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For some time, orbital simulations have predicted that fragments produced in very large collisions between asteroids may arrive on Earth considerably faster than the present meteorite transit rate. Now, a group of scientists in Zurich have offered evidence that may verify the predictions made by the simulation models.
The team analysed fossil meteorites discovered in limestone beds in Sweden. The beds, and therefore the meteorites they contain, were deposited during the Ordovician period, 480 million years ago. By producing extremely sensitive measurements of noble gases in chromite grains, the scientists were able to determine the period of time these fragments spent in space before impacting on Earth. This is because certain isotopes of some noble gases are created by cosmic ray bombardment, during the time between the large collision event, and the meteorite finally falling to Earth.
The team determined transit periods as low as 100,000 years, compared to present transit times of between 3 million to 60 million years. These data provide strong evidence that at least one route exists within the main asteroid belt that is capable of delivering meteorites into the inner Solar System within the short time periods predicted by simulations.
The fossil meteorites also have transit times that decrease with depth in the sediments; the rock beds in question being deposited over a period of around 2 million years. This supports the hypothesis that the meteorites are all derived from one parent body. In this scenario, the fragments with the shortest cosmic exposure ages impacted Earth first, and were thus incorporated into older rock beds. It follows, if the meteorites originate from the same collision event, that those with longer cosmic exposure times fell to Earth later and were thus incorporated into younger rock beds.
Further evidence points to the parent body of these meteorites being the precursor of a group of small fragments known as the Flora family, which orbits near 2.2 astronomical units from the Sun (An astronomical unit is the average distance between the Sun and Earth).
More info: Nature
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