17:15 〜 19:15
[PPS05-P02] UV/Visible Spectroscopic Observations of Venus with a Small Ground-based Telescope
キーワード：金星、分光観測
UV/Visible Spectroscopic Observations of Venus with a Small Ground-based Telescope
Rintaro Eguchi1*, Makoto Taguchi1*, Daisuke Kohno1*, Yasuhiro Shoi2*, Motoki Sarai2*
Toshihiko Nakano3*, Masataka Imai4*, Tatsuharu Ohno5*, Mitsuteru Sato5*, Seiko Takagi5*
Yukihiro Takahashi5*, Ko Hamamoto6* ,Masato Kagitani7*
1 Rikkyo University, Japan; 2 Kanazawa University, Japan; 3 National Institute of Technology, Oita College, Japan; 4 The University of Tokyo, Japan; 5 Hokkaido University, Japan; 6 JAXA, Japan; 7 Tohoku University
Venus, one of the terrestrial planets in the Solar System, exhibits an atmospheric circulation distinct from that of Earth, known as super-rotation. Super-rotation refers to the westward winds that extend across the entire Venusian atmosphere, with maximum wind speeds exceeding the movement speed of the planet’s solid surface due to its rotation. This suggests the existence of a mechanism that transports angular momentum from the solid surface to the upper atmosphere to sustain super-rotation. The energy source driving this mechanism is believed to be solar radiation. The upper layer of the Venusian cloud cover is heated by solar radiation, generating atmospheric gravity waves that carry eastward momentum. When these waves interact with the surface, a reaction force maintains super-rotation with westward momentum. The Venus orbiter Akatsuki has provided observational evidence of this mechanism by measuring wind speeds and temperature distributions at the cloud top.
Solar radiation absorption occurs in the wavelength range of 280–500 nm. While absorption between 280 nm and 320 nm is known to be caused by SO2, the absorber responsible for wavelengths longer than 320 nm remains unidentified. Spectroscopic observations of the reflection spectrum of sunlight by the Venusian atmosphere are effective for identifying this unknown absorber. However, ultraviolet observations from the ground are impossible due to the ozone layer, which exists at around 25 km altitude in Earth's atmosphere. To overcome this limitation, a spectroscopic imaging observation of Venus is planned using the balloon-borne telescope FUJIN-2, which can ascend to an altitude of 32 km. However, at wavelengths longer than 300 nm, some transmission is possible even from high-altitude ground-based sites. The objective of this study is to conduct preliminary ground-based spectroscopic observations of Venus to obtain clues for identifying the absorbing substance before the observations with FUJIN-2.
For ground-based observations, the T60 telescope of Tohoku University at Haleakalā Observatory in Hawaii, USA, was used. To investigate the absorption lines in the reflected light spectrum, the reflectance spectrum of Venus, which represents the fraction of sunlight reflected by the Venusian atmosphere, was examined. This required acquiring both the Venus spectrum and the solar spectrum. The reflectance spectrum was obtained by dividing the Venus spectrum by the solar spectrum.
It was essential to ensure that the atmospheric conditions were the same during observations. Here, "atmospheric conditions" refers to the transmittance of Earth's atmosphere. In this study, the transmittance was assumed to depend on airmass, and observations were conducted when Venus and the Sun were at the same altitude. Additionally, the same optical system needed to be used for both observations. To achieve simultaneous Venus and solar observations, a modification was made: a hole was drilled in the primary mirror cover of the telescope, reducing the aperture during solar observations. These modifications allowed observations in which the effects on the Venus and solar spectra were identical. When the effects of Earth's atmosphere and the optical system are the same, dividing the spectra cancels out these effects.
In this study, observational instruments were installed on the T60 telescope in September 2024, followed by online observations to accumulate and analyze reflectance spectra. The absorption lines in the reflectance spectra were examined. This presentation will discuss the analysis results of data observed on October 31, 2024, and after January 18, 2025, focusing particularly on the absorption lines in the reflectance spectra and temporal variations in ultraviolet reflectance.
Rintaro Eguchi1*, Makoto Taguchi1*, Daisuke Kohno1*, Yasuhiro Shoi2*, Motoki Sarai2*
Toshihiko Nakano3*, Masataka Imai4*, Tatsuharu Ohno5*, Mitsuteru Sato5*, Seiko Takagi5*
Yukihiro Takahashi5*, Ko Hamamoto6* ,Masato Kagitani7*
1 Rikkyo University, Japan; 2 Kanazawa University, Japan; 3 National Institute of Technology, Oita College, Japan; 4 The University of Tokyo, Japan; 5 Hokkaido University, Japan; 6 JAXA, Japan; 7 Tohoku University
Venus, one of the terrestrial planets in the Solar System, exhibits an atmospheric circulation distinct from that of Earth, known as super-rotation. Super-rotation refers to the westward winds that extend across the entire Venusian atmosphere, with maximum wind speeds exceeding the movement speed of the planet’s solid surface due to its rotation. This suggests the existence of a mechanism that transports angular momentum from the solid surface to the upper atmosphere to sustain super-rotation. The energy source driving this mechanism is believed to be solar radiation. The upper layer of the Venusian cloud cover is heated by solar radiation, generating atmospheric gravity waves that carry eastward momentum. When these waves interact with the surface, a reaction force maintains super-rotation with westward momentum. The Venus orbiter Akatsuki has provided observational evidence of this mechanism by measuring wind speeds and temperature distributions at the cloud top.
Solar radiation absorption occurs in the wavelength range of 280–500 nm. While absorption between 280 nm and 320 nm is known to be caused by SO2, the absorber responsible for wavelengths longer than 320 nm remains unidentified. Spectroscopic observations of the reflection spectrum of sunlight by the Venusian atmosphere are effective for identifying this unknown absorber. However, ultraviolet observations from the ground are impossible due to the ozone layer, which exists at around 25 km altitude in Earth's atmosphere. To overcome this limitation, a spectroscopic imaging observation of Venus is planned using the balloon-borne telescope FUJIN-2, which can ascend to an altitude of 32 km. However, at wavelengths longer than 300 nm, some transmission is possible even from high-altitude ground-based sites. The objective of this study is to conduct preliminary ground-based spectroscopic observations of Venus to obtain clues for identifying the absorbing substance before the observations with FUJIN-2.
For ground-based observations, the T60 telescope of Tohoku University at Haleakalā Observatory in Hawaii, USA, was used. To investigate the absorption lines in the reflected light spectrum, the reflectance spectrum of Venus, which represents the fraction of sunlight reflected by the Venusian atmosphere, was examined. This required acquiring both the Venus spectrum and the solar spectrum. The reflectance spectrum was obtained by dividing the Venus spectrum by the solar spectrum.
It was essential to ensure that the atmospheric conditions were the same during observations. Here, "atmospheric conditions" refers to the transmittance of Earth's atmosphere. In this study, the transmittance was assumed to depend on airmass, and observations were conducted when Venus and the Sun were at the same altitude. Additionally, the same optical system needed to be used for both observations. To achieve simultaneous Venus and solar observations, a modification was made: a hole was drilled in the primary mirror cover of the telescope, reducing the aperture during solar observations. These modifications allowed observations in which the effects on the Venus and solar spectra were identical. When the effects of Earth's atmosphere and the optical system are the same, dividing the spectra cancels out these effects.
In this study, observational instruments were installed on the T60 telescope in September 2024, followed by online observations to accumulate and analyze reflectance spectra. The absorption lines in the reflectance spectra were examined. This presentation will discuss the analysis results of data observed on October 31, 2024, and after January 18, 2025, focusing particularly on the absorption lines in the reflectance spectra and temporal variations in ultraviolet reflectance.