Oceans of the Solar System - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 225

detected yet); if so, this is the most likely habitat for any simple life
forms that may exist today on Mars.
There is also no doubt that a lot of water has flowed across the sur-
face, in the geological past, in the first billion years or so of Mars's
history (see Plate 6). The evidence for that has become ever clearer as
the images from the successive space missions have accumulated
(three are currently in orbit: the Mars Reconnaissance Orbiter , the Mars
Express , and the Mars Odyssey —the last of these already more than a
decade in harness and still sending back data). There is now a cornu-
copia of different kinds of imagery: photographic, infrared, radar,
gamma ray, and more. Much of it is freely available, so anyone with a
computer and Internet connection can fly virtually across the surface
of Mars and gaze down. What does one see?
The evidence of a more dynamic past is clear. The planet has a great
divide. Its southern hemisphere is dominated by rugged highlands—
within which the greatest mountain in all the solar system, Olympus
Mons, rises to over 23,700 metres. By contrast, the north forms a
smooth low-lying plain, much of which is extraordinarily flat. This
contrast, referred to as the 'Martian dichotomy', is probably very old.
The lowlands may have formed by the titanic impact of a Pluto-sized
object more than 4 billion years ago, during the violent early history
of our solar system. 129 The border zone between these provinces, and
the adjacent parts of the highlands, are traversed by canyon- and val-
ley-like structures, large and small, which grade down towards the
northern plain. In places, these valleys end in delta-like structures, the
surfaces of which have small distributory channels on them.
They look like the dried-up remnants of rivers flowing down to a
sea. And, indeed, the northern lowlands have been interpreted as the
remains of a substantial northern ocean, christened the Oceanus
Borealis. Such an ocean, in the first billion years of the history of
Mars, could have driven a hydrological cycle that created the rain