Micrometeorites from the deep sea and the deserts
The first micrometeorites ever recognised came from the bottom of the ocean. The deep-sea floor and the world's driest deserts are still two of the best places to find them, because both gather dust over very long periods.
On the deep ocean floor, mud builds up so slowly that space dust makes up a noticeable share of it. Scientists drag magnets across the seabed or study cores of mud to find black, magnetic spherules. In deserts, the wind blows away fine sand and leaves heavier particles behind, including micrometeorites that may have lain there for thousands of years.
Key facts
- First recognised
- In red clay dredged by the Challenger expedition of 1872–1876
- Why the deep sea works
- Red clay builds up at about a millimetre per thousand years
- Typical finds
- Cosmic spherules, especially iron-rich I-types
- Why deserts work
- Wind removes fine grains, leaving dense particles concentrated on old surfaces
A discovery from the Challenger expedition
Between 1872 and 1876 the British ship HMS Challenger sailed around the world on the first great scientific survey of the oceans, dredging mud from the deep-sea floor. In the reddish-brown clay from the deepest parts of the ocean, one of the expedition's naturalists, John Murray, and the Belgian geologist Alphonse Renard found tiny black magnetic spheres, some with a bright metallic core, and brown spheres with a crystalline texture like the chondrules in meteorites.
In reports published in the 1880s and 1890s they argued that the spheres were cosmic. They were found far from any volcano or land, they did not resemble the spheres produced in furnaces, and their metal cores contained nickel, like meteoritic iron and unlike volcanic iron. It was the first scientific identification of micrometeorites, though the name came much later.
Why red clay is rich in spherules
In the deepest parts of the ocean, below about 4 to 5 km, the shells of tiny sea creatures dissolve before they reach the bottom, and very little sediment arrives from the land. The red clay that forms there builds up at only about a millimetre per thousand years, sometimes less. Space dust keeps falling at its steady rate, so in such slowly forming mud it becomes relatively concentrated.
Magnetic rakes and sediment cores
In the 20th century, researchers dragged magnets along the seabed to pick out magnetic particles. In 1979 one such magnetic sled, towed across the Pacific floor, recovered thousands of cosmic spherules from the top few centimetres of red clay. Today scientists also study cores of sediment drilled from the seabed, which preserve a record of dust falling over millions of years, layer by layer.
Sediment cores do not need visible particles to tell the story. Space dust carries helium-3, a rare form of helium implanted by the solar wind, in far higher concentration than Earth rocks. Measuring helium-3 down a core reveals how the dust supply has changed through time. See fossil micrometeorites.
What sea water does
Deep-sea micrometeorites can be very old, but they are weathered. Glass dissolves, olivine is eaten away or replaced by clay, and fragile unmelted particles are mostly lost. Many old stony spherules end up as shells of magnetite with hollow cores. I-type spherules, made of iron oxides, survive best, and because they are also the easiest to pull out with magnets, they make up a much larger share of deep-sea collections than they do of the dust that actually falls.
Deserts
In very dry deserts, wind slowly removes fine dust and sand from flat, stable surfaces and leaves denser grains behind, a process called deflation. On surfaces that have been undisturbed for a very long time, micrometeorites accumulate along with the heavy minerals. Researchers have recovered cosmic spherules from desert surfaces and sand traps on several continents. The same dryness preserves them better than in wetter places, though desert particles are still more weathered than those from polar ice.
Deserts are also some of the richest hunting grounds for ordinary meteorites, for the same reasons: old surfaces, little rain and few Earth rocks of similar appearance.
Other natural traps
Anywhere material is sorted by weight can concentrate micrometeorites: beach sands where waves have left dark, heavy minerals behind, the sediment at the bottom of lakes, and soils that have been forming undisturbed for a long time. Ancient rocks hold them too, especially salt deposits and limestone, which can be dissolved away to free the particles inside. The oldest known micrometeorites were found in limestone.
Did you know?
Samples of the Challenger expedition's deep-sea clay are still kept in museum collections, more than 150 years after they were dredged.
Micrometeorites in red clay are mixed with sharks' teeth and the ear bones of whales, which also survive for a very long time on the deep-sea floor.
Helium-3 in deep-sea mud can be used as a clock: if the dust supply is steady, measuring it tells geologists how fast the sediment built up.