  The Late Heavy Bombardment (LHB) period is the narrow time interval between
3.8 and 3.9 Gyr ago, where the bulk of the craters we see on the Moon formed.
Even more craters formed on the Earth.
  During a field expedition to the 3.8 Gyr old Isua greenstone belt in
Greenland, we sampled three types of metasedimentary rocks, that contain direct
traces of the LHB impactors by a seven times enrichment (150 ppt) in iridium
compared to present day ocean crust (20 ppt).
  We show that this enrichment is in agreement with the lunar cratering rate,
providing the impactors were comets, but not if they were asteroids. Our study
is a first direct indication of the nature of the LHB impactors, and the first
to find an agreement between the LHB lunar cratering rate and the Earth's early
geochemical record (and the corresponding lunar record).
  The LHB comets that delivered the iridium we see at Isua will at the same
time have delivered the equivalent of a km deep ocean, and we explain why one
should expect a cometary ocean to become roughly the size of the Earth's
present-day ocean, not only in terms of depth but also in terms of the surface
area it covers.
