Helium helps trace the origin of Martian meteorite

While cleaning out his rock collection in 1999, Los Angeles resident and rock-enthusiast Bob Verish came across two unusual looking stones he had picked up on a hike in the Mojave Desert 20 years earlier. The stones, which Verish named “Miguel” and “Gabriel,” were soon recognized by UCLA scientists as remnants of a meteorite from Mars.

Four researchers from Yale’s Geology and Geophysics Department, led by post-doctoral fellow Kyoungwon Kyle Min, have taken up the quest to uncover the story behind these unlikely space travelers, a story that began around three million years ago.

“We wanted to know the time-temperature history of meteorites, not just when they were formed,” said Peter Reiners, a member of the Yale team and a professor of geology and geophysics. “This tells us about their history in space and on other planets. We want to know how often things bump into each other and how hard.”

In order to draw such conclusions about the Martian meteorite, now referred to as the “Los Angeles” or “LA,” the team had to think outside the box — or, in this case, outside the planet. They identified a method of helium dating which is used on terrestrial rocks and applied it to the meteorite from Mars, Reiners said.

The team knew that a meteor had hit Mars prior to the time that the LA was ejected from the red planet, Min said. When this meteor hit Mars, it caused a change in the constitution of the rock on the planet, known as impact-related shock metamorphism. The team was fortunate that the interior of the LA meteorite preserved its original Martian signatures, including those from the original Martian impact, making the LA a virtual time capsule from the impact of the meteorite on Mars, Min said.

During the impact, helium atoms, products of spontaneous uranium and thorium decay, would have been completely diffused out of the rock, Reiners said. As a result, the team, which also included Yale researchers Stefan Nicolescu and James Greenwood, was able to determine when the meteorite hit Mars by measuring the uranium, thorium and helium content of the sample.

Min said the study was innovative in its use of helium measurements from only single grains of the meteorite. By employing this method, Min and his colleagues discovered that the impact occurred on Mars around three million years ago. They were also able to conclude that the maximum temperature of impact was between 450 and 500 degrees Celsius, Reiners said.

The helium age was then compared to the LA’s exposure age, the amount of time that the sample has been exposed to cosmic rays. Since the LA was underground until its ejection from Mars, the exposure age equals the amount of time since it left Mars, Min said. The exposure age was also found to be about three million years. He said a main conclusion of the study was the similarity of the helium and exposure ages.

“Thus, we showed that the impact that launched it [the LA] off the planet also heated it up to pretty high temperatures,” Reiners said.

According to the study, this new method of meteorite analysis allows scientists to better understand the time and temperature impact processes. The method has a variety of applications to terrestrial and extraterrestrial materials, higher thermal sensitivity than other techniques and an accessible age range that spans from the birth of the solar system to the beginning of modern human history, according to the study.

Although Min’s technique cannot directly answer the question of extraterrestrial life, it could aid scientists in this search.

“It could tell us about how material that could potentially contain traces of life from Mars could or could not be transported to earth,” Reiners said.

By accurately determining the temperature, pressure and time of shock metamorphism, scientists can gauge the probability of finding evidence of life in meteorites, Min said.

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