Scientists recognize five major mass extinctions in Earth’s history, episodes when a large fraction of life all over our planet went extinct in a very short time on geological time scales. Scientists blame various causes for these extinctions. Climate change was often the real killer, but the cause of climate change varied.
The
most recent mass extinction, the K-T extinction 66 million years ago, is
perhaps the most famous and best understood of the major extinctions. It led to
the end of the dinosaurs and most marine reptiles. At that time, an asteroid 6
miles in diameter slammed into Earth just off the coast of what is now the
Yucatan Peninsula in Mexico. It threw hot ash and molten rock into the air that
covered the planet, resulting in worldwide fires, killing off many species. The
resulting cloud of debris and smoke then reduced sunlight from reaching the
planet’s surface for a period of years, causing the death of many plant species
which dramatically diminished the entire food chain.
A
new study suggests an astronomical cause for two other extinction episodes. The
Ordovician extinction event occurred 443.8 million years ago. This was a time
known for rapid diversification in marine life and the appearance of the first
plants on land. Scientists estimate that 71% of species disappeared during this
event.
The
Late Devonian mass extinction occurred 372 million years ago leading to the
extinction of nearly 70% of species. It is the least understood of the five
major events, and scientists have offered several possible explanations for it.
The new study provides plausible astronomical explanations for both of these
events.
In
both the Ordovician and late Devonian extinction events, there is evidence that
Earth’s ozone layer was severely depleted. A new study led by Dr. Alexis
Quintana at the University of Alicante in Spain, including other researchers
from Keele University in England, puts the blame for both events on nearby
supernovas.
When
a massive star runs out of fuel, it explodes with so much energy that a single
supernova can temporarily release more energy than its entire host galaxy. The
debris includes not only high-energy radiation, like X-rays and gamma rays, it also includes cosmic rays, charged particles moving at nearly the speed of light. A
nearby supernova can blast enough energy to destroy our ozone layer. With that
protective shield gone, the high-energy cosmic rays and the deadly radiation
can bathe Earth’s surface at lethal levels.
Credit NASA, ESA, and the Hubble Space Telescope Institute.
Life
on our planet owes its existence to supernovas, as all elements heavier than
hydrogen and helium – including oxygen, carbon, calcium, iron, and all the
chemical elements that make us up – are created in those stellar explosions. As
the authors say in the study, it is "a great illustration for how massive
stars can act as both creators and destructors of life".
Dr.
Quintana states, "Supernova explosions bring heavy chemical elements into
the interstellar medium, which are then used to form new stars and planets. But
if a planet, including the Earth, is located too close to this kind of event,
this can have devastating effects."
It
seems the universe can give us life, but can also take it away.
Each month, I write an astronomy-related column piece
for the Oklahoman newspaper. After
it is published there, I post that same column to my blog page.
This is reprinted with
permission from the Oklahoman and www.Oklahoman.com.
