The deserts of Earth, compared with the vast wastelands of Mars, are puny. The Sahara, the Gobi, the deserts of the western United States and the vast desert regions of Antarctica would together make up only fraction of the total area covered by these frigid Martian wastelands.
The entire northern latitudes of Mars (above roughly 45 to 50 degrees north) are girdled with a vast desert called Vastitas Borealis. This desert merges indistinguishably with other, smaller desert wastelands, Arcadia Planitia, Acidalia Planitia, and Utopia Planitia (where the second Viking Lander set down on September 3,1976).
Vastitas Borealis is noteworthy in the fact that there is almost nothing there. We find very few impact craters, almost no mountain-building appears to have gone on, and volcanos are rare. The only notable chain of mountains in this region is called Phlegra Montes. It lies in the borderland between Utopia Planitia and Arcadia Planitia, south of the Vastitas region. These deserts are relatively young. Smooth and featureless, and with very few impact craters, these deserts show no evidence of the epochs of meteoric bombardment which occurred roughly 2.5 billion years ago.
This is in stark contrast to the southern hemisphere which bears the dramatic scars of heavy meteoric bombardment from this epoch. The huge impact basins (Argyre and Hellas Planitia) remain clear and relatively uneroded, and the southern hemisphere is characterized by rugged mountain chains, thousands of impact craters, and ancient volcanos. The volcanos of the southern hemisphere are much smaller than the volcanos of the Tharsis region, but are also generally much older, though now apparently dead, and their often delicate structures have survived through many hundreds of millions, perhaps even billions of years.
The deserts of the northern hemisphere of Mars have been extensively eroded and resurfaced. These regions may once have been the bottom of a Planet-girdling shallow Martian sea, or they may have been eroded during an epoch of glaciation similar to our own Ice Ages. Another theory suggests that extensive wind erosion may have worn away whatever prominences might have been a part of these plains. The remarkable flatness of these wastelands regions may also have come about Through some other mechanisms presently unknown.
Scientists are not in agreement . The forces that gave rise to the vast northern plains are unique to Mars, and nothing analogous has occurred here on Earth. The surface of the southern hemisphere is ancient, and has been untouched by the forces that eroded and resurfaced the northern regions. Which all makes the extensive resurfacing of the northern latitudes even more mysterious. It would be as if all the continents on planet Earth were clustered together south of our equator, with a few island chains north of the equator, until we reached the temperate zones, where there was nothing but a vast northern sea (or, without our oceans, a vast desert with a general elevation lower than the elevation of the southern hemisphere).
My own suspicion is that these measureless deserts are indeed the bottoms of an ancient Martian sea. The periodic dust storms of Mars are global events, making no distinction between hemispheres. Had it been wind erosion only that gave rise to these infinite northern plains, it becomes hard to guess the mechanism that would have limited the effect to the northern latitudes. We should expect not to find the many ancient and very delicate features we do in the southern hemisphere. And glaciers leave scars. Though we find some areas of scarring in the northern hemisphere that do suggest glacial erosion, this scarring is sparse and undramatic. What we find are flat and ceaseless plains, suggesting the silting effect brought about by shallow and possibly stagnant seas.
We do find the remnants of one huge volcano, Alba Patera, which in its days of grandeur must have surpassed even Olympus Mons. It lies flaccid now, having exhausted all its magma, barely rising above the general elevation like the wasted skin of a lanced boil. Its lateral area is greater than Olympus, and when active it may have stood several times higher than Olympus does today, its caldera spewing hot lava into the freezing cold of space. Perhaps it was once a small continent rising above the shallow seas of Mars, the only feature to be seen in millions of square miles of an unnavigable ocean. But now it lies silent, barely distinguishable in its elevation from the featureless plains surrounding it.
At one time Mars generated thermal heat, as the Earth does, but this internal heating mechanism now appears to be ceased. It is possible that two or three billion years ago the internal heat was sufficient to compensate for Mars' distance from the sun, and a temperature permitting free-standing water may have obtained for many millions or even a billion years or so. But if the seas of Mars were limited to the northern latitudes, a global cycling of water as happens here on Earth may not have occurred, and the Martian seas may have been stagnant. We find no trenches no valleys or chasms, no chains of mountains similar to the Mid-Atlantic Ridge of Earth.
The flat and shallow bottoms suggest that these seas may have been untroubled by the vast and powerful currents that agitate the Earth's oceans, with the exception of those currents generated by the heat and lava flows of Alba Patera and other possible volcanic vents. But even in its volcanos, Mars differs from Earth. Volcano building on the surface of the Earth is dramatic. When the magma of the Earth's mantle breaks through to the surface, it does so in relatively short periods of time, with often spectacular displays. Volcanos have appeared on the surface of Earth, where none were in evidence before, in periods of time as short as weeks or months. But the volcano building on the surface Mars appears to have been a slow process, and the volcanos appear to result in steady pileups of lava over periods of millennia. Alba Patera and Olympus Mons though huge, may have been relatively peaceful giants, releasing their lava in steady flows spanning hundreds of millions, or even billions of years.
The image that comes to us is one of still and shallow silent seas. Because they are so tiny, the moons of Mars, Phoebos an Deimos, would have exerted only minor tidal pulls upon these Seas, and the length of the Martian year (twice the length of a year on Earth) Suggests that the annual tidal changes would have been slow and subtle. But with the gravitational pull of both the Martian moons, these tides may have been extremely complex phenomena.
Where is the water now (if water, indeed, it was)? It may have frozen, in part, into the permafrost which makes up these limitless deserts. Probably the greater part of it evaporated into space, for the gravity of Mars is weak compared to Earth's. Possibly these seas were evaporating even as they reach their greatest extent, and the cycle of evaporation, condensation and rainfall which keeps the Earth's oceans alive may never have occurred on Mars to any notable extent But this is questionable. It is also possible that this water found its way into the vents and lava tunnels of Alba Patera, and the other, younger volcanos of Mars. There, most of it would have boiled and spewed into the atmosphere as clouds of hot vapors, which gives us the image of a mighty rusted island surrounded by an eternal fog of boiling steam. But, as Alba Patera cooled, some of the water may have remained below the surface, finding its way into chasms, giving us a network of subsurface waterways, tributaries winding below the surface of Mars many dozens of miles deep, and possibly as extensive in area as the state of Texas.
When finally we stand on the surface of these vast deserts of Mars, our eye will be met with infinite monotony, a ceaseless and rusty plain beneath a featureless sky. Few clouds will punctuate this sky, and its appearance will be as monotonous as the ground at our feet. This monotony will be profound. We may find boulders, tumbled detritus from ancient flooding, perhaps some shallow effluvial valleys barely distinguishable from the general elevation, possibly dunes of rusted Martian sand, but there are no more than a half dozen or so significant craters, no chasm and even the Phlegra Montes are small and undistinguished. Otherwise we will find overwhelming sameness, an arid and desolate waste, a ceaseless an billion-year old monotony of permafrost and packed Martian dust.
I expect that Vastitas Borealis will be a profoundly humbling experience for us, even more than our experiences with Valles Marineris or Olympus Mons. These features are dramatic by virtue or their hugeness, and this hugeness is something that human beings find both humbling and inspiring. Confusing humility with modesty, we often feel proud of what we call our humility ("What a great piece of work I must be, to feel so profoundly humbled," is the unconscious thought that runs through our minds).
And the theatrical majesty of Olympus Mons resonates with our natural megalomania. We may be small, but the universe is a dramatic place, which suits our inclinations well. But the vast wastelands of Mars offer no such drama. They, more than the monstrous volcanos of Mars, are completely indifferent to any human scale of things. They are vast and they are wastelands, and they exist for no purpose, which can only emphasize the vast and fundamental purposeless of all being.