New study suggests that an encounter of the solar system with a giant molecular dark cloud could have driven an environmental catastrophe leading to mass extinction in the last 8 million years of the Cretaceous period.

Our galaxy the Milky Way has a disk with a radius of 32000 light-years and a thickness of 650 light years. Our solar system is located inside at ~ 28000 light years from its center. Many dark clouds are distributed throughout the disk. But first, what is a dark cloud? A dark cloud is a nebula that is so dense it obscures the light from objects behind it. It consists of high-density particles (protons) and low-temperature (8–40 K) neutral gas. Its size could range between 0.65–560 light years.

Scientists estimated that more than 150 clouds encounter likely occurs in the age of the sun. A number of previous studies have suggested that a nebula encounter may lead to an environmental catastrophe and thus to a mass extinction. But how could a nebula trigger some of Earth’s most dramatic environmental? A nebula encounter could lead to a global cooling by the enhanced flux of cosmic dust particles. They remain in the stratosphere for several months or even years and work as sunscreens to reflect back the sunlight. Therefore, it causes a global winter and possibly thousands-year-long cooling episode with devastating consequences.

But how can the scientists be sure about these encounters? The answer is iridium. Iridium is important because it is used by scientists as a “measurement tool”. Iridium is rare in Earth’s crust but it is abundant in many space objects as meteorites, asteroids, dark nebula etc. So finding such iridium-rich layer (subsurface layer which are enriched in iridium) in many sites around the world means one thing, a truly a worldwide event. This is how scientists proceed.

Through the Ocean Drilling Program (ODP), a core sample of sediment was taken from a site located in the central North Pacific. This site has been in that area at least since the end of the Cretaceous — a period usually noted for being the last portion of the “Age of Dinosaurs”.

Using analysis through a Co–Ir diagram, the result shows an abundance of iridium in the core sample of that site. This abundance cannot be explained by any mixture of the materials on the surface of the Earth, and thus requires contribution from extraterrestrial materials. Scientists data shows that this enhanced flux of the extraterrestrial material can be explained by a dark cloud encounter.

The encounter leads to a “Nebula Winter” in which an environmental catastrophe of the Earth is driven by an enhanced flux of cosmic dust particles and cosmic rays, which cause global cooling and destruction of the ozone layer. The long period of climate cooling forced by a dark-cloud encounter may account for a mass extinction at the end of the Cretaceous (Cretaceous–Paleogene boundary). It is also consistent with a trend of the reduction in generic number of dinosaur as well as ammonite 66 million years ago.

Chicxulub impact 66 million years ago The mass extinction that happened 66 million years ago is widely thought to have been caused by an impact of an asteroid. However, a complete extinction by just one asteroid impact seems rather difficult because of the following two reasons.

First, a severe perturbation in the global environment of Earth would finish nearly 5 years after the impact, the solid particles launched by the asteroid impact would settle down in only a few months to a few years and a negative radiative forcing would become negligible a few years subsequent to the impact.

For example, the effect of the sunshield by launched solid particles into the atmosphere of the Earth from the eruptions of the Pinatubo (June 1991) volcanoes completely vanished after 3–5 years. Even if 100 times larger than the Pinatubo eruption, as is the estimated case of the Chicxulub impact, the sunshield effect would not persist beyond about 5 years, because the retention time is determined by convection time of stratosphere.

Individuals of those few percentages of surviving species would recover completely after the environmental catastrophe was over.

Second, there have been impact events of similar scale to that of Chicxulub without catastrophe of the global environment, such as the Woodleigh impact cratering events (the formation of the 120-kmdiameter Woodleigh crater in western Australia which is about ~ 80% the size of the Chicxulub crater).

Some scientists go even further, say that an encounter with the dark cloud perturbs the orbit of asteroids and comets through its gravitational pull, causing showers of asteroids and/or comets, and likely associated impact events. The asteroid impact of Chicxulub therefore, may be one of the consequences of the dark cloud encounter.

We conclude that the cause of the cold climate at the EndCretaceous was driven by an encounter with a giant molecular cloud, with such an encounter and related perturbation in global climate a more plausible explanation for the mass extinction than a single impact event, Chicxulub.

said Tokuhiro Nimura Japan Spaceguard Association, lead author of the paper — you can find the preprint on arxiv.org.