Calculations
indicate that it could take hundreds of millions of years for an
object a few tens of meters across to vanish by this process
beneath the dust and regolith [10,11]. However, gardening
also serves to re-expose buried objects over a long period of
time. Third, the moon is tectonically inactive, so any activity
caused by an artifact (heat, strong magnetic field, radio-
activity, etc.) will show up clearly, rather than being hidden
by other processes suchasmightoccuronEarth.Thus Any
messages or traces of technological activity left on the moon
will likely still be around today, and should be possible to
locate without the vast expense that would go into searching
more distant bodies, such as Mars. The question, then, is
what forms these artifacts might take, and why they might
have been left.
There are many conceivable reasons that aliens would
leave artifacts on the moon, including some that humans
may not even be able to guess at. If for the sake of argument
we assume an alien species with roughly human-like reason-
ing and motivation, there are four main classes of artifacts or
technological modifications that we should consider