Earth Sweeps Up 5,200 Tons of Dust a Year

[Mercury]

by Sid Perkins

These micrometeorites come from both comets and asteroids.

Link: Science News

[neufer]

by Sid Perkins

These micrometeorites come from both comets and asteroids.

Link: Science News

Earth Sweeps Up an estimated 40,000 ± 20,000 tonnes per year (t/yr) of cosmic dust enters the upper atmosphere each year of which less than 10% (2700 ± 1400 t/yr) is estimated to reach the surface as particles.

This cosmic dust comes from asteroids, comets, Mars and our Moon.

<<Micrometeoroids are very small pieces of rock or metal broken off from larger chunks of rock and debris often dating back to the birth of the Solar System. Micrometeoroids are extremely common in space. Tiny particles are a major contributor to space weathering processes. When they hit the surface of the Moon, or any airless body (Mercury, the asteroids, etc.), the resulting melting and vaporization causes darkening and other optical changes in the regolith.

Sufficiently small micrometeoroids avoid significant heating on entry into the Earth's atmosphere. A micrometeorite (MM) is a micrometeoroid that has survived entry through the Earth's atmosphere. The IAU officially defines meteorites as 30 micrometers to 1 meter; micrometeorites are the small end of the range (~submillimeter). They are a subset of cosmic dust, which also includes the smaller interplanetary dust particles (10 to 300 micrometers) that cause the zodiacal light. According to Nesvorný and Jenniskens, when the dust grains are as small as about 150 micrometres in size, they will hit the Earth at an average speed of 14.5 km/s, many as slowly as 12 km/s. If so, they pointed out, this comet dust can survive entry in partially molten form, accounting for the unusual attributes of the micrometeorites collected in Antarctica.

An estimated 40,000 ± 20,000 tonnes per year (t/yr) of cosmic dust enters the upper atmosphere each year of which less than 10% (2700 ± 1400 t/yr) is estimated to reach the surface as particles. Therefore, the mass of micrometeorites deposited is roughly 50 times higher than that estimated for meteorites, which represent approximately 50 t/yr, and the huge number of particles entering the atmosphere each year (~10¹⁷ > 10 µm) suggests that large MM collections contain particles from all dust producing objects in the Solar System including asteroids, comets, and fragments from our Moon and Mars.

Fewer than 1% of MMs are achondritic and are similar to HED meteorites, which are thought to be from the asteroid, 4 Vesta. Most MMs are compositionally similar to carbonaceous chondrites, whereas approximately 3% of meteorites are of this type. The dominance of carbonaceous chondrite-like MMs and their low abundance in meteorite collections suggests that most MMs derive from sources different than those for most meteorites.

The influx of micrometeoroids also contributes to the composition of regolith (planetary/lunar soil) on other bodies in the Solar System. Mars has an estimated annual micrometeoroid influx of between 2,700 and 59,000 t/yr. This contributes to about 1m of micrometeoritic content to the depth of the Martian regolith every billion years. Measurements from the Viking program indicate that the Martian regolith is composed of 60% basaltic rock and 40% rock of meteoritic origin. The lower-density Martian atmosphere allows much larger particles than on Earth to survive the passage through to the surface, largely unaltered until impact. While on Earth particles that survive entry typically have undergone significant transformation, a significant fraction of particles entering the Martian atmosphere throughout the 60 to 1200-μm diameter range probably survive unmelted.>>