NASA: Earth's ice ages may have been triggered by the Sun's passage through interstellar clouds

NASA researchers have proposed a cosmic explanation for two key mysteries in the planet's climate history: the causes of ice ages and the existence of liquid water on early Earth. The first study, carried out at NASA's SHIELD research center and published in the journal Annual Review of Astronomy and Astrophysics, links glaciations to the Solar System's passage through dense interstellar clouds. The second, prepared by scientists at NASA's Goddard Space Flight Center, explains how oceans persisted on Earth when the young Sun shone noticeably more weakly than it does today. Both studies rely on computer modeling and laboratory experiments rather than direct observations.

The heliosphere as a living shield

The entire Solar System is enclosed in the heliosphere — a giant protective region formed by the solar wind of charged particles. It shields the planets from a significant portion of cosmic radiation and interstellar matter. Specialists at the SHIELD center reproduced the trajectory of the Solar System's motion over the last several million years and showed that over its 4.6 billion years of existence the system has repeatedly crossed various regions of the Galaxy, meaning the conditions under which Earth found itself were far from constant.

Three passages through cold clouds

According to computer modeling, over the last several million years the Sun has passed at least three times through cold, dense accumulations of interstellar gas and dust. The pressure of these clouds was so strong that the protective shield — the heliosphere — was compressed to a size smaller than Earth's orbit. At such moments, scientists estimate, roughly 2–3, 6–7, and 13–14 million years ago, Earth was directly exposed to the space environment, stripped of its previous level of protection.

Geological traces and the cooling mechanism

Geological data support this picture: in samples of seabed sediments, Antarctic snow, and lunar soil, scientists have detected elements characteristic precisely of interstellar dust from the corresponding periods. The influx of hydrogen from the cold clouds into the atmosphere, according to the researchers' model, increased the amount of water vapor and changed the dynamics of the upper atmospheric layers. This, in turn, led to a sharp cooling of the surface and could have triggered ice ages. The authors emphasize that this is a probabilistic model, not the only possible scenario.

The mystery of liquid water 3 billion years ago

A separate group at NASA's Goddard Space Flight Center examined another mystery: how liquid water existed on Earth about 3 billion years ago if the Sun then shone roughly 30 percent more weakly than it does now. A clue came from observations by the Kepler telescope of young stars similar to the early Sun. It turned out that the young Sun daily erupted powerful streams of high-energy particles — superflares — that bombarded the ancient atmosphere.

Nitrous oxide as an ancient greenhouse gas

In laboratory modeling of the ancient atmosphere — a mixture of nitrogen, ammonia, carbon dioxide, and carbon monoxide — and its irradiation with protons, nitrous oxide was produced. This is a greenhouse gas 300 times more powerful than carbon dioxide. The scientists note that even if ultraviolet radiation destroyed most of the compound, the remaining 10 percent of nitrous oxide was enough to warm equatorial regions to plus 5 degrees Celsius. By their calculations, this was sufficient for the existence of a liquid ocean and the rapid synthesis of amino acids that became the basis for life.

What this means for understanding climate

Together, the two studies expand the picture of Earth's climate history: external cosmic factors — from the compression of the heliosphere to the activity of the young Sun — may have played a notable role alongside internal geological and atmospheric processes. At the same time, the authors of both works emphasize the model-based nature of their conclusions: the mechanisms have been reconstructed on the basis of modeling and laboratory experiments, not direct measurements, so the details and time frames remain subject to further verification.