During the last 15 years, over 450 exoplanets (planets outside our own Solar System) have been discovered. Due to the nature of the discovery techniques, most of these planets are large — more like Jupiter than Earth — and orbit close to their parent star. While a small number of smaller, rocky planets (Super Earths) have been found, the discovery of a true ‘exoearth’ in the habitable zone around its parent star is still beyond the capabilities of our technology. However, in preparation for such a discovery which is likely within the next decade or so, there is considerable interest in how to characterize such planets and their atmospheres.

It took thousands of years, following the first questioning gaze at the sky, to understand why the clear daytime sky is blue [1]. The theory of Rayleigh scattering, however, does not explain the entire palette of sky colors that delight us at dawn or dusk. It was only realized in the middle of the last century [2]that the unstable oxygen molecule ozone (O3) has a profound impact on twilight colors.

It is well known that the ozone layer protects the surface of our planet from the damaging effects of ultraviolet radiation but few people appreciate that, without ozone, the color of the zenith (overhead)sky at twilight would be a pale green/straw yellow rather than the deep, steely blue that we observe. The electronic Chappuis absorption band of ozone is intrinsically weak and has little effect on the color of the daytime sky. This band, extending from 450 to 850 nm, only becomes significant when the pathlength of sunlight through the atmosphere is dramatically increased around sunrise and sunset. At these times, the Chappuis band becomes by far the strongest feature in the visible spectrum of the sky or the setting sun.

In preparation for future ‘transit spectroscopy’ (analysis of the light transmitted by the atmosphere of a planet transiting its parent star) we have performed relative spectrophotometry of the sky during twilight. This can be compared with the observations of the eclipsed Moon [3] which examine light that has grazed the Earth’s atmosphere during a lunar eclipse. The eclipse geometry results in a much stronger influence of Rayleigh and aerosol extinction in the light reflected from the eclipsed Moon than in scattered light from the horizon sky at twilight, resulting in strong suppression of the blue end of the spectrum.

The twilight spectra appearing in this report were obtained with an Ocean Optics JAZ spectrometer covering 350–1000 nm and using a single optical fiber input pointed about 10° above the western sky in overcast conditions. The altitude of the observing site was 560 m at a latitude of +47.8°.

Figure 1 The experimental setup for the twilight observations showing the JAZ spectrometer, the fiber feed and the data-taking laptop. These photographs, taken around the time of sunset with a ‘daylight’ white balance setting on a Canon 5D MkII, illustrate the high color temperature of the ambient light at this time and caused predominantly by the ozone absorption (see Fig 2).

Figure 2 The variation of sky brightness at 700 nm, in units of counts per 8192 ms, and color temperature, measured with a Gossen 2F COLORMASTER.

Figure 3 A sequence of ‘Relative Irradiance’ spectra of a cloudy western sky at approximately 10 min intervals from an hour before, to 15min after, sunset. Also plotted is the Lunar eclipse spectrum from [3]. The rapid development of the ozone Chappuis absorption, centered at 600 nm, is apparent together with the dramatic bluening of the sky color during this period. The somewhat irregular behavior of the blue end of the spectrum is due to the variable cloud thickness during these measurements. The reference spectrum is with a solar altitude of+13° and the final spectrum of the sequence is with an altitude of -3°.

Figure 4 The observed ratio (dark blue), with a normalized continuum, of a spectrum taken with a solar altitude of-3° to one at +13°. Atmospheric models [4], using the HITRAN database [5], with a similar but pure transmission geometry, are over plotted (red: pure ozone absorption; light blue: ozone + O2 + H2O). The models have been scaled in intensity by a factor of 1.7 as a way of accounting for the simplification of the model geometry.

This simple experiment clearly illustrates the profound influence of the ozone Chappuis absorption on the color of the twilight sky (the color temperature of the western sky varied from 6,000K to14,000K during the course of the observations) and the very strong visible spectral signature that could be expected in transit spectroscopy of an oxygen-rich exoearth atmosphere.

The absence of strong signatures of O2 and H2O in the spectral ratios probably indicates that most of the light entering the spectrometer has passed above the cloud layer, in the stratosphere (~25km), where ozone dominates the absorption.

An interesting exercise with these data is to invert them to derive the ozone Chappuis absorption cross-section times the column density of the atmospheric pathlength that is characteristic of these twilight observations.

Figure 5 The inversion of the observation (ln[observed ratio] * const) compared with the data in the radiative transfer code we used for the modeling, remembering that we have neglected the water and diatomic oxygen contributions which probably accounts for some of the deviations in the band wings.

This project, designed for high-school education, will go on to explore different illumination geometries involving long absorption pathlength through the atmosphere.

Acknowledgments: We thank Enric Pallé for providing the Lunar eclipse digital data.

References

1              Pesic, P., “Sky in a bottle”, The MIT Press

2              Hulbert, E. O., 1953, “Explanation of the Brightness and Color of the Sky, Particularly the Twilight Sky”, Journal of the Optical Society of America, 43, 113-118

3              Pallé, E. et al. 2009, “Earth’s transmission spectrum from lunar eclipse observations”, NATURE, 459, 814-816

4              Clough, S.A., et al. 2005, “Atmospheric radiative transfer modeling: a summary of the AERcodes”, Journal of Quantitative Spectroscopy and Radiative Transfer, , 91, 233

5              Rothman L.S., et al. 2009, “The HITRAN 2008 molecular spectroscopic database”, Journal of Quantitative Spectroscopy and Radiative Transfer, 110, 533

Robert (Bob) Fosbury [a] & Andreas Seifahrt [b]

a. Space Telescope – European Coordinating Facility, Garching bei München, 85748, Germany(rfosbury@eso.org)

b. Physics Department, Univ. of California, Davis, CA 95616, USA (seifahrt@physics.ucdavis.edu)

Tags: ozone

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