������ý

Like tiny skydivers, particles of space dust use water to “parachute” safely to Earth. This helps wet particles withstand the plunge to Earth better than dry ones, meaning we may be overestimating the wetness of nearby asteroids.

The area around Earth is full of cosmic dust particles, so Earth is full of them too. “Cosmic dust is everywhere: it is on our streets, it’s in our homes, and it’s quite possibly on our clothes as well,” says at Imperial College London, who has studied these micrometeorites for decades. “You’re probably no more than a few feet away from a cosmic dust particle that parachuted through the atmosphere.”

In his latest study, Genge ran simulations showing that as it hurtles through the atmosphere, cosmic dust can reach temperatures in excess of 1000°C. Any water in the particles will boil, forming bubbles in the rock.

Popcorn puffs

“They are really sort of dramatic little particles,” says Genge. “They contain so many bubbles, they’re basically magma foams.”

These bubbles cause the dust to become bigger and less dense, slowing its descent through the atmosphere.

“As it gets hot, popcorn pops and opens. It becomes bigger and less dense, and due to this, it will slow down in the air like a feather and will not burn,” says at the University of Helsinki in Finland. “The same thing happens with the micrometeorites.”

Genge’s study found that hydrated dust particles that go through this popcorn-like process are twice as likely to survive as their drier counterparts, which melt or are incinerated in the atmosphere.

This explains why most of the unmelted micrometeorites that we find on the ground come from asteroids with water on them.

“It is clear this effect skews our sampling of micrometeorites,” says at the European Centre for Research and Education in Environmental Geosciences (CEREGE). So when researchers use micrometeorites to study asteroids near Earth, they will have to keep in mind that more wet dust doesn’t necessarily mean more wet asteroids.