It is the oldest telescope in the world - and it lies at the bottom of the ocean. Ancient sea floor sediments have revealed that a supernova exploded during the Pliocene era and may have caused a minor extinction event on Earth.海底の堆積物は、過去の記録としてとてもいいものらしい。
これは世界最古の望遠鏡であって、海底にある。古代の海底堆積物から、鮮新世(500万年前〜160万年前)における小規模な絶滅が起きた頃に、超新星爆発が起きていたことが明らかになった。
Levels of radioactive iron-60 suggest the supernova was between 60 and 300 light years away, says Brian Fields of the University of Illinois at Urbana-Champaign. "It didn't hit us or we wouldn't be here." Radiation from the blast could have weakened Earth's atmosphere, he says, exposing organisms to the sun's ultraviolet radiation. This coincides with an extinction peak, but Fields says there is no direct evidence of a link. The work was reported at a meeting of the Geological Society of America in Denver, Colorado, this week.
「鉄60放射性同位元素の比率は、60〜300光年彼方で超新星爆発が起きたこと示唆している。それは我々を直撃していない。していたとしたら、我々は今ここにいないあだろう。超新星爆発からの放射は地球の大気圏を弱め、生物は太陽からの紫外線放射に曝された。絶滅とピークとの偶然の一致は、関係があることの直接の証拠にはならない。」と、University of IllinoisのBrian Fieldsは語った。この研究成果は、今週コロラド州Denverで開かれたGeological Society of Americaで発表された。
[Ancient sea mud records supernova blast (2007/11/04) on New Scientist Space]
で、この米国地質学会(Geological Society of America)の2007年度学会での発表というのが、これ:
Brian Fields: "When stars attack! Live undersea radioactivities as signatures of neaby supernova explosions", Paper No. 218-10 Presentation Time: 4:00 PM-4:15 PM at 2007 GSA Annual Meeting & Exposition (2007/10/28-31)この太陽系近傍の超新星爆発については、2005年頃の成果で、Brian Fieldsはまとめページを作っている。
The lifespans of the most massive stars are a symphony of the fundamental forces, culminating in a spectacular and violent supernova explosion. While these events are awesome to observe, they can take a more sinister shade when they occur closer to home, because an explosion inside a certain "minimum safe distance" would pose a grave threat to life on Earth. We will discuss these cosmic insults to life, and ways to determine whether a supernova occurred nearby over the course of the Earth's existence. We will then present recent evidence that a star exploded near the Earth about 3 million years ago. Radioactive iron-60 atoms have been found in deep-ocean ferromanganese crusts, and are likely to be debris from this explosion. Recent data confirm this radioactive signal, and for the first time allow sea sediments to be used as a telescope, probing the nuclear reactions that power exploding stars. Furthermore, an explosion so close to Earth was probably a "near-miss," which emitted intense and possibly harmful radiation. The resulting environmental damage may even have led to extinction of species which were the most vulnerable to this radiation.
ここで言及しているKnie et al.[2004]は、Phys Rev Lett掲載という、とっても立派なもの:
Fossils of Exploding Stars: Radioactive Supernova Debris in the Deep Ocean (2005/12 updated) on Brian Fields: "When Stars Attack!"
Moreover, both the deep-ocean sample, and the laboratory techniques, were better suited for a high-resolution measurement. Knie et al (2004) showed that the iron-60 atoms are isolated in a single layer 2.8 million years old. They estimated that a supernova exploding at that time, and a distance of about 120 light years, could lead to the radioactive signal they found.
さらに深海底堆積物サンプルおよび実験室のテクニックで、より解像度の高い計測がなされた。Knie et al.[2004]は、60Fe原子が280万年前の層で際立っていることを示した。彼らは超新星爆発がそこときに120光年彼方で起きたと推定した。この超新星爆発が彼らの発見した放射性同位元素のシグナルにつながった可能性がある。
K. Knie, G. Korschinek, T. Faestermann, E.A. Dorfi, G. Rugel, and A. Wallner: "60Fe Anomaly in a Deep-Sea Manganese Crust and Implications for a Nearby Supernova Source", Phys. Rev. Lett. 93, 171103, 2004 [Abstract]もちろん、Brian Fieldsもこの道にかけてはベテランである:
A nearby supernova (SN) explosion in the past can be confirmed by the detection of radioisotopes on Earth that were produced and ejected by the SN. We have now measured a well resolved time profile of the 60Fe concentration in a deep-sea ferromanganese crust and found a highly significant increase 2.8 Myr ago. The amount of 60Fe is compatible with the deposition of ejecta from a SN at a distance of a few 10 pc. The well defined time of the SN explosion makes it possible to search for plausible correlations with other events in Earth's history.
http://www.bl.physik.uni-muenchen.de/gams/va13data.jpg
John Ellis, Brian D. Fields, David N. Schramm: "Geological Isotope Anomalies as Signatures of Nearby Supernovae", Astrophys. J., 470, 1227-1236, 1996 [Abstract]
Brian D. Fields, Kathrin A. Hochmuth, John Ellis: "Deep-Ocean Crusts as Telescopes: Using Live Radioisotopes to Probe Supernova Nucleosynthesis", Astrophys. J., 621, 902-907, 2005 [Abstract]
ちなみに、宇宙のステルヴィアでは、太陽系から21光年の距離にある「みずへびβ」が、超新星爆発を起こしたことになっている。