May 1, 2012 | David F. Coppedge

Planetary Radiometric Dates 1/3 Younger

The half-lives of radioactive isotopes may not be as well-known as thought.  One decay rate frequently used to date solar system objects had to be adjusted down to 66% of its former assumed value, impacting theories of planet formation.

PhysOrg headlined, “‘Faster-Ticking Clock’ Indicates Early Solar System May Have Evolved Faster Than We Think.”  The old decay rate for samarium-146 (146Sm) was re-evaluated by a team from Argonne National Laboratory, Hebrew University, two Japanese universities and the University of Notre Dame.  The old value of 103 million years for its half-life was recalculated at 68 million years, two-thirds of its previously measured value.

146Sm has become the main tool for establishing the of the solar system over its first few hundred million years. This by itself owes to a delicate geochemical property of the element 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 smaller value, “previously adopted as 103 million years, to a much shorter value of 68 million years,” the article continued.  It “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 article said.  The story was reported a month ago by Science Daily.

The article put a positive spin on this adjustment, saying, “The new time scale, interestingly, is now consistent with a recent and precise dating made on a lunar rock and is in better agreement with the dating obtained with other chronometers.”  It seems they could just as well have said that the other chronometers are now cast into doubt by the adjustment of 146Sm, which was also considered a precise chronometer till now.

In any case, it is disturbing that a physical value that is “out there in the world” could be found to be so far off by human measurement.  How many published papers are affected by this change?  Papers often quote radiometric dates to 4 or more significant figures.  Theorists rely on these values.  If values are not discovered but “adopted,” is it possible there was motivation by theorists to “adopt” a different value to create consistency with other chronometers?  Does the new value make the “assessed chronology of events in the early system” more or less plausible?  What will be the ripple effect from here on for a chronometer that ticks 33% faster than previously thought?  Who will go back and correct theories based on the previous value?  These are questions the press releases never ask.



Leave a Reply