Unfortunately, his entry point was not quite right. By the end of 1894, after more than a year of drifting, it was clear from astronomical observations that the ship would not pa.s.s over the pole. In fact, there was little chance it would even get beyond 85 north, some 350 miles from the pole. With the possibility of the ship achieving the pole already lost, the thought of spending yet another year or even two aboard the ice-locked drifting Fram Fram before she was released to the sea was too painful for Nansen to consider. before she was released to the sea was too painful for Nansen to consider.

Not one to give up the polar quest easily, he decided to leave the ship in the care of his crew and, with a single companion, Fredrik Hjalmar Johansen, set off across the ice with dogs, skis, a sledge, and kayaks, in an attempt to reach the pole. Nansen and Johansen left Fram Fram in late February of 1895 and seemingly were making good progress, but by early April, observations of the position of the Sun showed that the pole was still 250 miles away. Six weeks of hard travel on the ice had brought them only 100 miles closer to their goal. An awful realization hit them: as they were struggling north, the ice was drifting south, erasing some of their daily progress toward the pole. On April 8, they recognized the futility of the attempt, and turned around. in late February of 1895 and seemingly were making good progress, but by early April, observations of the position of the Sun showed that the pole was still 250 miles away. Six weeks of hard travel on the ice had brought them only 100 miles closer to their goal. An awful realization hit them: as they were struggling north, the ice was drifting south, erasing some of their daily progress toward the pole. On April 8, they recognized the futility of the attempt, and turned around.

Nansen and Johansen knew they would have little chance of finding Fram Fram, because in the six weeks that had elapsed since they left the ship, she had continued to drift with the ice. They would have to make the homebound journey back to Norway, a distance of some 1,400 miles over the Arctic ice, entirely on their own. After many more months of adventure, hardship, and an improbable encounter with another polar adventurer, Nansen and Johansen set foot on Norwegian land in mid-August of 1896. Meanwhile, Fram Fram had continued her long and slow drift across the Arctic Ocean, to break out of the ice on the very day that Nansen and Johansen reached Norway. Within a week the entire expedition had been reunited. had continued her long and slow drift across the Arctic Ocean, to break out of the ice on the very day that Nansen and Johansen reached Norway. Within a week the entire expedition had been reunited.

The drifting of sea ice also figured dramatically in the saga of Ernest Shackleton's Imperial Trans Antarctic Expedition of 1914-16. Shackleton had already partic.i.p.ated in two unsuccessful attempts to reach the South Pole, the Discovery Discovery expedition led by Robert Falcon Scott, in 1901-3, and the expedition led by Robert Falcon Scott, in 1901-3, and the Nimrod Nimrod expedition under his own leadership, in 1907-9. The first fell short by 530 miles; the second reached 8823' south, only 97 miles from the pole. The overland segments of both expeditions turned back shy of the polar goal because of deteriorating health, diminishing provisions, and concerns that if they pressed ahead they might perish. expedition under his own leadership, in 1907-9. The first fell short by 530 miles; the second reached 8823' south, only 97 miles from the pole. The overland segments of both expeditions turned back shy of the polar goal because of deteriorating health, diminishing provisions, and concerns that if they pressed ahead they might perish.

In 1911, Robert Falcon Scott and Roald Amundsen mounted rival expeditions to reach the South Pole. The ship that brought Amundsen to Antarctica was none other than Fram Fram, the one Fridtjof Nansen had used in his unsuccessful attempt to drift over the North Pole. Amundsen and Scott chose different pathways to the pole, with Scott's slightly longer but better known. The race to the pole was won by Amundsen, by more than a month; his small team reached 90 south on December 11, 1911. Scott arrived at the pole thirty-five days later, only to endure the great disappointment of seeing the Norwegian flag atop a small tent left by Amundsen. But that disappointment was eclipsed by the death of Scott and the other four in his polar party during their return from the pole.

With the attainment of the South Pole then in Amundsen's book of laurels, and Scott's as well posthumously, Shackleton was forced to envision a different kind of exploration, a bold expedition that he hoped would ensure his own lasting prominence in Antarctic exploration. He devised a plan to cross the entire Antarctic continent, on a route from the Weddell Sea to the Ross Sea, with a stopover at the South Pole as a midway point. This expedition was to become known as the Imperial Trans-Antarctic Expedition, and the ship that carried Shackleton into the Weddell Sea was the Endurance Endurance.

Just as Fram Fram did intentionally in the Arctic, did intentionally in the Arctic, Endurance Endurance did unintentionally in the Antarctic-it became locked in the ice of the Weddell Sea in January of 1915, and began a slow clockwise drift, following the current gyre in the sea beneath. The drift carried did unintentionally in the Antarctic-it became locked in the ice of the Weddell Sea in January of 1915, and began a slow clockwise drift, following the current gyre in the sea beneath. The drift carried Endurance Endurance within some sixty miles of the ice shelf at the southern margin of the Weddell Sea, where Shackleton had planned to disembark and begin the overland trans-Antarctic trek. But the ice took them no closer-in fact, it began to carry them slowly away from the southern coast of the Weddell Sea and toward the long finger of the Antarctic Peninsula. By April, the drift exceeded two miles per day to the northwest, and the possibility of launching the overland segment of the expedition was rapidly receding. As author Alfred Lansing writes, " within some sixty miles of the ice shelf at the southern margin of the Weddell Sea, where Shackleton had planned to disembark and begin the overland trans-Antarctic trek. But the ice took them no closer-in fact, it began to carry them slowly away from the southern coast of the Weddell Sea and toward the long finger of the Antarctic Peninsula. By April, the drift exceeded two miles per day to the northwest, and the possibility of launching the overland segment of the expedition was rapidly receding. As author Alfred Lansing writes, "Endurance was one microscopic speck . . . embedded in nearly one million square miles of ice that was slowly being rotated by the irresistible clockwise sweep of the winds and currents of the Weddell Sea." was one microscopic speck . . . embedded in nearly one million square miles of ice that was slowly being rotated by the irresistible clockwise sweep of the winds and currents of the Weddell Sea."20 Fram and and Endurance Endurance had both been built by Norwegian shipwrights for service in polar regions. But had both been built by Norwegian shipwrights for service in polar regions. But Fram Fram was designed specifically for a long sea ice drift-her rounded hull would accommodate horizontal pressure from the ice by popping upward as the ice closed in around it. was designed specifically for a long sea ice drift-her rounded hull would accommodate horizontal pressure from the ice by popping upward as the ice closed in around it. Endurance Endurance was built very st.u.r.dily to push ice around, but her less rounded hull did not fare well when surrounded and squeezed by ice. Before she could be released from the ice as she moved northward toward a lat.i.tude (and season) of sea ice breakup, the ice crushed her, and she sank- was built very st.u.r.dily to push ice around, but her less rounded hull did not fare well when surrounded and squeezed by ice. Before she could be released from the ice as she moved northward toward a lat.i.tude (and season) of sea ice breakup, the ice crushed her, and she sank-Endurance did not endure the harshness of sea ice entrapment. The survival of Shackleton and his entire crew after the loss of did not endure the harshness of sea ice entrapment. The survival of Shackleton and his entire crew after the loss of Endurance Endurance is one of the moving stories of polar exploration and rescue. is one of the moving stories of polar exploration and rescue.

ICE, WATER, AND LIFE.

Water is considered an essential ingredient of life as we know it. The human body is approximately 90 percent water, so we are mostly hydrogen and oxygen. Add a little carbon and nitrogen, and you have the materials for 96 percent of our ma.s.s. Earth occupies a special location in the solar system because it is the only place in the neighborhood that has abundant liquid H2O, that essential ingredient for life, at its surface. For this reason it is sometimes called the "Goldilocks planet"-not too close to the Sun to lose all its water, not too far to have only ice. The third rock from the Sun is just about the right temperature for the purpose of life.

But Earth almost did not qualify for hosting water at its surface. Although the amount of radiant energy the Sun delivers to Earth is adequate to make our planet hospitable to liquid H2O, Earth reflects about 30 percent of that energy back into s.p.a.ce, thereby rejecting a sizable fraction of the solar gift of warmth. But Earth's climate system has another important player-an atmosphere that compensates for the reflective loss. The atmosphere is 99 percent nitrogen and oxygen, but the remaining 1 percent includes very small amounts of water vapor, carbon dioxide, and methane-heat-trapping gases that block heat trying to escape from Earth in the form of long-wavelength infrared radiation. This blanketing is called the natural greenhouse effect, as opposed to the contemporary warming of the planet due to anthropogenic greenhouse gases-those of human origin-now being added to the atmosphere through the burning of fossil fuels.

The natural greenhouse effect is a characteristic of Earth's atmosphere that has been around since Earth's earliest days as a planet, whereas the anthropogenic greenhouse effect is a phenomenon of just the last few centuries of Earth history. We should be very grateful for the natural greenhouse effect, because Earth is some sixty Fahrenheit degrees warmer than it would be without it. It has made Earth the water planet, the blue planet, instead of just another white s...o...b..ll in orbit around the Sun.

IS EARTH UNIQUE in the solar system as a place where life has emerged? The elements that comprise life and the rocks of our planetary host-hydrogen, oxygen, carbon, nitrogen, iron, magnesium, and silicon-are among the ten most abundant elements in the entire universe. It is clear that nature uses the most common raw materials around for both planetary architecture and life. With such materials widely available, might there be other places hospitable to life?

Of the other planets in the solar system family, Mercury and Venus are too close to the Sun to retain water, let alone to enable ice to form. But farther out in the solar system ice abounds, giving rise to fascinating possibilities of life, provided energy is available to change ice to water. Mars displays very obvious ice caps in its polar regions, two little white skullcaps sitting on the red planet. The northern cap is much larger than the southern, with a diameter some three times greater. But there is a second asymmetry that is more amazing-the two polar caps reveal a different composition, at least at their respective surfaces.

The surface of the southern cap of Mars is made up of solid carbon dioxide (CO2), which we call "dry ice," because when it warms, the solid transforms directly into a vapor, without first pa.s.sing through the liquid stage that characterizes the behavior of H2O on Earth. Thus on Mars the transformation of solid CO2 is a "dry" one, to a vapor, whereas the conversion of solid H is a "dry" one, to a vapor, whereas the conversion of solid H2O on Earth is a "wet" one, to a liquid, water. It is perhaps no surprise that Mars's south polar cap shows a surface composition of carbon dioxide-CO2 is the princ.i.p.al component of the Martian atmosphere, and the south polar temperatures are sufficiently cold to allow solid CO is the princ.i.p.al component of the Martian atmosphere, and the south polar temperatures are sufficiently cold to allow solid CO2 to condense out of the atmosphere onto the surface. to condense out of the atmosphere onto the surface.

The big surprise is that the surface of the larger northern cap is not CO2, but instead solid H2O, the ice that we know and love on Earth. The atmosphere of Mars is much thinner than Earth's, and exerts less than 1 percent of the pressure of Earth's atmosphere. At such a low pressure, H2O can also change from solid to vapor directly, a "dry" transformation, which can be seen indirectly as a summertime increase of water vapor over the north polar cap.

Both of the Martian polar caps probably contain substantial H2O ice, but the H2O in the southern cap is likely deep beneath the CO2 ice. The reason for this compositional asymmetry is not well established, but may be related to a north pole that is warmer than the south pole, because of the presence of more summertime dust on the north polar surface, which absorbs more sunshine. ice. The reason for this compositional asymmetry is not well established, but may be related to a north pole that is warmer than the south pole, because of the presence of more summertime dust on the north polar surface, which absorbs more sunshine.

The surface of Mars away from the poles, despite having a very low atmospheric pressure on it and a temperature well below freezing today, shows many well-developed river channels that indicate that water once flowed over the surface. Layers of sedimentary rock, typically formed as deposits in lakes, rivers, and oceans on Earth, can be seen in the Martian landscape. And there is reason to suspect that some of the water that once flowed on Mars may still be there, in the form of ice just beneath the Martian surface and possibly in liquid form at greater depths. Ground-penetrating radar surveys conducted in 2008 from the Mars Reconnaissance Orbiter s.p.a.cecraft returned signals indicative of ma.s.sive ice deposits in the southern mid-lat.i.tudes of Mars, suggesting that there may be large glaciers buried beneath just a few feet of dust and rock debris.21 With so much ice on Mars, the presence of water and life on the red planet is an intriguing possibility. With so much ice on Mars, the presence of water and life on the red planet is an intriguing possibility.

A TRIP TO MARS.

In August of 2007, I had the special experience of watching the launch of a s.p.a.cecraft to Mars called the Phoenix Phoenix Mars Lander. On launch day at Cape Canaveral I arose well before dawn and made my way to a viewing site, on the beach a short distance from the launchpad. The tense countdown proceeded flawlessly, right to the moment of ignition. It was a thrill to see the main rocket engines ignite and roar, and, a few seconds later, to watch this huge taxi lift its small s.p.a.cecraft pa.s.senger into the atmosphere, accelerate to a velocity sufficient to escape Earth's gravity, and send it on a nine-month journey to Mars. Although Mars's...o...b..t is only 50 million miles beyond Earth's...o...b..t, a distance half again as far as Earth is from the Sun, the actual journey of the Mars Lander. On launch day at Cape Canaveral I arose well before dawn and made my way to a viewing site, on the beach a short distance from the launchpad. The tense countdown proceeded flawlessly, right to the moment of ignition. It was a thrill to see the main rocket engines ignite and roar, and, a few seconds later, to watch this huge taxi lift its small s.p.a.cecraft pa.s.senger into the atmosphere, accelerate to a velocity sufficient to escape Earth's gravity, and send it on a nine-month journey to Mars. Although Mars's...o...b..t is only 50 million miles beyond Earth's...o...b..t, a distance half again as far as Earth is from the Sun, the actual journey of the Phoenix Phoenix Lander covered 422 million miles, as the s.p.a.cecraft had to chase Mars in its...o...b..t, a veritable moving target. Lander covered 422 million miles, as the s.p.a.cecraft had to chase Mars in its...o...b..t, a veritable moving target.

The instruments aboard the Lander were designed to send back information about the atmosphere and soil of Mars, and in particular to seek evidence that might confirm the presence of some form of microbial life, if indeed any were present. To be sure the instruments would perform as designed, they were tested in the frigid desert environment in the Dry Valleys of Antarctica, a short helicopter ride from McMurdo Station, a U.S. base in the Ross Sea sector of the Antarctic. This cold, dry, windy setting is probably very close to the actual operating conditions on Mars.

The Phoenix Phoenix s.p.a.cecraft arrived at Mars in late May of 2008, and after a breathtaking descent through the thin Martian atmosphere, landed safely at the target landing site close to the north polar cap. I am always amazed at the precision of such operations-a NASA engineer likened it to a rocket launched in California successfully reaching its distant target-home plate in Chicago's Wrigley Field! I "watched" the landing, too, so to speak, over NASA TV's live Internet broadcast. It was an emotional moment, tying together both ends of this 269-day journey across the vastness of the solar system. s.p.a.cecraft arrived at Mars in late May of 2008, and after a breathtaking descent through the thin Martian atmosphere, landed safely at the target landing site close to the north polar cap. I am always amazed at the precision of such operations-a NASA engineer likened it to a rocket launched in California successfully reaching its distant target-home plate in Chicago's Wrigley Field! I "watched" the landing, too, so to speak, over NASA TV's live Internet broadcast. It was an emotional moment, tying together both ends of this 269-day journey across the vastness of the solar system. Phoenix Phoenix, never distracted from duty, announced its own arrival, waited a few minutes for the dust to settle, unfolded its solar panels to power its instruments, and, like an enthusiastic tourist, began taking pictures to send home.

The first snapshots showed "patterned ground," almost identical to earthly terrains in Siberia, Alaska, and northern Canada-low ridges of rock fragments squeezed upward into polygonal patterns by cycles of freezing and thawing. Soon Phoenix Phoenix deployed its soil scooper, and brought into its mini-laboratory some Martian soil containing white nuggets, later confirmed to be H deployed its soil scooper, and brought into its mini-laboratory some Martian soil containing white nuggets, later confirmed to be H2O ice. The soil also contained hints of calcium carbonate, the princ.i.p.al mineral of limestone, which on Earth is usually precipitated in water. Also in the soil were traces of perchlorate, an oxidizing agent that could provide nourishment for any microbial life that might be present.

But Phoenix Phoenix's main instrument, the one designed to detect organic compounds, proved balky, and only about half the experiments on the schedule were able to be conducted. Even though Phoenix Phoenix continued to operate well beyond its design lifetime of three months, providing photos of dust storms and the distant faint Sun, there was no confirmed detection of an organic molecule. Of course, absence of evidence is not the same as evidence of absence, and so the possibility of some form of life on Mars remains. At the end of October of 2008, as the Sun reached the horizon, continued to operate well beyond its design lifetime of three months, providing photos of dust storms and the distant faint Sun, there was no confirmed detection of an organic molecule. Of course, absence of evidence is not the same as evidence of absence, and so the possibility of some form of life on Mars remains. At the end of October of 2008, as the Sun reached the horizon, Phoenix Phoenix's solar panels no longer could gather enough light to power the Lander, and by mid-November it had fallen totally silent. There is little likelihood it will survive the freezing darkness of the Martian winter.

ICE (AND LIFE?) BEYOND MARS.

In the more distant and frigid reaches of the solar system are the planets Jupiter, Saturn, Ura.n.u.s, and Neptune-very large bodies mostly of gaseous hydrogen and helium without a solid surface of ice or rock anywhere close to their visible cloud tops. The temperature of Jupiter's cloud tops is around -250 Fahrenheit, and it only gets colder farther out. These giant planets have many small satellites...o...b..ting about them, made up mostly of rock and ice, in varying proportions and vertically segregated-a rocky interior covered by a sh.e.l.l of ice hundreds of miles thick. Just as Earth's rocky crust displays faults and folds and ancient granite intrusions as evidence of a long geologic and tectonic history, so also can features in the icy crust of these distant satellites provide an archive of the processes that shaped them over time. Some of these processes may have melted ice to form water and provide an environ ment conducive to life.

Early in the history of the solar system there was a lot of debris flying around and colliding, creating heavily cratered surfaces such as we see on Earth's moon. If nothing else ever happened to bodies such as the moon, their surfaces today should look much as they did three to four billion years ago-heavily cratered. But if other processes are active over geologic time, surfaces can undergo alteration and modification. On Earth the ancient cratered surface has been almost totally obliterated by the process of plate tectonics, which recycles most of Earth's crust back into the planetary interior every two hundred million years or so. And what has not been recycled, what is left at the surface, is deeply eroded over time, or covered up by younger layers of sedimentary rock. In effect, Earth gets a periodic resurfacing, just as many roads-potholed and broken up during a harsh winter-receive a new layer of tarmac in the spring.

Many of the icy satellites around Jupiter and Saturn are indeed heavily cratered-the telltale signature of an ancient surface covering a dead interior. But other satellites display a very smooth surface with very few craters. Such a juvenile complexion, unblemished by the pockmarks of impacts, is an indication that the icy surfaces have experienced "resurfacing" events in their history.

How does an icy surface get rejuvenated? During a hockey game, the ice surface gets cut and scored by the players' skate blades, but between the periods of play, the surface is restored by a Zamboni, which spreads a sheet of water over the degraded ice that quickly freezes into a new smooth playing surface. So what is the great Zamboni resurfacing machine that operates in the distant reaches of the solar system? Perhaps, surprisingly, it is water coming from the interior of these planetary satellites, water that derives from melting at the base of the thick surface layer of ice. But the source of the heat that melts the ice is one that is not so familiar to Earth-dwellers-the heat derives from tidal forces exerted on the small satellites by the giants in the neighborhood, the planets Jupiter and Saturn. The nature of this heat-generating mechanism is very interesting.

TIDAL HEATING.

Many people are vaguely aware of tides, but don't think much about them unless they live on the seacoast. Tides in the ocean-the rhythmic rise and fall and sloshing around of the sea-are due to interactions between the orbits and gravity fields of the Sun, moon, and Earth. What most people are unaware of is that the tidal forces move and distort not only the water on Earth, but also the solid rock of the planet, although not very much. The physics of tides tells us that the strength of the tidal force depends on the distance between the co-orbiting bodies and on how big they are. Tidal effects on Earth are small because the moon, although relatively close to Earth, is small-only 1/80 of Earth's ma.s.s-and the Sun, although 332,000 times more ma.s.sive than Earth, is more than 400 times more distant from Earth than is the moon. These same tidal interactions take place wherever planets have satellites...o...b..ting nearby.

Orbiting closely around Jupiter are four natural satellites, each about the size of Earth's moon. These satellites-called Io, Europa, Ganymede, and Callisto-are known as the Galilean satellites, because they were first sighted and described by Galileo in 1610. The tidal forces on these small bodies...o...b..ting close to Jupiter are much greater than the tidal forces on Earth or the moon, because of the proximity and size of Jupiter, the largest planet in the solar system-the eight-hundred-pound gorilla of the neighborhood. These tidal forces distort and heat the interiors of the Galilean satellites, enough to melt rock on the closest, and to melt ice on the next.

The flexing and heating of these satellites by Jupiter can be envisioned in a simple experiment. Take a metal coat hanger, the kind that is shaped like a triangle with two sharply angled corners. Grasp the hanger by the long sides leading to one of the corners, and move the pieces apart and back together rapidly, thus flexing the corner. With only a few rapid flexes, the corner will get very hot to the touch. The energy that was expended in flexing the corner was converted to heat where the metal was being distorted. The tidal forces exerted by Jupiter on its nearby small satellites have the same effect on the satellite interior-the tidal energy is converted to heat.

Io is the closest to Jupiter, orbiting at a distance where the tidal heating is so intense that it melts rock. Volcanoes erupt at many places over its surface almost continuously. Europa is the next Galilean satellite of Jupiter, not as close to the giant planet as Io, but still close enough to show the effects of tidal flexing and heating. Europa displays much evidence of crustal resurfacing, particularly in the almost complete absence of craters on its smooth, icy surface. The water is not supplied externally, as with a Zamboni on an ice rink; it comes from a zone of melting ice far below the surface, and reaches the surface through major fissures and fractures in the icy crust. The outermost of the Galilean satellites is Callisto, beyond the reach of intense heating by Jupiter. Its icy surface displays the dense impact cratering of the early days of the solar system, with no evidence of a subsequent resurfacing that would indicate crustal melting and the presence of water.

Europa is a target in the search for extraterrestrial life, because of the likely existence of water in its interior.22 But could life develop deep within Europa, without benefit of the life-giving energy of the Sun? If Earth can serve as an example, the answer is certainly yes. At the bottom of Earth's oceans, along the seams of the tectonic plates, bizarre but vibrant biologic communities have evolved in total darkness, energized completely by hot springs emanating from the oceanic crust. And on land, within caves near the surface, evolution has produced organisms without eyes, endowing them with other sensory organs that let them navigate their dark environment. NASA has already conceptualized a mission to Europa that envisions a robotic penetrator that could reach the zone of liquid water-but that might not even be necessary. If the resurfacing of Europa has used interior water, it may be that evidence of deeper life is sitting at the surface, frozen in the newer ice that has given Europa its smooth surface. But could life develop deep within Europa, without benefit of the life-giving energy of the Sun? If Earth can serve as an example, the answer is certainly yes. At the bottom of Earth's oceans, along the seams of the tectonic plates, bizarre but vibrant biologic communities have evolved in total darkness, energized completely by hot springs emanating from the oceanic crust. And on land, within caves near the surface, evolution has produced organisms without eyes, endowing them with other sensory organs that let them navigate their dark environment. NASA has already conceptualized a mission to Europa that envisions a robotic penetrator that could reach the zone of liquid water-but that might not even be necessary. If the resurfacing of Europa has used interior water, it may be that evidence of deeper life is sitting at the surface, frozen in the newer ice that has given Europa its smooth surface.

Saturn, the ringed planet beyond Jupiter, also has a host of satellites that display icy surfaces and are susceptible to tidal flexing and heating. One of these satellites, Enceladus, has recently revealed extraordinary features that hint at the possibility of primitive life getting a start in its interior. The unmanned Ca.s.sini Ca.s.sini s.p.a.cecraft, operating in the vicinity of Saturn since 2005, has photographed large cracks in the ice near Enceladus's south pole, and instruments aboard s.p.a.cecraft, operating in the vicinity of Saturn since 2005, has photographed large cracks in the ice near Enceladus's south pole, and instruments aboard Ca.s.sini Ca.s.sini have detected evidence of active H have detected evidence of active H2O venting from the cracks, in geyser-like features that also yielded carbon dioxide, nitrogen, and methane.23 Even closer flybys of Enceladus, one at only fifteen miles above the surface, detected traces of other hydrocarbons such as acetylene, ethane, propane, benzene, and even formaldehyde. Clearly, the interior of Enceladus is an environment where organic molecules can form, and therefore it, too, may be a home to microbial life. Our solar system is proving to be far more diverse than ever imagined. Even closer flybys of Enceladus, one at only fifteen miles above the surface, detected traces of other hydrocarbons such as acetylene, ethane, propane, benzene, and even formaldehyde. Clearly, the interior of Enceladus is an environment where organic molecules can form, and therefore it, too, may be a home to microbial life. Our solar system is proving to be far more diverse than ever imagined.

WATER AND LIFE BENEATH EARTH'S ICE One need not travel to the far reaches of the solar system to seek life deep beneath ice. There are opportunities right here on Earth. The Antarctic ice blanket is on average about a mile and a half thick-at its thickest it is more than two and a half miles, about twelve Empire State Buildings stacked atop one another. And even though the surface temperature averages about -50 to -60 Fahrenheit over the year, at the base of this ice pile the temperature is warm enough to melt the ice. The heat comes from deeper within Earth, and although it is only a trickle of heat compared to what the Sun supplies, over time it has been enough to melt the base of the ice. Where does that melt.w.a.ter go? Effectively, nowhere-it fills in low-lying topography in the rock surface to form what are called subglacial lakes.

Two and a half miles beneath the Russian Vostok scientific station sits the largest subglacial lake in Antarctica-Lake Vostok, about the size of Lake Ontario, and on average about a thousand feet deep. The ice beneath Vostok Station has been drilled and cored to a depth just a hundred or so feet above the lake surface, and at that depth the ice is about 450,000 years old.24 That means that if there is any life in the lake, it has been isolated from life elsewhere on Earth for nearly a half million years. Just as Australia, because of its long isolation as an island, has many animals unique to the territory-the kangaroo, the koala bear, the platypus-so might Lake Vostok show some microbial evolutionary products that reflect its long isolation. That means that if there is any life in the lake, it has been isolated from life elsewhere on Earth for nearly a half million years. Just as Australia, because of its long isolation as an island, has many animals unique to the territory-the kangaroo, the koala bear, the platypus-so might Lake Vostok show some microbial evolutionary products that reflect its long isolation.

This presents a very interesting experiment-drill through the last remaining ice to reach the lake, draw samples of the water to examine for life, and compare what is found to life forms in a.n.a.logous settings elsewhere. But the challenge of this experiment is to ensure that no present-day life forms from the surface are introduced into the lake by the drilling process. Very careful thought has been given to how to achieve a clean entry, but full agreement is not yet at hand.

H2O ON THE MOVE.

H2O on Earth is continually moving from one reservoir to another. Water evaporates from the oceans, and some of it falls as rain or snow on the continents. Some of the precipitation infiltrates the ground and moves slowly through the subsurface reservoir. Some flows overland in rivers, returning to the sea only a month or two after precipitation. Most snowfall lasts only a season on land before rejoining the ocean reservoir. But the snow that falls on the Antarctic ice cap, and is later compressed into a layer of ice, may remain on the great white continent for hundreds of thousands of years before it creeps back to the sea in one or another of the many glaciers that drain ice from Antarctica.

Over short intervals of time, there is an equilibrium between the relative sizes of the reservoirs, but over long periods of time, large transfers between the two biggest reservoirs, the oceans and ice, occur. When water leaves the oceans to become a temporary icy resident on the continents, the sea level falls, the continental shelves are exposed, and the continents experience an ice age. The spread of ice over the land surface overrides the gra.s.ses, plants, and trees, and eliminates the vegetable component of diet for a wide variety of omnivores and herbivores. When the climate ameliorates and ice sheets melt, the water returns to the oceans, the sea level rises, and the newly exposed land surface is reoccupied biologically. These temporary loans of ocean water to the continents in the form of ice have taken place some twenty times during the past three million years of Earth's history alone, and several other times in the more distant past.

The fact that ice ages come and go tells us that ice on Earth is always on the cusp of existence-a push one way, and ice grows and spreads; a push the other way, and ice retreats and disappears. There is also a temporal component to the cusp with present-day relevance-on one side of the cusp are the ice ages of the recent past; and on the other side, the ascendancy of Earth's human population as a major player in the global climate system, a force that is pushing ice toward disappearance. Today we are perilously poised on the peak of that cusp.

CHAPTER 3.

WHEN ICE RULED THE WORLD.

Three feet of ice does not result from one day of cold weather.

-CHINESE PROVERB

Some 120,000 years ago, in what would one day be known as Finland, caribou grazed in the waning days of an unusually cool summer. The previous winter had delivered heavy snowfall, and as the brisk winds and shorter days of fall set in, there were pockets of last winter's snow remaining in sheltered crevices and shadowed valleys. This residual snow gave a head start to the next annual white blanketing of the land, and reflected some of the Sun's rays back to s.p.a.ce even earlier than usual, thus discouraging those occasional warm fall days before the onset of winter. And that next winter lasted a little longer as well, and springtime melting got a late start; at the end of the foreshortened summer there was even more residual snow to turn away the radiant energy of the autumnal Sun. It did not take many such downward-spiraling years to yield summertime snow cover over the entire region, forcing the caribou and the woolly mammoth to find new grazing farther south. The growth of an ice sheet had begun, one that would, over the next hundred thousand years, cover much of the land of Europe and North America with a blanket of ice two miles thick, and freeze the surface of high-lat.i.tude ocean water.

At its maximum extent the ice covered Canada, Greenland, Iceland, and Scandinavia completely. Most of the British Isles, Germany, Poland, and Russia, all the way to the Ural Mountains and beyond into the West Siberian Plain, were beneath the ice sheet. The ice extended into what is now the United States as far south as the modern Missouri and Ohio rivers and east over New England. High mountains beyond the margin of the ice sheets-the Rockies and Sierra Nevada in North America, the Alps and Pyrenees in Europe, and the high ranges of Asia-also developed glaciers.

Maximum extent of ice over North America during the most recent ice age, from about 120,000 to 20,000 years ago

In the Southern Hemisphere, Antarctica was completely blanketed, and the high peaks of the Patagonian Andes, in South America, and on the South Island of New Zealand were strongly shaped by ice that flowed all the way to the sea. Even in Africa, astride the equator, Mounts Kilimanjaro and Kenya and the peaks of Uganda's Ruwenzori Range hosted extensive ice.

At sea, ice was widespread as well. More or less permanent sea ice covered the entire Arctic Ocean and reached all the way to Iceland, in the Atlantic. In the Pacific, ice extended south into the Bering Sea, between Alaska and the Russian Far East. The Southern Ocean surrounding Antarctica also had a year-round icy surface, reaching northward to the sixtieth parallel of south lat.i.tude. Seasonal sea ice in both hemispheres extended the range of ice even farther, but it is unlikely that the Drake Pa.s.sage, between the Antarctic Peninsula and South America, ever was completely closed, because the strong winds and currents blowing through there did not let ice take hold.

Thick piles of ice, widely spread over Earth's surface just twenty thousand years ago-that is quite a concept. In fact, it was not until the late nineteenth century that scientists embraced the idea of widespread ice in the recent geological past. And geologists have since learned that this was only the most recent great ice excursion to spread over the Earth-one of a score or more that rhythmically advanced and retreated during the last three million years.

WHAT IS IT that geologists see that leads them to envision such a different world, a time in the history of Earth when there was much more ice on the planet than there is today? The evidence comes from both the land and sea, underfoot and underwater. On the land are multiple signatures in the landscape of an earlier presence of ice, and in the sea is evidence of the complementary signature-a reduction in water. The latter tells where the H2O came from; the former tells where it went.

CLUES FROM THE LAND.

When agriculture expanded from its beginnings in the Fertile Crescent of Mesopotamia into the plains of central and northern Europe, settlers found a terrain strewn with rocks and boulders, of all sizes, shapes, and colors. It was as if a giant spice bottle had been shaken overhead and had laid down a motley layer of pepper and paprika, sage and saffron, cloves and cinnamon over the ground. These silicate mega-spices were embedded in fertile silt and clay, and were so abundant and presented such formidable obstacles to plowing and planting that farmers were forced to clear boulder fields in order to gain enough area for crops. When Europeans came to the New World, they discovered that much of the northern United States and Canada was similarly strewn with a blanket of debris. Early farmers worked long days to move the boulders to the periphery of the fields or to stack them along the property line between neighbors, thus providing the setting for Robert Frost's poem "Mending Wall," with the memorable phrase "Good fences make good neighbors."

These boulders, heterogeneous in size, shape, and composition, are very different from the sediment in other geological layers. The mechanism for transporting sediment most familiar to early geologists is running water, which tends to segregate materials of different sizes-moving small-size particles along while leaving larger and heavier pebbles and boulders behind. Moreover, the tumbling of pieces of rock in a stream-bed tends to break off the sharp corners, eventually leaving the stones very well rounded. Similar rounding occurs on ocean beaches, where the incessant pounding of the surf produces well-rounded grains of sand. No, this blanket of mixed-up rock, of all sizes and shapes, was not laid down by water. It was simply dropped by whatever transported it so widely-and even "widely" is an understatement. This blanket of rock rubble can be found draped over some six million square miles on three continents of the Northern Hemisphere.

Ice was eventually recognized as the distributor of the debris. Unlike moving water, which sorts, rounds, and winnows the rock fragments it encounters, ice does none of these things. The key to understanding how ice picks up and delivers rocks to new locations was found by observing mountain glaciers, such as those seen in the European Alps and the Rocky Mountains of North America. But first, a few explanatory words about the mechanics of mountain glaciers: As snow acc.u.mulates in high areas year after year, the deeper layers get compressed into ice by the newer snow above. As the ice thickens, it slowly spills out of its catchment basin and begins to creep downhill, in a river of ice that flows a few tens of feet each year, truly at a "glacial pace." The ice descends to lower elevations, where it meets warmer air, and at some point the temperature reaches the melting point of ice. Beyond that point the glacier is steadily transformed into an ever-growing stream of melt.w.a.ter.

As glacial ice descends from the heights, it erodes whatever bedrock it encounters, plucking and sc.r.a.ping rock from the walls and floor of the valley through which it flows. The debris is rafted along with the flowing ice to lower elevations. The load is transported to the terminus of the glacier, that is, the place where ice melts as fast as it is delivered from above. At the terminus, the rock is deposited as an arcuate mound called a terminal moraine. The ice front, the snout of the glacier, may seem to be stationary, but in fact newer ice is always moving to the front, to meet the ultimate fate of melting. On its flow to the front, the glacial ice continues to carry rock debris for delivery to the moraine. The process is somewhat like placing suitcases on a descending escalator-they are transported to the bottom, where they are unceremoniously dumped, and simply pile up. As the glacier melts, the rocks are abandoned as a jumble of rough-around-the-edges newcomers from the heights, now relocated to a lower landscape. If the climate warms, the ice front will melt back farther up the valley, but the terminal moraine will remain where it was deposited, offering testimony to the greater reach of the glacier in earlier, colder times.

Large glacier spilling off the south polar plateau through the Transantarctic Mountains, in Antarctica LIKE A HOT KNIFE THROUGH b.u.t.tER.

Ice is a powerful shaper of the landscape, and not only by conveying rock from one place to another. The great boulder fields strewn widely over large areas give testimony to the vast extent of the ice sheets, and today's mountain glaciers demonstrate the ability of ice to transport rock from myriad source areas. As impressive as those characteristics of ice are, the description of ice streams and ice sheets is incomplete without drawing attention to the t.i.tanic erosive power of moving ice. It is the power to excavate, bulldoze, break, crush, and pulverize rock as it moves over the terrain, the power to sculpt mountains and carve out valleys. There is almost nothing in the terrain that can withstand the prolonged pa.s.sage of ice or forestall its reshaping of the landscape.

As ice succ.u.mbs to the pull of gravity and is drawn downward, it a.s.saults the mountaintops where it has acc.u.mulated. The slow but persistent flow of ice in many directions away from the summit pulls rock from all sides, sharpening the top into a pointed and angular shape, recognized in the descriptive names humans have given them: the Matterhorn, the Beartooth, the Sawtooth. Once rock has been trapped beneath moving ice, it acts like coa.r.s.e sandpaper on the bedrock below it, gouging and grooving the bedrock, leaving behind long striations that indicate the direction in which the ice moved. These scratches can be seen on glaciated terrains everywhere, from Glacier National Park in Montana, to the Upper Peninsula of Michigan, to Central Park in New York City, a clue that ice pa.s.sed over these places.

But running water and even wind also modify the landscape, so geologists have had to observe carefully to identify characteristics unique to each type of landscape. The deep main valley of Yosemite National Park in California is U-shaped in cross-section, a characteristic of erosion by ice, in contrast with V-shaped valleys, which have been cut into mountainous terrain by streams of water, such as in the headwaters of the Ganges and Indus rivers high on the Tibetan Plateau.

Erosion by ice and water differ in other respects. Water is exceptionally agile, and can find and follow low pathways in virtually any topography. It can zigzag through the terrain, giving rise to valley systems that are anything but straight. Ice, on the other hand, is like a big, stiff, lumbering giant, making its way downhill with much less meandering. Rather than go around obstacles in the terrain, as water does, ice just plows through them, straightening, smoothing, and widening the terrain like a bulldozer blazing a new highway. The resulting valleys carved by ice have steep sides and smooth, round bottoms-the U shape mentioned earlier. They are deep, and offer a long and un.o.bstructed view. The Finger Lakes of upstate New York, so named because of their long and narrow configurations and parallel development, are examples of valleys scoured and straightened by ice.

When valley glaciers erode deep U-shaped valleys all the way to the sea, the trough will become a fjord when seawater enters the valley after the ice has melted. Given the ample depth of fjords, oceangoing vessels can cruise tens and sometimes hundreds of miles "inland" along these valleys. These elegant topographic features are special remnants from the recent ice ages. They do not occur just anywhere; they form under only a certain set of conditions. A look at a world map, one that shows enough topographic detail, including fjords, reveals that they are well developed only along the west coasts of panhandle Alaska, Canada, Greenland, and Norway in the Northern Hemisphere, and along the west coasts of southern Chile and the south island of New Zealand. What do these places have in common?

U-shaped valleys carved by mountain glaciers, Torres del Paine National Park, Chile

First, they all are located where the prevailing winds generally pa.s.s over long stretches of ocean before reaching land, picking up substantial moisture along the way. As the wind encounters the higher and cooler elevations of the land, it gives up its cargo of water vapor as precipitation. Second, these "fjorded" coastlines are all at lat.i.tudes sufficiently far away from the equator (about 55 north or 45 south) so that during the recent ice ages, when Earth was colder by some ten to fifteen Fahrenheit degrees, the precipitation would fall as snow, and acc.u.mulate to form glacial ice. Third, at those lat.i.tudes or even farther toward the poles, it is cold enough even at sea level for glaciers to reach the sea instead of melting away while still flowing overland. Finally, because sea level is lower during an ice age, the glaciers flowed beyond today's high sea level coastline, and continued to carve deep valleys across the temporarily exposed continental shelves. When sea level later rose at the end of the ice age, as melt.w.a.ter returned to the sea, the deep glacial valleys became inundated, giving us these long scenic waterways. Fjords are truly a gift of the ice ages, and a reminder of the tremendous erosive power of flowing ice.

ICE TO WATER.

The piles of debris carried and ultimately dropped by the ice sheets created an irregular and rumpled surface. When the ice melted, the earth-moving ceased. The terrain was simply left as a work in progress, much like a road construction site on a weekend or holiday. But the glacial "workforce" left the site, never to the return, at least not for tens or hundreds of thousands of years. The low places filled with water. Many thousands of small lakes dot the debris-blanketed surface of Minnesota, Wisconsin, Michigan, and the Canadian provinces to the north. In Europe there is an equivalent "land of lakes" in Finland, parts of Sweden, and the far northwest of Russia. Flying over the lake-dotted landscape late in the day, one sees the low Sun reflecting off the surface of these lakes, little jewels glistening as far as the eye can see.

The effects of glaciation go well beyond the areas overridden by ice. If the geographic extent of ice cover on land is defined by the deposits of glacial debris and the lakes nestled within, the extended reach of glaciation can be seen in the water produced as the ice melted. The volume of water tied up in the continental ice sheets is immense-major glaciations withdraw enough water from the oceans to lower sea level by six hundred feet the world over. As the ice melted, streams of water transported and deposited sediment to form the many local sand and gravel quarries now exploited in construction, road building, and manufacturing. Quite in contrast to the unsorted character of the glacial deposits, the sediment in water-laid deposits is much more uniform in size, shape, and composition.

Melt.w.a.ter also acc.u.mulated to form large lakes in areas peripheral to the ice. The North American Great Lakes and the Great Salt Lake of Utah are the diminished remnants of the last big melt-off, elements of today's landscape that give testimony to the earlier widespread ice. Great Salt Lake was once much larger, deeper, and fresher. The broad depression in the Great Basin of Utah received melt.w.a.ter from the ice cover of the Rocky Mountains, to form what geologists call ancient Lake Bonneville. At its highest stand about seventeen thousand years ago, the surface of Lake Bonneville was about one thousand feet above the level of the present-day Great Salt Lake, and its water spread over much of western Utah, an area some twelve to thirteen times greater than today's Great Salt Lake.25 Why is Great Salt Lake saline, while the Great Lakes are fresh? The difference arises because Lake Bonneville occupied a closed depression from which there was no outlet-the only loss of water possible was through evaporation. Over most of the time since the highest level of Lake Bonneville, evaporation has exceeded precipitation and stream flow into the lake. The result has been a progressive decrease in lake level and a growing concentration of dissolved salt in the diminishing water volume. Large areas of the former lake bed of Lake Bonneville are now exposed, and reveal a "pavement" of salt that makes up today's Bonneville Salt Flats, where racing cars set world speed records in excess of six hundred miles per hour. On the sides of the mountain ranges that rise out of the Great Basin one can easily see older sh.o.r.elines of Lake Bonneville, like giant bathtub rings, recording pauses in the fall of the lake level.

The five Great Lakes of North America-Lakes Superior, Michigan, Huron, Erie, and Ontario-border eight states and Canada. Together they contain 85 percent of the surface water of North America, and 20 percent of the world's freshwater. They are all connected by rivers-the St. Marys River carries water from Lake Superior to Lakes Huron and Michigan; the Detroit River links Lake Huron to Lake Erie; and the Niagara River connects Lake Erie to Lake Ontario, with one giant leap over Niagara Falls. Lake Ontario empties into the St. Lawrence River, the final long waterway that delivers the Great Lakes water to the Atlantic Ocean.

The depressions that these lakes occupy were shaped by the North American ice sheet, which sc.r.a.ped away at weak bedrock to create low regions that would later host the lakes. The Lake Michigan and Lake Huron basins, which nearly surround the lower peninsula of Michigan, sit in relatively weak rocks, Paleozoic shale that was no match for a mile-thick ice sheet grinding its way south. By contrast, the Niagara dolomite, a very tough rock formation composed of magnesium carbonate, stood up better to the erosive power of the ice. The Niagara formation forms a sweeping arc around the Great Lakes-it is the backbone of Wisconsin's Door Peninsula, behind which lies Green Bay, and of Manitoulin Island and the Bruce Peninsula of Ontario, which separate Georgian Bay from Lake Huron. As the name Niagara suggests, this rock layer also forms the durable platform on which the Niagara River flows out of Lake Erie-its lip creates the ledge where the river plunges 190 feet at Niagara Falls, on its way to Lake Ontario and the sea.

Although the lake basins resulted from glacial erosion, the water now filling them is not glacial melt.w.a.ter; the water in the lakes has been replaced many times since the end of the last ice age. Rain and snow falling each year in the upper Great Lakes catchment basin replenishes the water that flows over Niagara Falls, through Lake Ontario, and outward to join the Atlantic Ocean. The annual loss and replenishment is about 1 percent of the volume of water in the lakes, so it takes about a hundred years to totally exchange the waters in the Great Lakes-pollution introduced into the lakes takes a century of flushing to purge.

There are similarities and differences between the Great Lakes and Lake Bonneville of Utah. As with Lake Bonneville, which had bigger volumes and higher lake levels in the past, so have the Great Lakes had higher stands. But unlike Lake Bonneville, the Great Lakes basins were covered by the North American ice sheet at the time of its maximum extent, some twenty thousand years ago. As the ice front retreated northward from the region, for a time the ice actually formed a barrier that prevented melt.w.a.ter from exiting through the St. Lawrence River. The melt.w.a.ter, blocked by the ice from flowing northeast, instead found its way into the Mississippi River and the Gulf of Mexico. Effectively the ice was a dam along the north margin of the Great Lakes, and for a while it led to higher lake levels than exist today.

Satellite photos reveal several former sh.o.r.elines along the margins of Lakes Michigan, Huron, and Erie. Each of these lakes had spread over a greater area than they occupy today-and the relict lake beds, flat and blanketed with fine sediment that settled out from the ancient lakes, are put to good use. These large, level, and featureless plains make for easy use in agriculture, and are attractive sites for airports. Detroit's Metropolitan Airport is located on the vast flat exposed lake bed of an earlier and bigger Lake Erie, twenty miles away from the western sh.o.r.e of today's Lake Erie.

Another large melt.w.a.ter lake once covered much of the Canadian province of Manitoba, but extended also into Ontario, Saskatchewan, North Dakota, and Minnesota. This lake, named Lake Aga.s.siz after the nineteenth-century Swiss geologist Louis Aga.s.siz, was big-some seven hundred miles north to south and two hundred miles across. At its maximum extent some thirteen thousand years ago, it was bigger than either California or Montana, almost two thirds the size of Texas. Its water covered an area 80 percent greater than all of the modern Great Lakes combined. This was the great lake of its time.

But Lake Aga.s.siz returned most of its glacial melt.w.a.ter to the sea. Abandoned sh.o.r.elines on hillsides and a giant exposed lake bed across the plains of southern Canada and adjacent north-central United States provide evidence of the lake's former extent. Today its much-diminished remnant is Lake Winnipeg-between Lake Erie and Lake Ontario in size-big by today's standards, but a shadow of its former self.

As the ice sheet melted, big rivers developed to drain the immense volume of melt.w.a.ter and return it to the sea. The ultimate margin of North America's last great ice sheet can be identified by the major rivers established on the periphery. Today we know them as the Missouri and Ohio rivers, along the southern boundary of the ice sheet, and the Mackenzie River to the west. The Mackenzie flowed northward to the Arctic Ocean, but the Missouri and Ohio joined the Mississippi to drain much of the early melt.w.a.ter to the Gulf of Mexico. As the ice sheet melted back, other outlets to the sea opened. When the St. Lawrence River began to drain the Great Lakes Basin, the melt.w.a.ter no longer coursed south to the Gulf of Mexico-it was delivered to the North Atlantic Ocean, with profound, albeit temporary, consequences to the Atlantic circulation and climate.

LOUISIANA OF THE NORTH.

In the early nineteenth century the Ohio and Missouri rivers were the pathway to the interior of the continent for the explorers Meriwether Lewis and William Clark. The Lewis and Clark Expedition had been authorized by President Thomas Jefferson shortly after he concluded the big territorial acquisition known as the Louisiana Purchase in 1803. Jefferson had purchased from Napoleonic France a huge swath of land in the interior of North America west of the Mississippi River, land that today includes all or part of fifteen states, representing almost a quarter of the area of the United States. The Lewis and Clark journey of 1804-6 had a simple purpose: to see what it was that we had just acquired from France. Wrote Thomas Jefferson: The object of your mission is to explore the Missouri river, and such princ.i.p.al stream of it as by its course and communication with the waters of the Pacific Ocean whether the Columbia, Oregon, Colorado or any other river may offer the most direct and practicable water communication across this continent for the purposes of commerce.

The scholarly Jefferson was interested in much more than simply commerce, and he instructed Lewis and Clark to make friendly contact with native peoples encountered along the way, and to observe the flora, fauna, and mineral resources of the different regions. The expedition followed the Missouri River to its headwaters along the present-day Montana-Idaho border, and then crossed the Continental Divide, the great watershed that separates westward drainage to the Pacific from waters headed eastward. There they joined the Snake River and followed it to the Columbia River, their pathway to the Pacific Ocean.

The Columbia River basin owes much of its landscape to the outflow of glacial melt.w.a.ter. The Columbia crosses both Washington and Oregon, flowing over and through a rock formation known as the Columbia River Basalt. These are volcanic rocks-lava flows that spilled over much of the Pacific Northwest during the mid-Miocene period, around fifteen million years ago. On top of the basalt sit a few hundred feet of wind-blown dust. This was the terrain that at the end of the last ice age was exposed to one of the most catastrophic floods in human history.

The scenario for this ma.s.sive flood, which came to be known as the Spokane, or Missoula, Flood, began with the formation of a temporary ice dam in the narrows of the Clark Fork River, in western Montana, around fifteen thousand years ago. This dam caused a large lake, Lake Missoula, to form behind it, in the same way that the ice front in the mid-continent led to temporarily higher levels of the Great Lakes. When the ice dam failed suddenly, the impounded lake water burst into the drainage ways that fed the Columbia River, and poured downriver with a velocity nearing fifty miles per hour. Some estimates of the volume of water that cascaded over the terrain suggest that it exceeded the total flow of all the rivers of the world, at least for a few days.

This torrent shaped the landscape in extraordinary ways. It cut deep canyons into the basalt caprock, leaving occasional large escarpments that drop from one basalt flow to another. The lips of these escarpments display a scalloped shape, much like the Horseshoe Falls at Niagara. Indeed, giant waterfalls did cascade over these cliffs, carrying immense volumes of water that scoured deep plunge pools at the cliffs' base. Large tabletop "islands" capped with basalt, similar to the mesas in the desert landscape of Utah and Arizona, stand isolated by channels scoured around them. This churning, turbulent sheet of water eroded huge boulders of basalt, much bigger than a house, tumbling them miles downstream and eventually dropping them in a plain marked by gigantic ripple marks, so large that the rhythmic rise and fall of the topography can be fully appreciated only from the air.

The surge of water continued toward the Pacific coast. It cut a canyon across the narrow continental shelf, at that time still exposed because of the lower sea level of the ice age. When the flood entered the sea, it dropped its sedimentary load on the ocean floor. Some of the debris had come all the way from Montana. And if one such flood was not sufficiently cataclysmic, geologists suggest that this scenario was repeated many times over the next two thousand years. Eventually, the ice retreated far enough north so that ice dams no longer formed in Montana, and outbursts no longer washed over the Columbia Plateau.