As the cometary "grit" from Swift-Tuttle strikes our atmosphere, it burns up, often creating streaks of light across the sky.

BBC: Perseid meteor show reaches peak

They were all blasted off the Red Planet by some asteroid or cometary impact, and then spent millions of years travelling through space before falling to Earth.

BBC: Mars meteorite 'Black Beauty' is in a class of its own

There might be even more: the measurements probe only the top half-metre of regolith, but it is thought that cometary bombardment could have left ice in the top two metres.

ECONOMIST: At least lunar gold-diggers will be able to take a shower