NASA's Hubble Space Telescope snapped this shot of Mars on Aug. 26, 2003, when the Red Planet was 34.7 million miles from Earth. The picture was taken just 11 hours before Mars made its closest approach to us in 60,000 years Image: NASA/ESA
Planetary scientists studying Mars have a slightly embarrassing secret: They don’t really know how old most of the planet’s surface is. They do have decent estimates, mostly based on counting craters pockmarking the Martian crust—more craters equate to a greater age. Yet the only way to pin down an age with something approaching absolute certainty is to closely analyze rock samples, and none of the rovers and landers set down on the Red Planet has carried the necessary equipment. Without precise ages the entire history of the planet is blurred, making it more difficult to answer important questions about when and whether Mars was ever truly habitable.
Fortunately, there are Martian rocks right here on Earth. Asteroids or comets can hit Mars hard enough to hurl chipped-off fragments of crust on interplanetary voyages to our world. Some 120 specimens out of the more than 60,000 meteorites in collections around the globe contain mixtures of minerals and microscopic air bubbles that match what we know of the Martian surface and atmosphere. Researchers can date these rare samples by measuring certain radioactive isotopes within them, because the isotopes decay into other elements at rates set by the laws of physics. With most igneous rocks, which begin life as molten material, calculating the ratio of a long-lived isotope, such as uranium 238, to its decay product, lead 206, yields a very good estimate of just how old that rock is—how long ago its isotopes became locked in minerals crystallizing out from a molten mass.
The trouble is that different isotopic tracers yield wildly different dates for the most common variety of Martian meteorites, hunks of igneous rock called shergottites. Grind up a whole shergottite, and the ratio of lead isotopes in the powder will suggest the rock is about four billion years old. If you instead look at various isotopes isolated within microscopic mineral grains inside the shergottite, you will conclude the rock is relatively youthful—only hundreds of millions of years old. This conundrum has flummoxed researchers for years, leaving them divided about the timing and duration of Martian volcanic activity, or when the consolidation of the Martian core and mantle occurred. Now, however, the matter seems to be settled: In a report appearing in the July 25 edition of Nature, a team of scientists lead by Desmond Moser of the University of Western Ontario has presented substantial new evidence that shergottites are young. They base their conclusions on a half-kilogram fragment of Mars known only as Northwest Africa 5298 (NWA 5298). (Scientific American is part of Nature Publishing Group)
“Lots of groups just jump to [isotope] dating right away,” says Kim Tait, a study co-author and mineralogist at the Royal Ontario Museum (ROM) in Toronto, which provided the sample of NWA 5298. “For us, the dating came absolutely last. We first looked very carefully at the minerals, scanning grain by grain so that we could really understand everything in context.... Every rock has a story to tell. Interpreting clues to uncover that story is where we come in.”
“These tiny meteorites are packed with stories on the evolution of a whole planet, stories that we can’t currently get from rovers,” Moser says. “What we’ve done is sort out the ‘page numbers’ for the stories preserved in these rare fragments from space.”
According to Tony Irving, a University of Washington geochemist who first identified NWA 5298’s Martian provenance, a desert nomad discovered the meteorite in 2008 outside the Moroccan village of Bir Gandouz. An anonymous Moroccan middleman purchased the rock from the nomad and eventually sold it for an undisclosed sum to David Gregory, a Canadian physician who later donated it to the ROM. Analyzing a small sample of the meteorite, Irving recognized it as a shergottite based on its characteristic chemical composition as well as its network of “shock metamorphosed” glassy veins and vesicles formed by the immense pressure of an ejecting impact. He also noted the presence of micron-scale grains of baddeleyite, a highly durable zirconium-rich mineral that is often used in uranium–lead dating.