Scientists search the earth and sky for clues to the climate history of our planet. Powerful and persistent volcanic eruptions can change the climate over long periods of time, and the sun’s output can change Earth’s climate over millions of years.
But what about interstellar hydrogen clouds? Could these gas and dust regions change Earth’s climate if the planet encounters them?
Interstellar clouds are not all the same. Some are diffuse, others are much denser. New research in Nature Astronomy says that our solar system may have passed through one of the dense clouds two to three million years ago.
The effect could have changed the chemistry of Earth’s atmosphere, affecting cloud formation and climate.
The research is “A possible direct exposure of Earth to the cold, dense interstellar medium 2-3 million years 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 environment.” – Avi Loeb, co-author, Harvard University’s Institute for Theory and Computation
The Sun moves through a large cavity in the interstellar medium (ISM), the local bubble. Within the LB, the sun’s solar energy creates a cocoon called the heliosphere. It protects the solar system from cosmic radiation.
There is more than just the sun within the LB. It also contains other stars and the Local Interstellar Cloud (LIC). The sun has moved through the LIC and will leave it in a few thousand years. The LIC is not very compact.
But over the past few million years, as the Sun crossed the local bubble, it encountered clouds much denser than the LIC. The researchers investigated what effect these encounters had 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 not only shows that they move, but that they undergo drastic changes.” – Merav Opher, professor of astronomy, BU College of Arts & Sciences
‘Here we show that the ISM that has crossed the Sun over the past few million years contains cold, compact clouds that could have drastically affected the heliosphere. We investigate a scenario where the solar system passed through a cold gas cloud. few million years ago,” Opher and her colleagues write.
Most of what the Sun travels through is thin ISM. The sun moves continuously through the thin ISM without any effect.
‘These clouds are abundant around the Sun, but are too low in density to contract the heliosphere to great distances
However, the denser clouds in the ISM are compact enough to dramatically affect the protective heliosphere.
“The ISM near the Solar System also hosts some rare, dense, cold clouds called the local ribbon of cold clouds,” they write.
One of the clouds in that 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 temperature. 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 article say there is a small chance, about 1.3%, that the sun passed through the tail of the LLCC.
“We call that part the Local Lynx of Cold Clouds (LxCCs). The LxCCs represent almost half of the total mass of the LRCC and are more massive than the better studied LLCC,” 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 origins and physics are not well understood,” the authors write. Their work is based on the assumption that they have not changed substantially in the two million years since the alleged encounter.
“We have assumed here that these clouds have not undergone any substantial change over the past 2 million years, although future work may provide further insight into their evolution.”
The researchers used simulations to study the effect of the dense cloud on the heliosphere and, by extension, on our planet. They say the cloud’s hydrogen density pushed the sun back, making the heliosphere smaller than the Earth’s orbit around the sun.
It brought both the Sun and 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 mainly produced in supernovae and has a half-life of 2.6 million years.
Previous research linked the 60Fe to a supernova explosion, where it anchored itself in dust grains and was then delivered to Earth. It is also present on the moon. 244Pu was delivered simultaneously, also in supernovae ejecta.
Although there is much uncertainty, the researchers say that the deposition of 60Fe on Earth corresponds to our solar system’s hypothetical passage through a dense cloud that compresses the protective heliosphere, allowing the isotopes to reach Earth.
“Our proposed scenario is consistent with the geological evidence from 60Fe and 244Pu isotopes that Earth was in direct contact with the ISM during that period,” they write.
But if a supernova delivered the radioisotopes, it would have to have been quite close, and other evidence casts doubt on 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 delivering them specifically to Earth, as does the scenario with nearby supernova explosions.”
The question at the heart of this issue is: what impact has this had on the Earth?
An in-depth investigation into the consequences is beyond the scope of this study. The team commented on some possibilities, but also cautioned that very little research has been done on this issue.
“Very few works have quantitatively investigated the climatic effects of such encounters in the context of encounters with dense giant molecular clouds. Some argue that such high densities would deplete the ozone in the mid-atmosphere (50-100 km) and ultimately cool the atmosphere.” Earth,” they write.
It’s a leap, but some research suggests 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, Earth was directly exposed to the ISM,” they write.
In their conclusion they remind us that the probability that this meeting took place is low. But not zero.
“Stars move, and now this paper shows not only that they are moving, but that they are undergoing drastic changes,” said Opher, professor of astronomy in the BU College of Arts & Sciences and member of the university’s Center for Space Physics.
‘While the coincidence of the Sun’s past motion with these rare clouds is truly remarkable, the turbulent nature of the ISM and the small current angular size of these clouds mean that the past error ellipse is much larger than the clouds and, in the absence of other information, the probability of their encounter is considered low,” they write in their conclusion.
It is for future work to delve deeper into the matter.
Even if this particular encounter may not have happened, the research is still fascinating. There seems to be a bewildering number of variables that led to us, and it’s not hard to imagine that passing through dense clouds in the ISM may have played a role at some point.
“Only rarely does our cosmic environment outside the solar system affect 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 discover that our passage through dense clouds a few million years ago could have exposed Earth to a much greater flow 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 environment.
“We hope that our current work will stimulate future works detailing the climate impacts resulting from an encounter of the heliosphere with the LRCC and possible consequences for Earth evolution,” the authors conclude.
This article was originally published by Universe Today. Read the original article.