J. Brit. Astron. Assoc., 107, 1, 1997, p. 38
by Richard Baum
A high resolution image of Mercury on 1995 October 22, taken by Johan Warell with the Swedish solar telescope at La Palma. Geocentric distance of planet 1.02 AU. Central meridian longitude 283°. Resolution about 200km. Bright spots near the limb may be ray centres.
What are probably the highest resolution images of Mercury ever obtained from an earth-based site were taken on 1995 October 22 by Johan Warell with the Swedish solar telescope at La Palma, when the apparent diameter of the 62 percent illuminated disk was 6.5 arcseconds. The images, which cover the hemisphere opposite to that scanned by Mariner 10 in 1974–75, show two bright isolated spots near the limb, inferred to be 'crater ray ejecta from the most recent impacts'. If the interpretation is correct, as seems likely, then it is of interest to recall that visual observers pre-empted the observation well over a century ago.
In his Celestial Objects for Common Telescopes[1] the Rev. T. W. Webb notes on 1867 June 11 'A bright spot with faint lines diverging from it, NE, and S., was seen... a little S. of the centre, in unusually clear air' by Prince [probably C Lesson Prince of Uckfield who was active around that time]. Again on 1870 March 13 'Birmingham was pretty certain that there was a large white spot near E. limb'. [2] In the same context Webb writes that 'Vogel at Bothkamp observed spots,' 1871 April 14 and 22. [3] Writing in the Astronomische Nachrichten, L. de Ball at Bothkamp spoke of 'starry portions which seemed brilliant' at the planet's limb. [4]
'In 1882 Nov. 5, 6, 8, and 9,' W. F. Denning tells us, 'I observed Mercury at about the time of sunrise, employing a 10-in. reflector, power 212. Dark patches and bright spots and veins were detected on Nov. 5, 18h 20m–50m' [here the time system is GMAT beginning at noon]. Significantly, 'The planet showed a well-marked luminous area between the ENE part of the limb and terminator... On Nov. 6, 18h 25m–55m the appearance was very similar... Nov. 8, 18h 35m–19h 30m, definition was excellent and the markings readily observed. The bright area seen on previous nights was still visible, and placed centrally within it appeared a small brilliant point with luminous radiations. Nov. 9, 18h 40m–19h 39m, definition was again very good, and most of the features were re-detected, though they were placed obviously to the eastwards of their positions on previous mornings, proving that the rotation-period is somewhat greater than 24 hours... the bright area was decidedly fainter'. [5]
H. McEwen writing in 1895 spoke of 'distinct, small brilliant patches, almost points'. [6] On 1898 April 3, 3h 15m local time, P. B. Molesworth, Ceylon, 12-in Calver at powers 165–230, was 'certain of the existence of a bright spot on S. p. limb as seen at 3h 15m. A fainter smaller spot on N. p. limb.' April 5, 4h 35m, 'Bright spot seen on S. p. limb.' April 9, 4h 0m, 'Very bright spot on S. p. limb,' power 340. Denning, who had the opportunity of seeing Molesworth's notes and drawings for 1898, says this same spot was also seen on April 7, 9 and 10. May 24, 17h 50m, 'White spot on N. f. limb was very striking and semi-oval in outline, almost like snow-cap on Mars. P. 230.' May 25, 18h 0m, 'Same spot on N. f. limb unchanged in place and very bright. June 1, 18h 10m, 'Limb bright with two brighter patches near centre.' Powers 230 and 340. June 2, 18h 0m. 'Bright patch on N. f. limb. P. 230 and 340.' Denning says this particular spot was seen from May 23 to June 17. Sept. 20, 17h 35m 'Brilliant oval patch surrounded by faint shade midway between the N. cusp and centre of limb.'[7]
On 1904 June 8 a Mr Lepper of Maritzburg, Natal saw 'a bright spot... in the S. Hemisphere about halfway from the equator to the pole and midway between the limb and terminator.'[8] G. V. Schiaparelli also gave notice he had seen brilliant white spots on the disk. Writing in his 1889 paper 'Sulla rotazione di Mercurio', [9] he noted they '...often last for more than a day in a particular place [and] occur for the longest periods near the limb of the planet, where they sometimes become very splendid; but they are not rare even on the inner parts of the disk, only they are then less bright and harder to recognise.' He thought them 'probably related' to the white veils, and suggested that together with the 'variable intensity of the dark spots, [they] are due to more or less opaque condensations, produced in the atmosphere of Mercury.'
Denning was more cautious. Schiaparelli had responded to him in 1882 to agree, 'that Mercury is much easier to observe than Venus, and that his aspect resembles Mars more than any other of the planets of the solar system.' He also remarked, 'It has some spots which become partially obscured and sometimes completely so; it has also some brilliant white spots in a variable position.' In his popular Telescopic Work for Starlight Evenings[10] Denning observed 'these deductions of the eminent Italian astronomer require corroboration, and this is not likely to be soon forthcoming owing to the obstacles which stand in the way.' Indeed, Denning believed 'the atmosphere of Mercury is of great rarity, and incapable of reflection in the same high degree as the dense atmosphere of Venus.' Nevertheless from his own experience he conceded the overall accuracy of Schiaparelli's observations.
Two years later G. Muller at Potsdam concluded from photometric evidence that Mercury has a surface analogous to that of the Moon, rough and uneven. [11] Nevertheless Mercury was still seen through the ideas of Schiaparelli. The white spots continued to intrigue Denning. 'One of these... was very interesting,' he had written in 1884, referring back to November 1882, and in a reprint of material originally published in The Observatory, [5] he again adverted to those observations but in much greater detail. Though he does not appear to have speculated about its cause, it is obvious he was profoundly impressed by the phenomenon, and it is tempting to ask, as did David Graham in 1995, [12] if he had seen 'enough to make him suspect the true nature of the bright patches on the disk.'
Richard Baum
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1. Webb T. W., Celestial Objects for Common Telescopes, Longmans, Green & Co., London, 4th ed., 1881, p. 47
2. ibid., p. 47
3. ibid., p. 47
4. de Ball L., Astronomische Nachrichten 104, no. 2479, cols. 97–102 (1883)
5. Denning W. F., The Planets Mercury and Venus, 1906–07, p. 13
6. ibid., p. 17
7. ibid., pp. 15–16
8. ibid., p. 17
9. Schiaparelli G. V., Astronomische Nachrichten 123, no. 2944, cols. 241–250 (1889)
10. Denning, Taylor & Francis, Telescopic Work for Starlight Evenings, London, 1891
11. Muller G., Astronomische Nachrichten 133, cols. 47–52 (1893)
12. Graham D. L., J. Brit. Astron. Assoc., 105(2), 59–64 (1995)