No. 117: May-June 1998
Mars has two puzzling "faces": (1) That human-like visage at Cydonia; and (2) The whole northern hemisphere or "face." The latter is definitely real and consists of an immense, low-lying plain centered roughly on the planet's North Pole. The opposite face of Mars is occupied by rough, cratered highlands. This sharp, profound crustal dichotomy has been known for many years and has resisted explanation.
In SF#113, T. Van Flandern advanced the theory that the rugged southern highlands are composed of the debris from an exploded planet which Mars once attended as a satellite. Be that as it may, there is something puzzling about the northern plains.
We now have information from the Mars Global Surveyor (MGS), now orbitting the red planet, that the low-lying northern plains are much flatter than thought. For thousands of kilometers, they are smooth on a scale of hundreds of meters. This is flatter than the lava flows of the lunar maria; flatter than the smoothest central Sahara. These startling data come from the MGS's laser altimeter that can measure elevation of the terrain below it with 10meter accuracy averaged over the beam width of 150 meters.
The only known terrain in the entire solar system that can match this flatness is the abyssal, sediment-filled floor of the South Atlantic. Hmmm! Does this imply that the northern plains of Mars were once an ocean floor? That's the favorite interpretation today. Of course, they might also be lava plains created by a colossal ancient impact, made smoother by blown dust.
(Kerr, Richard A.; "Surveyor Shows the Flat Face of Mars," Science, 279:1634, 1998.)