A Martian meteorite picked up in Africa came from one of the longest-lived volcanic centres in the solar system, scientists announced this week.
Writing in Science Advances, University of Houston scientist Tom Lapen and his colleagues analysed a couple of grams of material extracted from the meteorite NW Africa 7635, which was collected in Algeria in 2012.
The palm-sized striped stone is a form of volcanic rock called a shergottite. Chemically similar samples have been found before and dated to between 327 and 600 million years ago. This one, however, dated at more than 2.4 billion years, is much older.
The meteorite was ejected into space when something slammed into a lava field or volcano on Mars. After drifting around the solar system for a million years or so, the pebble eventually crossed the Earth's orbit and made a fiery journey to our own planet's surface.
The houston team used mass spectrometry to compare the ratios of different chemical elements in the material. These elements are radioactive and, as they decay, the relative amounts of each changes. This provides scientists with an "atomic clock" against which to age the sample.
Lapen and his colleagues were also able to look for signature signs of cosmic irradiation: particles of exotic elements produced when space-faring bodies are zapped by high-energy cosmic rays; measuring the amounts of these can tell scientists how long an object has been journeying through space. This is how Lapen and his team know that their meteorite spent about a million years drifting in cosmic limbo.
Together these findings show that the region of Mars from which this, and the other shergottites recovered so far, originated was geologically active for more than 2 billion years - over half the red planet's lifetime. This makes it one of the oldest and longest-lived volcanic regions in the solar system. It's also important because it fills a current void in our knowledge of what was happening on Mars at the time when this object formed.
"Most Mars meteorites have ages 0.2-1.3 billion years. ALH84001 [another Mars meteorite] is nearly 4.5 billion yr. This one," says Open University planetary geologist David Rothery, commenting on the work, "at 2.4 billion years old, fills a gap."
The composition and crystal structure of the object call also, Rothery explains, tell us something about the conditions on Mars when it forms. "Like all shergottites, this one has basaltic composition, and a grain size small enough for it to be classed as basalt (less than 250 microns). That is consistent with the relatively fast cooling that happens when erupted as a lava flow."
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