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Spectroscopic Atlas for Amateur Astronomers 168

29 Reflectance Spectra of Solar System Bodies

29.1 Overview

The objects in our solar system are not self-luminous, and visible only by reflected sunlight.
Therefore, with exception of comets, these spectra always show, not surprisingly, the ab-
sorption lines of the Sun. On the other hand the spectral continua of the reflected profiles
are overprinted, because certain molecules, e.g. CH4 (methane), in the atmospheres of the
large gas planets, are reflecting or absorbing the light differently strong within specific
wavelength ranges (wavelength-dependent albedo). Already 1871, Angelo Secchi had dis-
covered these dark, at that time not identifiable bands. This was achieved not until 1930 by
Rupert Wild and Vesto Slipher.

29.2 Commented Spectra

According to their characteristics the planetary reflectance spectra are distributed here to
three tables, and compared with the continuum of the sunlight. All profiles (200L grating)
have been recorded at an elevation of some 30 – 40° above the horizon and are equally
normalised to unity.

29.3 Reflectance Spectra of Mars and Venus

Table 90:

The extremely dense atmosphere of Venus generates on the surface a pressure of about 90
bar, ie approximately 90 times as high as on Earth. It consists of about 96% carbon dioxide
(CO2). The remaining shares are mainly nitrogen (N2), water vapour (H2O), and sulphur
compounds in the form of sulphur dioxide (SO2) and sulphuric acid (H2SO4). The extremely
thin atmosphere of Mars consists similar to Venus, to about 96% of CO2, but here under a
surface pressure of only 0.006 bar, i.e. <1% of the value on the surface of the Earth. Here
particularly the rocky surface of the planet might determine the reflectance properties. In
the displayed range the spectra neither of Venus nor Mars show significant deviations from
the shape of the Sun’s spectral continuum. In higher resolved spectra, of course experts
can recognise and analyse differences.

29.4 Reflectance spectra of Jupiter and Saturn

Table 91:

The outer atmosphere of Jupiter consists of about 89% hydrogen and10% helium, the rest
mainly of methane and ammonia. These gases have hardly any influence on the reflectance
characteristics (continuum course), in contrast to the small rest which mainly consists of
methane (CH4) and ammonia (NH3).

Saturn's outer atmosphere is composed slightly different. It consists of about 93% hydro-
gen and only close to7% of helium. Further some traces appear of methane, ammonia and
other gases. Impressive to see here are, concentrated in the near-infrared range, the very
broad methane (CH4) and ammonia (NH3) absorption gaps in the spectral continuum. In this
wavelength domain, these differences appear most pronounced in the areas of 6200 and λ
7300.

29.5 Reflectance spectra of Uranus, Neptune and Saturn-Moon Titan

Table 92:

The atmospheres of Uranus and Neptune show a similar composition like Jupiter and Sat-
urn. Due to the much greater distance from the Sun their temperatures are so low that a
majority of components, such as ammonia and methane, are below their specific freezing