Viva Origino
Online ISSN : 1346-6933
Print ISSN : 0910-4003
51 巻, 3 号
  • 小松 勇
    2023 年 51 巻 3 号 p. 4-
    発行日: 2023年
    公開日: 2023/12/23
    ジャーナル フリー

    For the detection of extraterrestrial life from extrasolar planets, the identification of potential biosignatures as gaseous molecules such as oxygen and surface spectral features originating from life is the most promising way. In the latter case, the vegetation red edge (VRE), which is the steep difference in the vegetation reflectance spectrum, has been conventionally considered. However, predicting the wavelength at which the edge of "exovegetation" appears would be difficult, e.g., around M dwarf stars, which are the current targets of observations and whose radiation is far from that of the Sun. Here we present the possible detection of photosynthetic fluorescence as a biosignature in addition to VRE, based on a recent paper of ours summarized by an interdisciplinary discussion from biology to astronomy. The fluorescence has recently been detected by remote sensing of the Earth. We investigated how the signal appears in the planetary spectrum. Photoabsorption, reflection, and fluorescence are consistently modeled because they are physically related. A mock observation of an Earth-Sun system at 10 pc from a future space telescope shows the enormously long exposure time required to identify the fluorescence. However, around ultracool dwarfs such as TRAPPIST-1, which has strong stellar absorption of VO and FeH, the apparent reflection of an Earth-like planet with fluorescence whose spectrum is that of bacteriochlorophyll b-bearing purple bacteria was greatly enhanced, which could be a promising signal for high-dispersion spectroscopy using future ground-based telescopes such as TMT. Possible false positive/negative detection of the fluorescence would be better eliminated by simultaneous detection with the VRE and/or nonlinearity of the photosynthetic fluorescence.

  • 黒川 宏之
    2023 年 51 巻 3 号 p. 5-
    発行日: 2023年
    公開日: 2023/12/23
    ジャーナル フリー

    Remote sensing observations and returned sample analysis in Hayabusa2 mission revealed the origin and evolution of the asteroid 162173 Ryugu and its parent body and provided insights into the early solar system history. Here first we review findings of the Hayabusa2 mission, mostly focusing on Ryugu’s physical, chemical, and mineralogical properties. Spectral properties, elemental composition, and mineralogy of the asteroid Ryugu informed from remote-sensing observations of the asteroid and analysis of its returned samples show that Ryugu closely resembles CI chondrite meteorites, while its properties more primitive than CI chondrite meteorites are also identified. The presence of CO2 fluid inclusions and nucleosynthetic isotopic characteristics point to a distant formation location distinct from those of other carbonaceous chondrite parent bodies for Ryugu’s and CI chondrite parent bodies, possibly beyond Jupiter’s formation location. The homogeneity of Ryugu’s surface suggests that the parent body may be an undifferentiated body. Next, we place those findings in broader context with the solar system explorations, astronomical observations, and theoretical studies to highlight unsolved issues on the origin and evolution of the asteroid Ryugu and the early solar system. While Ryugu’s origin beyond Jupiter’s orbit has been widely discussed in previous studies, combined evidence including Jupiter’s envelope composition, asteroid taxonomy, and theoretical studies do not rule out other possibilities. Observational evidence and theoretical studies suggest water-rock differentiation of large asteroids (parent bodies) and its role in chemical evolution. We conclude with discussion on future studies to better understand the origin of the asteroid Ryugu and the evolution of the early solar system.