Iridium is a scarce metal on Earth, but it's relatively common
in comets and asteroids. According to the team's calculations,
iridium levels in the rocks around an asteroid impact should be
about 18,000 parts per trillion. A comet impact, meanwhile,
should leave behind only about 130 parts per trillion. That's
because comets would carry less metal, since they're mostly made
of loosely packed water ice with some rocky debris. Comets also
strike Earth at higher speeds, because of their longer orbits
around the sun. As a result, "the explosion formed by a comet is
more violent than from an asteroid, and the amount of
material—including iridium—thrown back into space is larger,"
Jorgensen said. The team found that the Greenland rocks
contained about 150 parts per trillion of iridium, supporting
the idea that comets were the main players in the Late Heavy
Bombardment. All that ice from the comet swarm then thawed to
create a global ocean more than half a mile (about a kilometer)
deep, the team calculates. The moon