Earth\u2019s protective atmosphere has sheltered life for billions of years, creating a haven where evolution produced complex lifeforms like us. The ozone layer plays a critical role in shielding the biosphere from deadly UV radiation. It blocks 99% of the Sun\u2019s powerful UV output. Earth\u2019s magnetosphere also shelters us.<\/p>\n
But the Sun is relatively tame. How effective are the ozone and the magnetosphere at protecting us from powerful supernova explosions?<\/p>\n
<\/p>\n Every million years\u2014a small fraction of Earth\u2019s 4.5 billion-year lifetime\u2014a massive star explodes within 100 parsecs (326 light-years) of Earth. We know this because our Solar System sits inside a massive bubble in space called the Local Bubble. It\u2019s a cavernous region of space where hydrogen density is much lower than outside the bubble. A series of supernovae explosions in the previous 10 to 20 million years carved out the bubble.<\/p>\n Supernovae are dangerous, and the closer a planet is to one, the more deadly its effects. Scientists have speculated on the effects that supernova explosions have had on Earth, wondering if it triggered mass extinctions or at least partial extinctions. A supernova\u2019s gamma-ray burst and cosmic rays can deplete Earth\u2019s ozone and allow ionizing UV radiation to reach the planet\u2019s surface. The effects can also create more aerosol particles in the atmosphere, increasing cloud coverage and causing global cooling. <\/p>\n A new research article in Nature Communications Earth and Environment examines supernova explosions and their effect on Earth. It is titled \u201cEarth\u2019s Atmosphere Protects the Biosphere from Nearby Supernovae.\u201d The lead author is Theodoros Christoudias from the Climate and Atmosphere Research Center, Cyprus Institute, Nicosia, Cyprus.<\/p>\n The Local Bubble isn\u2019t the only evidence of nearby core-collapse supernovae (SNe) in the last few million years. Ocean sediments also contain 60<\/sup>Fe, a radioactive isotope of iron with a half-life of 2.6 million years. SNe expel 60<\/sup>Fe into space when they explode, indicating that a nearby supernova exploded about 2 million years ago. There\u2019s also 60<\/sup>Fe in sediments that indicate another SN explosion about 8 million years ago. <\/p>\n Researchers have correlated an SN explosion with the Late Devonian extinction about 370 million years ago. In one paper, researchers found plant spores burned by UV light, an indication that something powerful depleted Earth\u2019s ozone layer. In fact, Earth\u2019s biodiversity declined for about 300,000 years prior to the Late Devonian extinction, suggesting that multiple SNe could\u2019ve played a role. <\/p>\n Earth\u2019s ozone layer is in constant flux. As UV energy reaches it, it breaks ozone molecules (O3) apart. That dissipates the UV energy, and the oxygen atoms combine into O3 again. The cycle repeats. That\u2019s a simplified version of the atmospheric chemistry involved, but it serves to illustrate the cycle. A nearby supernova could overwhelm the cycle, depleting the ozone column density and allowing more deadly UV to reach Earth\u2019s surface. <\/p>\n But in the new paper, Christoudias and his fellow authors suggest that Earth\u2019s ozone layer is much more resilient than thought and provides ample protection against SNe within 100 parsecs. While previous researchers have modelled Earth\u2019s atmosphere and its response to a nearby SN, the authors say that they\u2019ve improved on that work. <\/p>\n They modelled Earth\u2019s atmosphere with an Earth Systems Model with Atmospheric Chemistry (EMAC) model to study the impact of nearby SNe explosions on Earth\u2019s atmosphere. Using EMAC, the authors say they\u2019ve modelled \u201cthe complex atmospheric circulation dynamics, chemistry, and process feedbacks\u201d of Earth\u2019s atmosphere. These are needed to \u201csimulate stratospheric ozone loss in response to elevated ionization, leading to ion-induced nucleation and particle growth to CCN\u201d (cloud condensation nuclei.)<\/p>\n \u201cWe assume a representative nearby SN with GCR (galactic cosmic ray) ionization rates in the atmosphere that are 100 times present levels,\u201d they write. That correlates with a supernova explosion about 100 parsecs or 326 light-years away. <\/p>\n \u201cThe maximum ozone depletion over the poles is less than the present-day anthropogenic ozone hole over Antarctica, which amounts to an ozone column loss of 60\u201370%,\u201d the authors explain. \u201cOn the other hand, there is an increase of ozone in the troposphere, but it is well within the levels resulting from recent anthropogenic pollution.\u201d<\/p>\n But let\u2019s cut to the chase. We want to know if Earth\u2019s biosphere is safe or not.<\/p>\n The maximum mean stratospheric ozone depletion from 100 times more ionizing radiation than normal, representative of a nearby SN, is about 10% globally. That\u2019s about the same decrease as our anthropogenic pollution causes. It wouldn\u2019t affect the biosphere very much.<\/p>\n \u201cAlthough significant, it is unlikely that such ozone changes would have a major impact on the biosphere, especially because most of the ozone loss is found to occur at high latitudes,\u201d the authors explain. <\/p>\n But that\u2019s for modern Earth. During the pre-Cambrian, before life exploded in a multiplication of forms, the atmosphere had only about 2% oxygen. How would an SN affect that? \u201cWe simulated a 2% oxygen atmosphere since this would likely represent conditions where the emerging biosphere on land would still be particularly sensitive to ozone depletion,\u201d the authors write.<\/p>\n \u201cOzone loss is about 10\u201325% at mid-latitudes and an order of magnitude lower in the tropics,\u201d the authors write. At minimum ozone levels at the poles, ionizing radiation from an SN could actually end up increasing the ozone column. \u201cWe conclude that these changes of atmospheric ozone are unlikely to have had a major impact on the emerging biosphere on land during the Cambrian,\u201d they conclude.<\/p>\n What about global cooling? <\/p>\n Global cooling would increase, but not to a dangerous extent. Over the Pacific and Southern oceans, CCN could increase by up to 100%, which sounds like a lot. \u201cThese changes, while climatically relevant, are comparable to the contrast between the pristine pre-industrial atmosphere and the polluted present-day atmosphere.\u201d They\u2019re saying that it would cool the atmosphere by about the same amount as we\u2019re heating it now. <\/p>\n The researchers point out that their study concerns the entire biosphere, not individuals. \u201cOur study does not consider the direct health risks to humans and animals resulting from exposure to elevated ionizing radiation,\u201d they write. Depending on individual circumstances, individuals could be exposed to dangerous levels of radiation over time. But overall, the biosphere would hum along despite a 100-fold increase in UV radiation. Our atmosphere and magnetosphere can handle it. <\/p>\n \u201cOverall, we find that nearby SNe are unlikely to have caused mass extinctions on Earth,\u201d the authors write. \u201cWe conclude that our planet\u2019s atmosphere and geomagnetic field effectively shield the biosphere from the effects of nearby SNe, which has allowed life to evolve on land over the last hundreds of million years.\u201d<\/p>\n This study shows that Earth\u2019s biosphere will not suffer greatly as long as supernova explosions keep their distance. <\/p>\n![\"These](\"https:\/\/www.universetoday.com\/wp-content\/uploads\/2024\/06\/43247_2024_1490_Fig3_png-1024x894.jpg\")
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