries contributed 2,391 observations: 1,316 drawings, Autumn Equinox N. hemisphere/ 13 photos and 1,062 CCD images. The number of days ob- Spring Equinox S. hemisphere 1997 Sep 12 Ls= 180° served per month by original observations was as follows: Perihelion 1998 Jan 5 Ls= 250° 1996 Aug 4(actual)/31(possible), Sep 5/30, Oct 10/31, Nov 9/30, Dec 13/31, 1997 Jan 27/31, Feb 28/28, Mar 31/31, Solar conjunction 1998 May 12 Apr 30/30, May 30/31, Jun 28/30, Jul 21/31, Aug 15/31, Sep 0/30, Mars was closest to the Earth on 1997 March 20, when the Oct 6/31, Nov 1/30, Dec 2/31, 1998 Jan 1/31. In addition there disk diameter was just 14".2, and the distance 98.6 million were very useful summaries by members of the OAA.13 km. Opposition occurred at declination +4° 40' in the con- Particular mention should be made of the fine European stellation of Virgo, when the latitude of the apparent disk CCD work by Dijon,9 Platt,21 Quarra and colleagues,8 and centre was +23°. Although the disk diameter was fractionally Warell, as well as that by Parker in the USA (Figure 1), and larger than in 1995, the planet’s considerably lower N. decli- the images of Higa and Miyazaki from Japan. Dr Akabane nation diminished the number of observations and margin- contributed CCD images from his professional programme at ally reduced the quality of the CCD images. Observers were the Kwasan and Hida Observatories. Especially good series also distracted by the simultaneous apparition of the great of visual records were made by Devadas, Gaskell, Gray, Heath, Comet Hale−Bopp in the spring of 1997. Nevertheless, the Mettig, P. A. Moore, Schmude, Shirreff, Teichert, Troiani, J. Br. Astron. Assoc. 116, 4, 2006 169 McKim: The opposition of Mars, 1997 Warell, the Director, and by the late Tom Cave and the late 1995 the regions adjacent to the spring NPC had appeared Dr Rowland Topping. Biver and Tanga made good use of uniformly dark and bland, but in 1997 the far northern maria large apertures, whilst Adamoli did sterling work with a small were lightened in several areas (due to dust deposition dur- refractor in 1997 July−August when most contributors had ing the previous martian N. winter). This fading of the north- ceased observing. Butler reported naked eye magnitudes of ern maria with the onset of summer was recognised a cen- Mars. As usual the Director was glad to receive copies of the tury ago by Antoniadi,36 in connection with Mare Acidalium work of members of other organisations. and its environs. The HST had a very extensive program of Mars obser- Another feature of the apparition was the rather striking vations,9 concentrating upon the meteorological condi- difference in apparent colour between the northern and south- tions of the Pathfinder landing site. Many images were ern maria. Such differences have been seen at many previ- uploaded to the internet.22 Both academic and popular ous aphelic oppositions. The Director easily spotted the dif- magazines gave much coverage to the space missions as ference around opposition. On March 21 (CML= 286°) Syrtis well as to the claimed discovery of ‘life-forms’ in a martian Major−Mare Tyrrhenum was distinctly blue-grey to him, meteorite.23 Some authors reviewed the HST and ground- whilst Casius−Utopia looked reddish-brown. On March 10 based work accomplished.24 A second International Mars and April 15 (CML= 21−26°) Sabaeus Sinus−Margaritifer Telescopic Observations workshop25 was held in Arizona Sinus−Mare Erythraeum−Aurorae Sinus appeared blue- in 1997 whilst Pathfinder was active on the martian sur- grey, contrasting with the brownish Mare Acidalium in the face; the Director attended and presented BAA data.26 north. CCD images showed that the central part of Syrtis Many papers appeared concerning the Pathfinder27 and Major was visibly bluer than any other albedo feature. Col- Mars Global Surveyor28 results. our differences seem to be related to recent mineralogical The present report is a continuation of that for 1995.1 analyses that reveal hemispherical differences in iron-bear- Seasonally comparable apparitions during BAA history have ing minerals. The apparent blueness of the Syrtis − at least been: 1982,29 1965,30 1950, 1933,31 1918,32 and 1903.33 towards the edge of the disk − must have been enhanced by Figure 1. (opposite) Colour CCD images of Mars (to correct relative scale) by D. C. Parker. Tricolour filter work with Schott glass filters (original images: red with RG610; green with VG9 + IR rejection filter; blue with BG12 + IR Observations rejection filter), 410mm refl., F/43, and Spectrasource Lynxx camera. Top row, left to right: (A) 1996 September 18d 10h 20m, CML= 17°. NPH present. Figure 1 presents a selection of composite CCD images by (B) 1996 October 31d 10h 34m, CML= 324°. NPC with dark border. Parker. Figure 2 is an apparition map, and Figure 4 gives (C) 1996 November 8d 10h 04m, CML= 239°. some of the Director’s drawings. These illustrate the chief (D) 1996 December 7d 10h 03m, CML= 319°. (E) 1997 January 4d 08h 52m, CML= 36°. features of the apparition. Of special interest here, Bell et al.34 − 2nd row: have reviewed 1995 ’97 HST near-infrared images to chart (F) 1997 January 29d 07h 53m, CML= 148°. Early images of the albedo variations since the Viking era. evening orographic clouds. (G) 1997 February 1d 08h 14m, CML= 117°. As (F); Nix Olympica at the CM. (H) 1997 February 4d 08h 38m, CML= 104°. Orographic clouds. (I) 1997 February 10d 06h 30m, CML= 19°. 3rd row: Surface features (J) 1997 March 7d 06h 13m, CML= 153°. Orographic clouds (com- pare Figure 8A); small ‘oases’ west of Propontis I; Ierne p. the CM and Olympia f. it Apparition map (K) 1997 March 13d 04h 42m, CML= 78°. ECB. Structures in Tithonius Lacus and Nilokeras. (L) 1997 March 19d 04h 42m, CML= 26°.