When the meteorite NWA 7533 was discovered in 2011, it was recognized to be rare and precious, a piece of Mars knocked off by an asteroid impact. Now planetary scientists have discovered it's more than rare, it's unique — the only object we've found forged deep inside Mars, providing an unprecedented opportunity to learn about the Martian interior.
A great deal of what we know about the Earth's geologic past comes from zircons, tiny crystals that trap uranium and thorium as they form, revealing their age by the proportion of these elements that have undergone radioactive decay. Sadly, the bits of Mars we have examined, either with rovers or from other meteorites, have very few zircons. Without plate tectonics, the Red Planet hasn't experienced the conditions that form most zircons on Earth.
NWA 7533's color helped it stand out against the Moroccan desert sands where it had landed, earning it the nickname “Black Beauty”. The first scientists to examine it knew they had something special but had no idea just how extraordinary it was.
When 15 grams (0.5 ounces) of Black Beauty was crushed for analysis, 57 zircons were found, most from 4.5 billion years ago when Mars was just forming. This delighted those studying the meteorite, with Professor Martin Bizzarro of the University of Copenhagen saying in a statement: "We were quite surprised and excited when we found so many zircons in this martian meteorite... Having access to so many zircons is like opening a time window into the geologic history of planet.” The authors think older zircons were formed when large bodies collided with Mars back when the Solar System was just getting started.
However, Bizzarro reports in Proceedings of the National Academy of Sciences that eight zircons formed in the last 1.5 billion years, the most recent just 300 million years ago.
"These young ages were a great surprise", Bizzarro said. "The Black Beauty meteorite is believed to come from the southern hemisphere of Mars, which does not have any young volcanic terrains. The only possible source for these young zircons is the Tharsis volcanic province located in the northern hemisphere of the planet, which contains large volcanoes that were recently active,"
The Tharsis volcanoes are the largest in the Solar System, with one being 21 kilometers (13 miles) from base to peak. They are so enormous not only because the weak Martian gravity allows it but because of the planet's lack of plate tectonics. Each of Hawaii's volcanoes can only get so big before the Pacific plate's migration removes them from the mantle plume that nourishes them, beginning the process at another location. It is thought similar forces produced Tharsis, but with no crustal migration the region kept growing until the plume ran out of heat.
The more recent zircons give us the first view we have had into the internal workings of this immense volcanic system. They may allow a better understanding of how much water Mars had and where it went. So far, however, the discovery raises as many questions as it answers, particularly how these young zircons got incorporated into the breccia that makes up most of NWA 7533.
The authors believe the resemblance of the younger zircons to chondrite meteorites, the most primitive parts of the Solar System, confirms the “stagnant lid” model of Mars where little changes in the Martian interior. They also argue finding and studying more zircons should be a priority for future Martian missions.