24The Spectra of Saturn and its Ring
Even a small telescope can see the ringed shape of Saturn. It is the most recognizable of all of the Solar System planets.
The spectrum in Figure 1 is an observation where the main body of Saturn and the associated ring were imaged on the slit of the spectrograph. As has been introduced repeatedly in this “Universe of Spectroscopy” series, the speed and movement of an object can be learned from its spectrum. Saturn’s light is primarily reflected sunlight. The vertical dark lines in this spectrum are absorption lines (wavelengths where the light has been absorbed) contained in the Sun’s light. The thickest line is one of the lines produced by hydrogen (Balmer Line.)
The absorption lines in the spectrum of the main body are slanted. This shows that the upper limb of Saturn is approaching us, while the lower limb is receding away from us. In other words, this tilt in the spectral lines is produced by Saturn’s rotation. From this wavelength difference, we know that the part which fell on the slit, the equator, is rotating at a speed on the order of 10 km/s. According to the Rikanenpyo (Chronological Scientific Tables), Saturn’s equatorial radius is about 60,000 km and the rotation period is about 10.6 hours. From these, the rotation speed can be calculated to be 9.8 km/s. In the measurements there is some ambiguity about what you take to be the “surface” of Saturn, but it can be said that these results are in good agreement.
On the other hand, looking at the spectra for the ring, the wavelengths are offset in the upper and lower sections. From this we know that the ring is rotating in the same direction as the main body of Saturn. In addition, the spectra for each section of the ring are a little slanted. This slant is in the opposite direction as for the main body. This means that the outer part of the ring rotates slower than the inner part of the ring.
Moving on, if you look closely at the spectrum of the main body of Saturn, you can see that actually there are 2 more types of absorption lines (Figure 2.) The straight vertical ones are absorption lines due to the Earth’s atmosphere. In contrast, the thick, slanted absorption lines are the solar spectrum introduced above; because they are reflected from Saturn’s surface, they are affected by the rotation twice (coming and going), and the tilt becomes large. The absorption lines with the intermediate tilt are absorption lines due to the gasses in Saturn’s atmosphere (like methane and ammonia.) These are affected by the rotation only once. In this way, high-wavelength-resolution observations are an extremely valuable research method capable of elucidating essential astronomical information like the motion of an object or the absorptions/emissions in its atmosphere. (The data acquisition and image production were performed by Akito Tajitsu of the Subaru Telescope.)