By using the same approach as for the Full spectrum of Uranus commented, it is possible to understand many things about the behavior of the filters used to image the planet. First article of a serie dedicated to Venus imaging.
Perceived as uniformy white when viewed through a telescope, Venus is a planet rather difficult to observe. We know that it is mainly in the shorthest wavelengths (UV, violet) that some contrasted details can be detected. Spectroscopy will allow us to understand why.
As for any of the planets of the Solar system, Venus is only shining to our eyes because it is illuminated by the Sun. This means that when you take a spectrum of Venus, what you get is in the first place, a spectrum of the Sun… However, some differences will appear when comparing this reflected Sun spectrum by Venus, with a direct spectrum of our day star. This is when things become interesting!
This is first a “colour spectrum” of Venus, corrected from the instrumental response (this means the influence of the telescope, the camera and the Star Analyzer grating), and from the transmission of our own atmosphere – but not from the influence of the Sun (Click here for full size) :
This is a relative comparison between the spectrum of Venus (blue line) and one of a solar star (orange line) very similar to the Sun (this is HD9986 in Pisces). Both profiles look quite close but there are some interesting differences. Let’s put aside the large absorption bands in the deep red due to our own atmosphere (water and dioxygen). We clearly see that the two profiles look more “spaced” in the UV/violet. This is where is active an unknown chemical component generally called the “UV-absorber” (some recent studies however are implying the role of some sulfur dioxyd polymeres). This component absorbs the sunlight and so the Venus spectrum is much less intense. This is why we see details there (see image at right).
Now this is a set that show the colour spectrum of Venus along with images taken at different wavelenghts on March 25th, 2020. We see that only the UV and violet images are showing significant details. In the visible (RGB) and in the near infrared (IR), the absorption is both fainter and more homogeneous around the globe, and so the details that can be seen are of very low contrast. Click here for full size
Finally, it is possible to extract the proper spectrum of Venus from that of the reflected Sun. This is easily done by diving the raw spectrum of the planet by the raw spectrum of a solar star observed at the same airmass (for this study I used CHI01 Orionis). Here is the result:
The profile is rather simple and this is the reason why wee see Venus as white: it almost perfectly reflects the Sunlight (if reflection was absolutely perfect, the profile would be a straight horizontal line). The curve however is enlightening again the absorption in the UV of the venusian atmosphere. This absorption is lingering into visible blue light. This shows that the real colour of the planet is not white, but a quite subtle yellow. The saturation of the tint is probaly too faint to be perceptible by the human eye. However, digital imaging is providing images were colours are enhanced and so when doing well calibrated true colour images of the planet, the yellow hue will appear, as can be seen on the left images.
Next article: in depth analysis of the filters!