  The existence of possible early oceans in the northern hemisphere of Mars has
been researched and debated for decades. The nature of the early martian
climate is still somewhat mysterious, but evidence for one or more early oceans
implies long-lasting periods of habitability. The primary evidence supporting
early oceans is a set of proposed remnant shorelines circling large fractions
of the planet. The features are thought to be older than 3.6 Ga and possibly as
old as 4 Ga, which would make them some of the oldest large-scale features
still identifiable on the surface of Mars. One question that has not been
thoroughly addressed, however, is whether shorelines this old could survive
modification and destruction processes like impact craters, tectonics,
volcanism, and hydrology in recognizable form. Here we address one of these
processes -- impact cratering -- in detail. We use standard crater counting age
models to generate synthetic, global populations of craters and intersect them
with hypothetical shorelines, tracking portions of the shoreline that are
directly impacted. The oldest shorelines (>= 4 Ga) are at least 70 % destroyed
by direct impacts. Shorelines of any age >3.6 Ga are dissected into relatively
short, discontinuous segments no larger than about 40 km when including the
effects of craters larger than 100 m in radius. When craters smaller than 500 m
in radius are excluded, surviving segment lengths can be as large as ~1000 km.
The oldest shorelines exhibit fractal structure after impacts, presenting as a
discontinuous collection of lines over a range of scales. If the features are
truly shorelines, high-resolution studies should find similar levels of
destruction and discontinuity. However, our results indicate that observing
shorelines as old as 4 Ga, should they exist, is a significant challenge and
raises questions about prior mapping efforts.
