Before we actually visited our neighboring worlds, prevailing viewpoints held that their climates were not so different from that of Earth. Many scientists argued that Venus was a water world, that the thick clouds enshrouded a lush, tropical, and possibly verdant planet. Mars was believed to be somewhat colder than Earth, with a thinner atmosphere, but pre–space age descriptions of the Red Planet often hinted or stated that the seasonally shifting surface features seen through telescopes were signs of vegetation.
Spacecraft data woke us rudely from these sweet dreams of almost-home. Early results from other planets carried sobering hints about the extremes that climate change can take. The very first thing we learned from any spacecraft at another planet, the first result from Mariner 2 at Venus, was that Earth’s sister is absurdly hot. Venus was radiating a frightening amount of heat from its surface. It took years, and several more missions, before we realized the true extremity of the Venusian climate. The Soviet Union had a remarkable string of fruitful Venus missions in the 1970s, including the only successful landings on that planet (still, to date), which left no doubt that Venus is an oven world, with a surface where no liquid water or living matter could exist. Heat is the enemy of complex organic molecules, and the stuff we’re made of doesn’t stand a chance anywhere within twenty-five miles of that searing surface. Yet early spacecraft results also hinted that Venus is a planet with a past, one that was likely cooler and wetter. We began to see Venus as a place where planetary climate had started off like Earth’s but had gone completely off the rails, into the hot zone.
In July 1965, Mariner 4 became the first Earth craft to fly by Mars. The pictures showed a lunar-like, barren landscape of craters and—not much else, just craters. Again, naïve expectations of a place where our kind of life could thrive were found wanting. The exploration continued, and after several flyby missions that only snapped pictures of small areas, Mariner 9 was launched to Mars in 1971 with a promise to become the first spacecraft to orbit another planet, allowing us systematically to photograph the entire surface.
Mariner 9 made it there and entered orbit successfully—a bold new feat. Its global view had long been eagerly anticipated. So when the camera was finally turned on and the first images beamed down to Earth, scientists were amazed to see… absolutely nothing: just a bland, featureless, fuzzy disk.
There was nothing wrong with the camera. Rather, Mars was in the throes of a global dust storm. Every Martian year, when it is Southern Hemisphere summer, large storms erupt. Afternoon winds stir up thick clouds of dust, much as they do in Arizona. Dust absorbs sunlight, which further heats the summer air, driving faster winds, which whip up more dust. A Martian dust storm can quickly grow into a vast regional tempest, visible from orbit or even from Earth. Once in a while, every few years, a storm grows and grows until it becomes a planet-shrouding monster, engulfing and obscuring all of Mars.
We didn’t know about this when Mariner 9 showed up at the peak of one of these events, much to the astonishment of the mission scientists. They watched in wonderment and relief as, over a period of several weeks, the dust settled, revealing first the tops of a few giant volcanoes and, gradually, the rest of the Martian surface. This episode demonstrated dramatically that weather and climate on other worlds can be complex and changeable. As we’ll see, the study of dust storms on Mars soon proved invaluable for understanding some big mysteries of Earth history as well as some troubling changes that human technology might yet cause on our planet.
Mariner 9’s orbital mapping mission revealed a much more storied and mysterious world than earlier missions had suggested. Seen in its global fullness, Mars is varied, wind-whipped, ice-capped, and carved by ancient dried-up river channels, giant extinct volcanoes, and vast antediluvian eroded canyons. These all spoke of a long and dramatic evolutionary history, and hinted at ancient oases, wet and fertile, lost across the red sands of time, a promising but vanished Martian past that, half a century later, we are still seeking.
We began to understand that Mars, like Venus, had suffered through a climate catastrophe that long ago turned a once-more-Earth-like planet into an entirely different and more forbidding kind of place.
In comparison, Earth emerges as the true oddball of local space. Forested continents, rippling streams, and a flagrantly oxygenated atmosphere are, we have learned, far beyond the norm in this solar system. What happened here?
A New Science
Suddenly the planets were no longer just wandering lights in the sky, but diverse and mysterious locales. They were not just pixels but places, and the information was pouring in. This data explosion created a problem. Who was going to interpret it and figure out what it all means? Who was going to do the science? Nobody had ever studied other planets up close. It was not something astronomers did. They were good at using telescopes and studying stars and galaxies. Interpreting ancient Martian rivers or Venusian clouds would require—what? Geology? Meteorology? Chemistry? Geologists didn’t think about Venus and Mars. Nobody was trained to work on these questions.
Making matters worse, science in the twentieth century had exploded and splintered into a heap of separate fields, each with its own specialized knowledge, techniques, culture, and language. Yet here was a task that required minds ready to bridge these conceptual fences. This challenge (and the new funding available from NASA) attracted a small group of intellectually adventurous, broadly educated young scientists. Gradually the new hybrid field developed a unified identity and took on a lasting name: planetary science.
It was still a young field in the early 1980s when I showed up in Tucson, one of the early hotbeds, to start work on my doctorate. Arizona’s clear, dry, star-studded skies had drawn Dutch astronomer Gerard Kuiper, the founder of modern planetary astronomy and one of Carl Sagan’s mentors, to establish his Lunar and Planetary Laboratory there, where in 1973 the University of Arizona started the first Department of Planetary Sciences in the United States.
None of my professors there had degrees in planetary science. They were the first generation: chemists, physicists, meteorologists, and geologists; veterans of Apollo and the audacious first missions to the planets. They were still figuring out what planetary science was. Now, a generation later, most professionals in the field have planetary science degrees, so it seems to have become a real thing.
Arizona was a magical place to study planets. In addition to those profoundly deep skies, the geology is endlessly rich. A highlight of grad school was the weekend geological field trips spent camping out in the volcanic fields, steeply faulted mountains, and vast erosional canyons; learning to connect the outcrops, debris flows, and cliffs along which we hiked with the bigger, hidden picture of underground structures, geological maps, rock types, and planetary histories.* Having lived my whole life in flat, forested New England, for me these were like visits to a raw, exotic planet, or a whole series of them. I’ve spent most of my subsequent career poring over spacecraft data on computer screens, writing code, and running models of distant worlds, but I always feel grounded by these formative experiences clambering over Arizona rocks and dirt.
Comparative planetary geology1 has allowed us to recognize and make sense of myriad Earth forms found on other planets: volcanoes, faults, landslides, frost heaves, folded mountain belts, and braided streams. These comparisons also sometimes yield fresh insights about Earth.
In the last half century, we’ve had several major conceptual breakthroughs in understanding our home planet, several big “aha” moments for science where the picture abruptly comes into focus. It’s not a coincidence that these were the same decades when we took our first tentative forays out into the unknown darkness beyond the terrestrial village. Yet note that “terrestrial” has two opposites, the other being “marine.” The paleo-space age of the 1960s was also the decade when we completed much of the initial exploration and mapping of the deep ocean floor,* revealing a previously hidden half of Earth’s surface. In visiting these terrae incognitae above and b
eyond the surface and the land, we first saw Earth whole, and could begin to see the path that planetary evolution took here as only one of many possible paths. This unleashed a burst of self-discovery. From this convergence of new knowledge and perspective emerged three new “big picture” insights into the nature of our world.
The first of these big ideas was the theory of plate tectonics, a once-fringe concept that has become the key to understanding how Earth works. Before we had this unifying vision, geologists studying disparate parts of our planet were like the proverbial blind men puzzling over an elephant. Now we see it as one beast, its seemingly separate mountain ranges, canyons, and ocean ridges, and its apparently independent patterns of earthquakes, volcanoes, uplift, and erosion, all revealed to be connected in one global system. The outer skin of our planet is broken up into about a dozen rigid pieces: the plates, the shifting shards of a broken sphere. These slowly drift around the planet, colliding, jostling, sliding against and elbowing into one another. Most geological activity can be explained by these interactions, the tectonics.
At the dawn of the space age, when Sputnik spooked America into jumping skyward, plate tectonics was still a suspect idea, considered controversial and largely rejected by mainstream geologists. Just over a decade later, by the time Apollo astronauts were first driving buggies and birdies over lunar landscapes, and sending back the first whole Earth selfies, the idea was rapidly taking root as the unifying theory of the earth sciences. Like continental drift, which any child can see explains the neat puzzle fit of Africa and the Americas, once you get it, it seems obvious. You wonder how generations of brilliant scientists could have missed or doubted it. Not only did plate tectonics explain, under one theory, the history and geographical distribution of devastating earthquakes in Turkey, California, and Japan; the conical volcanoes and steep ocean trenches facing off across the west coast of South America; and the continuing, trembling uplift of the high Himalaya, but it went much deeper, showing how all these surface activities manifest hidden forces from Earth’s insides. The tectonic plates, these sluggishly gliding rock rafts on which we ride out our hurried lives, are pulled along by currents arising far below in Earth’s mantle, a vast realm of rock that is solid but squishy like butter. These inner rock flows are the convection pattern of Earth’s interior, the original lava lamp where hot continent-size blobs rise and cool slabs of ocean floor sink into the mantle. Now we see that all Earth’s major landscapes and their accumulated changes are part of one coherent, if chaotic, system, driven by the heat emanating from Earth’s marrow.
A second big new idea involved the deep, integral role of life. The more we study the entangled history of Earth and its biosphere, the more we see how many features of our planet result from a complex relationship between life and the “nonliving” world. Our planet has been brought to a strange, anomalous state by a force that is (as far as we can tell) absent on the neighbors but that has come to dominate here: the life force. The interplanetary perspective helped us to see the deep, pervasive, planet-altering role of life. Perhaps life can even be best defined as a kind of transformation that might happen to some planets. This is the topic of the next chapter. But first…
The third big new idea about Earth, born of the space age, was the realization that our planet has not been so isolated from the rest of the solar system as earth scientists had long assumed. Ancient extraterrestrial collisions watered our world and seeded it with organic molecules. Occasional large impacts have continued to disturb and prod the evolution of Earth and life. As was the case with plate tectonics, this discovery encountered fierce resistance before being widely accepted. Once we broadened our perspective to include other worlds, and widened our temporal view to include the immense swaths of time laid bare on the many more dormant and aged surfaces we found out there, we realized that Earth, with its restless activity and eternally youthful surface, had been hiding something.
Worlds in Collision
One of our best grad school geology field trips was visiting Meteor Crater, the well-preserved, fifty-thousand-year-old, mile-wide hole in the Northern Arizona desert that has played a key role in allowing us to connect the geology of Earth with that of other planets. It’s a surreal and sensational place to visit, but what made it especially memorable for us was scrambling through the crater with Eugene Shoemaker, the brilliant, amiable man who had unlocked its mysteries. As Kuiper was to planetary astronomy, Shoemaker was to planetary geology. He foresaw that the space age would revolutionize earth science, and he led the charge. He knew that crater as if it were his backyard, which it sort of was. As a grad student at Princeton in the 1950s, Shoemaker did the pioneering fieldwork that proved that, yes, the giant hole in the ground had been caused by an object crashing down from space, and not by a volcanic steam explosion. Later he established the astrogeology branch of the U.S. Geological Survey in nearby Flagstaff, and he recruited (or simply attracted through his magnetic, genial intellect) many of the first generation of American planetary geologists. What a treat to visit the crater with the man himself. Gene seemed familiar with every cobble and shrub. He had the geologic map burned into his brain, and could effortlessly point out the telltale signatures of the ancient impact. He showed us the upside-down sequences of sedimentary and volcanic rock, where the explosion had blasted out layers of bedrock and folded them back over the desert like a sheet. Belying the magnitude of his accomplishments and influence, he was an unassuming, humble, gentle, and extraordinarily nice guy, always listening to students and young scientists and offering helpful, constructive responses to their ideas. One of his intellectual legacies is a wider appreciation of the continuing influence of impact explosions on Earth and other planets. His loss in a car crash while out investigating a crater near Alice Springs, Australia, in July 1997 was deeply mourned by our community.
Gene Shoemaker’s proof that Meteor Crater was the scar from a space impact provided the crucial link between Earth’s surface and the craters we see on the Moon. This helped us realize the extent to which Earth has been repeatedly hit and changed by such events. The exploration of the Moon and planets made it obvious that all worlds endure these insults and that Earth could not have escaped. So geologists began to scour orbital and aerial photos and maps for circular features on Earth. Craters, it turns out, are not that rare, but many are no longer obviously visible from space. On a planet this geologically restless, craters do not stay in their pristine bowl shape for long. Many are squished, warped, scoured, partially filled, or fully buried. After a while, planetary geologists became adept at recognizing the signs of craters that had been altered almost beyond recognition. Often satellite imagery gives hints of possible craters, but it takes field expeditions on the ground to identify the mineral and geologic markers that Shoemaker and others discovered to discriminate an impact from volcanic processes that can also make circular features.
It wasn’t until the 1980s that we started to fully recognize the important role of impacts in Earth history. The watershed was the “Alvarez hypothesis”—the proposal that a large asteroid struck Earth sixty-five million years ago, causing the “end-Cretaceous extinction.” This was a controversial new solution to the long-standing mystery of what killed off those most famous of all extinct organisms, the dinosaurs. Yet it wasn’t just the dinosaurs who got snuffed out at that moment in Earth’s history. More than 70 percent of all species suddenly went extinct. Something happened to Earth that wiped out nearly everything. The mystery was solved by physicist Luis Alvarez, with his geologist son, Walter, and a team of other scientists. All around the world there is a thin layer of clay, about a centimeter thick, separating the older Cretaceous rocks, in which dinosaur bones are plentiful, from the younger Paleogene rocks, in which dinosaur bones are absent. The Alvarezes examined samples of that clay layer from a site in Gubbio, Italy, and found them to be heavily laced with the element iridium, which is known to be a marker of extraterrestrial origin. They proposed that this layer of sediment spike
d with iridium was actually the fallout of dust thrown around the world by a huge impact explosion that caused the mass extinction. In June 1980 they published a paper in Science entitled “Extraterrestrial Cause for the Cretaceous-Tertiary Extinction.” This idea crashed into the established geological worldview like a hypersonic rock out of the blue.
Earth, it seemed, was less isolated than we thought. Maybe large impacts from space had had a repeated and significant influence on the evolution of our planet. People started referring to this as the “new catastrophism.” This phrasing implied a reversal of the dogma of “uniformitarianism,” the geological principle that holds that features on Earth result from the slow accumulation of gradual changes. The uniformitarian mind-set was itself the result of an earlier liberating, revolutionary triumph in the nineteenth century: the discovery of deep time. Geologists back then realized that the major features of Earth could be explained without biblical floods or other cataclysms, but rather as the accrued result of many smaller events and changes (storms, earthquakes, volcanoes, erosion, and subsidence) occurring over thousands of millennia. The uniformitarian mantra is “The present is the key to the past.” You don’t need abrupt or miraculous events to explain the origin of mighty mountains and valleys. You just need previously unimaginable expanses of years, not the hundreds or thousands of years of recorded history but the millions and billions of years recorded in rock layers and the nuclear clocks hidden inside minerals. Once we learned about deep time, then, we no longer needed catastrophic changes to explain the world, and catastrophism, when it was mentioned at all in our college courses, was taught as a relic of ancient, ignorant, pre-Enlightenment thinking.
Earth in Human Hands Page 3
