The next decade is expected to bring real gold 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 group from 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 called “Black Beauty” and were able to quickly and accurately date it by scanning it with a small laser beam — significantly an improvement over previous techniques that involved much more work and destroyed parts of the sample.
“We are very excited about this demonstration study because we think we will be able to use the same approach to date rocks that will be returned by multiple space missions in the future,” said Nicolas Dofas, the Louis Block Professor of Geophysical Sciences at University of Chicago and first author of a study reporting the results. “The next decade is going to be mind-blowing in terms of planetary exploration.”
A rock of ages
Scientists have been using isotopes to determine the age of specimens for more than a century. This method takes advantage of the fact that certain element types are unstable and will slowly transform into other types at a slow, predictable rate. In this case, scientists are tapping into the fact that rubidium-87 will turn into strontium-87 – so the older the rock, the more strontium-87 it will have.
Rubidium dating can be used to determine the age 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 plumbing beneath volcanoes, and to trace human migration and trade in archaeology.
Previously, however, the way to make this measurement took weeks – and would 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 ultraclean lab to process them, and then take it to a mass spectrometer to measure the isotopes,” explained the co-author of the study by Maria Valdes, a postdoctoral fellow at the Field Museum of Natural History’s Robert A. Pritzker Center for Meteorites and Polar Studies.
But Thermo Fisher Scientific has developed a new machine that promises to significantly reduce time, toxicity and the amount of sample destroyed in the process. He used a laser to vaporize a small part of the sample – the hole created is the size of a human hair – and then analyzed the rubidium and strontium atoms with a mass spectrometer that uses new technological advances to precisely measure strontium isotopes.
Daufas, Valdes and several other collaborators wanted to test the new technique—and they had a perfect candidate: a piece of meteorite that had fallen to Earth from Mars.
This particular meteorite is nicknamed “The Black Beauty” because of its magnificent dark color. It is interspersed with lighter fragments that represent even older rocks embedded in the bedrock.
However, these fragments were rolled into another rock at some point much later in Mars’ history. It’s a little like when you bake cookies, Valdez explained; the chocolate chips and nuts are made at different times and places, but all the components come together when you bake the cookie.
Scientists want to know the age of everything from those steps along the way, because the composition of each set tells them what conditions on Mars were like at the time, including the composition of the atmosphere and volcanic activity on the surface. They can use this information to compile a chronology of Mars.
Until now, however, parts of the story have been disputed; different studies have given different answers about the age when all the components of Black Beauty came together and formed a single rock – so scientists believe that the meteorite will be the perfect candidate to test the capabilities of the new technique. They took a sample of Black Beauty to Germany to try it out.
In hours, not weeks, the instrument returned its answer: 2.2 billion years. The team believes this represents the time it fused into its final form.
What’s more, to perform the test, the scientists were able to put the entire meteorite piece into the machine and then precisely select a small spot to test the age. “It was a particularly good tool to resolve this dispute,” Dofas said. “When you cut off a piece of rock to test the old way, it’s possible to get other fragments mixed in, 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 Daufas and Valdes are particularly interested in it to understand everything from the history of water on the surface of Mars to how the solar system itself formed.
Over the next decade, scientists expect a wealth of new samples from places other than Earth. US and China plan new missions to the moon; intercept missions to an asteroid called Bennu will land in 2023 with payloads of dirt scraped from its surface; another mission will bring samples from the Mars moon Phobos in 2027; and by the early 2030s, NASA hopes to return samples that the Perseverance rover is now collecting on Mars.
With all these samples, scientists expect to learn a lot more about the planets and asteroids around us.
“It’s a huge advance,” Daufas said. “There are many valuable meteorites and artifacts that you do not want to destroy. This allows you to significantly minimize the impact you have during your analysis.”
The meteorite was provided by the Robert A. Pritzker Center for Meteoritics and Polar Studies at the Field Museum of Natural History. Other UChicago-affiliated scholars in the article include Timo Hopp, Zhe Zhang, Philip Heck, Bruce LA Charlier, and Andrew Davis. Other co-authors on the study include those from Thermo Fisher Scientific, Victoria University of Wellington in New Zealand, the University of California, Los Angeles, and Washington University in St. Louis.
Quote: “In situ 87Rb–87Sr analyzes of terrestrial and extraterrestrial samples by double Wien filter LA-MC-ICP-MS/MS and collision cell technologies.” Dauphas et al, Journal of Analytical Atomic Spectrometry, 10 October 2022.
Funding: NASA, National Science Foundation, US Department of Energy.