Everyone knows about the extinction of the dinosaurs some 66 million years ago, the result of a mountain-sized space object colliding with Earth and wreaking havoc all over the planet. However, what most people don't know is that this extinction was not the most devastating in Earth's history.
The dinosaur extinction is one of the “Big Five†extinctions paleontologists know about based on geological evidence. The most destructive extinction of the Big Five occurred about 252 million years ago. This extinction, known as the Permian-Triassic mass extinction, or “The Great Dying,†wiped out 9 out of 10 of the marine species, and 7 out of 10 of the land species on Earth.
Most mass extinctions come about through multiple natural disasters occurring at the same time, say, a giant asteroid impact along with an erupting supervolcano. Are most extinctions therefore due to simple bad luck for planet Earth? Maybe, but recent studies suggest there is some element of predictability to mass extinctions: patterns in large impacts, volcanic activity, and declines in biodiversity are all extremely similar. The period of impacts and volcanic activity, meaning how much time has passed between each event, is extremely close to the period of mass extinctions.
What is causing the similarity in the patterns?
The Milky Way galaxy is essentially a giant spiral laid out over a relatively flat plane with a large central bulge. Just like how the moon orbits the Earth and the Earth orbits the sun, the entire solar system orbits the center of the galaxy.
This orbit is not a perfect circle, however. While the solar system travels forward along its path, it also bobs up and down like a wave, traveling in and out of the galactic plane. The time it takes between crossing the plane, known as the period, is about 32 million years. This number is very close to the amount of time (the period) between increased comet impacts on Earth. The dinosaur extinction occurred roughly 66 million years ago, and around this same time period, the solar system would have been traveling through the galactic plane.
When scientists noticed the correlation between the time that passed between extinctions and between comet impacts, they began to investigate further. There were initially two hypotheses as to why comets were getting thrown out of their home territory of the Oort Cloud, a giant sphere of comets, space rocks, ice, and dust that envelops the entire solar system, and flying toward the Earth.
The first hypothesis: There must be an undiscovered star or massive planet outside of the solar system that is disturbing the Oort Cloud comets. This makes sense, but even after a massive search, this suspected star/planet was not found. This hypothesis was ruled out.
The second hypothesis: Since scientists searched for some kind of object that was gravitationally affecting the comets and did not find such a thing, the next hypothesis was that there was something unseen doing the deed. The prime suspect: dark matter! Normal matter gives off light (whether it is visible or not), but dark matter does not at all.
Dark matter interacts with the rest of the universe mainly through the gravitational force. Everything with mass interacts with other objects that have mass through the force of gravity, and since dark matter has mass, it is included in these interactions. It makes up about 85% of the mass of the universe, and a lot of it probably exists in central planes of spiral galaxies like the Milky Way. In fact, observations of small satellite galaxies orbiting Andromeda, our neighboring spiral galaxy, support the existence of a dark matter disk. This suggests that our galaxy has one as well.
Dark matter interacts with the rest of the universe mainly through the gravitational force.
The gravitational influence on the Oort Cloud may be from a dense dark matter disk in the plane of the Milky Way Galaxy affecting the entire solar system. The gravitational effect of the normal matter in the disk does not explain the correlation with increased Oort Cloud activity, simply because normal matter is not dense enough since it is mostly gas and dust, which has little effect on the solar system, let alone the Oort Cloud. In order for the density to be great enough to throw comets out of the Oort Cloud and into the inner solar system as the solar system crosses the plane of the galaxy, dark matter must be present.
Thus, there must be a thin disk of dark matter within the plane of the Milky Way galaxy that perturbs the comets' paths and sends them flying into the inner solar system, increasing the probability of a collision with Earth every 32 million years or so.
Thus, a period of increased comet bombardment due to the suspected dark matter disk is likely what led to the demise of the dinosaurs. (For more on dark matter, check out a previous Athens Science Observer blog post.)
Concluding thoughts
If this hypothesis is true, then scientists must begin to consider the histories of both the Earth and the solar system in a galactic context. This is because the Milky Way's “invisible dark matter architecture†is probably the cause of many key events in a planet's life.
Within a decade or so, scientists will be able to determine if dark matter truly exists in a thin, clumpy disk within the plane of the galaxy. The European Space Agency's Gaia space mission, launched in 2013, will study the Milky Way in unprecedented ways. It will observe about a billion stars, measuring their distances and velocities, which will in turn allow astronomers to map the surface density of the galaxy's disk as a function of height.
This means that they will be able to check if there is a dark matter disk within the plane of the galaxy with more mass than can be accounted for with only “normal†matter models. Evidence of this disk would provide better models of dark matter and the structure of our galaxy, and this would ultimately help us to better understand important events that affect our planet.
About the Author
 Paige Copenhaver is an undergraduate studying Physics and Astronomy at the University of Georgia. When she is not studying solar-type stars, she can be found playing ukulele or reading Lord of the Rings. You can email her at pac25136@uga.edu or follow her on twitter: @p_copenhaver. More from Paige Copenhaver. |
