Noctilucent clouds were once thought to be a relatively modern phenomenon. A team of researchers recently calculated that Earth and the entire solar system may have passed through two dense interstellar clouds that caused global noctilucent clouds that may have triggered an ice age. The event is thought to have occurred 7 million years ago and compressed the heliosphere, exposing Earth to the interstellar medium.
Interstellar clouds are huge regions of gas and dust that lie between the stars in galaxies. They consist mainly of hydrogen, but also a little helium and trace amounts of heavier elements. They are an important part of the life cycle of stars, as they provide the material for the formation of new stars and are replenished with elements after stars die. The clouds vary considerably in size, density and location and are an important part of the evolution of the galaxy.
An annotated illustration of the interstellar medium. The solar gravitational lens marks the point at which a conceptual spacecraft in interstellar space could use our Sun as a giant lens to provide magnified close-up views of planets orbiting other stars. Image credit: Charles Carter/Keck Institute for Space Studies
Earth's journey around the galaxy is not for the impatient, as it takes about 250 million years to complete a complete orbit at a speed of 828,000 kilometers per hour. Currently, the solar system is in the Orion Arm, one of the spiral arms of our galaxy. During the journey, Earth passes through different regions, encountering stars and different densities of the interstellar medium. It experiences gravitational interactions with nearby stars and nebulae, which sometimes have subtle interactions. Despite the tremendous journey, the stars of our galaxy remain relatively unchanged over the course of a human lifetime.
The Milky Way is a spiral galaxy with several prominent arms that contain stellar nurseries shrouded in pink clouds of hydrogen gas. The Sun is seen below in Orion's spur. Image credit: NASA
A team of astronomers led by Jess A. Miller of Boston University's Institute for Astronomy has traced the Sun's path through time. They have identified two occasions when the Earth and the solar system passed through two dense interstellar clouds. One of the passages occurred 2 million years ago, the other 7 million years ago. The team studied the properties of the clouds and found that the clouds are dense enough to compress the solar wind into Earth's orbit.
The solar wind is a constant stream of charged particles, mainly electrons and protons, emitted from the upper layer of the Sun's atmosphere, the corona. The particles move through the solar system at speeds between 400 and 800 kilometers per second. The edge of our solar system is defined as the point where the solar wind merges with the interstellar medium.
A composite image of the Sun's surface, corona, and digitally added coronal loops, rendered by Andrew McCarthy. (Credit: Andrew McCarthy)
Previous teams have analyzed climate changes due to these interactions of the interstellar medium and found similar results. The result was global cooling that triggered an ice age. The study by Miller and his team has revisited this very topic using modern technologies and techniques.
The team found that the interactions did indeed play a role in the changes to Earth's atmosphere. They found that hydrogen levels in the upper atmosphere would have increased significantly. The newly gained hydrogen would be converted to water molecules in the lower atmosphere and would also have led to a reduction in ozone levels in the mesosphere. These processes would have led to the formation of global noctilucent clouds in the mesosphere. They would not have been permanent, but they may have blocked 7% of sunlight from reaching Earth and plunged our planet into an ice age.
Source: Earth's mesosphere during possible encounters with massive interstellar clouds 2 and 7 million years ago
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