Scientists try to settle an age-old question about Martian meteorites
Aug. 22, 2013
LOS ANGELES — Planetary scientists dream of sending a geologist to Mars to study its rocks by hand. Until then, they have to settle for examining meteorites — chunks of the Red Planet that land on Earth after hurtling through space and surviving the searing fall through our atmosphere.
Though a little banged up, these meteorites provide a vital up-close view of our rust-hued neighbor. But it can be hard for geologists to interpret what they see when they can’t agree on how old a rock is. Conflicting age estimates for certain rocks differ by up to 4 billion years — the vast majority of Mars’ planetary existence.
Do these rocks tell the beginning, or the end, of Mars’ story?
In a paper last month in the journal Nature, an international team of scientists sought to sort this out for a particular group of meteorites known as shergottites. Martian space rocks tend to be very old, but shergottites are thought to be remarkably young.
Formed When Dinosaurs Roamed Earth
Most scientists believe the shergottites were formed as a result of volcanic activity about 150 to 250 million years ago. This would place their formation during Earth's Jurassic period, when dinosaurs roamed the Earth.
If so, shergottites are a sign that Mars might have been geologically active fairly recently. That in turn means it might still have been hospitable to some form of life, said Munir Humayun, a planetary geochemist at Florida State University who was not involved in the study.
But a few researchers recently estimated that shergottites are actually about 4 billion years old — a finding that had far-reaching implications for scientists’ understanding of Martian history. “Most of us understand that if there is to be life on Mars, it’s not going to be roaming on the surface. ... but it could be thriving underground with the heat energy provided by volcanism,” Humayun said.
However, “if there hasn’t been [volcanic activity in] 4 billion years, ... it would be very difficult to believe there was life left on Mars.”
To settle the question, Desmond Moser of Western University in Ontario, Canada had his team examine a meteorite known as Northwest Africa 5298. They looked at very tiny deposits within it called baddeleyites. These tough, zirconium-rich minerals are “10 times smaller than the diameter of a human hair,” said study co-author Axel Schmitt, a geochemist at the University of California, Los Angeles.
A Tough Tiny Mineral In Meteorites
Researchers generally measure relative amounts of pairs of radioactive elements in rocks to gauge how long a rock has been around since it fully crystallized. But that isotopic ratio can be reset by a big impact that melts the rock down, erasing the geochemical trail.
That’s why scientists picked the tiny baddeleyites: Once they form, they are durable little minerals. It’s not easy to melt them down and reset their isotopic clock. By looking at the isotopic ratios of radioactive uranium and lead in this tough rock, the scientists found that it formed about 187 million years ago, give or take 33 million years. This confirmed that the baddeleyites were young.
The scientists could also see evidence of the damage done to the meteorite by the event that may have released it from the Martian surface sometime around 22 million years ago. The shock from that impact muddled up the mineral structure inside, while keeping the chemicals themselves mostly intact. The result was something like shaking up a sealed gift box of chocolates.
“It looks good on the outside, but all the pieces are dislodged on the inside,” Schmitt said. The study further shows that the shergottites weren’t old rocks, Humayun said. But that was what he had expected. “If they had found that it was 4 billion years old, I would eat my hat," he said.
The researchers say they can use their method of dating these rocks to study other Martian meteorites. Their hope is that it will reveal new information about the Red Planet’s geophysical inner life.