Just over a million years ago, an asteroid or comet hit Mars with such force that it threw many rocks into space. Some of these eventually collided with Earth, surviving the trip through the atmosphere. So far, we have found 11 of these, with presumably more to come. These objects, among the class of meteorites known as shergottites, have given us important insights into Martian geology, since we can examine them with equipment far more sophisticated than what the rovers currently carry on Mars.
Similarities in their composition indicate these 11 shergottites come from the same region of Mars, while cosmic radiation has left traces on these rocks, allowing us to confirm they were all hurled into space at the same time approximately 1.1 million years ago. Nevertheless, they were actually formed far earlier and at different dates from each other. Ten of the meteorites from this impact have been dated as forming between 327 million and 574 million years ago. However, a paper in Science Advances has revealed that the 11th is much older, with an estimated date of 2.4 billion years, making it an exceptionally valuable time capsule of Martian geology.
The meteorite in question is named NWA 7635, not after the hip-hop group, but because it came to Earth in Northwest Africa. At just 196 grams (7 ounces), it's hardly imposing, but first author Professor Thomas Lapen of the University of Houston shaved off two grams for analysis of the array of elements within.
Using the known rate of decay of the rare Earth isotope 147 samarium to 143 neodymium, Lapen calculated that the crystals in the rock formed 2.4 billion years ago, placing it in what is known as Mars' early Amazonian age. In the context of the other, much younger meteorites thrown up at the time, this means that volcanic activity at this one location lasted at least 2 billion years.
Moreover, similarities between the shergottites blasted into space in this impact and other Martian meteorites indicate all were formed from the same, very long-lasting mantle plume. Each carry a fingerprint that can be distinguised from the plumes that caused the formation of other classes of Martian rocks we have had the chance to examine.
“The big discovery is that we have confirmed the long lived nature of volcanoes on Mars,” Lappin said in a statement. This keeps with recent evidence that the giant volcanoes of Elysium and Tharsis have erupted over periods that span 3 billion years. In contrast, volcanic activity on Earth moves around with continental drift, so no site stays active for that long.
Only around 0.2 percent of known meteorites come from Mars, but their scientific value is one of the reasons scientists are constantly seeking new rocks from space.