|The Planet Mars: A History of Observation and Discovery|
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As it turned out, the belief in the drab and moonlike Mars that followed in the wake of the flyby Mariners was premature, and quite as unjustified as the belief in the Earthlike Mars that had preceded it. The close-up photographs taken by Mariners 4, 6, and 7 had covered only 10 percent of the Martian surface and had somehow managed to miss the most exciting features. In 1968, Clark R. Chapman, James Pollack, and Carl Sagan warned: "If substantial aqueous-erosion features---such as river valleys---were produced during earlier epochs on Mars, we should not expect any trace of them to be visible on the Mariner IV photographs unless they were of greater extent than typical features on the Earth. . . . [A]ny conclusions . . . that the apparent absence of clear signs of aqueous erosion excludes running water during the entire history of Mars . . . must certainly be regarded as fallacious."1 Their comments could hardly have been more prophetic, although they made little impression at the time.
The next phase in the spacecraft exploration of Mars began in 1971, when five spacecraft---three Russian and two American---were readied for launch. American plans called for putting two spacecraft into closed orbits around Mars. The first, Mariner 8, was to enter a highly inclined orbit that would allow coverage of about 70 percent of the surface, including the polar areas. The emphasis was to be on mapping topographic rather than albedo features, thus requiring photography under conditions of low Sun elevation (when shadows would bring out relief). The second spacecraft, Mariner 9, would be placed in a more nearly equatorial orbit, from which it could carry out a careful study of albedo variations, best seen under conditions of high Sun.
On May 8, 1971, Mariner 8 lifted off from Cape Canaveral (actually Cape Kennedy, as it was called for a brief time; the name was changed back to Canaveral a few years later). The second stage of the Atlas-Centaur launch vehicle failed to ignite, however, and the spacecraft crashed ignominiously into the Atlantic Ocean. At once, the mission engineers made changes in their plans for the second spacecraft, Mariner 9, in order to try to fulfill with one spacecraft as many as possible of the objectives originally designed for two. The revised plans called for Mariner 9 to enter an orbit inclined 65° to the Martian equator, with a minimum altitude of 1,350 kilometers. This meant both a greater altitude and higher Sun conditions than were ideal for the topographic mapping, while the elevation of the Sun would not really be high enough for the albedo variations study. All in all, however, it was a reasonable compromise. Mariner 9 was launched on May 30, 1971, and successfully placed in its transfer orbit for Mars. This spacecraft would ultimately revolutionize all our ideas about the planet.
In the meantime, two days after Mariner 8 had aborted its mission, the Russians launched their first spacecraft from Baikonur, in central Kazakhstan, but it failed to leave Earth orbit owing to "a most gross and unforgivable mistake" in a command sent to the on-board computer. This was to be an orbiter-only mission, which the Russian space planners had hoped to send on a faster trajectory to Mars so that it would arrive ahead of the American Mariners. The Russians followed up with two more spacecraft, both orbiter-landers. After entering Martian orbit, the landers were supposed to separate from the orbiters, brake their descent through the Martian atmosphere partly by rocket thrusters and partly by parachutes, and touch down gently on the surface. Mars 2 was launched flawlessly on May 19, and Mars 3 was away on May 28.
Thus, three spacecraft, one American and two Russian, traversed their transfer orbits across interplanetary space, taking about five months to reach Mars. The American spacecraft arrived in mid-November, some two weeks ahead of the first of the Russian vehicles.
Meanwhile, however, fateful events had been taking place on Mars itself. Although they took most astronomers by surprise, these developments were not entirely unanticipated. In February 1971, Charles F. Capen at Lowell Observatory had made a prediction in an article about the dust clouds, or "yellow clouds" as they were still known at that time, because they show up best when Mars is observed with a yellow filter:
|Though yellow clouds have been recorded in all Martian seasons, the largest outbreaks seem to occur during Martian perihelic oppositions, when the insolation is greatest on the planet and the thermal equator is far south of the geometric equator. . . . If a bright yellow cloud again develops in the Hellespontus region [as was the case in 1956], . . . it will likely do so after opposition. A vast atmospheric disturbance could interfere with . . . the first Mariner orbiter spacecraft mission, which is planned to begin reconnaissance of the planet in November.2|
The actual events followed Capen's prediction to an uncanny degree. On September 21, six weeks after opposition, the great dust storm of 1971 began unobtrusively with a small yellow cloud over Hellespontus. One of the first observers to see it was Alan W. Heath, an English amateur using a 12-inch (30-cm) reflector. The storm expanded rapidly. In the 6-inch (15-cm) reflector that was my usual instrument at the time, the dark markings still showed their normal appearance in late September, but by early October they had become ill-defined as a continuous belt of dust clouds spread through the mid-southern latitudes. Within three weeks the entire planet was covered.
On November 10, Mariner 9 had drawn to within 800,000 kilometers of Mars and switched on its television cameras. The planet remained hopelessly obscured, and the first pictures showed no detail whatever except for the bright south polar cap at the bottom of the disk and four mystifying smudgy spots near the equator.3 The situation could hardly have seemed worse: the spacecraft had traveled all the way to Mars, only to be clouded out. (It was even suggested in some quarters that the storm was not coincidental, that the Martians were hiding from the spacecraft cameras.)
Under the circumstances, it was indeed fortunate that Mariner 9's mission plan had been kept flexible in order to allow for last-minute changes. On November 14, after slowing itself down by firing its thrusters, the spacecraft entered a closed orbit around Mars inclined some 65° to the equator and shut off its television cameras in order to conserve energy until the storm cleared.
On November 27, Mariner 9 was joined by the first of the Russian spacecraft, Mars 2. Shortly before entering orbit Mars 2 released a descent module, but the lander failed to function properly and crashed onto the surface. Its sole claim to fame is that in doing so it became the first man-made object to reach the surface of Mars (at a site just north of Hellas: latitude 44.2° S, longitude 313.2° W), where it deposited a pennant bearing the insignia of the Soviet Union. Mars 3 went into orbit on December 2. Again a descent module was released, and this time the capsule actually reached the surface safely, but it remained intact just long enough to turn on its television camera---within a few seconds, contact was lost. At the time it was suggested that the lander might have been blown over by the galelike winds scouring the surface.
The two Russian orbiters fared little better than the landers; they had been preprogrammed to carry out their imaging sequences automatically, and consequently could not wait out the dust storm as Mariner 9 was doing. Heedless of the dust, they sent back a series of blank and completely uninformative images. But other instruments on board did succeed in sending back some useful information, including measurements of the temperature at various points on the surface---the coldest point proved to be the north polar cap, where the temperature was -110°C (-166°F); elsewhere the values ranged from -93° to 13°C (-135°--55°F), depending on the latitude and time of day.
Of the flotilla of five spacecraft that had set out for Mars in 1971, all hopes had thus come to center on Mariner 9. A month after the spacecraft had entered Martian orbit, the pall of dust had cleared sufficiently for the systematic mapping of the surface to begin. The large dark spots that had at first defied explanation were the first features to emerge from the dust, and proved to be huge shield volcanoes. The largest, with a complex summit caldera, was in the position of Schiaparelli's Nix Olympica (the Snows of Olympus); it has since been renamed Olympus Mons. With its summit towering 25 kilometers above the surrounding plains, Olympus Mons is the tallest mountain in the solar system; since it measures 600 kilometers at the base, however, its slope is very gradual. (By comparison, the largest shield volcano on Earth, Mauna Loa, measures only 120 kilometers across at its base, and the summit rises 9 kilometers above the ocean floor.) The other great volcanoes in this part of Mars are known as the Tharsis Montes---Ascraeus Mons, Pavonis Mons, and Arsia Mons (corresponding in position to the telescopic patches Ascraeus Lacus, Pavonis Lacus, and Nodus Gordii). They are spaced about 700 kilometers apart and aligned southwest-northeast along the crest of the great rise known as the Tharsis bulge; their summits reach 17 kilometers above the surrounding plains.
Mariner 9's first mapping cycle took place predominantly over the southern hemisphere---the region between 25° and 65° S. The most prominent features seen in this cycle included Hellas and Argyre, which are great impact basins. That this is ancient, heavily cratered terrain had been known since the flyby Mariners; but some completely unexpected features emerged as well. Mariner 9 found networks of channels and tributaries that looked for all the world like runoff channels and dry riverbeds, and which strongly suggested that conditions on Mars must once have been very different from what they are today; once, running water had existed on that surface. The next mapping cycle included regions as far north as latitude 25° N, and revealed the enormous canyon system known as Valles Marineris---the Grand Canyon of Mars as it has been called, though it much bedwarfs its terrestrial counterpart. Valles Marineris extends along the equator for 4,000 kilometers, one-fourth of the way around the planet! Its origin is close to the summit of the Tharsis bulge, at Syria Planum, where it consists of a series of short, deep gashes intersecting at all angles known as Noctis Labyrinthus. In its middle section the canyons become more continuous and run in parallel as three main branches (Ophir, Candor, and Melas Chasmata), which are separated by intervening ridges. The combined width across all three canyons reaches 700 kilometers, and the depth, in places, is as much as 7 kilometers. These canyons connect with Coprates Chasma, which runs eastward and joins Eos Chasma, which finally merges with the large area of blocky "chaotic terrain" near the classical dark area Margaritifer Sinus, which is associated with the large Ares, Tiu, and Simud outflow channels, of which more later.
Other important results included the discovery of the etched, pitted, and laminated terrain around the south polar cap, which consists of sediments overlying an older crater topography and eroded chiefly by winds---and at some past stage possibly by flowing ice. This again seemed to attest to past cycles of climatic change. The spacecraft also returned the first close-up images ever taken of the two Martian moons. By the time Mariner 9 finally ran out of fuel, on October 27, 1972, it had obtained 7,239 images. The red planet was revealed as never before, and it was neither another Earth nor another Moon; it was Mars---"itself alone."
I do not propose to go into great detail about Martian geology, a vast subject that has been ably treated elsewhere.4 However, I must at least summarize some of the main points that emerged from a close study of the Mariner 9 images.
First, with few exceptions, the boundaries of the classical albedo features do not correlate at all well with Martian topography (among the exceptions are the impact basins Hellas and Argyre, which had been well known from Earth as large, circular bright regions). For example, instead of being a dried-up sea basin as was long believed, Syrtis Major turns out to be an elevated plateau; on the other hand, some of the other dark areas, such as Mare Acidalium, are relatively flat plains. The low areas of the planet---the large impact basins---are bright, but so too are the highest, the Tharsis and Elysium rises. In short, the Schiaparellian nomenclature that had long served for the albedo features would simply not do for the topographic features, and it became necessary, in 1973, for the International Astronomical Union to introduce some new terms:
Catena: chain of craters
Fossa (pl. fossae): long, narrow valley(s)
Labyrinthus: intersecting valley complex
Mensa (pl. mensae): flat-topped elevation(s)
Mons (pl. montes): mountain(s)
Patera: shallow crater with scalloped edges
Tholus: small, domical mountain or hill
Vallis (pl. valles): valley(s)
Vastitas: widespread lowlands
Broadly speaking, the Martian surface is divided into two main regions. This division has been referred to as the great "crustal dichotomy." South of a circle inclined by roughly 35° to the planet's equator are ancient, heavily cratered highlands; north of this circle are younger, relatively smooth plains and volcanic features. The boundary between the two regions is formed by a gentle, irregular scarp and low, knobby hills. On average, the southern highlands are some 2.1 kilometers higher in elevation than the northern lowlands.
The southern highlands, which include most of the subequatorial parts of the planet as well as a rather wide tongue extending northward beyond Sinus Sabaeus--Sinus Meridiani, are profuse with craters, so much so that the view looks superficially very much like that of the highlands on the Moon---thus the discouraging results of the flyby Mariners. In the case of the Moon, there was a long and heated debate about the origin of these craters; on one side were those who believed them to be impact features, on the other were those who maintained that they had been formed by internal processes of some kind---broadly speaking, by volcanism. During the 1960s and early 1970s, the question was finally settled decisively in favor of the impact theory, and there can be no doubt that the Martian craters were formed in the same way.
The impact process has now been worked out in considerable detail. When an object---say, a small asteroid---plunges into the surface of a planet, it produces two interacting shock waves. The first shock wave engulfs the asteroid, vaporizes it, and melts rock at the immediate point of impact. This part of the process absorbs a relatively small fraction of the energy of impact; the much greater share goes into producing a second shock wave, which travels radially away from the point of impact, excavates the crater, and throws a rim of disintegrated material around it (the ejecta blanket).
Craters of different diameters have different forms. Very small craters are simply bowl-shaped pits that have a fairly constant depth-to-diameter ratio of about 0.20. Larger craters are more complex. Violent rebound of the floor from the shock of impact gives rise to a central peak or peaks. In addition, many of the larger craters have terraced walls caused by landslip---the slumping in of rim materials toward the center of the crater. This partial filling in with wall material explains why the more complex craters become shallower with increasing diameter.
Even after the flyby Mariners it was obvious that in general the Martian craters are flatter and more subdued than their lunar counterparts, and usually lack the latters' hummocky surrounds. There is also a relative paucity of smaller pits on Mars (less than about 20 km across). These findings are readily explained as being due to the fact that on Mars, unlike the Moon, there has been considerable weathering over time by water, air, and (possibly) glacial erosion. There are other important differences between Martian and lunar craters as well. Summit pits on the central peaks of Martian craters are much more common than on the Moon, and though smaller pits have lunarlike ejecta blankets that have been laid down with ballistic trajectories, those with diameters greater than about 5 kilometers tend to have a different pattern of overlapping sheets of ejecta with lobate margins---the latter being telltale signs of formation by flow across the surface. It has been suggested that the 5-kilometer diameter transition between ballistic and flow patterns may correspond to the minimum depth that needs to be excavated in order to release subsurface water ice.5
Martian impact craters with diameters greater than about 50 to 70 kilometers are referred to as basins. The very largest basins, greater than 300 kilometers or so across, are multiringed features similar to those already known on the Moon, though again, because of erosive forces the condition of the Martian features is much less pristine.
The Hellas basin, which measures 2,300 kilometers from rim to rim, is the most imposing topographical feature of the Martian southern hemisphere. Its floor is the lowest point on Mars, located some 5 kilometers below the Martian datum. (On Mars, of course, there is no true "sea level"; instead, the reference datum is defined as the level at which the partial carbon dioxide pressure is 6.1 millibars, which marks the triple point of water. For partial pressures of water greater than 6.1 millibars, liquid water can exist under some temperature conditions; for partial pressures lower than this, it is always unstable.)6 In the winter Hellas is often covered with carbon dioxide frost, and at such times can appear brilliant white. Other basins clearly visible in the Mariner 9 images are Isidis and Argyre---the latter measures 1,900 kilometers across and still shows its rim mountains, the Nereidum and Charitum Montes. This basin, too, is often covered with frost. Another major mountain range, the Phlegra Montes, is located in Elysium; these mountains have been identified as remnants of another basin rim that was later inundated by volcanoes. In recent years a number of other large basins have been identified, most of them well-nigh obliterated by Martian weathering.
When the frequencies of craters of different sizes are plotted for the highlands of the Moon, Mercury, and Mars, the resulting distributions, although not identical, demonstrate the existence of two distinct cratering populations: an older population, made during a period of heavy bombardment in which the great basins and the majority of the craters of the rugged highlands were formed; and a younger population that may be traced in the more recent plains and was formed by post--late heavy bombardment.
When the solar system began to form 4.6 billion years ago, a rotating disk of gas and dust began, grain by grain, to accrete into larger objects known as planetesimals; these in turn accreted into the planets. Some of the planetesimals became rather large in their own right and collided with the planets, with fateful consequences. For instance, a smash-up involving a Mars-sized body and the Earth is believed to have given rise to the Moon. Another collision between a large object and Mars about 4.2 billion years ago may well account for the Martian crustal dichotomy. In the low-lying plains of the northern hemisphere, which are now largely covered with sedimentary debris, a gigantic impact basin has been tentatively identified. The Borealis basin, as it is called, is 7,700 kilometers across and is centered in Vastitas Borealis (50° N, 190° W); it is indeed vast---altogether it covers some 80 percent of the northern hemisphere plains.7
Obviously space was much more crowded in the early history of the solar system than it is now, and impacts were more frequent. The residue of accretional material subjected the Moon and planets to a massive late bombardment in which the impacts occurred at such high rates that their surfaces became saturated with craters---in other words, the formation of new craters could take place only by obliterating preexisting ones. In the case of the Moon, this violent period came to an end about 3.8 billion years ago. It may have ended at about the same time on Mars, although there is evidence to suggest that it continued until somewhat later. In any case, the objects derived from accretional debris became extinct. Henceforth, craters were added at a much lower rate and were caused by the occasional impacts of asteroids and comets (for Mars, which lies very near the asteroid belt, it has been estimated that asteroids create seven times more craters than comets do). It is these comets and asteroids that are responsible for the post--late heavy bombardment cratering.
Various areas on the Martian surface show widely different cratering densities; the most heavily cratered areas are the oldest. Thus the relative ages of surface units (stratigraphic relationships) can be worked out, and by making certain assumptions about when the late heavy bombardment phase ended and the rates of cratering since, one can even go so far as to estimate their absolute ages (though really reliable values must await the return of actual surface materials from Mars). The oldest units make up the so-called Noachian system, which is represented by the ancient cratered terrain centered on Noachis Terra (approximate ages 4.60 to 3.80 billion years). Overlying the rocks of Noachian age are those of the Hesperian system, whose units are characterized by ridged plains material, of which examples are found in Hesperia Planum and Vastitas Borealis (approximate ages 3.80--3.55 billion years). Finally, the Amazonian system consists of largely smooth plains material such as that which covers Acidalia, Amazonis, and Elysium Planitia (ages less than 3.55 billion years).
Areologists once believed that Mars was cool early in its history, and that it formed a hot core at a relatively advanced stage, after heating due to decay of radioactive materials warmed it sufficiently to initiate melting of rock. This would have implied a late onset of volcanism. We now know otherwise, and from an unimpeachable source: through direct analysis of material from Mars itself.
This material exists in the form of several unusual meteorites---one fell at Chassigny, France, in 1815; others fell at Shergotty, India, in 1865 and at Nakhla, Egypt, in 1911; and several others have been identified as well. Their Martian origins have, however, been suspected only since 1981. The meteorites are classified into three groups: shergottites, nakhlites, and chassignites (collectively known as SNCs). All are of the common stony type, but they are very young compared with the 4.5-billion-year age of most meteorites. Gases captured in shock-generated glassy nodules within them were analyzed and proved to have the exact composition of the same gases in the Martian atmosphere; they also contain small amounts of water and water-altered minerals. There can be little doubt that they are Martian. They were blasted into space in one or (more probably) several impacts, with such force that they reached escape velocity and eventually reached Earth.
Detailed analyses of the SNC meteorites made it clear that not long after Mars accreted into a world, its interior was already hot and had differentiated into a nickel-iron core, mantle, and crust. Much of the heat at this stage must have come from the energy of the impacts themselves. Thus, Martian volcanism began early. During late Noachian and early Hesperian times, melted rock (magma) began to reach the surface. Extensive ridged plains were laid down in areas of the southern hemisphere; the so-called highland paterae also emerged, of which four are located near the Hellas basin and probably were formed in relation to deep-seated fractures produced during the impact that formed it. The best-known example is Patera Tyrrhena (at 23° S, 255° W), which seems to have been a volcano of the explosive type, as its irregular summit caldera (40 km long by 12 km wide) is surrounded by large quantities of ash. The plateau of Syrtis Major Planitia (10° N, longitude 290° W) is another early volcanic region; its activity seems to have begun during postimpact adjustments of the crust around the Isidis basin. The dark materials that cover it originated from a low-relief volcanic shield.
This early volcanic period was characterized by the rapid escape of heat from the interior to the surface. Inevitably, the planet began to cool; as it did, convection of the mantle decreased, the overlying crust steadily thickened, and surface volcanism came to be concentrated in ever more limited regions. For some reason not yet entirely understood, the main volcanic activity came to center on two areas: Elysium and Tharsis. These areas marked the locations of "hot spots," or mantle plumes---places where a column of heated material rose from the mantle. Why there should have been only two such plumes on Mars is not known, but the consequences are obvious enough. Lava flooding occurred in these regions on an enormous scale, producing a domical buildup of material which stretched the overlying crust and produced belts of intense fracturing. Along the equator, Valles Marineris began to open when a series of deep troughs formed, oriented radially to the Tharsis rise; later, these troughs were eroded into spurs and gullies.
In Elysium, the first volcano to appear was Hecates Tholus, a shield structure some 180 kilometers across and 6 kilometers wide. The center of the eruptions then shifted some 850 kilometers to the south to form the dome-shaped Albor Tholus. Still later eruptions gave rise to Elysium Mons, a shield volcano some 500 kilometers across at its base and standing some 9 kilometers above the surrounding plain; its summit is marked by a 14-kilometer-wide caldera.
By far the greatest activity took place in Tharsis, to the west. The Tharsis bulge stands out like an enormous hump relative to the ellipsoidal shape that the planet would have were it in equilibrium with itself; this hump extends 4,000 kilometers north-south from the plains bordering Mareotis Fossae to Solis Planum, and 3,000 kilometers east-west from Lunae Planum to Amazonis and Arcadia Planitia. Its average level stands some 8--10 kilometers above the Martian datum. In late Hesperian time, the volcanic activity in the region centered on Alba Patera, which lies on the bulge's north flank in an extensively fractured landscape; the caldera itself shows little vertical relief, but measures 1,500 kilometers across. By early Amazonian time, a fault line running on the northwest flank of the Tharsis bulge had become active, giving rise to the three great shield volcanoes of the Tharsis Montes. Several smaller shields and volcanic domes lie close to the same line---Biblis, Ulysses, and Uranius Paterae, and Uranius, Ceraunius, and Tharsis Tholi. Finally, on the southeastern flank of the Tharsis bulge, some 1,200 kilometers northeast of the Tharsis Montes, arose Olympus Mons, whose slopes make up what is probably the youngest surface on Mars. The eruptions here continued long after cooling of the planet's interior had extinguished active volcanism elsewhere---the most recent may have occurred as little as 300 million years ago.
Of the many discoveries of Mariner 9, by all odds the most exciting was the recognition of valley networks and outflow channels, which can hardly have formed otherwise than by the action of running water at or near the surface. Their existence has provided the strongest evidence that Mars may have undergone major climatic changes over time.
The valley networks are in general the older features. They have tributaries, so that they look very much like dry riverbeds, and they lie almost entirely (about 98 percent) in the heavily cratered highlands of the southern hemisphere---and so must be as ancient as they. They are typically 1 to 2 kilometers wide, and not very long; even including their tributary systems, networks seldom go on for more than a few hundred kilometers. The longest---Ma'adim Vallis (centered at 20° S, 182° W), Al Quahira (at 18° S, 196° W), and Nirgal Valis (north of Argyre at 28° S, 40° W)---range in length from 400 to 800 kilometers. At first it was hoped that the valley networks might be proof that precipitation took place on early Mars---rain, in other words---though more recent studies have shown that they give every indication of having been formed by groundwater sapping due to melting of an ice-rich permafrost.8
Some valleys formed after the end of the period of heavy bombardment; for example, the system located on the northern flanks of Alba Patera. It is certainly young, and may well be of early Amazonian age. In every way it looks similar to the fluvial valleys produced by runoff on the flanks of the Hawaiian volcanoes, and like the latter is believed to have been formed by surface runoff (though again produced by a sapping process rather than by rainwater).9
Compared with the comparatively tiny valley networks, the outflow channels are features on a grand scale; typically they measure hundreds of kilometers long and tens of kilometers wide. They generally emerge in areas of the surface that have undergone collapse, such as chaotic terrain or canyons. Several outstanding examples---the Ares, Tiu, and Simud Valles---originate in the chaotic terrain in Aurorae Sinus at the eastern end of Valles Marineris; Kasei Vallis extends from Echus Chasma, a canyon just west of Lunae Planum; and the Maja, Vedra, and Bahram Valles all arise from Juventae Chasma, on the opposite flank of Lunae Planum. All of these channels then converge on and disappear into the southern floor of Chryse Planitia on the eastern margin of the Tharsis bulge. There are also many outflow channels in Elysium, northwest of the volcanic province, whence they debouch into the low-lying northern plains; still others are found in Memnonia, Amazonis Planitia, and on the rim of Hellas.
These enormous channels lack tributaries; some, such as Mangala Vallis in the upland region of Memnonia on the border of Amazonis Planitia, are characterized by sculpted landforms such as teardrop or lemniscate islands. The closest terrestrial analogy to these features is the Channeled Scabland of eastern Washington, in the United States, which was formed at the end of the last Ice Age when much of western Montana was covered by a glacial meltwater lake (Lake Missoula). The lake was held in check by an ice dam lying across northern Idaho; when the dam suddenly broke, it released the captured water in a flash flood that drained the whole Columbia Plateau as far as the Pacific Ocean over a period of several days.10 There is every reason to believe that the Martian channels were also formed by catastrophic flooding, but on an even more monumental scale.
The outflow channels are generally younger than the valley networks, and all were formed after the period of heavy bombardment; the circum-Chryse channels have been dated to Hesperian times, and Mangala Vallis is of Amazonian age. Many of the channels seem to record multiple flooding events over a long period, and apparently they could even form under present conditions; although liquid water is unstable on the surface of Mars because of the low atmospheric pressure (any that formed there would rapidly boil away), this poses no obstacle to massive floods of brief duration.
What was the source of all the water? It is probable that early in its history Mars had a much more substantial atmosphere than it does now. Over time, the oxygen present bonded with rock, turning it red, and the water seeped down through the meteorite-fractured regolith. In the upper parts of the regolith the water froze, but farther down the temperature was still warm enough for the water to remain liquid, and seas formed deep within Mars, pooling beneath an ice-rich permafrost layer perhaps several kilometers thick. Volcanism such as that in Tharsis caused extensive melting of the permafrost cap, releasing the water through permeable volcanic rocks, out along the great fracture systems such as Memnonia Fossae and Valles Marineris, and finally onto the surface in vast catastrophic floods.
The implications of the valley networks and outflow channels are still being debated, and I shall have more to say about them in the next chapter, but there can be little doubt that their discovery was the single most important revelation of Mariner 9. To astronomers, geologists, and laypersons alike they suggested the distinct possibility that Mars, in having once allowed running water on its surface, may not always have been so forbiddingly severe as it is now. This in turn gave a tremendous impetus to the next American mission, Viking, whose lofty goal was nothing less than to commence the in situ search for evidence of life on Mars.
The odds of finding living organisms on Mars were obviously very slim, but it seemed that they might not be nil. Such a search was not without its chimerical aspects, but it had been all but inevitable ever since Percival Lowell had sat at the eyepiece of his telescope and, scanning the little globe of orange-yellow spotted by transverse stripes of color, had savored the thought: Here there be life!