The cosmic event that rewrote Earth's climate history

New astrophysical research sheds light on an important cosmic event that occurred two million years ago when the solar system passed through a dense interstellar cloud. This may have changed Earth's climate by exposing it to enhanced cosmic radiation, supported by increased isotopes found in the geological record.

Earth was a very different place about two million years ago, with our early human ancestors living alongside saber-toothed tigers, mastodons, and huge rodents. Depending on where they were, they might have been cold: the ground had fallen In deep freeze, with multiple ice ages coming and going until about 12,000 years ago. Scientists theorize that ice ages occur for a number of reasons, including the planet's tilt and rotation, shifting tectonic plates, volcanic eruptions, and levels of carbon dioxide in the atmosphere.

But what if such drastic changes are a result not only of Earth's environment, but also of the Sun's position in the galaxy?

The effect of the Sun's journey into the galaxy

In a new paper published today (June 10) in natural astronomy, Lead author and astrophysicist Merav Ofer — a professor of astronomy at Boston University and a fellow at the Harvard Radcliffe Institute — found evidence that about 2 million years ago, the solar system encountered an interstellar cloud so dense that it could have interfered with the sun's solar wind. Over and her colleagues believe this indicates that the Sun's position in space may shape Earth's history more than previously thought.

The role of the heliosphere in protecting the Earth

Our entire solar system is coated with a protective layer plasma The shield that emanates from the Sun, known as the heliosphere. They are made of a continuous stream of charged particles, called the solar wind, which extends far beyond PlutoWhat do planets cover? NASA is calling “Giant bubble.” It protects us from radiation and galactic rays that can change DNAScientists believe this is part of the reason why life on Earth evolved as it did.

According to the latest research, the cold cloud compressed the heliosphere in a way that briefly placed Earth and other planets in the solar system outside the influence of the heliosphere.

Effects of galactic encounter on Earth

“This paper is the first to quantitatively show that there was a collision between the Sun and something outside the solar system that would affect Earth's climate,” says Over, a heliospheric expert.

Her models have shaped our scientific understanding of the heliosphere, and how the bubble is formed by the solar wind that presses on the interstellar medium, the space between stars and beyond the heliosphere in our galaxy. Her theory is that the heliosphere resembles a puffy croissant, an idea that has shocked the space physics community. It now sheds new light on how the heliosphere, where the Sun moves through space, affects the chemistry of Earth's atmosphere.

“Stars are moving, and now this paper shows not only are they moving, but they are experiencing dramatic changes,” Over says. I first discovered this study and began working on it during a year-long fellowship at the Harvard Radcliffe Institute.

Simulation insights into cosmic interactions

To study this phenomenon, Ofer and her collaborators looked back in time, using sophisticated computer models to visualize where the Sun was, along with it, the heliosphere, and the rest of the solar system two million years ago.

They also mapped the path of the local band system of cold clouds, a series of large, dense, very cold clouds made up mostly of hydrogen atoms. Their simulations showed that one of the clouds near the end of this band, called the Local Lynx of Cold Cloud, could collide with the heliosphere.

Geological and cosmological evidence

If that had happened, Over says, Earth would have been completely exposed to the interstellar medium, where gas and dust mix with atomic elements left over from exploding stars, including iron and plutonium. Normally, the heliosphere filters out most of these radioactive particles. But without protection, they can easily reach the ground.

According to the paper, this is consistent with geological evidence showing an increase in isotopes of 60Fe (iron 60) and 244Pu (plutonium 244) in the ocean, on the Moon, Antarctic snow, and ice cores from the same time period. The timing also matches temperature records that indicate a period of cooling.

Long-term galactic influences

“Our cosmic neighborhood outside the solar system rarely affects life on Earth,” says Avi Loeb, director of the Institute for Theory and Computation at Harvard University and a co-author of the paper. “It is exciting to discover that our passage through dense clouds a few million years ago could have exposed Earth to a much greater flux of cosmic rays and hydrogen atoms. Our results open a new window on the relationship between the evolution of life on Earth and our cosmic neighborhood.

The outward pressure from the local lynx in the cold cloud could have continuously blocked the heliosphere for anywhere from a couple of hundred years to a million years, depending on the size of the cloud, Over says. “But as soon as Earth moved away from the cold cloud, the heliosphere engulfed all the planets, including Earth,” she says. This is the case today.

Future research and its implications

It is impossible to know the exact effect of cold clouds on Earth, as they could trigger an ice age. But there are a few other cold clouds in the interstellar medium, which the Sun has likely encountered in the billions of years since its birth, Over says. It will likely falter further in another million years or so.

Ofer and her collaborators are now working to trace where the sun was seven million years ago, and even further back. The position of the Sun millions of years ago, as well as the cold cloud system, has been made possible by data collected by the European Space Agency's Gaia mission, which is building the largest 3D map of the galaxy and giving an unprecedented look at the galaxy. The speed of movement of stars.

Explore the sun's past path

“This cloud was already in our past, and if we crossed something that massive, we would be exposed to the interstellar medium,” Over says. The effect of crossing paths containing a lot of hydrogen and radioactive material is unclear, so Ofer and her team at Boston University NASA-Funded SHIELD (Solar Wind with Hydrogen Ion Exchange and Large-Scale Dynamics) DRIVE Science Center They are now exploring the impact this could have on Earth's radiation, as well as the atmosphere and climate.

“This is just the beginning,” Ofer says. She hopes this paper will open the door to further exploration of how the solar system was affected by external forces in the deep past and how these forces in turn shaped life on our planet.

Reference: “Possibility of direct exposure of Earth to the cold, dense interstellar medium 2-3 million years ago” June 10, 2024, Nature astronomy.
doi: 10.1038/s41550-024-02279-8

This research was supported by NASA.