Using mid-infrared data from the 8.2-m Subaru Telescope, a research team led by Dr. Hideaki Fujiwara, Subaru scientist at the National Astronomical Observatory of Japan, has measured the brightness and temperatures of Saturn’s main rings.
This is a three-color composite image of Saturn and its rings taken January 23, 2008 with the COMICS instrument 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. Image credit: NAOJ.
The beautiful appearance of Saturn and its rings has always fascinated people.
The rings consist of countless numbers of ice particles orbiting above the gas giant’s equator. However, their detailed origin and nature remain unknown.
Dr. Fujiwara and his colleagues from the University of California, Los Angeles, NASA’s Jet Propulsion Laboratory, and the National Astronomical Observatory of Japan, collected and analyzed mid-infrared images of Saturn’s rings obtained in January 2008 and April 2005 with the COoled Mid-Infrared Camera and Spectrometer (COMICS) mounted on the Subaru Telescope at Mauna Kea, Hawaii.
They found that the appearance of the rings in the mid-infrared part of the spectrum is 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 scientists explained.
“The 2008 image shows that the Cassini Division and the C ring are brighter in the mid-infrared than the B and A rings appear to be.”
“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.”
The researchers also measured the temperatures of the rings from the Subaru images, which revealed that the Cassini Division and the C ring are warmer than the B and A rings.
“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,” they said.
“On the other hand, in the visible light, we 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.
“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 researchers said.
The 2005 Subaru image shows 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.
Dr. Fujiwara and co-authors 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,” they explained.
“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 team’s findings were published in the March 2017 issue of the journal Astronomy & Astrophysics.
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Hideaki Fujiwara et al. 2017. Seasonal variation of the radial brightness contrast of Saturn’s rings viewed in mid-infrared by Subaru/COMICS. A&A 599, A29; doi: 10.1051/0004-6361/201527529
This article is based on a press-release from the National Astronomical Observatory of Japan.
