Ocean Origins on Earth - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 30

followed, high numbers of large asteroids and comets continued to
fall out of the sky and impact upon the Earth. (Some still do, of course,
although much more rarely now.)
This planet has lost almost all evidence of those early violent events.
The continual renewing of the Earth's surface, thanks to plate tecton-
ics, has erased those enormous early craters. Evidence of that bom-
bardment can be seen on the face of the Moon, where the absence of
air, water, and plate tectonics has left the impact craters in almost
pristine condition. All that has modified them are more recent impacts
and, in places, the basalt lava that has oozed out to form the dark
'seas' that partly cover them. The surface is so heavily cratered that it
is difficult to extract a clear history from it (Fig. 4). However, the Moon
has recently been mapped in great detail using lasers by an orbiting
NASA instrument rather fetchingly called 'LOLA' (for Lunar Orbiter
Laser Altimeter). The results showed that the ancient highlands
were hit by a range of meteorites, from small to very large, while
the meteorites that impacted the main basalt 'mare' (that are about
3.6 billion years old) were small to medium sized.
The Earth must have been yet more fiercely pelted, being both a
larger target and having a much stronger gravitational field to draw
in meteorites. Several times over the first half-billion years of the
Earth's history there must have been meteorite impacts large
enough to vapourize all of the Earth's oceans, and sterilize any life
that had managed to evolve within them. Could anything survive at
all? Perhaps—even if all the oceans did repeatedly boil away. Today,
microbes extend down into a 'deep biosphere', inhabiting tiny rock
fractures to depths of a kilometre or more. If that was the case in the
Hadean, then maybe there could be survivors (at least of the less
cataclysmic of the large impacts), to emerge to recolonize the newly
condensed oceans, as the Earth cooled over the millennia following
a major impact. 28