Famed meteorite reveals early water on Mars—and an early outer space bombardment

Mars suffered an early bombardment before settling in to a prolonged calm, a new meteorite record indicates.
DETLEV VAN RAVENSWAAY/SCIENCE SOURCE

It was a tough fundraising pitch: Martin Bizzarro, a cosmochemist at the University of Copenhagen, needed some $500,000 to buy—and then grind up—material from one of the oldest and most valuable rocks in the world. Only this rock wasn’t originally from Earth. It came from Mars.

Bizzarro’s bet has now paid off handsomely. With just 15 grams of the 4.4-billion-year-old “Black Beauty” meteorite, discovered in 2011 in the western Sahara, his team has revealed a record of asteroid impacts and volcanic eruptions spanning nearly all of martian history.

One of the most surprising findings: After Mars underwent a pummeling early in its life, all went quiet—even during a time, nearly 4 billion years ago, when our Solar System was thought to have suffered a cataclysmic assault. That’s based on the ages of 51 zircon crystals found within the meteorite that likely formed under the extreme heat of an impact. The ages of the zircons largely cluster around 4.5 billion years ago—indicating a period of planetary assault—but the planet seems relatively calm after that, Bizzarro and colleagues report today in the Proceedings of the National Academy of Sciences.

The discovery lines up with recent work that casts doubt on the so-called Late Heavy Bombardment. When scientists first analyzed the rocks brought back by the Apollo astronauts, they seemed to indicate multiple devastating impacts on the Moon nearly 4 billion years ago. Solar System models suggested fluctuations in the orbits of Jupiter and Saturn around that time stirred up asteroids and sent them careering into the inner Solar System.

But reanalysis has since made it clear that many of the Moon rocks came from a single impact, and other strikes came as far back as 4.3 billion years ago. That suggested no spike in impacts, but rather a gradual decline in bombardment through time.

Meanwhile, updated computer models suggested the migration of the giant planets to their present orbits also came earlier, near the Solar System’s start 4.56 billion years ago, triggering an early heavy bombardment that systematically declined from there. The idea is now further confirmed by the new study, says Desmond Moser, a geochronologist at Western University who was not involved in the work. The work, he says, paints “a compelling picture” of martian history.

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