The next decade is expected to be a veritable bonanza for planet science, with space missions returning samples of rock from the moon, Mars, the Martian moon of Phobos, and a primitive asteroid. A new technique for determining the age of rocks, meteorites, and even artifacts, according to scientists, could help usher in a new era of discovery.
A team from the University of Chicago and the Field Museum of Natural History used a Thermo Fisher Scientific instrument to quickly and precisely date a piece of a Martian meteorite nicknamed "Black Beauty" by probing it with a tiny laser beam—a significant improvement over previous techniques, which required far more work and destroyed parts of the sample. Their findings appear in the Journal of Analytical Atomic Spectrometry.
"We are very excited by this demonstration study because 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, the Louis Block Professor of Geophysical Sciences at the University of Chicago and the study's first author. "In terms of planetary exploration, the next decade will be mind-boggling."
For more than a century, scientists have used isotopes to estimate the ages of specimens. This method takes advantage of the fact that certain elements are unstable and will gradually transform into other types at a slow, predictable rate. Scientists use the fact that rubidium-87 will change into strontium-87 in this case, so the older the rock, the more strontium-87 it will have.
Rubidium dating can be used to determine the ages of billions of years old rocks and objects; it is widely used in archaeology to understand how the moon, Earth, and solar system formed, to understand the magma plumbing system beneath volcanoes, and to trace human migration and trades.
Previously, however, this measurement would take weeks and would destroy a portion of the sample.
To conduct those tests using the traditional method, "you take your piece of rock, crush it with a hammer, dissolve the minerals with chemicals and process them in a special ultra-clean laboratory, and then take it to a mass spectrometer to measure the isotopes," explained study co-author Maria Valdes, a postdoctoral researcher at the Field Museum of Natural History.
However, Thermo Fisher Scientific developed a new machine that promised to significantly reduce the amount of time, toxicity, and sample destroyed in the process. It vaporizes a tiny portion of the sample with a laser—the hole created is the size of a single human hair—and then analyzes the rubidium and strontium atoms with a mass spectrometer that cleanly measures strontium isotopes.
Dauphas, Valdes, and several other collaborators wanted to put the new technique to the test, and they had the perfect candidate: a piece of Mars meteorite that had landed on Earth.
Because of its stunning dark color, this meteorite is known as Black Beauty. It is flecked with lighter fragments that are embedded with even older rocks.
These fragments, however, were rolled up into another rock much later in Mars' history. It's similar to baking cookies, explained Valdes; the chocolate chips and nuts were made at different times and locations, but all the components come together when the cookie is baked.
Scientists want to know the ages of all of these steps along the way because the composition of each set reveals information about the conditions on Mars at the time, such as the composition of the atmosphere and the volcanic activity on the surface. They can use this data to piece together a Mars timeline.
However, different studies had returned different answers for the age when all the components of Black Beauty came together and formed one rock—so the scientists thought the meteorite would be an ideal candidate to test the capabilities of the new technique. They sent a sample of Black Beauty to Germany for testing.
The instrument returned its answer in hours rather than weeks: 2.2 billion years old. The team believes this is when it came together into its final form.
Furthermore, the scientists were able to perform the test by placing the entire meteorite chunk in the machine and then precisely selecting a tiny site to test the age. "This was a particularly good tool for resolving this controversy," Dauphas said. "When you chip out a piece of rock to test it the old way, other fragments may get mixed in, affecting your results. That is not an issue with the new machine."
The technique has the potential to be extremely useful in a variety of fields, but Dauphas and Valdes are particularly interested in using it to learn about everything from the history of water on Mars's surface to the formation of the solar system itself.
Scientists anticipate a flood of new samples from places other than Earth over the next decade. The United States and China are planning new moon missions; one will intercept an asteroid called Bennu and land in 2023 with payloads of dirt scooped from its surface; another will bring back samples from Mars' moon Phobos in 2027, and NASA hopes to bring back samples that the Perseverance rover is now collecting on Mars by the early 2030s.
Scientists hope to learn a lot more about the planets and asteroids that surround us with all of these samples.
"This is a huge step forward," Dauphas said. "You do not want to destroy many valuable meteorites and artifacts. This allows you to significantly reduce the impact of your analysis."