Fastest moving dust on Earth
They found S type (silicate-dominated) cosmic spherules which were melted into disk and other non-spherical shapes — an effect of the extreme temperatures they experienced during atmospheric entry. Cosmic dust specks are usually incredibly tiny, measuring around 0.01 millimeters (0.003 inches) in size, but the ones the team found were larger, measuring about 0.03 millimeters. Based on the shape and size, Genge believes they fell to Earth with speeds around 12 km/s (7.5 miles/s), which would make them the fastest-ever dust particles on Earth.
While they’re the fastest, they’re probably not the oldest dust specks we’ve seen. The crystal structures found in these samples resemble those of particles dating from medieval times — by contrast, older samples that date back millions of years which were found in Antarctica show a different crystal make-up. Exactly why these differences arose is still unknown, but the team speculates it’s the effect of planetary orbit changes in the Solar System. Over millions of years, gravitational fluctuations cause planets to shift their orbits around the sun slightly, which in turn affects their gravitational effect on the matter around them.
They could in fact be the most recently-crashed bits of cosmic dust on Earth. Since the rooftops of commercial buildings are cleaned regularly and there was little rusting of the dust’s metallic content (which only started after they got to Earth,) the team estimates that they fell down sometime within the past six years.
The team points out that while they found these particles only in cities, they could be found anywhere on the planet. And the more of it we can collect and analyze, the more we’ll understand about how the Solar System and Earth evolved from birth all the way up to today.
“This find is important because if we are to look at fossil cosmic dust collected from ancient rocks to reconstruct a geological history of our Solar System, then we need to understand how this dust is changed by the continuous pull of the planets,” says Genge.
“The obvious advantage to this new approach is that it is much easier to source cosmic dust particles if they are in our backyards.”
The full paper “An urban collection of modern-day large micrometeorites: Evidence for variations in the extraterrestrial dust flux through the Quaternary” has been published in the journal Geology.