Ordinary Meeting, 2007 March 28
BAA Studies of the Venusian Atmosphere
Dr McKim opened by remarking that Venus was a very easy planet to see, but a tricky one to observe. Visual telescopic images revealed little more than its phase. To begin, he looked back in history to the work of Charles Boyer, by day a magistrate in French equatorial Africa and by night an amateur astronomer, who in 1957 had observed that distinct features became apparent on Venus' disk when a violet filter was used. He had gone on to observe that these repeated with a four-day period, and so had concluded that Venus had a four-day rotation period.
It was now known that violet light did not show the surface of Venus, but rather high-altitude clouds, and so this rotation period was now ascribed to the atmosphere rather than to the planet itself. Boyer had subsequently observed its rotation speed to be surprisingly constant, refining it to 3.99525 days. Two features were especially clear, one Y-shaped and the other Ψ-shaped, separated by around 90°. Both were orientated with their stems approximately parallel to the equator.
Within the past few years, it had become possible for amateurs to take high-resolution ultraviolet images for the first time. In these, the disk markings first noted by Boyer were especially clear. This advance in technology had led the speaker to suggest that the BAA should now make new measurements to refine Boyer's estimate of their rotation speed. He had begun by taking all of the UV images which had been submitted to the Mercury and Venus Section in 2004, and using the fork of the Y-shaped feature as a reference point with which to calculate its rotation. He had derived a rotation period of 3.995 days, which showed their rotation speed today to match with remarkable precision that measured half a century earlier.
Dr McKim reported that he was also now receiving near-infrared images from amateurs. Whilst these were often featureless, banding and other markings occasionally showed up. Analysing images received in 2004, he had estimated their rotation period to be 5.0 days; he suggested that the cloud structures being seen here were at low altitudes, as described by Prof Taylor earlier.
On six occasions in 2004 May, Christophe Pellier had imaged the 1-μm near-infrared emission of the night-side of Venus. These showed faint albedo markings which appeared stable over several days and seemed therefore to correspond to actual surface topology. If real, these ground-breaking observations represented the first amateur observations of the surface of Venus.5,6
A more detailed account of the observations described by Dr McKim can be found in his report of the activities of the Mercury and Venus Section in 2004.6
Following the applause, Mr Nick James asked why the rotation rate of the upper atmosphere of Venus had been so constant over the past 50 years. Dr McKim invited Prof Taylor, in the audience, to respond. Prof Taylor agreed that this was mysterious, especially given that the rotation rate seen was around 50 times faster than that of the surface below. Some recent theoretical studies had tried to reproduce this behaviour; they had shown that the gas heating which resulted from the absorption of solar radiation by clouds in the upper atmosphere could actually drive fast winds of precisely the kind seen. However, an explanation of why these winds were so constant was less forthcoming. Prof Taylor offered the thought that Venus had no seasons – it was in a near-circular orbit with little axial inclination – and so perhaps there were no seasonal variations to drive large-scale changes to wind patterns?
The President then introduced the evening's final speaker, Dr Stewart Moore, Director of the Deep Sky Section, who would be presenting this month's Sky Notes.