Entry and survival: how a speck of dust reaches the ground
A micrometeoroid hits the top of the atmosphere at more than 11 km/s. Within a few seconds it is braked, heated and either vaporised, melted or, if it is lucky, left almost untouched.
Big meteoroids glow and blast apart because they cannot get rid of heat fast enough. Tiny grains are different: they have a lot of surface for their weight, so they radiate heat away quickly and slow down high in the air. Whether a grain survives depends mainly on its speed, its size and the angle at which it comes in. Slow, small grains arriving at a shallow angle survive best.
Key facts
- Entry speed
- At least 11.2 km/s; most survivors probably arrived at about 12–16 km/s
- Where the heating happens
- Mostly between about 75 and 100 km up
- How long it lasts
- A few seconds of strong heating
- Melting starts
- When the grain passes roughly 1,300 °C
Why speed matters so much
Nothing can hit the Earth slower than 11.2 km/s, the speed an object gains by falling in from far away under the pull of Earth's gravity. Dust that is overtaking the Earth gently may arrive close to that minimum; dust meeting the Earth head-on, such as grains from long-period comets, can arrive at up to 72 km/s.
The energy of motion grows with the square of speed, so a grain at 22 km/s carries four times the energy of one at 11 km/s. Much of that energy turns into heat as the grain ploughs into the air. Fast grains are almost always vaporised, which is why most micrometeorites on the ground are thought to come from slow-moving dust.
Why small grains survive
In 1950 the astronomer Fred Whipple showed why small particles can reach the ground. Heating happens at a particle's surface, as it collides with air molecules, and a particle can lose heat by glowing, which also happens at the surface. A tiny grain has a large surface compared with its weight, so it sheds heat quickly and is braked by the air within a few seconds, high in the atmosphere, where the air is extremely thin. A large meteoroid, by contrast, reaches much denser air before it slows, and its surface melts and streams away in the process called ablation.
The strongest heating usually happens between about 75 and 100 km up and lasts only a few seconds. After that, the grain has lost almost all its cosmic speed and falls gently, taking from hours to weeks to reach the ground, depending on its size and the weather.
Four possible fates
What happens in those seconds depends on the grain's speed, size, density, composition and entry angle. A grain coming in at a shallow, grazing angle is braked over a longer path and stays cooler than one diving steeply. The result is a sequence from cool to hot:
- Unmelted: the grain warms but never melts. It keeps its original minerals, though water-bearing clays may dry out and a thin rim of magnetite often forms on its surface.
- Partly melted (scoriaceous): the grain begins to melt and gases such as water vapour, sulphur and carbon compounds bubble out, leaving a frothy, cinder-like particle.
- Fully melted (cosmic spherule): the whole grain becomes a droplet, surface tension pulls it into a sphere, and it freezes again in a fraction of a second.
- Vaporised: the grain turns entirely to vapour. This is the fate of most of the incoming mass, especially the fast grains.
Chemistry in the fireball
Melting changes a grain's chemistry. The most volatile elements, such as sodium and potassium, evaporate first, followed by iron, silicon and magnesium. The most strongly heated spherules lose so much that they are left enriched in the elements that are hardest to evaporate: calcium, aluminium and titanium. Iron metal melts and reacts with oxygen in the air, turning into the iron oxides magnetite and wüstite, and the rare platinum-group metals it contained are squeezed together into a nugget that can be smaller than a micrometre.
What the vapour does
The vaporised dust does not vanish. Its atoms form permanent thin layers of metals, such as sodium, iron, magnesium and calcium, between about 80 and 105 km up. Large telescopes use the sodium layer to make artificial guide stars: they shine a laser into it to make it glow, then measure how the air blurs that glowing spot so that the blurring can be corrected.
The metal atoms also join together into extremely fine particles called meteoric smoke, only a few millionths of a millimetre across. These are thought to help seed the high, shining noctilucent clouds seen on summer nights, and they slowly drift down to the poles and into the ice.
On other worlds
On the Moon, with no air at all, micrometeoroids strike the surface at full speed, melting and vaporising rock and slowly gardening and darkening the soil. On Mars, the thin air means that more grains survive unmelted than on Earth, over a wider range of sizes.
Did you know?
Fred Whipple also designed a way to protect spacecraft from dust: a thin outer sheet that shatters an incoming grain before it reaches the main wall. Shields like this still protect the International Space Station.
A cosmic spherule can melt and freeze again in less than a few seconds, which is why many contain crystals that grew extremely fast, shaped like bars, feathers and branches.
Some unmelted micrometeorites still contain clay minerals that formed when water soaked through their parent body long ago. They survived the fiery entry because the heating was so brief.