The Ancient Martian Rock that Fell to Earth
NWA 7533 traveled a long way to get into the lab of Munir Humayun at Florida State University. The ancient meteorite was recovered in 2011 after its long interplanetary journey. Soon the space rock was sold off to a collectors in the United States and France, but eventually a sample found its way into Humayun’s hands.
Once he got a look at it, Humayun knew he had something rare. His team’s analysis of the meteorite, published today in Nature, reveals that this chunk of Mars is the oldest known Martian meteorite. Humayun says the 4.4 billion-year-old rock is a sample of the heavily cratered southern highlands of Mars and retains a similar composition to the samples that the Curiosity rover is analyzing.
“This is very exciting,” Humayun tells PopMech. “It’s the first time that we have a sample that is in situ with what’s happening on Mars. We can ground truth from the space missions.”
A Martian Time Machine
NWA 7533, and four other pieces including NWA 7034, were once part of a bigger space rock that broke apart. Together they form a time capsule—a snapshot of Mars’ geological history and what the Red Planet might of looked like in its younger years. One thing is for certain: Mars had abundant amounts of water. When heated, meteorites give off water vapor trapped in their minerals. A type of Martian meteorite more commonly found on Earth, known as a Shergottite, gives off a couple hundred parts per million. This sample had 6000.
“That’s a lot of water to come out of a rock,” says Carl Agee professor of Earth and Planetary Science at the University of New Mexico and the scientist who originally identified NWA 7034 (and subsequently Humayun’s sample as well) as a piece of Mars. “When that rocked formed, it had to have been in an environment that was water rich. The water permeates in the rock, the minerals interact with it, and are locked away.”
The rock’s age was at first difficult to obtain. Original estimates placed the meteorite around 2.1 billion years old, which is ancient by historical standards, but not in planetary science. However, researchers realized the meteorite is an amalgamation of several different kinds of rock, and the original age was only an average. Scientists arrived at its true age after isolating certain minerals known as zircons.
“Zircon is common on Earth and occurs in a lot of different rocks,” says Harry McSween, a Mars geologist and professor at the University of Tennessee. But, he says, zircon is incredibly uncommon in the 60 or so known Martian samples on Earth. “[Zircon] is the best vehicle we have for determining the ages of rocks because it’s a very tough mineral.”
Scientists used a focused beam to get the formation age of the grain. The sample’s age has since been confirmed by other universities and space agencies and NWA 7533 was officially declared ancient with a 4.4 billion year age. Because of this ancient evidence, Humayun was able to determine that the Mars’ crust formed in the first 100 million years of its history.
The path to understanding this meteorite, which splintered into NWA 7533 and 7034 when it entered Earth’s atmosphere, begins with Agee. “I received the meteorite from a collector that purchased it from a Moroccan meteorite dealer,” Agee says. “He didn’t know what it was, and I didn’t know what it was either, and I see meteorites every day.” The rock then sat on Agee’s shelf for several months until one day he decided to take a closer look. He began cutting into the rock with a diamond saw and discovered a spectacular rock mixture. This was a martian meteorite—unlike any other that’s ever been found.
Agee’s sample was nicknamed “Black Beauty” and weighed about 11 ounces. Once Moroccan meteorite hunters (it’s a productive trade in some corners of the world ) heard of the meteorite’s value, they surveyed the Sahara again and recovered NWA 7533, the sample that would eventually find its way to Humayun. Agee guesses that the entire meteorite before it broke apart must have weighed close to 2 or 3 lbs.
Recovering and analyzing the five fragments of this meteorite has been an international effort. Moroccan meteorite hunters, french collectors, american scientists, Australian researchers, NASA specialists, and a dozen of other universities and institutions have pitched in to unlock the small rock’s secrets, an opportunity that Agee admits might come only once in a lifetime as NWA 7533 and the four other pieces allow geologists to survey the Martian surface like never before. “This is only the beginning and we hope to be working on it for several more years,” Agee says. “We’ve open people’s minds...sometimes I used to say ‘Oh, that’s just a rock from the Earth.’ I always give it a second look now to see if it's a Black Beauty.”