Fingering the Culprit that
Polluted the Solar System
Washington, D.C.
— For decades it has been thought that
a shock wave from a supernova explosion triggered the formation of our Solar
System. According to this theory, the shock wave also injected material from the
exploding star into a cloud of dust and gas, and the newly polluted cloud
collapsed to form the Sun and its surrounding planets. New work from Carnegie’s
Alan Boss and Sandra Keiser provides the first fully three-dimensional (3-D)
models for how this process could have happened.
Traces of the supernova’s
pollution can be found in meteorites in the form of short-lived radioactive
isotopes, or SLRIs. SLRIs—versions of elements with the same number of protons,
but a different number of neutrons—found in primitive meteorites decay on time
scales of millions of years and turn into different, so-called daughter,
elements. A million years may sound like a long time, but it is actually
considered short when compared to other radioactive isotopes studied by
geochemists and cosmochemists, which have half-lives measured in billions of
years.
When scientists find the
daughter elements distributed in telltale patterns in primitive meteorites, this
means that the parent SLRIs had to be created just before the meteorites
themselves were formed. This presents a timing problem, as the SLRIs must be
formed in a supernova, injected into the presolar cloud, and trapped inside the
meteoritic precursors, all in less than a million years.
The telltale patterns prove
that the relevant daughter elements were not the ones that were injected. This
is because the abundances of these daughters in different mineral phases in the
meteorite are correlated with the abundances of a stable isotope of the parent
element. Different elements have different chemical behaviors during the
formation of these first solids, and the fact that the daughter elements
correlate with the parent elements means that those daughters had to be derived
from the decay of unstable parent elements after those solids were
crystallized.
One of these SLRIs, iron-60, is
only created in significant amounts by nuclear reactions in massive stars. The
iron-60 must have come from a supernova, or from a giant star called an AGB
star. Boss and Keiser’s previous modeling showed that it was likely that a
supernova triggered our Solar System’s formation, as AGB star shocks are too
thick to inject the iron-60 into the cloud. Supernova shocks are hundreds of
times thinner, leading to more efficient injection.
Now Boss and Keiser have
extended those models to 3-D, so they can see the shock wave striking the gas
cloud, compressing it and forming a parabolic shock front that envelopes the
cloud, creating finger-like indentations in the cloud’s surface. The fingers
inject the SLRI pollution from the supernova. Less than 0.1 million years later,
the cloud collapses and forms the core of the protostar that became the Sun and
its surrounding planets. The 3-D models show that only one or two fingers are
likely to have caused the SLRI pollution found in primitive
meteorites.
“The evidence leads us to
believe that a supernova was indeed the culprit,” said Boss. However, more
detective work needs to be done: Boss and Keiser still need to find the
combination of cloud and shock wave parameters that will line up perfectly with
observations of exploding supernovae.
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This research was supported by NASA. The software used in
this work was, in part, developed by the DOE-supported ASC/Alliances Center for
Astrophysical Thermonuclear Flashes a the University of Chicago
The Carnegie Institution for Science (carnegiescience.edu) is a private,
nonprofit organization headquartered in Washington, D.C., with six research
departments throughout the U.S. Since its founding in 1902, the Carnegie
Institution has been a pioneering force in basic scientific research. Carnegie
scientists are leaders in plant biology, developmental biology, astronomy,
materials science, global ecology, and Earth and planetary science.
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