Noctilucent clouds were once thought to be a relatively modern phenomenon. A team of researchers has recently calculated that Earth and the entire Solar System may have passed through two dense interstellar clouds that caused global noctilucent clouds that could have triggered an ice age. The phenomenon is thought to have occurred 7 million years ago and would have compressed the heliosphere, exposing Earth to the interstellar medium.
Interstellar clouds are vast regions of gas and dust that extend between stars within galaxies. They are made up mostly of hydrogen, along with some helium and traces of heavier elements. They are a key part of the life cycle of stars, providing the materials for new stars to form and being seeded with elements after stars die. Clouds vary significantly in size, density, and location and are an important part of galaxy evolution.
Earth's journey around the Galaxy is not for the faint of heart, as it takes about 250 million years to complete a full orbit at a speed of 828,000 kilometers per hour. Currently, the Solar System is located in the Orion Arm, one of the spiral arms of our Galaxy. During the journey, Earth travels through different regions, encountering stars and different densities of the interstellar medium. It experiences gravitational interactions with nearby stars and nebulae that sometimes exert subtle interactions. Despite the immense journey, the stars of our Galaxy remain relatively unchanged over the course of a human lifetime.
A team of astronomers led by Jess A. Miller of Boston University’s Department of Astronomy has traced the Sun’s path through time. In doing so, they have identified two occasions when Earth and the Solar System passed through two dense interstellar clouds. One of the crossings occurred 2 million years ago, the other 7 million years ago. By exploring the properties of the clouds, the team claims 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, mostly electrons and protons, emitted from the upper layer of the Sun's atmosphere, the corona. The particles travel through the Solar System at speeds of 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.
Previous teams have looked at climate change events due to these interactions of the interstellar medium with similar findings. The result was a global cooling that triggered an ice age. Miller and his team's study has re-addressed this same issue using modern technology and processes.
The team has found that interactions did indeed play a role in changes to Earth’s atmosphere. They found that hydrogen levels in the upper atmosphere would have increased substantially. The newly acquired hydrogen would have been converted into water molecules in the lower atmosphere and would have also led to a reduction in mesospheric ozone levels. These processes would have led to the appearance of global noctilucent clouds in the mesosphere. They would not have been permanent, but they could have blocked 7% of sunlight reaching Earth, plunging our planet into an ice age.
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