Figure 1: A three-color composite of the mid-infrared
images of Saturn on January 23, 2008 captured with COMICS on the Subaru
Telescope. The Cassini Division and the C ring appear bright. Color
differences reflect the temperatures; the warmer part is blue, the
cooler part is red. (Credit: NAOJ)
A team of researchers has succeeded in measuring the brightnesses and
temperatures of Saturn's rings using the mid-infrared images taken by
the Subaru Telescope in 2008. The images are the highest resolution
ground-based views ever made. They reveal that, at that time, the
Cassini Division and the C ring were brighter than the other rings in
the mid-infrared light and that the brightness contrast appeared to be
the inverse of that seen in the visible light (Figure 1). The data give important insights into the nature of Saturn's rings.
Subaru Views Saturn
The 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 lightSaturn'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.
On the other hand, in the visible light, observers see sunlight being
reflected by the ring particles. Therefore, the B and A rings, with
their dense populations of particles, always seem bright in the visible
wavelengths, while the Cassini Division and the C ring appear faint. The
difference in the emission process explains the inverse brightnesses of
Saturn's rings between the mid-infrared and the visible-light views.
Changing Angles Change the Brightnesses
It turns out that the Cassini Division and the C ring are not always
brighter than the B and A rings, even in the mid-infrared. The team
investigated images of Saturn's rings taken in April 2005 with COMICS,
and found that the Cassini Division and the C ring were fainter than the
B and A rings at that time, which is the same contrast to what was seen
in the visible light (Figure 3).
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 data taken with the Subaru Telescope revealed that the Cassini
Division and the C ring are sometimes bright in the mid-infrared though
they are always faint in visible light. "I am so happy that the public
information activities of the Subaru Telescope, of which I am in charge,
led to this scientific finding," said Dr. Fujiwara. "We are going to
observe Saturn again in May 2017 and hope to investigate the nature of
Saturn's rings further by taking advantages of observations with space
missions and ground-based telescopes."
This research is published in Astronomy & Astrophysics, Volume 599, A29 and posted on-line on February 23, 2017 (Fujiwara et al., 2017, "Seasonal variation of the radial brightness contrast of Saturn's rings viewed in mid-infrared by Subaru/COMICS"). This work is supported JSPS KAKENHI Grant Numbers JP23103002 and JP26800110.
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
Source: Subaru Telescope
