Even among rare and precious Martian meteorites, NWA 7034 stands out, having been formed 4.5 billion years ago, when the red planet was first producing a crust. It is the only brecciated (composed of multiple rock fragments) Martian meteorite ever found, making where on Mars it came from particularly interesting to planetary scientists.
NWA (referring to its discovery in North West Africa not the hip hop band) 7034 was such a large and significant space rock it was named Black Beauty. Ground parts of it have been used to prove Earthly organisms can survive for a while on Martian soil.
Black Beauty's Martian origins were quickly identified, thanks to its chemical composition, but it has taken 11 years and the help of a supercomputer for its site of origin to be revealed in Nature Communications.
Like all Martian meteorites, NWA 7034 was kicked into space when an asteroid slammed into Mars with such force the impact threw chunks of the planet's surface into space, eventually colliding with Earth.
Mars has millions of craters large and intact enough to be identifiable from orbiting satellites, so finding the one a particular rock was knocked out of millions of years ago might seem an impossible task. However, Dr Anthony Lagain of Curtin University explained to IFLScience the steps his team went through to narrow the search down.
“We understand all Martian meteorites were ejected recently,” Lagain said. A lump of rock knocked off the surface of a planet is unlikely to find a stable orbit and then be knocked out of it. If it collides with a planet it is usually in the first 20 million years after being launched into space. Moreover, Lagain told IFLScience: “We think a crater needs to be larger than 3 kilometers [1.9 miles] in diameter to eject rocks above escape velocity.”
Martian satellites mean we can measure crater size precisely but the age can be harder to discern. Lagain and co-authors created a new method. Large impacts throw up rocks at a range of speeds, and those below escape velocity produce secondary craters around the initial site. Erosion on Mars is slow compared to on Earth, but small craters do get filled in by dust storms, so a big crater surrounded by smaller ones is newly formed.
The authors created a file of 94 million Martian craters and sought locations where they are clustered, indicating abundant fresh secondary impacts. Using this method, they identified 19 craters large enough and young enough to be responsible for meteorites.
Only one of these is at a site with thorium, iron, and potassium concentrations matching those in Black Beauty, which they named Karratha after a Western Australian town. For added proof, NWA 7034 retains traces of the long-gone Martian magnetic field from when it is formed. Karratha lies in one of the few similarly magnetized parts of Mars.
Only one other Martian meteorite's origins have been traced using similar methods, and in that case, two craters matched the requirements.
“The more complex history a meteorite has, the easier it is to track its origin,” Lagain told IFLScience. Black Beauty certainly qualifies, having formed 4.5 billion years ago, been thrust to the surface in an impact three billion years later, and then knocked off the planet around five million years ago.
Karratha lies near Tharsis, home to the Solar System's largest volcanoes. This is also where the Martian crust is thickest, which Lagain told IFLScience may represent the remains of a continent formed through similar processes to those on Earth. “It's probably the most interesting region of Mars if you're interested in the early evolution of Mars' and other planets' crust,” he said. These may be the oldest rocks we can retrieve and analyze.
“Finding the region where the ‘Black Beauty’ meteorite originates is critical because it contains the oldest Martian fragments ever found, aged at 4.48 billion years old, and it shows similarities between Mars’ very old crust, aged about 4.53 billion years old, and today’s Earth continents,” said Lagain in a statement.
“The region we identify as being the source of this unique Martian meteorite sample constitutes a true window into the earliest environment of the planets, including the Earth, which our planet lost because of plate tectonics and erosion.”