Approximately two million years ago a
star exploded in a supernova close to our solar system: Its traces can
still be found today in the form of an iron isotope found on the ocean
floor. Now scientists at the Technical University of Munich (TUM),
together with colleagues from the US, have found increased
concentrations of this supernova-iron in lunar samples as well. They
believe both discoveries to originate from the same stellar explosion.
A dying star ends its life in a cataclysmic
explosion, shooting the majority of the star's material, primarily new
chemical elements created during the explosion, out into space.
One or more such supernovae appear to have occurred close to
our solar system approximately two million years ago. Evidence of the
fact has been found on the earth in the form of increased concentrations
of the iron isotope 60Fe detected in Pacific ocean deep-sea crusts and
in ocean-floor sediment samples.
This evidence is highly compelling: The radioactive 60Fe
isotope is created almost exclusively in supernova explosions. And with a
half-life of 2.62 million years, relatively short compared to the age
of our solar system, any radioactive 60Fe originating from the time of
the solar system's birth should have long ago decayed into stable elements and thus should no longer be found on the earth.
Lunar samples from the Apollo missions
This supernova hypothesis was first put forth in 1999 by
researchers at the Technical University of Munich (TUM) who had found
initial evidence in a deep-sea crust. Now their claim has received
further substantiation: Physicists at the TUM and their colleagues from
the US have succeeded in demonstrating an unusually high concentration
of 60Fe in lunar ground samples as well.
The samples were gathered between 1969 and 1972 during
Apollo lunar missions 12, 15 and 16, which brought the lunar material
back to earth.
It's also conceivable that 60Fe can occur on the moon as the
result of bombardment with cosmic particles, since these particles do
not break up when colliding with air molecules, as is the case with the
earth's atmosphere. Instead they directly impact the lunar surface and
can thus result in transmutation of elements. "But this can only account
for a very small portion of the 60Fe found," explains Dr. Gunther
Korschinek, physicist at TUM and scientist of the Cluster of Excellence
Structure and Origin of the Universe.
Deposits of newly produced stellar matter
"We therefore assume that the 60Fe found in both terrestrial and
lunar samples has the same source: These deposits are newly created
stellar matter, produced in one or more supernovae", says Dr.
Korschinek.
Since the moon generally provides a better cosmic record
than the earth, the scientists were also able to specify for the first
time an upper limit for the flow of 60Fe that must have reached the
moon. Among other things this also makes it possible for the researchers
to infer the distance to the supernova event: "The measured 60Fe-flow
corresponds to a supernova at a distance of about 300 light years," says
Korschinek. "This value is in good agreement with a recently
theoretical estimation published in Nature."
The lunar samples were investigated using the high-sensitivity accelerator mass spectrometer of the Maier-Leibnitz Laboratory near Munich.
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