Scientists have calculated a new value for the half-life of samarium, an isotope used to track how our solar system—including the sun, pictured above—came into being. Image: NASA/GSFC/SDO
The early days of our solar
system might look quite different than previously thought, according to
research at the United States Department of Energy’s (DOE) Argonne National
Laboratory published in Science. The study used more
sensitive instruments to find a different half-life for samarium, one of the
isotopes used to chart the evolution of the solar system.
“It shrinks the chronology of
early events in the solar system, like the formation of planets, into a shorter
time span,” said Argonne physicist Michael Paul. “It also means some of the oldest rocks on Earth would have formed even
earlier—as early as 120 million years after the solar system formed, in one
case of Greenland rocks.”
According to current theory,
everything in our solar system formed from star dust several billion years ago.
Some of this dust was formed in giant supernovae explosions which supplied most
of our heavy elements. One of these is the isotope samarium-146.
Samarium-146, or Sm-146, is
unstable and occasionally emits a particle, which changes the atom into a
different element. Using the same technique as radiocarbon dating, scientists
can calculate how long it’s been since the Sm-146 was created. Because Sm-146
decays extremely slowly—on the order of millions of years—many models
use it to help determine the age of the solar system.
The number of years it takes for
an isotope to decrease by half is called its half-life. Since Sm-146 emits
particles so rarely, it takes a sophisticated instrument to measure this
The Argonne Tandem Linac
Accelerator System, or ATLAS, is a DOE national user facility for the study of
nuclear structure and astrophysics, and is just such an instrument. “It’s easy
for the ATLAS, used as a mass spectrometer, to pick out one Sm-146 atom in tens
of billions of atoms,” said physicist Richard Pardo, who manages the facility
and participated in the study.
By counting Sm-146 atoms with
ATLAS and tracking the particles that the sample emits, the team came up with a
new calculation for its half-life: just 68 million years.
This is significantly shorter
than the previously used value of 103 million years.
The new value patches some holes
in current understanding, according to Paul. “The new time scale now matches up
with a recent, precise dating taken from a lunar rock, and is in better
agreement with dates obtained with other chronometers,” Paul said.