A Japanese geologist says he's discovered the oldest known bits of micrometeorites -- space dust -- to have fallen on the Earth, 240 million years ago.

Tetsuji Onoue of Kagoshima University was studying a form of microcrystalline quartz from Ajiro Island off the southern coast of Honshu, Japan, when he made the discovery, PhysOrg.com reported Tuesday.

About 30,000 tons of space dust, mostly from comets and asteroids, falls on the Earth every year but is very hard to find due to the particles' small size and the way they scatter about after falling.

To have survived intact for 240 million years, Onoue's specimens would have had to first survive being burned up completely in the atmosphere, the fate of 90 percent of space dust arriving at Earth, then would have had to fall into some sediment that would encase them and hold their delicate structures safely in place for all those thousands of centuries.

Onoue's microscopic iron rich spheroids are about 50 million years older than any other space dust ever found on Earth.

The samples will help geologists piece together the puzzle of accretion -- the addition of new material added to the surface of the Earth -- and whether there are, or were, patterns of different stuff falling over time, and if so, how much, researchers said.