What would happen to the climate if Earth passed through an interstellar cloud?

Noctilucent clouds were once thought to be a relatively recent phenomenon, but a team of researchers recently calculated that Earth and the entire solar system may have passed through two dense interstellar clouds, which could have triggered the Earth-wide noctilucent clouds and sparked the ice age.

This event, thought to have occurred 7 million years ago, would have compressed the heliosphere and exposed Earth to interstellar material.

Interstellar clouds are vast regions of gas and dust between the stars in our galaxy. They consist mainly of hydrogen, with smaller amounts of helium, and trace amounts of heavier elements.

Clouds are an important part of a star’s life cycle, providing materials for new stars to form and supplying elements after stars die. Clouds vary greatly in size, density and location and are an important part of the evolution of galaxies.

Earth’s galactic orbit is not for the impatient, as it takes about 250 million years to complete one revolution at a speed of 828,000 kilometers per hour.Currently, our solar system is located in the Orion Arm, one of the spiral arms of the Milky Way galaxy.

During its journey, Earth passes through different regions, encountering stars and interstellar matter of different densities. It experiences gravitational interactions with nearby stars and nebulae, sometimes resulting in subtle interactions. Despite its epic journey, the stars of our galaxy remain relatively unchanged during a human lifetime.

A team of astronomers led by Jess A. Miller of the Boston University Department of Astronomy traced the Sun’s orbit back in time. In the process, they identified two occasions when Earth and the solar system passed through two dense interstellar clouds. The study is published in Geophysical Research Letters.

One crossing occurred 2 million years ago, the other 7 million years ago. The team investigated the properties of the cloud and claim that it is dense enough to compress solar wind inside Earth’s orbit.

The solar wind is a constant stream of charged particles, mainly electrons and protons, that emerges from the corona, the upper layer of the Sun’s atmosphere. The particles travel through the Solar System at speeds between 400 and 800 kilometers per second. The edge of the Solar System is defined as the point where the solar wind merges with interstellar material.

Previous research teams have analyzed these interstellar interactions to produce climatic changes that have yielded similar results, resulting in global cooling and the onset of ice ages. Miller and his team’s study revisits this exact problem using modern techniques and processes.

The team found that this interaction did indeed affect changes in Earth’s atmosphere: They concluded that hydrogen levels in the upper atmosphere increased significantly, and that this newly acquired hydrogen was converted into water molecules in the lower atmosphere, which may have also led to a decrease in ozone levels in the mesosphere.

These processes would have led to the appearance of global noctilucent clouds in the mesosphere. Although these clouds may not have been permanent, they could have blocked 7% of the sunlight reaching Earth, plunging the planet into an ice age.