By measuring the size
of the largest raindrop imprints (inset) in ash that solidified soon after an eruption 2.7 billion years ago (pocked slab, main
image) and
comparing them to the imprints made by drops
of various sizes and momentums in lab tests, the team estimates that the density
of Earth's oxygen - free atmosphere 2.7 billion years ago most likely ranged between 50 % and 108 %
of today's air and was certainly less than twice its modern density — a thickness insufficient to offset the dimness
of the
sun at the time.