Reflecting on Mars: Moroccan black rock connection

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  • Published: Mar 1, 2015
  • Author: David Bradley
  • Channels: UV/Vis Spectroscopy
thumbnail image: Reflecting on Mars: Moroccan black rock connection

Black Beauty

New spectroscopic analysis of “Black Beauty,” a meteorite found in the Moroccan desert, has given scientists a better picture of the crust beneath Mars’ red dust. Public domain image

A chunk of shiny black rock found in the Moroccan desert in 2011 and given the name NWA 7034, is Martian in origin, a meteorite that fell to Earth having spent the first 4.4 billion years of its existence as an extraterrestrial. Now, reflectance spectroscopy and other techniques reveal this meteorite to be like no other rock on earth and that it was part of the crust of Mars that lies beneath its red, dusty surface.

Planetary scientists were well aware that the so-called Red Planet has dark plains, areas where its coating of red dust is thin and the crustal rocks beneath are partially exposed. Now, Jack Mustard from Brown University in Providence, Rhode Island and Carl Agee of the University of New Mexico, USA, explain in the journal Icarus how analysis of NWA 7034 matches orbital analysis of those Martian dark plains. The work suggests that the meteorite, which also goes by the much slicker nickname of Black Beauty, could represent the type of rock that is the "bulk background" of rocks on the Martian surface.

NWA 7304

Graduate student Kevin Cannon at Brown University is lead author on the paper and was well aware that the first scientists to analyse Black Beauty in 2011 knew that they had found something special. The chemical composition of the meteorite proved that it originated on Mars and was presumably dispatched from the planet's surface and into an Earth-crossing orbit by an asteroid or cometary impact long ago. But, Black Beauty, while definitively Martian was unlike any other Martian meteorites found on Earth classified as SNC meteorites (shergottites, nakhlites, or chassignites), those are generally igneous rocks formed from cooled volcanic material. Black Beauty by contrast is a breccia, a conglomerate of different rock types fused together in a basalt matrix. The special meteorite contains sedimentary components that match the chemical makeup of rocks analysed by NASA's Mars rovers. And scientists have concluded that it might be a representative piece of Martian crust - as such the first known sample to have reached Earth and survived entry.

The team hoped that Black Beauty might help them solve a longstanding puzzle. The spectral lines for SNC meteorites are always slightly different to those obtained by spectrometers orbiting Mars. "Most samples from Mars are somewhat similar to spacecraft measurements," Mustard explains, but at the same time, "annoyingly different."

Cannon and Mustard obtained a tiny chip off the old Black Beauty from Agee and carried out an array of analyses including hyperspectral imaging that allowed them to obtain a detailed spectral image of the chip as a whole. "Other techniques give us measurements of a dime-sized spot," Cannon explains. "What we wanted to do was get an average for the entire sample. That overall measurement was what ended up matching the orbital data."

What a mess!

The team points out that the spectroscopic match helps put a face on the dark plains, suggesting that the regions are dominated by brecciated rocks similar to Black Beauty. Because the dark plains are dust-poor regions, they're thought to be representative of what hides beneath the red dust on much of the rest of the planet. "This is showing that if you went to Mars and picked up a chunk of crust, you'd expect it to be heavily beat up, battered, broken apart and put back together," Cannon adds. That the surface of Mars would be rich in Black Beauty-like breccias makes a lot of sense, given what we know about Mars, the researchers say.

Mars is pock-marked with almost half a million impact craters greater than 1 kilometre in diameter that have accrued on its surface over millions of years. The team explains that because brecciation has to be a natural consequence of such impacts, future probes or explorers could expect to find materials that have a similar profile to NWA 7034 across the whole of the planet. In other words, Mustard says, the bulk of rocks on the surface of Mars probably look a lot like Black Beauty: "dark, messy and beautiful."

"We're working on a new meteorite, NWA 6963, that may have come from deeper down in Mars' crust," Cannon told SpectroscopyNOW. "Ultimately we're trying to use orbital and laboratory spectroscopy to piece together the history of the planet, and what conditions were like there billions of years ago when life was emerging on Earth."

Related Links

Icarus 2015, 252, 150-153: "Evidence for a widespread basaltic breccia component in the martian low-albedo regions from the reflectance spectrum of Northwest Africa 7034"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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