- 3216 - LIFE - asteroids and snowball Earth? Scientists know that the Earth was bombarded by huge asteroids in our early history. This analysis suggest that the number of these impacts may have been x10 higher than previously thought
------------------ 3216 - LIFE - asteroids and snowball Earth?
- This barrage of asteroids collisions was similar in scale to that of the asteroid strike which wiped out the dinosaurs, on average every 15 million years between 2,500 and 3,500 million years ago.
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- Scientists are considering what effect these impacts may have had on the Earth's evolving near-surface chemistry.
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- Earth's early years were unimaginably violent. Scientists believe that Earth was struck by a significant number of large asteroids greater than 10 kilometers in diameter. This would have had significant effect on the Earth's near-surface chemistry and ability to support life.
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- The effect of just one such collision was shown comparatively recently by the Chicxulub impact 66 million years ago, which led to the extinction of the dinosaurs.
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- Impact craters from similar collisions can be seen on the Moon and other rocky planets, but atmospheric weathering and plate tectonics have tended to mask any direct evidence for ancient impact craters on Earth.
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- However, echoes of these distant impacts can be seen in the presence of "spherules" found in ancient rocks. The huge impacts threw up molten particles and vapors which then cooled and fell to earth to be embedded in rock as small spherical glassy particles, called spherules.. The greater the impact, the more these particles would have spread from the impact site, so global distribution of a thick spherule layer shows a huge impact.
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- Current models of Earth's early bombardment severely underestimate the number of known impacts, as recorded by spherule layers. The true impact flux could have been up to a factor of 10 times higher than previously thought in the period between 3.5 and 2.5 billion years ago. In this early period, we were being hit by a Chicxulub-sized impact on average every 15 million years.
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- The number and magnitude of these cosmic collisions altered the Earth's surface and atmospheric evolution. These impacts may have also affected the evolution of atmospheric oxygen.
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- Oxygen levels would have drastically fluctuated in the period of intense impacts. Chemical markers suggest there were "whiffs" of oxygen in the early atmosphere, before a permanent rise around 2.5 billion years ago. We tend to focus on the Earth's interior and the evolution of life as controls on Earth's oxygen balance, but bombardment may be another key element.
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- Also a key factor for life is our planets was its tilt on its axis making it more capable of evolving complex life.
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- Since the first discovery of exoplanets, planets orbiting distant stars, in 1992, scientists have been looking for worlds which might support life. It is believed that to sustain even basic life, exoplanets need to be at just the right distance from their stars to allow liquid water to exist; the so-called 'Goldilocks zone'.
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- Oxygen plays a critical role in respiration, the chemical process which drives the metabolisms of most complex living things. Some basic life forms produce oxygen in small quantities, but for more complex life forms, such as plants and animals, oxygen is critical. Early Earth had little oxygen even though basic life forms existed.
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- The researchers found that increasing day length, higher surface pressure, and the emergence of continents all influence ocean circulation patterns and associated nutrient transport in ways that may increase oxygen production.
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- These relationships may have contributed to Earth's oxygenation by favoring oxygen transfer to the atmosphere as Earth's rotation has slowed, its continents have grown, and surface pressure has increased through time.
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- The most interesting result came when they modeled 'orbital obliquity', how the planet tilts as it circles around its star. Greater tilting increased photosynthetic oxygen production in the ocean by increasing the efficiency with which biological ingredients are recycled. The effect was similar to doubling the amount of nutrients that sustain life.
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- Earth's sphere tilts on its axis at an angle of 23.5 degrees. This gives us our seasons, with parts of the Earth receiving more direct sunlight in summer than in winter. However, not all planets in our Solar System are tilted like the Earth: Uranus is tilted at 98 degrees, Mercury is not tilted at all.
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- There are several factors to consider in looking for life on another planet. The planet needs to be the right distance from its star to allow liquid water and have the chemical ingredients for the origin of life.
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- But not all oceans will be great hosts for life as we know it, and an even smaller subset will have suitable habitats for life to progress towards animal complexity. Small tilts or extreme seasonality on planets with Uranus-like tilts may limit the proliferation of life, but modest tilt of a planet on its axis may increase the likelihood that it develops oxygenated atmospheres that could serve as beacons of microbial life and fuel the metabolisms of large organisms.
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- The first biological production of oxygen on Earth and its first appreciable accumulation in the atmosphere and oceans are milestones in the history of life on Earth. Studies of Earth teach us that oxygen may be one of our most important biosignatures in the search for life on distant exoplanets.
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- A planet's seasonality, could increase or decrease the possibility of finding oxygen derived from life outside our solar system.
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- Besides Earth’s tilt the changes in Earth's orbit may have allowed complex life to emerge and thrive during the most hostile climate episode the planet has ever experienced.
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- Researchers studied a succession of rocks laid down when most of Earth's surface was covered in ice during a severe glaciation, dubbed 'Snowball Earth', that lasted over 50 million years.
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- In the South Australian outback researches found kilometer-thick units of glacial rocks that formed about 700 million years ago. At this time, Australia was located closer to the equator, known today for its tropical climates. The rocks show unequivocal evidence that ice sheets extended as far as the equator at this time, providing compelling evidence that Earth was completely covered in an icy shell.
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- "Banded Iron Formations" are sedimentary rocks consisting of alternating layers of iron-rich and silica-rich material. These rocks were deposited in the ice-covered ocean near colossal ice sheets.
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- During the “snowball glaciation“, the frozen ocean would have been entirely cut off from the atmosphere. Without the normal exchange between the sea and air, many variations in climate that normally occur simply wouldn't have.
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- Highly variable rock layers appeared to show cycles that looked a lot like climate cycles associated with the advance and retreat of ice sheets. Such variability was thought to be at odds with a static Snowball Earth entombing the whole ocean in ice.
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- The iron comes from hydrothermal vents on the seafloor. Normally, the atmosphere oxidizes any iron immediately, so “Banded Iron Formations’ typically do not accumulate. But during the Snowball, with the ocean cut off from the air, iron was able to accumulate enough for them to form.
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- Using magnetic susceptibility which is a measure of the extent to which the rocks become magnetised when exposed to a magnetic field, scientists discoved that the layered rock archives preserve evidence for nearly all orbital cycles.
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- Earth's orbit around the sun changes its shape and the tilt and wobble of Earth's spin axis also undergo cyclic changes. These astronomical cycles change the amount of incoming solar radiation that reaches Earth's surface and as a result control the climate.
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- Even though Earth's climate system behaved very differently during the Snowball, Earth's orbital variations allowed the waxing and waning of ice sheets, enabling periodic ice-free regions to develop on snowball Earth.
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- The sedimentary rocks of this age show evidence for flowing water at Earth's surface when this water should have been locked up in ice sheets. Complex multicellular life is now known to have originated during this period of “climate crisis‘.
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- The existence of ice-free 'oases' in the snowball ocean provided a sanctuary for animal life to survive the most extreme climate event in Earth history, Snowball Earth.
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- July 13, 2021 LIFE - asteroids and snowball Earth? 3216
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