Scientists scour the earth and sky for clues to our planet’s climate history. Strong and persistent volcanic eruptions can change the climate for long periods of time, and the rising of the Sun can change the Earth’s climate for millions of years.
But what about interstellar hydrogen clouds? Could these regions of gas and dust change Earth’s climate when the planet encounters them?
Interstellar clouds are not all created equal. Some are scattered while some are much denser. New research in Astronomy of nature says that our solar system may have passed through one of the dense clouds two or three million years ago.
The effect could have changed the chemistry of Earth’s atmosphere, affecting cloud formation and climate.
The research is “Possible direct exposure of Earth to the cold dense interstellar medium 2–3 Myr ago.” The lead author is Merav Opher of the Radcliffe Institute for Advanced Study at Harvard University and the Astronomy Department at Boston University.
“Our results open a new window into the relationship between the evolution of life on Earth and our cosmic neighborhood.” – Avi Loeb, co-author, Institute for Theory and Computation, Harvard University
The Sun moves through a large cavity in the interstellar medium (ISM) called the Local Bubble. Inside the LB, the solar rise creates a cocoon called the heliosphere. It protects the solar system from cosmic rays.
There is more than just the Sun inside the LB. It also contains other stars and the Local Interstellar Cloud (LIC). The Sun moves through the LIC and will leave it in a few thousand years. LIC is not very dense.
But in the last few million years, as the Sun passed through the Local Bubble, it encountered clouds that are much denser than the LIC. Scientists have studied the effect of these encounters on the Sun’s ability to carve out a cocoon for the Solar System and what effect this has had on Earth.
“The stars are moving, and this paper now shows not only that they are moving, but that they are experiencing drastic changes.” – Merav Opher, Professor of Astronomy, BU College of Arts & Sciences
“Here we show that there are cold compact clouds in the ISM that the Sun has passed through over the past few million years, which could have drastically affected the heliosphere. We investigate a scenario where the Solar System passed through a cold cloud of gas and several million years ago,” Opher and her colleagues write .
Most of what the Sun travels through is the thin ISM. The Sun is constantly moving through the thin ISM without effect.
“There are many of these clouds around the Sun, but they are too low in density to pull the heliosphere out into the distance.”
However, the denser clouds in the ISM are dense enough to dramatically affect the protective heliosphere.
“The ISM near the Solar System also harbors several rare, dense, cold clouds called the Local Cold Cloud Ribbon,” they write.
One of the clouds in this belt is called the Local Leo Cold Cloud (LLCC). It is one of the largest clouds in the ribbon and has been studied extensively by astronomers. They know its density and temperature. Researchers haven’t paid as much attention to the other clouds in the ribbon, but expect them to be similar.
The authors of this paper say that there is a small chance, about 1.3%, that the Sun has passed through the tail of the LLCC.
“We name this fraction the Local Lynx of Cold Clouds (LxCCs). LxCCs account for nearly half of all LRCC mass and are more massive than the better-studied LLCCs,” they write.
There are questions about the nature of these clouds in the past.
“Note that these clouds are anomalous and unexplained structures in the ISM, and their origin and physics are not well understood,” the authors write. Their work is based on the assumption that they have not changed substantially in the 2 million years since the alleged encounter.
“We assume that these clouds have not undergone any substantial change over the last 2~Myr, although future work may provide more information about their evolution.”
Scientists used simulations to study the effect of the dense cloud on the heliosphere and, by extension, our planet. They say the cloud’s hydrogen density has shifted back toward the Sun, shrinking the heliosphere smaller than Earth’s orbit around the Sun.
It brought both the Sun and the Moon into contact with the dense, cold ISM. “Such an event could have had a dramatic impact on Earth’s climate,” they explain.
The encounter is supported by the presence of the radioisotope 60Fe on Earth. 60Fe is produced mostly in supernovae and has a half-life of 2.6 million years.
Previous research has linked 60Fe to a supernova explosion, where it became embedded in dust grains and then made its way to Earth. It is also present on the moon. 244Pu was delivered at the same time, also in supernovae ejections.
Although there are many uncertainties, scientists say that the deposition of 60Fe on Earth corresponds to the hypothetical passage of our solar system through a dense cloud that compressed the protective heliosphere and allowed the isotopes to reach Earth.
“Our proposed scenario agrees with geological evidence from 60Fe and 244Pu isotopes that the Earth was in direct contact with the ISM during this period,” they write.
But if the supernova delivered the radioisotopes, it would have to be pretty close, and the other evidence doesn’t discount the supernova source.
“A nearby supernova explosion contradicts the recent model of Local Bubble formation,” the authors explain. “The scenario does not require the absorption of 60Fe and 244Pu into dust particles to deliver them specifically to Earth, like the scenario with nearby supernova explosions.”
The question at the heart of this problem is how did it affect the Earth?
An in-depth study of the implications is beyond the scope of this research. The team commented on some of the possibilities, while cautioning that very little research has been done on the matter.
“Very few papers have examined the climatic effects of such encounters quantitatively in the context of encounters with dense giant molecular clouds. Some argue that such high densities would damage ozone in the middle atmosphere (50-100? km) and eventually cool the Earth,” they write.
It’s a leap, but some research suggests that this cooling may have contributed to the rise of our species.
“The hypothesis is that the emergence of our species, Homo sapiens, was shaped by the need to adapt to climate change. As the heliosphere contracted, the Earth was exposed directly to the ISM,” they write.
In their conclusion, they remind that the probability that this encounter took place is low. But not zero.
“Stars are moving, and this paper now shows not only that they are moving, but that they are experiencing drastic changes,” said Opher, a professor of astronomy in the BU College of Arts & Sciences and a member of the university’s Center for Space Physics.
“Although the coincidence of the Sun’s past motions with these rare clouds is indeed remarkable, the turbulent nature of the ISM and the small current angular size of these clouds mean that the past position error ellipse is much larger than the clouds and any other is missing.” information, the probability of their meeting is measured as low,” they write in their conclusion.
It is for future work to delve deeper into this matter.
While this particular encounter may not have happened, the research is still fascinating. There seems to be a bewildering number of variables that led us there, and it’s not hard to imagine that passage through dense clouds in the ISM played a role at some point.
“Our space neighborhood outside the Solar System rarely affects life on Earth,” said Avi Loeb, director of Harvard University’s Institute for Theory and Computation and co-author of the paper.
“It is exciting to find that our passage through dense clouds several million years ago may have exposed Earth to a much greater flux of cosmic rays and hydrogen atoms. Our results open a new window into the relationship between the evolution of life on Earth.” and our space neighborhood.
“We hope that our current work will stimulate future work detailing the climatic effects due to the heliosphere’s encounter with the LRCC and possible consequences for evolution on Earth,” the authors conclude.
This article was originally published by Universe Today. Read the original article.