Astronomical Science Ozone: Twilit Skies, and (Exo-)planet Transits Robert Fosbury1 the atmosphere, can be rich and varied. This article is about the effect of ozone George Koch 2 The processes that result in this palette on the colour of the twilit sky and, in the Johannes Koch 2 are geometrically complex, but comprise same vein, its appearance as the strong- a limited number of now well-understood est telluric absorption feature in the visi- physical effects. This understanding was ble spectrum of the Earth as it would be 1 ESO not gained easily. From the time when seen by a distant observer watching it 2 Lycée Français Jean Renoir, München, early humans first consciously posed the transit in front of the Sun. We present and Germany question: “What makes the sky blue?”, to analyse spectrophotometric observations the time when the processes of scatter- of sunset and also discuss observations ing by molecules and molecular density of the eclipsed Moon reported by Pallé et Although only a trace constituent gas fluctuations were elucidated, thousands al. (2009). in the Earth’s atmosphere, ozone plays of years passed, during which increas- a critical role in protecting the Earth’s ingly intensive experiments and theories surface from receiving a damaging flux were developed and carried out (Pesic, Ozone of solar ultraviolet radiation. What is not 2005). generally appreciated, however, is that Ozone (O3 or trioxygen) is an unstable the intrinsically weak, visible Chappuis Most physicists, if asked why the sky is allotrope of oxygen that most people can absorption band becomes an important blue, would answer with little hesitation: detect by smell at concentrations as influence on the colour of the entire sky “Because of Rayleigh scattering by mole­ low as 0.01 parts per million (ppm). When when the Sun is low or just below the cules.” With the Sun above the horizon present in the low atmosphere as a horizon. This effect has been explored in a clear sky, this is worth a good mark. pollutant, it has many damaging effects, using spectra of the sunset and also During twilight, however, things get more including to lung tissue. At higher alti- of the eclipsed Moon; phenomena that complicated. tudes, typically between 15 and 40 km, involve a similar passage of sunlight however, it produces the beneficial effect tangential to the Earth’s surface. This Twilight has a special place in the life of of preventing damaging ultraviolet (UV) geometry will also be relevant in future an observational (optical/near-infrared) radiation from reaching ground level. The attempts to perform transit spectros- astronomer who is privileged to witness it Hartley band, extending between 200 copy of exo-Earths. from some of the most spectacular sites and 300 nm, absorbs very strongly in this on the planet. The geometry of the illumi- region with a maximum at 255 nm. The nation of the atmosphere at twilight is UV absorption extends, more weakly, in Introduction also very pertinent to the study of transit- the Huggins band up to around 360 nm. ing exoplanets, when the path of star- In the visible spectrum, a radial path The colours seen by an observer of the light is tangential to the planetary sphere. through the atmosphere exhibits very little Earth’s sky, from within or from without ozone absorption but, as the column increases at greater zenith distances, the Chappuis band begins to have an appre- ciable effect by absorbing across the R. Fosbury entire visible spectrum with a maximum close to 600 nm. More absorptions, in the Wulf bands, appear in the infrared at 4.7, 9.6 and 14.1 μm. By absorbing red and orange light, the Chappuis band has the effect of impart- ing a pale blue colour to pure ozone gas in the laboratory. As the Sun ap- proaches the horizon, the increasing opti- cal depth in the Chappuis band begins to have an effect on the sky colour that, at twilight, dominates the effect due to Ray- leigh scattering (Hulbert, 1953). The colour of the clear zenith sky as the Sun Figure 1. A few minutes before sunset at the Apple- ton Water Tower in Norfolk on 27 June 2010. The white circle shows the approximate pointing and acceptance aperture of the fibre input to the spec- trometer. The deep red Sun and the haze at the horizon suggest a high atmospheric aerosol content which is reflected in the model fit to the spectrum. The Messenger 143 – March 2011 27 Astronomical Science Fosbury R. et al., Ozone: Twilit Skies, and (Exo-)planet Transits sets is far bluer than can ever be hour before sunset. In order to remove the spectral shape. The initial values were achieved by Rayleigh scattering alone the Fraunhofer lines originating in the chosen based on the geometry of the and simple models demonstrate that, solar atmosphere, the observed spec- observation, but then adjusted to give the in the absence of ozone, it would be a trum is divided by one taken five hours best match. The aerosol and ozone scal- pale grey–yellow shade at this time. before sunset. This shows the very ing factors can be chosen independently Observations of the sky and/or the set- strong telluric water and diatomic oxygen from those applied to the direct solar ting sun with a spectrometer show what absorptions expected at large zenith path. Each sky path has a source term a profound effect the ozone has on distances. It also shows the very broad that is proportional to the sum of the the telluric spectrum, as it far exceeds dip, centred at about 585 nm, due to Rayleigh and aerosol cross-sections and the effects produced by water and dia- the Chappuis band of ozone. Note the a negative exponential sink term that tomic oxygen absorption (so familiar coincidence of a pair of water bands represents ozone absorption and molec- to optical spectroscopists) on the visible with the central structure of the ozone ular and aerosol scattering out of the spectral energy distribution. Chappuis absorption between 560 and beam, both scaled for pathlength. Finally, 600 nm. This ozone feature is promi- the relative contributions of direct sunlight nent in the spectrum of sunset viewed and skylight are adjusted to take account Spectral measurements and models with a visual spectroscope. of the balance imposed by the large input aperture. We have made a series of specrophoto- In order to model the overall shape of this metric measurements of both the sky and spectrum we have used an analytic rep- Although this model serves the purpose the Sun, covering a period from several resentation of molecular Rayleigh scatter- of a rather complicated fitting function, hours before sunset up to a short period ing, taken from Allen (1973), expressed rather than an ab initio calculation, afterwards. These have been made under per atmo-cm (thickness of atmospheric it does give a very good feel for how the both clear and overcast skies using a layer in cm when reduced to standard skylight we see is influenced by the fibre input to a spectrometer, which re- temperature and pressure, STP). This geometry it has to negotiate on its path sults in an effective input aperture with a gives a scattering cross-section which through the atmosphere. We stress that full width half maximum (FWHM) of 25°. varies as wavelength to the power –4.05 the complexity derives from the geometry The spectrometer, an Ocean Optics Jaz, to take account of the scattering and rather than the physics of absorption and covers a wavelength range of 350– the wavelength variation of refractive scattering. 1000 nm in 2048 spectral channels with index. It is normalised to give an optical a spectral resolution of 1.5 nm (FWHM). depth of 0.098/atmo-cm at 550 nm. For Figure 2 (upper) shows the separate Exposure times range from 3 to over dust and aerosols, we use Allen’s wave- components of direct sunlight (brown 100 ms per integration with each obser- length exponent of –1.3, normalised to line) and the combination of a set of vation being the average of 32 or 64 in - give an optical depth of 0.195/atmo-cm at different paths involving a single scatter- dividual exposures. 550 nm, which corresponds to normally ing from the sky (dark blue line). The clear conditions. For the ozone cross- combination of the two is shown as a red section we use data from the project line, and the ratio of the data to the model Sunset Spectroscopy & Molecular Properties of gives a normalised telluric spectrum 2 Ozone , normalised to give an optical containing the O2 and H2O features that Reported here are data collected from depth of 0.0268 at 550 nm for 0.3 atmo- we do not model (light blue line). Figure 2 the top of a tower (the Appleton Water cm (taken as the standard ozone depth). (lower) shows the same model except Tower1) in Sandringham, Norfolk, Eng- for the ozone contribution that is set to land on the evening of the 27 June 2010 We consider two types of atmospheric zero for both the direct and the scattered with the axis of the input fibre tracking path to compute the spectrum ratios re - paths. the position of the Sun to within a few lative to the high Sun. The first is the degrees. The scene just preceding sun- direct line of sight to the Sun, where we set is shown in Figure 1, where it can be account for Rayleigh and aerosol scatter- Lunar eclipse seen that the sky is partially cloudy and ing out of the beam and ozone absorp- quite hazy on the horizon, suggesting tion.