A trusted equation for calculating the age of the solar system may need rewriting. New measurements show that one of the equation’s assumptions — that certain kinds of uranium always appear in the same relative quantities in meteorites — is wrong.
“Since the 1950s, or even before that, no one had been able to detect any differences” in the quantities of uranium, says Gregory Brennecka of Arizona State University, coauthor of a paper describing the work published online December 31 in Science. “Now we’re able to measure slight differences.”
Those differences could mean that current estimates of the age of the solar system overshoot that age by 1 million years or more. Historical estimates place the age at about 4.5 billion years—a number that is not precise enough to show a difference of one million—but more finely honed recent calculations place the age at more like 4.5672 billion years. One million years is still an eyeblink at this scale, representing the difference between 4.566 and 4.567, but this difference is important in understanding the infant solar system.
“The building blocks of planets all formed within the span of 10 million years at most,” says coauthor Meenakshi Wadhwa, also of Arizona State. “When you start to try to unravel the sequence of events within that 10 million years, it becomes important to resolve the time scales within a million years or less.”
The study also finds evidence bolstering the idea that a low-mass supernova exploded nearby shortly before the solar system was born, providing heavy elements to build planets.
Geochemists measure the ages of rocks by measuring the abundance of radioactive isotopes — versions of the same element that have different atomic masses — in parts of meteorites called calcium-aluminum–rich inclusions. These inclusions are thought to be the first solids to have condensed from the cooling cloud of gas that gave birth to the sun and planets.
Because a radioactive element decays from a parent isotope to a daughter isotope at a specific rate, scientists can infer the age of a rock by comparing the amounts of each isotope.
The currently accepted calculation of the solar system’s age is derived from comparing lead-206, a daughter isotope of uranium-238, to lead-207, a daughter isotope of uranium-235.
That comparison relies on knowing the ratio of uranium-238 to uranium-235. Earlier calculations of the ratio all came up with the same number, 137.88. The assumption that the ratio was constant simplified calculations greatly — it allowed scientists to combine both uranium values into a single number, eliminating one variable from the equation. Lead isotopes are easier to measure with high precision than uranium isotopes, so an age-estimation system based only on lead values was thought to be extremely precise.
“Everybody was sitting on this two-legged stool claiming it was very stable,” comments Gerald Wasserburg, emeritus professor of geology at Caltech who was involved in much of the early work in measuring uranium ratios. “But it turns out it’s not.”
There were reasons to doubt that the uranium ratio was constant. For one thing, no theoretical reasoning supports the assumption. What’s more, measurements that relied on other, less precise radioisotopes disagreed with the age derived from lead — but agreed with each other.
“It’s kind of been a black eye for a few people in geochronology,” Brennecka says. “To really say we know the age of the solar system based on the age of the rock, it’s essential that they all agree.”
To test whether the uranium ratio really was constant, Brennecka and colleagues took samples from calcium-aluminum–rich inclusions in the well-studied Allende meteorite and measured how much uranium-235 and uranium-238 they held. Technological innovations made their measurements more precise than previous efforts.
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Measurements at Brennecka’s lab and at a collaborator’s lab in Frankfurt, Germany, both showed an excess of uranium-235. This excess means that future geochemists will have to first measure the quantities of uranium-235 and uranium-238 in early solar system materials before determining their ages.
“It’s not as if this age dating process doesn’t work anymore,” says coauthor Ariel Anbar, also of Arizona State. “But if you want to push this isotope system to get ages that are really precise, suddenly we realize that there’s this variation you need to take into account.”
The team also determined that the extra uranium-235 comes from trace amounts of a radioactive element called curium present in the early solar system and formed only in certain types of supernova explosions.
“It’s an important step forward,” comments Andrew Davis of the University of Chicago. “There have been so many unsuccessful experiments in the past, but this one succeeded. I think it will be an important piece of the puzzle.”