An unusually pristine piece of Mars

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Blasted into space by a collision with an asteroid, the jagged hunk of Mars rock tumbled silently through the solar system for 7,000 centuries.

Finally, on July 18, 2011, the rock’s long journey ended as violently as it had begun: It plunged to Earth as a fireball that illuminated the Moroccan night, awakening soldiers and nomads with a sonic boom. One eyewitness said it turned from yellow to green before it finally split in two and vanished from view.

Such was the dramatic arrival of the so-called Tissint meteorite, named for a village where pieces fell. The unusually pristine specimen is one of only five Martian projectiles that have been observed entering Earth’s atmosphere and then recovered for study.

It turns out the meteorite has a great deal in common with other rocks that have made the trip from Mars to Earth. An international team of researchers examined its molecular structure and determined that Tissint was probably ejected from Mars by the same impact that launched another group of meteorites that also landed on Earth, many in Antarctica, after a shorter journey through the solar system.

They determined this by calculating the meteorite’s exposure to cosmic rays.

As the Mars rocks travelled through space, they endured constant bombardment by cosmic rays — high-energy protons that would penetrate the rocks and sometimes knock out protons and neutrons from their atoms. The process created rare isotopes that scientists use to determine how long it took the rocks to make their journeys.

Tissint has a cosmic ray exposure age of 700,000 years, give or take 300,000, authors wrote. This was consistent with the meteorites that arrived earlier, “suggesting that they were ejected from Mars during the same event.”

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Roughly 15 pounds of scorched and shattered rock was plucked from the Moroccan desert by meteor hunters in the months after Tissint landed last summer. Its relatively swift collection made it largely free of the earthly contamination that’s typical of most Martian meteorites.

The largest Tissint samples were covered with a shiny black fusion crust — the result of its super-heated entry into Earth’s atmosphere. Inside, the rocks were pale gray and studded with pale yellow olivine crystals. Researchers also observed small pockets and tiny veins of black glass.

Although numerous Martian meteorites have landed on Earth, it wasn’t until the 1980s that scientists were able to determine their planetary origin. Using mass spectrometry, they found that certain meteorites contained strikingly similar gases and isotope ratios as samples obtained by the Viking landers, which visited Mars in 1976.

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