One important question is sidestepped by both the impact theories: What has happened to the immense volumes of crust, three kilometers deep, that appear to have been scalped from the northern hemisphere? Scientists have calculated that this crustal material would be too massive simply to have eroded away, even over billions of years. As Michael Carr of the U.S. Geological Survey has observed:
The precise mechanism whereby the former ancient crust has been so extensively destroyed in the northern hemisphere is poorly understood…. Erosion alone cannot explain [its] disappearance … for there is no sink of sufficient size to accommodate the debris.8
The impact theories are also weakened because they call for a freak additional bombardment in the north but are unable to describe any mechanism that would persuasively account for such a bombardment. The best suggestion is that the material flung at Mars was drawn across its orbit because of “perturbations and collisions of bodies in the asteroid belt” probably caused by the atmospheric attraction of Jupiter.9 But critics say that such perturbations and collisions could not possibly have ejected enough material from the asteroid belt to inflict all the damage that is visible on Mars. Nor is it clear why the damage should have been focused on one hemisphere—the north—with such ferocity that its crust could have been stripped away to a depth of three kilometers. As critics have pointed out:
Any attempt to explain the dichotomy via impact depends on a statistical clustering of impacts in the northern lowland…. Unless impacts are significantly more numerous within the lowland than elsewhere, there simply is no reason to expect that the lowland will differ in any way from the rest of the planet.10
So could Mars have been hit by “significantly more numerous” impacts in the north than in the south?
There are some who suggest that everything might have been the other way around.
ASTRA
It is the consensus of astronomers that collisions between asteroids and the planets were frequent in the early history of the solar system and have been steadily declining ever since at a uniform and predictable rate. “On any given planet,” as a result, it is assumed that “the relative ages are clear, since heavily cratered areas are older than sparsely cratered ones.”11 It is for this reason that the heavily cratered southern highlands of Mars are always referred to as “older” than the “recently resurfaced” plains of the north.12
Geographer Donald W. Patten and engineer Samuel L. Windsor have other ideas. They argue that it was not the northern hemisphere of Mars that was the victim of a “freak additional bombardment” (as all other scholars have suggested) but the southern hemisphere. They say that this additional shower of cosmic debris is the only reason why the southern hemisphere is more heavily cratered than the north; that is, its surface is not older than the northern plains. And, though they do not make the connection themselves, their findings raise an intriguing possibility: The loss of the northern crust might not have resulted from direct impacts anywhere in the north but could instead have been a “domino” effect from devastating impacts in the south.13
At present there are nine planets in the solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Patten and Windsor’s theory is that there was once also a small tenth planet, orbiting between Mars and Jupiter—in the area where the asteroid belt is now found—and that it came into a collision course with Mars. They name this hypothetical planet Astra and believe that it was drawn toward Mars like a moth to a flame and then destroyed as it entered the larger planet’s “Roche limit,” a technical term used by astronomers for
the zone which surrounds any large object of appreciable mass producing a gravitational field at a distance of 2 to 3 radii of the object concerned. In effect it is a danger zone, and any object with a smaller mass or weaker gravitational field entering it will be either swiftly expelled from it electromagnetically, or, more commonly, be subjected to intolerable tidal stress and disintegrated.14
The Roche limit is a magical thing, an invisible force field. If its Roche limit is breached, a planet can be expected to defend itself—reaching out, almost like a living being, to destroy the intruder. When this happens the defending planet will suffer serious and perhaps irreversible damage from thousands of fragments of the invader, some very large, that rain down on it. But such damage is likely to be less severe than it would be if there was an actual collision between two intact bodies of planetary size.
Patten and Windsor believe that Astra approached to within 5,000 kilometers of Mars, well inside its Roche limit, and was then torn apart by gravitational and electromagnetic forces—splattering the Martian hemisphere that was facing it with a sudden burst of high-speed projectiles all coming in from the same direction at the same time. The two researchers find plentiful evidence for such an explosion over the southern hemisphere of Mars, pointing out that there is
an abrupt edge, or rim, for a dramatic drop-off in the density of craters on Mars. That rim [the line of dichotomy] is “where the buckshot ends.” It is where the Red Planet’s serene [northern] hemisphere begins. This rim is obvious to anyone who is thinking of fragmentation on the Roche limit of Mars. So far, astronomers who fail to think of planetary catastrophism also have failed to see the obvious. The rim rises farthest north on Mars in its northwest quadrant, at latitude 40 degrees north and at longitude 320 degrees west…. The southerly extremity of the rim is at latitude 42 degrees south and at longitude 110 degrees west. The rim of craters is not hard to identify if it is expected or anticipated. It is there as it ought to be if Mars experienced a sudden, intense, 15-minute blizzard of fragments bombarding it on one side only.15
Very like those who propose selective bombardment of the north, the two researchers’ weakest point is that they do not suggest a convincing mechanism that could have put their hypothetical tenth planet, Astra, on a collision course with Mars. Their ideas on this matter rest, in essence, on the belief that the solar system only recently organized itself into its present form and that the orbits of the planets were previously very different.16
That few scholars would agree with this aspect of Patten and Windsors hypothesis does not necessarily mean that they are wrong. Furthermore, even if they are wildly wrong about the mechanism, they still may be one hundred percent right about other things.
They could, for example, be right about the existence of Astra, or something like it. Certainly there is no objection in principle to the notion of an exploded tenth planet as the source of the countless thousands of rocky missiles, some large, some small, that orbit in the asteroid belt between Mars and Jupiter. Indeed, as long ago as 1978 the astronomer Tom Van Flandern of the U.S. Naval Observatory in Washington, D.C., made exactly this case in the planetary science journal Icarus.17 Although admitting that he could think of no means whereby a planet could blow up, he presented persuasive evidence that a tenth planet between Mars and Jupiter could indeed have been destroyed—he thought about five million years ago—and could have been the source not only of the asteroid belt but also of comets entering the inner solar system.18
Patten and Windsor’s other central idea is of a massive bombardment selectively focused on southern Mars. At the very least, this is no more inherently improbable than the widely accepted notion of a “statistical clustering of impacts” in the northern hemisphere. Moreover a growing body of evidence suggests that the south may indeed have been the target of just such a bombardment.
KILLER PROJECTILES
Hellas, Isidis, and Argyre, the three largest impact craters in the solar system, all lie south of the line of dichotomy.
Centered at 295 W, 40 S, Hellas is an elliptical basin, five kilometers deep, measuring 1,600 kilometers by 2,000 kilometers, so massive that even the ramparts of its rim are 400 kilometers wide.19 According to Patten and Windsor’s calculations, this immense crater formed as a result of an impact with an object measuring 1,000 kilometers in diameter20—“as big as Alaska with Washington and half of Oregon thrown in,
twice as big as Texas, bigger than most of western Europe.”21
The Isidis crater measures 1,000 kilometers across and was made, say Patten and Windsor, by an object 600 kilometers wide. Argyre has a diameter of 630 kilometers and was made by an object 360 kilometers wide.22
In Patten and Windsor’s reconstruction Hellas was the first of the three killer projectiles to reach Mars, screaming down through the atmosphere at a speed of 40,000 kilometers per hour toward a bull’s-eye point in the center of the hemisphere south of the line of dichotomy:
The Hellas fragment hit the crust of Mars a direct blow, from almost vertical. It passed into the internal magma of Mars, creating enormous pressure waves and shear waves. The Hellas fragment did not pass out through the other side of the crust…. But the angle of the hit and its velocity did cause sudden, immense internal distress, resulting in a huge pair of bulges in the opposite hemisphere…. The Hellas fragment continued to plunge onward, rotating all the way, through the magma of Mars. The Tharsis Bulge began to rise, with suddenness, about 100 minutes after Astra fragmented…. Simultaneously there were at least two other fragments that penetrated the Martian crust, Isidis and Argyre. In the vicinity opposite the Isidis crater is the second bulge of Mars—the Elysium Bulge.23
THE DEATH OF WORLDS
Among tens of thousands of smaller craters, and more than 3,000 craters with a diameter greater than 30 kilometers (including dozens with diameters up to 250 kilometers24) Hellas, Isidis, and Argyre are the dark, lurking monsters of Martian topography. Patten and Windsor’s estimates of the diameters of the three asteroids that caused these craters—respectively 1,000 kilometers, 600 kilometers, and 360 kilometers—are not correct. We know from studies of impacts on Earth that an object 10 kilometers in diameter can make a crater nearly 200 kilometers wide. More accurate estimates of the Martian impactors suggest diameters in the range of 100 kilometers for Hellas, 50 kilometers for Isidis, and 36 kilometers for Argyre.25
For a planet the size of Earth (and Mars is not much more than half the size of Earth), it is important to understand that a collision with any object wider than about one kilometer is a catastrophic event. Indeed, extensive damage has been caused on Earth by much smaller objects. The famous Barringer Crater in Arizona, which is 180 meters deep and just over a kilometer wide, was gouged out by an iron meteorite no larger than 50 meters in diameter.26 The so-called Tunguska Event of 30 June 1908 was an aerial explosion over Russia of a fragment of a comet or asteroid measuring 70 meters across and traveling at 100,000 kilometers per hour.27 Estimated to have occurred at an altitude of about six kilometers above the Siberian plains, this vast explosion flattened more than 2,000 square kilometers of forest, completely incinerated a central region of 1,000 square kilometers, and ignited people’s clothes as far afield as 500 kilometers from the epicenter.28 Seismic shocks from the Tunguska Event were measured at a distance of more than 4,000 kilometers, and so much dust was thrown up into the atmosphere, blocking out sunlight, that the earth’s surface temperature was measurably reduced for several years afterward.29
The Tunguska object was 70 meters across and, mercifully, exploded over an unpopulated area before colliding with Earth. Sixty-five million years ago, another object, this time 10 kilometers across, crashed into the northern end of the Yucatan Peninsula and the Gulf of Mexico with an explosive force that is estimated to have been a thousand times more powerful than all the nuclear bombs and missiles currently stockpiled on earth. It gouged out a crater 180 kilometers in diameter, sent up a dust cloud that blotted out the sun for five years, and created seismic instabilities that wracked the entire planet for decades with aftershocks and volcanic erruptions.30
The notorious “K/T boundary Event” wiped out the dinosaurs, and 75 percent of all other species then living on Earth.31 It has been aptly described as
one of the greatest disasters that has ever hit our planet…. It was the equivalent of a rock the size of Mount Everest, traveling ten times faster than the fastest bullet, producing an impact so severe that the entire Earth shifted in its orbit by a few dozen meters.32
That a “rock the size of Mount Everest,” with a diameter of just 10 kilometers, could have caused a planet-wide cataclysm that almost ended life on Earth is surely a chilling thought. Asteroids and comets measuring 10 kilometers or bigger are relatively common in the solar system, and we shall see in part 4 that many of them hurtle along on potentially disastrous Earth-crossing orbits.33 Astronomers refer to them as “Apollo objects”34 and believe that some may reach 100 kilometers in diameter.35 Such giants are thought to be rare, but it is widely understood that a collision with one of them would be a world-killing event in which it is unlikely that any form of life would survive.
It is worth repeating that the object that excavated the Hellas crater on Mars had a diameter of 100 kilometers. The Isidis object had a diameter of 50 kilometers. The Argyre object had a diameter of 36 kilometers.
As each of these huge interplanetary dumdum bullets was large enough to have killed Mars on its own, it is not hard to imagine what the global consequences of three such impacts must have been. Indeed, imagination is superfluous, because we have the NASA photographs of the ruined corpse of Mars to tell us the whole story. At the risk of overextending the metaphor, what these photographs suggest is that the “victim” was first hit from the south at point-blank range with the cosmic equivalent of a blast from a 12-bore shotgun—hence the thousands of craters clustered to the south of the line of dichotomy—and that the “killer” then finished off the job with three single shots from a large-caliber rifle.
ENERGY WAVES
Sixty-five million years ago, at the moment that the 10-kilometer-wide comet or asteroid that destroyed the dinosaurs hit Earth, tremendous shock waves were sent surging around the planet from the point of impact in the Gulf of Mexico. Geologists do not think it is an accident that almost exactly on the opposite side of the globe, at exactly the same time, an extraordinary burst of volcanic activity occurred in India. Wide-scale seepage of molten magma through fissures in the earth rapidly built up a great shield of basaltic lava—nearly a thousand meters high and thousands of square kilometers in area—which cooled to form the Deccan Traps. “Shock waves rippling out from the impact,” observe science writers John and Mary Gribbin, “would have tended to focus together again in just about that part of the world.”36
Patten and Windsor’s argument is that much the same thing, only a hundred times worse, happened on Mars—that the Tharsis Bulge swelled up in reaction to the Hellas impact and that the Elysium Bulge was a reaction to the Isidis impact. The shock waves are estimated to have been of such magnitude that they would not merely have passed around the planet but would have punched directly through it, ahead of the penetrating asteroids that cut into Mars like augurs. Indeed, it has been calculated that from their points of entry south of the line of dichotomy the Hellas, Isidis, and Argyre asteroids could possibly have traveled a distance of some 5,000 kilometers before coming to a halt inside the opposite, “serene” hemisphere north of the line of the dichotomy.37 There they would have released gigantic pressure waves that would have rushed upward to the surface at about 5,000 kilometers per hour.38
It is an entirely reasonable proposition, well supported by the Deccan Traps precedent on Earth, that this could have produced sufficient volcanic activity at the surface to account for Tharsis and Elysium—and probably for Olympus Mons as well. In addition, Patten and Windsor suggest that the sudden need for Mars to absorb and “digest” the mass and kinetic energy of the three large asteroids may have brought it close to total destruction. It was not enough for the planet to vent magma at Elysium and Tharsis. The pressure and expansion called for further release, and from the eastern edge of Tharsis the planet burst its seams along fully one-quarter of its circumference, forming the formidable gash that we know as the Valles Marineris.39 This vertiginous canyon system reaches depths of seven kilometers—too deep, according to authorities like P
eter Cattermole, to be explained by internal geological processes.40
Is it possible that one other thing—more devastating than all the rest—could have happened to Mars as a result of the three gigantic impacts it suffered? Is it possible that the hammer blows that it received from within, emanating from the south, could have transmitted sufficient energy to the north to shake loose the crust?
This was almost exactly the scenario envisaged by William K. Hartmann in Scientific American—that a collision with just one very large impactor could theoretically account for the Martian “asymmetry.” As we have seen, it has always been assumed that such a collision—or multiple collisions—would have occurred in the northern hemisphere. But recent research supports the notion that tremendous energy pulses transmitted from south to north during the Hellas, Isidis, and Argyre impacts could have done the job just as effectively. This research has shown that even Shock waves from relatively small impacts have caused the surface of Mars “to bounce, flicking boulders up to 15 meters across into space.”41
Hellas, Isidis, and Argyre were not small impacts. The possibility cannot be ruled out that their combined mass and momentum could have “bounced” the entire northern hemisphere vigorously enough to flick a three-kilometer-thick layer of its crust into space.
DISORDER AND DISTURBANCES
The Mars Mystery: The Secret Connection Between Earth and the Red Planet Page 5
