How Interstellar Clouds May Have Shaped Earth’s Climate: A Journey Through the Milky Way

Two million years ago, the Solar System – our Sun and planets – might have entered into a surprisingly intense encounter with the clouds between the stars. The effects of this cosmic brush with interstellar cloud material could have been dramatic, changing Earth’s climate in ways that have been lost, hidden in the geological and climatic record. Until relatively recently, interstellar encounters were at the fringes of astronomy and Earth science, widely considered to be theoretical and – if possible at all – incredibly difficult to detect. But, as a group of planetary scientists and I have come to realise, the path through interstellar space might have left its mark here on Earth’s rocks. In this piece, I explain the likelihood that our planet’s journey through the clouds between the stars has altered the climate on our world, and what we might now be able to discern about the secrets of interstellar space.

Understanding Interstellar Clouds

But how do we define the term, and what exactly constitutes space? To answer these questions, we must take a tour of the vast vacuum we call space. Our tour will begin here — with interstellar clouds. These clouds are massive clusters of gaseous particles. Most of the gas is composed of hydrogen, ranging from normal Singular ‘hydrogen’ with two protons and two electrons in the nucleus to hydrogen that has been ionised — or stripped of only one electron. Another significant component of these gaseous clouds is helium, a single-electron atom that follows a chemical pattern similar to hydrogen. Though interstellar clouds may appear to be empty from a human perspective, these gaseous clouds actually pack in the major elements essential for life — hydrogen, helium, oxygen, carbon, neon, nitrogen, and iron, to name a few.

Interstellar clouds are large accumulations of gas and dust between and, at times, within the stars of a galaxy. Some lie tens or hundreds of light years across. Most of the material in the giant molecular clouds is simple hydrogen – hydrogen molecules and then the next common element, helium – with a small fraction by mass of heavier elements. Although the clouds are often flattened, elongated and spade-like, still they are about 100,000 times thicker than interstellar space. When the elements of interstellar clouds begin to accumulate into a mass of gas that can collapse towards a star, this would be the very beginning of its journey to form new stars and often its planets as well. Most of the star-creation regions of the Milky Way exist in these dense cores within interstellar clouds. The clouds range in density and size, from relatively thin sheets to densely packed lumps and spheres.

When our Solar System travels through a more dense region of interstellar cloud these interactions with the filaments of cosmic dust and gas can change our planetary environment. The influx of interstellar particles could shift the shape or size of the heliosphere, the region of space that’s dominated by the solar wind and the Sun’s magnetic field. Such a change in the heliosphere would alter the level of cosmic radiation that penetrates past the Sun to reach the Earth.

Clouds in the space between the stars are important to study because they help researchers to understand the process through which stars and planets form, as well as the abundances of different chemical elements that are present in the Universe, and how they came to be. Interstellar clouds are our very own laboratory in the sky; when we analyse the chemical composition of these clouds, our findings reveal the mechanisms through which the vast oceans of hydrogen atoms that exist throughout the Universe evolve into galaxies and, within galaxies, their swirling discs of gas eventually collapse to form stars.

The Solar System’s Journey Through the Milky Way

First, the Solar System does not sit still — we travel with the rest of the stars, comets and dust of the Milky Way galaxy on a path through the galactic plane. This Galactic travel exposes the Solar System to different regions of interstellar material than before, some more dense with gas and dust than others. A journey through these environments takes around a few million years, giving enough time to shift the heliosphere into different galactic environments. This is where the ‘lost in the clouds’ motif comes back in bold.

About 2 million years ago, the Solar System might have passed through a particularly dense interstellar cloud. Researchers believe that such passages are common and therefore part of the Milky Way age’s natural seasonal changes. Moving through the galaxy, the Solar System falls into and out of these dense clouds of gas and dust, getting more and less cosmic dust and radiation along the way. It’s conceivable that such collisions might affect the orbital play of comets, and the mixing and mingling of interstellar matter.

Throughout these passages, the cosmic bubble created by the solar wind can be squeezed or distorted, allowing more of the cosmic rays to stream through the Solar System’s comfortable bubble and into the inner regions, even into our home planet’s atmosphere, which can influence Earth’s climate, too. And slipping through dense interstellar clouds may bring long periods of jet-boosted cosmic rays, which can affect many spheres, from cloud formation to weather patterns on Earth. This can help scientists understand how such interactions might foreshadow future, deeper disturbances as the Solar System moves through the vicissitudes of the Milky Way.

Geological and Climatic Evidence on Earth

This part will include geological and climatic evidences that support the hypothesis due to the interplay of interstellar cloud occurrences and climate changes in Earth.

Geological records can be crucial evidence of past changes in the climate of our Earth. By examining sediment cores and ice cores from glaciers or looking at fossilised rock formations, researchers can piece together information about the climate of times long past. About two million years ago, Earth underwent a period of climatic extremes as it completed its journey through a dense stellar cloud known as the Pleistocene epoch. One intriguing idea pitched by Krüger and others is that this climatic instability tracked the Sun’s ingress through a dense interstellar cloud. If you look deeply enough into geologic layers, you can discover unusual spikes in the deposition of extraterrestrial dust.

Perhaps the most compelling evidence for this hypothesis is the presence of isotopic signatures (numerical variations in the atoms of certain elements) such as beryllium-10 in the sedimentary record. (Beryllium-10, produced by cosmic ray collisions in the Earth’s atmosphere, increases in concentration when cosmic rays are more plentiful.) During the previous ice age, the Earth’s climate underwent similar shifts on similar timescales. A coincidence? We don’t think so.

Besides, the climatic models predict that heightened cosmic ray activity can affect cloud cover and modify Earth’s weather conditions. The scenarios suggest that as the Solar System passed through dense interstellar clouds, rising cosmic ray fluxes could have amplified cloud formation rates. In turn, this could have increased the Earth’s albedo (reflected light), cooling the planet and triggering glacial periods. Some of this can be tied to geological and palaeoclimatic data: an indicative but not definitive case.

The Role of Volcanic Activity

Great volcanic outpourings which injected massive amounts of ash and sulfur dioxide into the atmosphere can cause periods of global cooling. Such periods are referred to as volcanic winters. Volcanic winters are short-lived, but counterintuitively, very large eruptions might keep Earth cool for a longer period of time – while human abatement efforts would cause significant global heating in a shorter timeframe. Large volcanic events did occur during the transition period of the Solar System’s flight through a denser interstellar cloud, and increased volcanic activity could have elevated the effect of increased cosmic ray activity on the weather in our planet.

More drastic climatic changes might have been triggered by the interaction of volcanic activity and encounters with interstellar clouds. A mixture of ash and cloud cover produced by cosmic rays could have decreased the average temperatures by several degrees, which might have triggered the onset of glacials. During this ‘volcanic age’, Earth might have crossed a divide between a climate dominated by terrestrial and extraterrestrial forces that produced a rather distinctive climatic regime.

Looking at volcanic deposits and their chronology can help researchers pull together different strands of evidence, to see how oceans, continents, inner thermal Earth and cosmos acted together at different times. Because we know the exact calendar year in which each volcanic layer in a sedimentary record was formed, we can now correlate these eruptions to earlier times with heightened activity in space, when the Sun’s neighbours span close. Using the staging power of geology and climate to contextualise cosmology is not as facile as it might sound.

Implications for Earth’s Climate and Life

Namely, that over the next hundreds of millions (and possibly billions) of years, interstellar cloud encounters could represent a powerful, if sporadic, driver of climatic change and evolutionary innovation.

Paragraph 3: The astronomical effects of passing through a dense interstellar cloud also have substantial climatological implications. When interstellar clouds are stable, increased cosmic ray exposure to Earth’s atmosphere can also affect biological systems downstream, possibly altering the course of evolution. Cosmic rays – high-energy subatomic particles shoot across the galaxy, galaxy cluster and eventually the intergalactic space of the local Universe. With atmospheric shielding at a minimum near peaks in solar and stellar activity, increased radiation flux could lead to elevated mutation rates over evolutionary timescales. This interstellar end to climatic stability could have provided both challenges and opportunities for our direct ancestors, Homo erectushomines, and other species to develop and adapt along new evolutionary trajectories.

Whereas climate and environment would have rhymed with species’ distibution patterns, and climate and biological changes would have driven each other. Some species would have survived a cooler climate and a changed, wetter, windier weather. Changes in climate and environment would have favoured the evolution of some type of species that could tolerate cooler temperatures and changed weather, and spread in the environment, increasing biodiversity. The research of this episode of climatic variability is illustrating how cosmic events and biological evolution on Earth were interconnected.

Further, interstellar cloud conditions give scientists a sense of what might affect Earth’s thermodynamic conditions in the future. As the Solar System continues to move through the Galaxy, it will pass through more interstellar clouds. Having a sense of what has occurred before allows scientists to predict, and hopefully prepare for, what might happen next and how our climate and environment could be affected in the coming millennia. Understanding such processes can help scientists develop strategies to sustain life on this planet.

Modern Research and Future Directions

As for future directions, and what we can expect from scientists, this final section of the original will provide an overview of the studies that have been proposed and that can now start to be pursued.

What’s increasingly driving today’s research in astrophysics and geology is insight into the role of interstellar clouds in the Solar System and on Earth. From James Webb to other state-of-the-art telescopes and instruments, such as the ones operated by NASA and the European Space Agency, the last few years have witnessed a veritable boom in investigations on interstellar space, including its inhabitants. With these observations, researchers are able to derive the density of interstellar clouds on routes that the Sun might take in the future We can now map the spatial distribution of the gas and dust in various clouds that are found along the star’s paths, not least because it remembers its past.

Researchers are also developing increasingly complex models of the interactions between heliospheric restructuring and interstellar gas clouds, taking into account the density of the cloud, the speed of the wind and mass loss of the Solar System, the strength of cosmic rays, and, by coupling these models with terrestrial data on geology and climatology, they can construct complex scenarios of how all of this might affect the climate of Earth.

A next step may involve probing the geological archive for isotopic signatures that might reflect similar galactic events. Another new area of research will examine how to improve methods of detecting cosmic dust. Astronomers, geologists and other climate scientists will need to work together. At the same time, as we delve deeper into the processes of interstellar space, we learn not just about the cosmos, but about the highly interconnected tapestry that determines life on Earth, both in the past and the future.

Conclusion

But as we know, our passage through two of these dense interstellar clouds 2 million years ago points to interesting avenues for future investigation into Earth’s climatic history. Clouds and the constant input of metals from these events, as well as terrestrial volcanic activity, may have influenced Earth’s climate and directions in the evolution of life. It’s likely that, at best, our understanding of and the relevance of interstellar input into the terrestrial environment will emerge in our present understanding of the workings of our planet and its ability to support life as we know it. Wherever we sail in the Milky Way, the cosmos will always move in and send us interstellar waves, which will continue to shape and drift through the oceans of our planetary home.