A fascinating link between the number of nearby exploding stars, known as supernovae, and
life on Earth has been discovered. Evidence suggests a connection between changes
in supernova occurrence and the fraction of organic stuff buried in sediments.
This correlation can be seen over the last 3.5 billion years, and it can be seen in more
detail over the previous 500 million years. The correlation suggests that supernovae have
created the necessary conditions for life on Earth to exist.
An explanation for the observed link between supernovae and life is that supernovae influence
Earth's climate. How does it do that? A high number of supernovae result in a cold
climate on Earth, with a significant temperature difference between the equator and the poles.
That, in turn, leads to strong winds and ocean mixing, which are vital for delivering nutrients
to ecosystems. High concentrations of nutrients result in
more bio-productivity and more organic matter buried in sediments.
Warmer climates have weaker winds and less mixing of the ocean, a reduced supply of nutrients,
poorer bio-productivity, and less burial of organic matter.
An interesting consequence is that the process of moving organic matter to sediments indirectly
produces oxygen, which is the cornerstone of all complex life on Earth.
Photosynthesis produces oxygen and sugar from light, water, and carbon dioxide. But, if
organic matter is not moved into sediments, oxygen and organic matter become carbon dioxide
and water. This reverse reaction is prevented by burying organic material.
As a result, supernovae indirectly influence oxygen generation, and oxygen is the basis
of all complex life. Over the last 500 million years, a measure
of the concentration of nutrients in the ocean reasonably correlates with supernovae frequency
fluctuations. Measurements of trace elements in pyrite,
which is iron sulfide, popularly known as "fool's gold", buried in black shale, which
is sedimented on the seabed, are used to determine the concentration of nutrients in the oceans.
Measuring carbon-13 relative to carbon-12 can be used to estimate the fraction of organic
material in sediments. Because life prefers the lighter carbon-12
atom, the ratio between carbon-12 and carbon-13 measured in marine sediments fluctuates due
to the amount of biomass in the world's oceans. The new evidence points to a unique link between
life on Earth and supernovae, which is mediated by the effect of cosmic rays on clouds and
climate. According to previous studies conducted by
Svensmark and colleagues, ions aid in the production and growth of aerosols, thus influencing
cloud fraction. The cosmic ray/cloud relationship is vital
for climate because clouds can regulate the amount of solar energy reaching the Earth's
surface. Empirical evidence demonstrates that the climate
on Earth changes when the strength of cosmic rays changes. Supernovae frequency can fluctuate
by hundreds of percent on geological time scales, leading to significant climate changes.
During the explosion of heavy stars, they emit cosmic rays that are made of elementary
particles with immense energies. Cosmic rays travel to our solar system, and
some of them end their journey by colliding with Earth's atmosphere. Here, they are in
charge of ionizing the atmosphere. Also, a previous study suggests a faraway
supernova explosion could have contributed to a minor mass extinction here on Earth 2.59
million years ago. Cosmic rays that were blasted out by the supernova may
have played a role in the climatic changes that apparently led to a die-off at the end
of the Pliocene epoch and the start of the Pleistocene. Africa dried out, and a lot of
the forest turned into savanna. Around this time and afterward, the Earth
started experiencing glaciations over and over again, and it's not clear why that started
to happen. It's controversial, but maybe cosmic rays from the supernova had something to do
with it. The Pliocene-Pleistocene extinction was a
pretty minor one as far as these events go. Previous studies have suggested that a supernova
would have to be quite close to Earth, within 25 light-years or so, to trigger a major mass
extinction such as the one 65 million years ago that marked the end of the age of dinosaurs.