Scientists are scouring the Earth and skies for clues to our planet’s climate history. Powerful, sustained volcanic eruptions can change the climate over long periods of time, and the power of the Sun can change the Earth’s climate over millions of years.
But what about interstellar hydrogen clouds? Can these regions of gas and dust modify the Earth’s climate when the planet encounters them?
Not all interstellar clouds are the same. Some are diffuse, while others are much denser. New research in Natural astronomy says that our solar system may have passed through one of the dense clouds two or three million years ago.
This effect could have changed the chemistry of Earth’s atmosphere, affecting cloud formation and climate.
The research focuses on “A possible direct exposure of the Earth to the dense and cold interstellar medium 2 to 3 Myr ago”. The lead author is Merav Opher of the Radcliffe Institute for Advanced Study at Harvard University and the Department of Astronomy at Boston University.
“Our results open a new window on the relationship between the evolution of life on Earth and our cosmic neighborhood.” – Avi Loeb, co-author, Institute for Theory and Computation at Harvard University
The Sun moves through a large cavity in the interstellar medium (ISM) called a local bubble. Inside the LB, the Sun’s solar output creates a cocoon called the heliosphere. It protects the solar system from cosmic radiation.
Inside the LB, there is not only the Sun. It also contains other stars and the Local Interstellar Cloud (LIC). The Sun has passed through the LIC and will leave it in a few thousand years. LIC is not very dense.
But over the past few million years, as the Sun passed through the local bubble, it encountered clouds much denser than the LIC. Researchers examined the effect of these encounters on the Sun’s ability to create a cocoon for the solar system and what effect this had on Earth.
“Stars move, and now this paper shows not only that they are moving, but that they are undergoing drastic changes.” – Merav Opher, professor of astronomy, BU College of Arts & Sciences
“Here we show that in the ISM that the Sun has passed through over the past two million years, there are cold, compact clouds that could have significantly affected the heliosphere. We explore a scenario in which the solar system passed through a cloud of cold gas a few million years ago,” write Opher and his colleagues.
Most of what the Sun passes through is thin ISM. The Sun constantly moves through the thin ISM without effect.
“These clouds are numerous around the Sun but have too low a density to contract the heliosphere at distances
However, the denser clouds of the ISM are dense enough to significantly affect the protective heliosphere.
“The ISM near the solar system also hosts a few rare, dense, cold clouds, called the local ribbon of cold clouds,” they write.
One of the clouds in this ribbon is called the Local Leo Cold Cloud (LLCC). It is one of the largest clouds in the ribbon and astronomers have studied it extensively. They know its density and its temperature. The researchers haven’t paid as much attention to the other clouds in the ribbon, but they expect them to be similar.
The authors of this paper say that there is a small chance, about 1.3%, that the Sun will pass through the tail of the LLCC.
“We call this part the Local Lynx of Cold Clouds (LxCC). LxCCs make up almost half of the total mass of the LRCC and are more massive than the more well-studied LLCC,” they write.
Questions arise 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 much in the 2 million years since the alleged encounter.
“We have assumed here that these clouds have not undergone any substantial changes over the past 2 months, although future work may provide more information on their evolution.”
Researchers used simulations to study the effect of dense clouds on the heliosphere and, by extension, our planet. They say the cloud’s hydrogen density pushed the Sun away, shrinking the heliosphere to a size smaller than Earth’s orbit around the Sun.
This brought the Sun and Moon into contact with the dense, cold ISM. “Such an event could have had a dramatic impact on the Earth’s climate,” they explain.
The encounter is supported by the presence of the radioisotope 60Fe on Earth. 60Fe is mainly produced in supernovae and has a half-life of 2.6 million years.
Previous research has linked 60Fe to a supernova explosion, where it became encased in dust grains and then sent to Earth. It is also present on the Moon. 244Pu was delivered at the same time, also in the form of supernova ejecta.
Although there is much uncertainty, researchers say the deposition of 60Fe on Earth corresponds to our solar system’s hypothetical passage through a dense cloud that compressed the protective heliosphere, allowing the isotopes to reach Earth .
“Our proposed scenario is consistent with geological evidence from the 60Fe and 244Pu isotopes that Earth was in direct contact with the ISM during this period,” they write.
But if a supernova delivered the radioisotopes, it should have been pretty close, and other evidence rules out the supernova source.
“A near-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 that deliver them specifically to Earth, like the scenario with nearby supernova explosions.”
The question at the heart of this problem is: how has this affected the Earth?
A thorough study of the consequences is beyond the scope of this research. The team commented on some possibilities, while cautioning that very little research has been done on this question.
“Very little work has quantitatively studied the climatic effects of such encounters in the context of encounters with dense giant molecular clouds. Some argue that such high densities would deplete ozone in the middle atmosphere (50 to 100 km) and would eventually cool the Earth’s atmosphere,” they write.
It’s a step forward, but some research suggests that this cooling could 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. With the shrinking of the heliosphere, the Earth was exposed directly to the ISM,” they write.
In their conclusion, they recall that the probability of this meeting taking place is low. But not zero.
“Stars move, and now this paper shows not only that they move, but that they undergo drastic changes,” said Opher, professor of astronomy in the BU College of Arts & Sciences and member of the Center for Space Physics of the University.
“While the coincidence of the Sun’s past motion with these rare clouds is truly remarkable, the turbulent nature of the ISM and the current small angular size of these clouds mean that the past location error ellipse is much larger than that of clouds and, in the absence of any other information, the probability of their encounter is considered low,” they write in their conclusion.
It is up to future work to explore this issue further.
While this particular encounter may not have happened, the research remains fascinating. There seem to be a bewildering number of variables that led us to this, and it’s not a stretch to imagine that passing through dense clouds in the ISM played some role at some point.
“Our cosmic neighborhood beyond the solar system rarely affects life on Earth,” said Avi Loeb, director of the Institute for Theory and Computation at Harvard University and 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.
“We hope that our current work will prompt future work detailing the climatic effects due to an encounter of the heliosphere with the LRCC and the possible consequences for evolution on Earth,” the authors conclude.
This article was originally published by Universe Today. Read the original article.