Subaru Views SaturnThe Subaru Telescope also has observed Saturn several times over the years. Dr. Hideaki Fujiwara, Subaru Public Information Officer/Scientist, analyzed data taken in January 2008 using the Cooled Mid-Infrared Camera and Spectrometer (COMICS) on the telescope to produce a beautiful image of Saturn for public information purposes. During the analysis, he noticed that the appearance of Saturn's rings in the mid-infrared part of the spectrum was totally different from what is seen in the visible light
Saturn's main rings consist of the C, B, and A rings, each with different populations of particles. The Cassini Division separates the B and A rings. The 2008 image shows that the Cassini Division and the C ring are brighter in the mid-infrared wavelengths than the B and A rings appear to be (Figure 1). This brightness contrast is the inverse of how they appear in the visible light, where the B and A rings are always brighter than the Cassini Division and the C ring (Figure 2).
"Thermal emission" from ring particles is observed in the mid-infrared, where warmer particles are brighter. The team measured the temperatures of the rings from the images, which revealed that the Cassini Division and the C ring are warmer than the B and A rings. The team concluded that this was because the particles in the Cassini Division and C ring are more easily heated by solar light due to their sparser populations and darker surfaces.
Changing Angles Change the Brightnesses
The team concluded that the "inversion" of the brightness of Saturn's rings between 2005 and 2008 was caused by the seasonal change in the ring opening angle to the Sun and Earth. Since the rotation axis of Saturn inclines compared to its orbital plane around the Sun, the ring opening angle to the Sun changes over a 15-year cycle. This makes a seasonal variation in the solar heating of the ring particles. The change in the opening angle viewed from the Earth affects the apparent filling factor of the particles in the rings. These two variations – the temperature and the observed filling factor of the particles – led to the change in the mid-infrared appearance of Saturn's rings.
The research team:
- Hideaki Fujiwara: Subaru Telescope, National Astronomical Observatory of Japan, USA
- Ryuji Morishima: University of California, Los Angeles/Jet Propulsion Laboratory, California Institute of Technology, USA
- Takuya Fujiyoshi: Subaru Telescope, National Astronomical Observatory of Japan, USA
- Takuya Yamashita: National Astronomical Observatory of Japan, Japan