Martian meteorites have revealed that the Red Planet may have formed several million years later in the Solar System’s history than previously thought, scientists say.
These meteorites are rocks blasted off the surface of Mars millions of years ago, later to fall to Earth. At present, about 200 are known, some weighing several kilograms.
They are important because their nature has brought them to us, where they can be studied with instruments far beyond anything NASA has yet been able to put on a Mars rover.
“There is a lot of data from [the] rovers, but not in the depth needed to do detailed geochemical modelling,” says Melinda Hutson, curator of the Cascadia Meteorite Laboratory at Portland State University, US, who was not part of the new study. “We rely on the meteorites for that.”
Previous studies based on these rocks had suggested that Mars had largely formed two to four million years after the start of the Solar System. (Earth, by comparison, formed after about 60 million years.)
But the new work, says Simone Marchi, a planetary scientist from the Southwest Research Institute in Colorado, and first author of a study in the journal Science Advances, extends that date to as much as 20 million years after the start of the Solar System.
That’s important, he says, because “you have to think about the formation of Mars within the larger context of the Solar System”.
Scientists have long known that the planets formed out of a protoplanetary disc of gas and dust, circling the infant Sun. But that disc didn’t last long, with its gas believed to have dissipated in the first 10 million years.
“If you were to form Mars extremely fast, then Mars would have been embedded in this protoplanetary disc,” Marchi says. “But 20 million years is past the dissipation of the protoplanetary disc, so now you’re forming Mars in an environment that is very different.
“That could have implications for the way volatiles were retained and the thickness of the early atmosphere.”
Also, he says, knowing how quickly the planets formed is a clue to how it occurred. Rapid, early formation supports some models, while longer times favours others.
To sort it out, scientists rely on concentrations of trace elements in the Martian meteorites.
One is tungsten, of which two isotopes are important: tungsten-182, which is formed by the radioactive decay of an isotope of hafnium, and the more plentiful tungsten-184. The ratio of these two is a clue to how much halfnim-182 was left at the time the planet formed.
Also important are rare metals such as gold, iridium, osmium, and platinum. These are “siderophile” or “iron-loving” elements, meaning that when a planet is forming they easily mix with iron and settle to the core.
What’s interesting, Marchi says, is that Martian meteorites contain wildly varying amounts of these metals. Platinum, for example, ranges from “almost zero” (0.07 parts per billion) to 20 parts per billion. Not that either concentration is a lot, but, Marchi says, “[it’s] a wide range.”
That much had been known for a long time, he adds, but nobody understood why. But his team’s new research, he says, shows that it can be accounted for if the infant Mars had been clobbered, after it had formed, by one to three large asteroids, 1000 to 2000 kilometres in diameter.
This would bring in additional siderophile elements but not sufficiently melt the planet to cause them entirely to settle to its core. Furthermore, material from these impacts would not be evenly distributed through the Martian mantle and crust.
Rather, it would be concentrated in some spots and virtually absent in others, in a pattern Marchi analogises to a marble cake, where batters of two different flavours are stirred, but not thoroughly mixed.
In the process, he says, tungsten isotopes could also have been altered by contamination from the asteroids, making it look like Mars formed millions of years before it actually did.
Not that Marchi’s team is claiming to have proven that Mars formed at the later date. “We do not have such a definitive answer,” he says. “We are just saying that it is possible.”