by The coming decade is expected to bring a real bonus for planetary science: space missions are planned to bring back rock samples from the moon, Mars, the Martian moon Phobos and a primitive asteroid. And scientists say there’s a new technique for determining the age of rocks, meteorites and even artifacts that could help usher in a new era of discovery.
A team with the University of Chicago and the Field Museum of Natural History tested an instrument made by Thermo Fisher Scientific on a piece of a Martian meteorite nicknamed “Black Beauty” and was able to quickly and accurately date it by scanning it with a tiny laser . beam—a significant improvement over previous techniques, which involved much more work and destroyed parts of the sample.
“We are very excited about this demonstration study, as we believe we will be able to use the same approach to date rocks returned by multiple space missions in the future,” said Nicolas Dauphas, Louis Block Professor of Geophysical Sciences. at the University of Chicago and first author of a study presenting the results. “The next decade is going to be exciting in terms of planetary exploration.”
Rock of Ages
Scientists have been using isotopes to estimate the age of samples for more than a century. This method takes advantage of the fact that some element types are unstable and will slowly transform into other types at a slow, predictable rate. In this case, scientists take advantage of the fact that rubidium-87 will convert to strontium-87—so the older the rock, the more strontium-87 it will have.
Rubidium dating can be used to determine the ages of rocks and objects that are billions of years old. It is widely used to understand how the moon, Earth, and solar system formed, to understand magma hydraulics beneath volcanoes, and to trace human migration and trade in archaeology.
Previously, however, the way to make this measurement would take weeks – and destroy part of the sample.
To perform these tests with the conventional method, “you take your piece of rock, crush it with a hammer, dissolve the minerals with chemicals and use a special ultra-clean lab to process them, then take it to a mass spectrometer to measure the isotopes,” explained study co-author Maria Valdes, a postdoctoral researcher at the Robert A. Pritzker Center for Meteoritics and Polar Studies at the Field Museum of Natural History.
But Thermo Fisher Scientific developed a new machine that promised to significantly reduce the time, toxicity and amount of sample destroyed in the process. It uses a laser to vaporize a tiny section of the sample—the hole created is the size of a human hair—and then analyzes the rubidium and strontium atoms with a mass spectrometer that uses new technological advances to cleanly measure strontium isotopes.
Dauphas, Valdes and several other collaborators wanted to test the new technique—and they had a perfect candidate: a piece of a meteorite that landed on Earth from Mars.
This particular meteorite is nicknamed “Black Beauty” for its magnificent dark color. It is sculpted with lighter fragments representing even older rocks embedded in the rock.
However, these fragments became encased in another rock sometime much later in the history of Mars. It’s a bit like baking cookies, Valdes explained. the chocolate chips and nuts were made at different times and places, but all the ingredients come together when you bake the cookie.
Scientists want to know the ages all from these steps along the way, because the composition of each set tells them about what conditions were like on Mars at the time, including the composition of the atmosphere and volcanic activity on the surface. They can use this information to put together a timeline of Mars.
However, so far, parts of the story have been disputed. different studies had given different answers about when all of Black Beauty’s components came together and formed a rock—so scientists thought the meteorite would be the perfect candidate to test the new technique’s capabilities. They took a sample of Black Beauty to Germany to test.
Within hours, not weeks, the instrument returned its answer: 2.2 billion years. The team believes this represents the moment it merged into its final form.
Additionally, to perform the test, the scientists were able to place the entire piece of meteorite in the machine and then precisely select a tiny location to check the age. “This was a particularly good tool for resolving this dispute,” Dauphas said. “When you chip a piece of rock to test the old way, you’re likely to mix in other fragments, which can affect your results. We don’t have that problem with the new machine.”
The technique could be extremely useful in many fields, but Dauphas and Valdes are particularly interested in understanding everything from the history of water on the surface of Mars to how the solar system itself formed.
In the next decade, scientists expect a large amount of new samples from places outside of Earth. The US and China are planning new missions to the moon. A mission to intercept an asteroid called Bennu will land in 2023 with payloads of dirt removed from its surface. Another mission will bring back samples from the Martian moon Phobos in 2027. And in the early 2030s, NASA hopes to bring back samples now being collected by the Perseverance rover on Mars.
With all these samples, scientists expect to learn a lot more about the planets and asteroids around us.
“This is a huge advance,” Dauphas said. “There are many precious meteorites and artifacts that you do not want to destroy. This allows you to greatly minimize the impact you have during your analysis.”
The meteorite was provided by the Field Museum of Natural History’s Robert A. Pritzker Center for Meteoritics and Polar Studies. Other UChicago-affiliated scientists on the paper included Timo Hopp, Zhe Zhang, Phillip Heck, Bruce LA Charlier, and Andrew Davis. The study’s other co-authors included those from Thermo Fisher Scientific, Victoria University of Wellington in New Zealand, the University of California Los Angeles and Washington University in St.
Report: “In situ 87Rb–87Sr analyzes of terrestrial and extraterrestrial samples by LA-MC-ICP-MS/MS with double Wien filter and collision cell technologies.” Dauphas et al, Journal of Analytical Atomic Spectrometry, 10 October 2022.
Funding: NASA, National Science Foundation, US Department of Energy.
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