Illustration of the protective bubble around the sun (yellow dot) and Earth (blue dot)
Harvard Radcliffe Institute
Between 2 million and 3 million years ago, the solar system encountered turbulence on a galactic scale, colliding with a dense interstellar cloud that may have altered both the climate and evolution on Earth.
Researchers have only recently been able to map the path of the sun through our galaxy, particularly in relation to the relatively dense hydrogen clouds that also journey through the interstellar medium, the vast space between star systems.
Now, a team led by Merav Opher at Boston University in Massachusetts has uncovered evidence that one of these clouds, the Local Ribbon of Cold Clouds in the constellation Lynx, probably crossed paths with our sun’s heliosphere.
The heliosphere is a protective cocoon or bubble formed by solar winds pushing out to the edges of the solar system. Inside the heliosphere, planets are protected from the worst of the galaxy’s radiation.
The new study proposes that as the solar system passed through the interstellar cloud, the heliosphere retreated from it, moving inwards towards the sun. The researchers think the heliosphere shrunk so far that Earth was outside the protective cocoon provided by the solar winds, possibly for as long as 10,000 years.
Using the European Space Agency’s Gaia Satellite, Merav and her colleagues mapped the location of the dense cold cloud and the past trajectory of the sun.
Opher says the probable encounter between the heliosphere and the cold cloud aligns with the deposition of the elements plutonium-244 and radioactive iron-60 in Antarctic ice, deep ocean cores and lunar samples. These elements, which originated in distant supernovae, are captured within interstellar clouds and were probably deposited on Earth while it was outside the heliosphere.
“The indication of an increase in these elements around 2 [million] to 3 million years ago gives us compelling evidence that indeed the sun crossed that cloud around 2 million years ago,” says Opher. “The Earth’s exposure to cold interstellar medium clouds and the related massive increase of hydrogen in the atmosphere and increased radiation almost certainly had a substantial impact on our planet and its climate.”
Sarah Spitzer at the University of Michigan says the paper provides “compelling” evidence that the heliosphere was exposed to a much denser interstellar cloud 2 million to 3 million years ago. The result of the solar system passing through that dense cold cloud was that Earth would have been outside the heliosphere and directly exposed to the interstellar environment, she says.
“Understanding this helps us learn about the effects of the interstellar medium on life on Earth in the past,” says Spitzer. “But it also helps us better understand the current effects of the heliosphere on life on Earth, what might happen if the Earth is exposed to the interstellar medium again in the future, and when that might happen.”
Evan Economo at the Okinawa Institute of Science and Technology in Japan says it is intriguing to think about how encounters in “our local cosmic neighbourhood” may have affected the environment experienced by life on Earth.
“The heliosphere is part of the extended environment that organisms experience on the surface of the Earth, affecting climate and incoming radiation from space,” he says. “If we were outside the heliosphere for certain periods, this could have changed the evolutionary trajectories of a broad range of organisms, including humans. Such links are highly speculative at this point, but give us a new research direction.”
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