This is the Valles Marineris. Named after Mariner 9, the first spacecraft to photograph it, it is up to seven kilometers deep with a maximum width of more than 200 kilometers.22 By comparison it is four times deeper, six times wider, and more than ten times longer than the Grand Canyon.23
At its eastern end Marineris curves northward toward the equator and debouches into a morass of so-called chaotic terrain—a tortured and overturned landscape of blocky remnants, valleys, and fractures that seems like one of the lower circles of Dante’s Inferno. From the northern edge of this chaotic zone emerge the deeply etched channels of Simud Vallis, Tiu Vallis, and Ares Vallis (it was in Ares Vallis that NASA’s lander Global Surveyor touched down on 4 July 1997). All of these channels are very wide and long. They run across the floor of a huge basin known as the Chryse Planitia, where they are joined by other channels, notably Kasei Vallis, which runs out of the north of the central section of the Marineris canyons and is 3,000 kilometers long.24
What is striking about the channels, geologists unanimously agree, is that they could only have been caused by floods involving prodigious quantities of water. These floods flowed from the southern hemisphere of Mars into the northern hemisphere at a very rapid rate because they were draining downhill.
A DIVIDED PLANET
One of the great mysteries of Mars is that it has two quite distinct and clearly defined areas of relief—the heavily cratered southern uplands, most of which stand at two kilometers or more above datum, and the relatively smooth and uncratered northern lowlands, most of which lie at least one kilometer below datum.25 The highland and the lowland occupy approximately a hemisphere each, but these only roughly coincide with the present northern and southern hemispheres of Mars. As geologist Peter Cattermole explains:
The “line of dichotomy” separating these two elevation zones describes a great circle inclined at approximately 35 degrees to the Martian equator.26
The main exceptions to the subdatum topography in the “low” northern hemisphere are the Elysium Bulge, entirely inside the northern hemisphere, and a large part of the Tharsis Bulge, which straddles the line of dichotomy.27 The main exceptions to the above-datum topography in the “high” hemisphere are parts of the Valles Marineris and two stupendous craters, Argyre and Hellas, caused by impacts with comets or asteroids. Argyre is three kilometers deep with a diameter of 630 kilometers. Hellas is five kilometers deep with a diameter of nearly 2,000 kilometers.28
These craters together with a third, Isidis, are the largest on Mars. But the planet also has legions of other craters with diameters of 30 kilometers or more, many of which, including one at the south pole, are real behemoths that exceed 200 kilometers in diameter.29
All in all, among tens of thousands of smaller craters down to one kilometer in diameter, a grand total of 3,305 craters wider than 30 kilometers have been counted on Mars. Of these, it is difficult to explain why 3,068, or 93 percent, lie south of the line of dichotomy; only 237 such large craters are found north of the line of dichotomy.30 Equally curious is the fact that the uncratered hemisphere is so much lower in altitude—by several kilometers—than its cratered counterpart.
The reason for this lowland-highland dichotomy, as the geologist Ronald Greely observes, “remains one of the major unsolved problems of Mars.”31 All that is certain is that at some point in its history the planet was afflicted by a cataclysm of almost unimaginable proportions. In chapter 4 we will investigate the causes and consequences of this cataclysm—which a number of scientists suspect may also have been responsible for stripping Mars of its formerly congenial atmosphere and its once abundant resources of liquid water.32
WATER, WATER EVERYWHERE
Many of the largest and most damaging Martian craters in the range of 30 kilometers and upward show unmistakable signs of having been made when the planet had a wet and warm environment. Hellas, Isidis, and Argyre in particular have low, indistinct rims and flat floors that several authorities take as evidence of formation when Mars still had a dense atmosphere, rapid erosion, and a stronger magnetic field than it does today.33 In the same way, acted upon by erosion, craters of great size on Earth “can blend into the landscape in a period of a few hundred years to such an extent as to be practically unrecognizable from the surrounding landscape.”34
Other large Martian craters, typically measuring 30 to 45 kilometers in diameter, have central peaks, somewhat like gigantic stalagmites, with pits on the summits. Ronald Greely believes that the best explanation for these is that they are “splash” craters and that “water or the atmosphere of Mars, or both, may have been responsible for the form of ejecta.”35
Planetary scientists Jay Melosh and Ann Vickery have calculated that Mars “probably had an original atmosphere with about the same surface pressure as that of the earth today, and a correspondingly higher surface temperature above the melting point of ice.”36 Their research suggests that the atmosphere was torn away by repeated asteroid impacts: “Because the gravity of Mars is so weak, it is easy for the expanding cloud of vapor from a major impact to blast all of the atmosphere in its vicinity out into space.”37
In a graphic demonstration of warmer, wetter times, one of the Mars meteorites studied by NASA actually proved to contain a few milligrams of liquid water—the droplet is now kept on display in a sealed glass vial.38 Moreover, it has been calculated that frozen “subsurface water to a depth of 200 meters may exist on Mars at present.”39 There are even hints that at sufficient depths, close to the planet’s inner layers of molten magma, there may be underground hot springs.40 Theoretically these could vent superheated steam to the surface, and in August 1980, Dr. Leonard Martin of the Lowell Observatory in Arizona reported that two successive images taken by NASA’s Viking orbiter of an area just south of the Valles Marineris did “suggest an explosive water spout or steam vent.”41 Vincent DiPietro and Gregory Molenaar carried out computer enhancement of these images. They concluded: “Not only did we confirm Dr. Martins discovery, but we also found a circular compression ring around the center column…. The size difference between the images of the two frames indicates the cloud to be rising at a velocity of over 200 feet per second.”42
The “waterspout” is a controversial matter. But the evidence that Mars possessed vast resources of flowing water in the past is not disputed by scientists and can be seen in plain view in tens of thousands of NASA images. Recently this evidence was subjected to an intensive evaluation by a team in the Exobiology Program Office at NASA. The team included Dr. David Des Marais of NASA’s Ames Research Center, Dr. Michael Carr of the U.S. Geological Survey, Dr. Michael A. Meyer of NASA HQ, and the late Dr. Carl Sagan.43 Their conclusions, which represent the concensus of scientific opinion on this subject, are quoted here at length:
One of the most puzzling aspects of Martian geology is the role that water has played in the evolution of the planet. Although liquid water is unstable at the surface under present conditions, we see abundant evidence of water erosion. The most intriguing features are large dry valleys, interpreted as having been formed by large floods. Many of the valleys start in areas of what has been termed chaotic terrain in which the ground has seemingly collapsed to form a surface of jostled and tilted blocks 1–2 kilometers below the surrounding terrain…. [In Chryse Planitia the] valleys emerge from the chaotic terrain and extend northward down the regional slope for several hundred kilometers. Several large channels to the north and east of [the Valles Marineris] converge on the Chryse basin and then continue farther north, where they merge into the low-lying northern plains. The valleys emerge full size and have few if any tributaries. They have streamlined walls, scoured floors and commonly contain teardrop-shaped islands. All these characteristics suggest that they are the result of large floods…. Although most of the floods are around the Chryse basin, they are found elsewhere … near Elysium and Hellas. Others occur in Memnonia and western Amazonis….
Other fluvial features appear to be the result of slow erosion
of running water. Branching valley networks are found throughout the heavily cratered terrain…. They resemble terrestrial river valleys in that they have tributaries and increase in size downstream…. The most plausible explanation for the valleys is that they formed by erosion of running water.44
THE SUDDEN END OF A LUSH ENVIRONMENT
Although expressed in the dry language of science, the NASA report nevertheless concerns itself with matters of great significance. It confirms not only that Mars might once have had a wet and relatively warm environment—perhaps even an environment suitable for higher life-forms—but also that this environment seems to have been suddenly swept away.
Other studies have reinforced the same general picture. The major channel system in Chryse Planitia is up to 25 kilometers wide and more than 2,000 kilometers long.45 It was made by a sudden catastrophic flood that not only shaped its sheer walls but also gouged “cavernous potholes several hundred meters deep” and carved streamlined “teardrop” islands measuring 100 kilometers from end to end.46 The flood was traveling extremely fast,
so rapidly as to provide peak discharges of millions of cubic meters per second. Even the dense atmosphere of Earth cannot provide water fast enough to yield such discharges from comparable-sized catchment areas…. Only dam bursts have yielded flows of significant macro erosion.”47
The volume of water required to cut the channels has also been estimated. It was very large; Peter Cattermole calculates that it was equivalent to a global ocean more than 50 meters deep.48 Michael Carr of the U.S. Geological Survey believes that it was equivalent to an ocean 500 meters deep.49
Another major flood took place in Ares Vallis. Photographs sent back by NASA’s Pathfinder lander module in July 1997 show that this immense channel was once filled with “thousands of feet of churning water.”50 According to Pathfinder scientist Dr. Michael Malin:
This was huge. The comparable flood on Earth would be the flood that filled the Mediterranean basin.51
Layered deposits of stratified sedimentary material of the kind laid down by the largest terrestrial lakes have been identified in many different locations on Mars. In some places these deposits are five kilometers thick—confirming not only the former existence on Mars of a dense and warm atmosphere in which water could survive in a liquid state but also that the planet’s water must have been present for an extremely long period during which Earth-like sedimentation processes occurred.52 These deductions are strengthened by the compelling evidence, touched on in the NASA report, that rivers flowed in certain regions of the planet for hundreds of millions of years.53 Moreover, “the existence of run-off channels makes it likely that at one time there was even rainfall on Mars.”54
THE SHORELINES OF CYDONIA
It is generally believed that these warm and wet conditions last prevailed billions of years ago. However, Harold Masursky of the U.S. Geological Survey has shown that there may have been liquid water on Mars “as recently as a few million years ago.”55 In the U.K., Colin Pillinger and his team have gone further. Their study of Martian meteorites demonstrates that liquid water and primitive life could have existed on the Red Planet just 600,000 years ago.56 Other researchers, whose work we will consider in chapter 4, are prepared to consider a time frame that is even more recent, with a great cataclysm striking Mars and stripping it of its atmosphere and water less than 17,000 years ago.
Specialists increasingly accept that as well as extensive lakes, “deltas and seas may once have existed on Mars.”57 David Scott of the U.S. Geological Survey has examined “meandering channels, spillways and outlets, spits, terraces, deposits and shorelines” in a number of basins in Elysium, Amazonis, Utopia, Isidis, and Chryse, which he attributes to the presence of former lakes and seas. The Elysium basin, he believes, was once filled with water to a depth of 1,500 meters.58 Likewise Vic Baker and scientists at the University of Arizona suggest that a great ocean once covered much of the northern hemisphere59 and support their theory with evidence of ancient shorelines in the low-lying northern plains.60
Such features have been identified at latitude 41 degrees north, longitude 9 degrees west,61 close by the so-called pyramids and Face of Mars in the Cydonia region. According to environmental geologist James L. Erjavec, this region, which lies to the northeast of Chryse Planitia, contains
areas that look like they’re shoreline features, areas where there’s erosion, where landslides would occur at the edge of a shoreline, where there may be some erosion of material down below the base of the cliff and sediment has poured into it. Certain erosion features surely indicate that water may have been here in a sizeable quantity. As to what time in Martian history, that still remains to be seen.62
The surface of Mars is a palimpsest inscribed with layer upon layer of mysteries. Amid these layers is written the story of the death of a world. It may not have been billions of years in the past, and the fate that afflicted Mars may not have entirely bypassed Earth.
4
The Janus Planet
MARS is a planet of many mysteries, its history only guessed at, its true significance in the solar system as yet unknown. All that is certain is that it was once vibrant with rain and rivers, lakes and oceans, and that it is now barren and dead.
It is the scientific consensus that Mars was killed—executed would not be too strong a word—by a stupendous bombardment of asteroids or comets. Thousands of huge craters pockmarking its tortured surface are the silent witnesses to this. And it is thought likely that the same bombardment also caused the cataclysmic floods (described in chapter 3) and then stripped away the planet’s formerly dense atmosphere so that liquid water could no longer survive anywhere upon it.1
What kind of event could this have been? And what does it say about the nature of the universe in which we live—perhaps even about the predicament of Earth itself—that Mars was so completely rubbed out when it was in its prime?
CLUES FROM THE BODY
We are looking at a murder victim. All we have are photographs and measurements of the corpse and the results of certain scientific tests that have been done on it. These tell us a number of curious things about Mars.
Item 1: Its orbit is highly eccentric and elliptical, following a course that brings it close to the Sun and then very far away from it every year.2
Item 2: Its rate of rotation is much slower than it should be.
Item 3: It has almost no magnetic field.
Item 4: Over long periods of time its north-south spin axis seesaws wildly in space, radically changing the angle at which the planet is oriented toward the Sun.
Item 5: There is evidence that the Martian crust may have slipped in one piece around the inner layers of the planet on several occasions in the past—causing landmasses at the poles to be shifted into equatorial zones and vice versa.
Item 6: The vast majority of Martian impact craters, far more than should be the case statistically, are clustered in the hemisphere south of the so-called line of dichotomy (discussed in chapter 3).
Item 7: The northern hemisphere shows only light crater damage and is a vast basin, three kilometers lower in altitude than the south.
Item 8: The line of dichotomy between north and south is physically marked on the surface of Mars by the sheer edge of the upland escarpment. This unique feature runs all the way around the planet in a great ragged circle that crosses the equator at an angle of about 35 degrees.
Item 9: Also unique to Mars is the tremendous chasm of the Valles Marineris—seven kilometers deep, 4,000 kilometers long—that has been torn in its surface.
Item 10: Last but not least there are Hellas, Isidis, and Argyre, the deepest and widest craters in the solar system, weirdly compensated on the other side of Mars by the Elysium Bulge and by the immense Tharsis Bulge—from the eastern edge of which the Valles Marineris bursts forth.
IMPACTS
Let us start with the mystery of the dichotomy. Geologists admit that “despite an ever-increasing awareness of its importan
ce, manifested in intensive research into its nature, mode, and age of formation, there is still no firmly held hypothesis to account for it.”3
A few scientists favor purely internal, geological process,4 but the majority agree with William K. Hartmann, writing in the Scientific American in January 1977, who pointed out that “an asteroid 1,000 kilometers across striking a primordial planet could have given rise to a fundamental asymmetry in the planet, perhaps by knocking the crust off one side…. [This] kind of collision might be involved in the asymmetry of Mars where one hemisphere has many ancient craters and the other has been almost entirely modified by volcanism.”5
Since the Martian hemisphere lying north of the line of dichotomy has a lower altitude than the southern hemisphere, the automatic assumption has been that the northern hemisphere was struck and lost the outer layer of its crust. The only serious dispute is whether the dichotomy was produced by multiple large impacts in the north6 or by a “single mega impact,”7 although both theories present an essentially similar picture of collisions big enough to excavate a basin across an entire Martian hemisphere. Both also assume that there was a time when the north of Mars had a roughly equal number of craters to the south. Then it is supposed that a freak additional bombardment by asteroids (or by one mega asteroid) occurred, for some reason falling only on the north, breaking through its crust, lowering its altitude, and obliterating its preexisting craters. Next, fresh lava welled up from the planet’s interior and poured out over the flayed northern hemisphere, covering its wounds and effectively resurfacing it. Subsequently, although occasional asteroids have continued to strike, collisions have become much less frequent and neither hemisphere has experienced any further episodes of intense bombardment.