The vast solar system that contains Earth is not older than scientists had
thought, but it apparently developed faster than they surmised, according to an
international team of researchers from the Hebrew University of Jerusalem and
universities and laboratories in the US and Japan.
The astronomical team,
who published their findings in a recent issue of the prestigious journal
Science, said that the solar system indeed came into being some 4.5 billion
years after the Big Bang, but its formation took a shorter time than researchers
had thought.
Establishing chronologies of past events or determining ages
of objects need clocks that tick at different paces, according to how far back
one looks. Nuclear clocks, which are used for such dating, are based on the rate
of decay of an atomic nucleus expressed by a half-life – the time it takes for
half of a number of nuclei to decay. Each nuclear species has its own
half-life.
Radiocarbon dating, which was invented in Chicago more than
six decades ago and has been refined ever since, can determine when artifacts
even from prehistoric times originated because the half-life of radiocarbon
(carbon-14) is a few thousand years. The evaluation of ages of the history of
earth or of the solar system requires extremely “slow-paced” chronometers
consisting of nuclear clocks with much longer half-lives.
HU nuclear
physicist Prof. Michael Paul, as well as researchers from the University of
Notre Dame and the Argonne National Laboratory in the US and two Japanese
universities, conducted the research.
The main tool for establishing the
time evolution of the solar system over its first few hundred million years is
called 146Sm, which belongs to a family of nuclear species that were “alive” in
our sun and its solar system when they were “born.”
Events that occurred
later, within a few hundred million years, are dated by the amount of 146Sm that
was left in various mineral archives until its eventual
“extinction.”
This by itself is due to a delicate geochemical property of
samarium, a rare element in nature. It is a sensitive probe for the separation,
or differentiation, of the silicate portion of Earth and of other planetary
bodies.
The main result of the team’s work is a new determination of the
half-life of 146Sm, which was previously recognized as 103 million years, to a
much shorter value of 68 million years.
The shorter half-life value, like
a clock ticking faster, has the effect of shrinking the assessed chronology of
events in the early solar system and in planetary differentiation into a shorter
time span.
The new time scale is now consistent with recent and precise
dating made on a lunar rock and is in better agreement with the dating obtained
with other chronometers. The measurement of the tool’s half-life, performed over
several years by the collaborators, involved the use of the ATLAS particle
accelerator at the Argonne National Laboratory in Illinois.