The vast majority of the roughly 60,000 meteorites that have been collected from Earth’s surface—mostly in desert locales such as Antarctica and Morocco—are material sloughed off from asteroids. Such meteorites are critical for understanding the early days of the solar system, when a maelstrom of gas, dust, and rock circled the Sun.
However, about 200 of all known meteorites didn’t come from asteroids. They’re chunks of Mars believed to have been blasted off the planet by asteroid impacts. These unique meteorites are truly cause for excitement.
“They are actual samples of Mars.”
“They are actual samples of Mars,” said Chris Herd, a planetary geologist at the University of Alberta in Edmonton, Canada. And they can give scientists clues about the evolution of the Red Planet. Researchers have now pinpointed five craters that they believe birthed at least a third of all known Martian meteorites. These new results were published in Science Advances.
Are You My Crater?
Scientists can tell that a meteorite came from Mars rather than an asteroid by looking at its chemical composition. Tiny bubbles of air trapped within rocks reflect the local environment in which those rocks formed, and the air within Martian meteorites is a dead ringer, compositionally speaking, for Mars’s atmosphere.
Herd and his colleagues analyzed data from six Martian meteorites. That might seem like a small sample, but those rocks represent five out of the 10 so-called pairing groups of Martian meteorites found on Earth. Rocks within the same pairing group were likely ejected from the same place on Mars at the same time and therefore were birthed by a single impact event, Herd said.
“The craters that we link to these ejection events must be quite fresh.”
Linking Martian meteorites with their source crater provides information about provenance, which is key to better understanding these pieces of Mars, said Stephanie Werner, a planetary scientist at the University of Oslo in Norway who was not involved in the research. “We try to find where they come from so we get the context of the rock.”
Herd and his team mined chemical and structural measurements from each meteorite, along with estimates of when each rock originally formed and when it was ejected from Mars. Those data allowed the researchers to winnow down the millions of impact craters that pepper Mars’s surface.
Researchers generally agree that Martian meteorites were likely ejected from the Red Planet within the past 20 million years. That fact was an important starting point to this investigation, Herd said. “The craters that we link to these ejection events must be quite fresh.”
By looking for craters characterized by features that indicate youth—“secondary” craters created by ejected debris, for instance—Herd and his collaborators homed in on roughly 300 craters.
Damaged Goods
The team then looked at shock-related features in each meteorite. “We looked at specific physical damage,” Herd said. That information allowed the researchers to constrain the magnitude of the impact—and therefore the size of the resulting crater—necessary to have imparted that damage. The researchers used those data, along with estimates of the ages of each crater and the age of the surrounding terrain—based on the density of nearby craters—to link the five Martian meteorite pairing groups with their most likely source craters.
Herd and his colleagues pinpointed five craters ranging in diameter from 14 to 39 kilometers (9–24 miles). One, called Tooting, had previously been linked to a Martian meteorite pairing group, but the other four—Chakpar, Corinto, Kotka, and Domoni—are new discoveries.
It’s exciting to think about connecting craters on Mars with Martian meteorites, but there’s a lot of uncertainty in these types of analyses, Werner said. For starters, Herd and his colleagues assumed that the meteorites in their sample were all relatively young rocks, she said. But Werner and some other researchers believe that some Martian meteorites might be billions of years old. “There are different opinions about how old they are,” she said. And that affects where you’d look for a source crater, she added, because some lie on younger terrain and others lie on older terrain.
Herd and his collaborators also pinpointed several fresh craters of sufficient size to have ejected material from Mars’s gravitational field that don’t seem to be connected to any Martian meteorite pairing group. One example is Zunil, a crater 10 kilometers (6 miles) in diameter located in Mars’s northern hemisphere. Perhaps the impacts that created those craters didn’t end up ejecting any material that fell to Earth, or maybe those Martian meteorites are still waiting to be found, Herd and his colleagues concluded. “This is, to me, the most exciting thing about this whole study,” Herd said.
—Katherine Kornei (@KatherineKornei), Science Writer
This news article is included in our ENGAGE resource for educators seeking science news for their classroom lessons. Browse all ENGAGE articles, and share with your fellow educators how you integrated the article into an activity in the comments section below.
